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Atmospheric Chemistry and Physics

Artykuły z Atmospheric Chemistry and Physics (ACP)

Model-simulated trend of surface carbon monoxide for the 2001–2010 decadeAtmospheric Chemistry and Physics, 14, 10465-10482, 2014Author(s): J. Yoon and A. PozzerWe present decadal trend estimates of surface carbon monoxide (CO) simulated
using the atmospheric chemistry general circulation model ECHAM5/MESSy (EMAC;
ECHAM5 and MESSy stand for fifth-generation European Centre Hamburg general
circulation model and Modular Earth Submodel System, respectively) based on
the emission scenarios Representative Concentration Pathways (RCP) 8.5 for
anthropogenic activity and Global Fire Emissions Database (GFED) v3.1 for
biomass burning from 2001 through 2010. The spatial distribution of the
modeled surface CO is evaluated with monthly data from the Measurements Of
Pollution In The Troposphere (MOPITT) thermal infrared product. The global
means of correlation coefficient and relative bias for the decade 2001–2010
are 0.95 and −4.29%, respectively. We also find a reasonable
correlation (R = 0.78) between the trends of EMAC surface CO and full
10-year monthly records from ground-based observation (World Data Centre for
Greenhouse Gases, WDCGG). Over western Europe, eastern USA, and northern
Australia, the significant decreases in EMAC surface CO are estimated at
−35.5 ± 5.8, −59.6 ± 9.1, and
−13.7 ± 9.5 ppbv decade−1, respectively. In contrast, the
surface CO increases by +8.9 ± 4.8 ppbv decade−1 over southern
Asia. A high correlation (R = 0.92) between the changes in EMAC-simulated
surface CO and total emission flux shows that the significant regional trends
are attributed to the changes in primary and direct emissions from both
anthropogenic activity and biomass burning. 2014/10/03 - 11:51

Volatile and intermediate volatility organic compounds in suburban Paris: variability, origin and importance for SOA formationAtmospheric Chemistry and Physics, 14, 10439-10464, 2014Author(s): W. Ait-Helal, A. Borbon, S. Sauvage, J. A. de Gouw, A. Colomb, V. Gros, F. Freutel, M. Crippa, C. Afif, U. Baltensperger, M. Beekmann, J.-F. Doussin, R. Durand-Jolibois, I. Fronval, N. Grand, T. Leonardis, M. Lopez, V. Michoud, K. Miet, S. Perrier, A. S. H. Prévôt, J. Schneider, G. Siour, P. Zapf, and N. LocogeMeasurements of gaseous and particulate organic carbon were performed during
the MEGAPOLI experiments, in July 2009 and January–February 2010, at the
SIRTA observatory in suburban Paris. Measurements comprise primary and
secondary volatile organic compounds (VOCs), of both anthropogenic and
biogenic origins, including C12–C16 n-alkanes of intermediate
volatility (IVOCs), suspected to be efficient precursors of secondary organic
aerosol (SOA). The time series of gaseous carbon are generally consistent
with times series of particulate organic carbon at regional scale, and are
clearly affected by meteorology and air mass origin. Concentration levels of
anthropogenic VOCs in urban and suburban Paris were surprisingly low
(2–963 ppt) compared to other megacities worldwide and to rural continental
sites. Urban enhancement ratios of anthropogenic VOC pairs agree well between
the urban and suburban Paris sites, showing the regional extent of
anthropogenic sources of similar composition. Contrary to other primary
anthropogenic VOCs (aromatics and alkanes), IVOCs showed lower concentrations
in winter (< 5 ppt) compared to summer (13–27 ppt), which cannot
be explained by the gas-particle partitioning theory. Higher concentrations
of most oxygenated VOCs in winter (18–5984 ppt) suggest their dominant
primary anthropogenic origin. The respective role of primary anthropogenic
gaseous compounds in regional SOA formation was investigated by estimating
the SOA mass concentration expected from the anthropogenic VOCs and IVOCs
(I / VOCs) measured at SIRTA. From an integrated approach based on
emission ratios and SOA yields, 38 % of the SOA measured at SIRTA is
explained by the measured concentrations of I / VOCs, with a 2%
contribution by C12–C16 n-alkane IVOCs. From the results of an
alternative time-resolved approach, the average IVOC contribution to SOA
formation is estimated to be 7%, which is half of the average contribution
of the traditional aromatic compounds (15%). Both approaches, which are
based on in situ observations of particular I / VOCs, emphasize
the importance of the intermediate volatility compounds in the SOA formation,
and support previous results from chamber experiments and modeling studies.
They also support the need to make systematic the IVOCs' speciated
measurement during field campaigns. 2014/10/03 - 11:51

How sensitive is the recovery of stratospheric ozone to changes in concentrations of very short-lived bromocarbons?Atmospheric Chemistry and Physics, 14, 10431-10438, 2014Author(s): X. Yang, N. L. Abraham, A. T. Archibald, P. Braesicke, J. Keeble, P. J. Telford, N. J. Warwick, and J. A. PyleNaturally produced very short-lived substances (VSLS) account for almost a
quarter of the current stratospheric inorganic bromine, Bry.
Following VSLS oxidation, bromine radicals (Br and BrO) can catalytically
destroy ozone. The extent to which possible increases in surface emissions or
transport of these VSLS bromocarbons to the stratosphere could counteract the
effect of halogen reductions under the Montreal Protocol is an important
policy question. Here, by using a chemistry–climate model, UM-UKCA, we
investigate the impact of a hypothetical doubling (an increase of 5 ppt
Bry) of VSLS bromocarbons on ozone and how the resulting ozone
changes depend on the background concentrations of chlorine and bromine. Our
model experiments indicate that for the 5 ppt increase in Bry
from VSLS, the ozone decrease in the lowermost stratosphere of the Southern
Hemisphere (SH) may reach up to 10% in the annual mean; the ozone
decrease in the Northern Hemisphere (NH) is smaller (4–6%). The largest
impact on the ozone column is found in the Antarctic spring. There is a
significantly larger ozone decrease following the doubling of the VSLS burden
under a high stratospheric chlorine background than under a low chlorine
background, indicating the importance of the inter-halogen reactions. For
example, the decline in the high-latitude, lower-stratospheric ozone
concentration as a function of Bry is higher by about
30–40% when stratospheric Cly is ~ 3 ppb (present
day), compared with Cly of ~ 0.8 ppb (a pre-industrial or
projected future situation). Bromine will play an important role in the future
ozone layer. However, even if bromine levels from natural VSLS were to
increase significantly later this century, changes in the concentration of
ozone will likely be dominated by the decrease in anthropogenic chlorine. Our
calculation suggests that for a 5 ppt increase in Bry from VSLS,
the Antarctic ozone hole recovery date could be delayed by approximately 6–8
years, depending on Cly levels. 2014/10/03 - 11:51

Different contact angle distributions for heterogeneous ice nucleation in the Community Atmospheric Model version 5Atmospheric Chemistry and Physics, 14, 10411-10430, 2014Author(s): Y. Wang, X. Liu, C. Hoose, and B. WangIn order to investigate the impact of different
treatments for the contact angle (α) in heterogeneous ice nucleating
properties of natural dust and black carbon (BC) particles, we implement the
classical-nucleation-theory-based parameterization of heterogeneous ice
nucleation (Hoose et al., 2010) in the Community Atmospheric Model version 5
(CAM5) and then improve it by replacing the original single-contact-angle
model with the probability-density-function-of-α (α-PDF)
model to better represent the ice nucleation behavior of natural dust found
in observations. We refit the classical nucleation theory (CNT) to
constrain the uncertain parameters (i.e., onset α and activation
energy in the single-α model; mean contact angle and standard
deviation in the α-PDF model) using recent observation data sets for
Saharan natural dust and BC (soot). We investigate the impact of the time dependence of droplet freezing on mixed-phase clouds and climate in
CAM5 as well as the roles of natural dust and soot in different nucleation
mechanisms. Our results show that, when compared with observations, the
potential ice nuclei (IN) calculated by the α-PDF model show better
agreement than those calculated by the single-α model at warm
temperatures (T; T > −20 °C). More ice crystals can form at low
altitudes (with warm temperatures) simulated by the α-PDF model than compared to the single-α model in CAM5. All of these can be
attributed to different ice nucleation efficiencies among aerosol particles,
with some particles having smaller contact angles (higher efficiencies) in
the α-PDF model. In the sensitivity tests with the α-PDF
model, we find that the change in mean contact angle has a larger impact on
the active fraction at a given temperature than a change in standard deviation,
even though the change in standard deviation can lead to a change in freezing behavior. Both the single-α and the α-PDF model
indicate that the immersion freezing of natural dust plays a more important
role in the heterogeneous nucleation than that of soot in mixed-phase
clouds. The new parameterizations implemented in CAM5 induce more
significant aerosol indirect effects than the default parameterization. 2014/10/03 - 11:51

Constraining CO2 emissions from open biomass burning by satellite observations of co-emitted species: a method and its application to wildfires in SiberiaAtmospheric Chemistry and Physics, 14, 10383-10410, 2014Author(s): I. B. Konovalov, E. V. Berezin, P. Ciais, G. Broquet, M. Beekmann, J. Hadji-Lazaro, C. Clerbaux, M. O. Andreae, J. W. Kaiser, and E.-D. SchulzeA method to constrain carbon dioxide (CO2) emissions from
open biomass burning by using satellite observations of co-emitted
species and a chemistry-transport model (CTM) is proposed and
applied to the case of wildfires in Siberia. CO2 emissions
are assessed by means of an emission model assuming a direct
relationship between the biomass burning rate (BBR) and the fire
radiative power (FRP) derived from MODIS measurements. The key
features of the method are (1) estimating the FRP-to-BBR conversion
factors (α) for different vegetative land cover types by
assimilating the satellite observations of co-emitted species into
the CTM, (2) optimal combination of the estimates of α
derived independently from satellite observations of different
species (CO and aerosol in this study), and (3) estimation of
the diurnal cycle of the fire emissions directly from the FRP
measurements. Values of α for forest and grassland fires in
Siberia and their uncertainties are estimated using the Infrared
Atmospheric Sounding Interferometer (IASI)
carbon monoxide (CO) retrievals and MODIS aerosol optical depth
(AOD) measurements combined with outputs from the CHIMERE mesoscale
chemistry-transport model. The constrained CO emissions are
validated through comparison of the respective simulations with
independent data of ground-based CO measurements at the ZOTTO
site. Using our optimal regional-scale estimates of the conversion
factors (which are found to be in agreement with earlier
published estimates obtained from local measurements of experimental
fires), the total CO2 emissions from wildfires in Siberia in
2012 are estimated to be in the range from 280 to 550 Tg C,
with the optimal (maximum likelihood) value of
392 Tg C. Sensitivity test cases featuring different
assumptions regarding the injection height and diurnal variations of
emissions indicate that the derived estimates of the total
CO2 emissions in Siberia are robust with respect to
the modeling options (the different estimates vary within less than
15% of their magnitude). The CO2 emission
estimates obtained for several years are compared with independent
estimates provided by the GFED3.1 and GFASv1.0 global emission
inventories. It is found that our "top-down" estimates for
the total annual biomass burning CO2 emissions in the period
from 2007 to 2011 in Siberia are by factors of 2.5 and 1.8 larger
than the respective bottom-up estimates; these discrepancies cannot
be fully explained by uncertainties in our estimates. There are also
considerable differences in the spatial distribution of
the different emission estimates; some of those differences have
a systematic character and require further analysis. 2014/10/03 - 11:51

Worldwide biogenic soil NOx emissions inferred from OMI NO2 observationsAtmospheric Chemistry and Physics, 14, 10363-10381, 2014Author(s): G. C. M. Vinken, K. F. Boersma, J. D. Maasakkers, M. Adon, and R. V. MartinBiogenic NOx emissions from soils are a large natural source
with substantial uncertainties in global bottom-up estimates
(ranging from 4 to 15 Tg N yr−1). We reduce this range
in emission estimates, and present a top-down soil NOx
emission inventory for 2005 based on retrieved tropospheric
NO2 columns from the Ozone Monitoring Instrument (OMI). We
use a state-of-science soil NOx emission inventory
(Hudman et al., 2012) as a priori in the GEOS-Chem chemistry transport
model to identify 11 regions where tropospheric NO2 columns
are dominated by soil NOx emissions. Strong correlations
between soil NOx emissions and simulated NO2 columns
indicate that spatial patterns in simulated NO2 columns in
these regions indeed reflect the underlying soil NOx
emissions. Subsequently, we use a mass-balance approach to
constrain emissions for these 11 regions on all major continents
using OMI observed and GEOS-Chem simulated tropospheric NO2
columns. We find that responses of simulated NO2 columns to
changing NOx emissions are suppressed over low NOx
regions, and account for these non-linearities in our inversion
approach. In general, our approach suggests that emissions need to
be increased in most regions. Our OMI top-down soil NOx
inventory amounts to 10.0 Tg N for 2005 when only
constraining the 11 regions, and 12.9 Tg N when
extrapolating the constraints globally. Substantial regional
differences exist (ranging from −40% to +90%), and
globally our top-down inventory is 4–35% higher than the
GEOS-Chem a priori (9.6 Tg N yr−1). We evaluate
NO2 concentrations simulated with our new OMI top-down
inventory against surface NO2 measurements from monitoring
stations in Africa, the USA and Europe. Although this comparison
is complicated by several factors, we find an encouraging improved
agreement when using the OMI top-down inventory compared to using
the a priori inventory. To our knowledge, this study provides, for
the first time, specific constraints on soil NOx emissions
on all major continents using OMI NO2 columns. Our results
rule out the low end of reported soil NOx emission
estimates, and suggest that global emissions are most likely around
12.9 ± 3.9 Tg N yr−1. 2014/10/03 - 11:51

The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)Atmospheric Chemistry and Physics, 14, 10333-10362, 2014Author(s): R. Dlugi, M. Berger, M. Zelger, A. Hofzumahaus, F. Rohrer, F. Holland, K. Lu, and G. KrammAn inhomogeneous mixing of reactants causes a reduction
of their chemical removal compared to the homogeneously mixed case
in turbulent atmospheric flows.
This can be described by the intensity of segregation
IS being the covariance of the mixing ratios of two species
divided by the product of their means.
Both terms appear in the balance equation of the mixing ratio and are discussed for the
reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest.
For most of these data, IS is negatively correlated with the fraction of mean OH mixing ratio
reacting with isoprene.
IS is also negatively correlated with the isoprene standard deviation.
Both findings agree with model results discussed by Patton et al. (2001) and others.
The correlation coefficient between OH and isoprene
and, therefore, IS increases with increasing mean reaction rate.
In addition, the balance equation of the covariance between isoprene and OH
is applied as the theoretical framework
for the analysis of the same field data.
The storage term is small, and, therefore, a diagnostic
equation for this covariance
can be derived.
The chemical reaction term Rij is dominated by the variance of
isoprene times the quotient of mixing ratios of OH and isoprene.
Based on these findings a new
diagnostic equation for IS is formulated.
Comparing different terms of this equation,
IS and Rij show a relation also to the normalised isoprene
standard deviation.
It is shown that not only chemistry but also turbulent and convective
mixing and advection – considered in a residual term – influence IS.
Despite this finding, a detection of the influence of coherent eddy transport
above the forest according to Katul et al. (1997)
on IS fails, but a relation to the turbulent and advective
transport of isoprene variance is determined.
The largest values of IS are found for most unstable conditions with
increasing buoyant production, confirming qualitatively model predictions
by Ouwersloot et al. (2011). 2014/10/03 - 11:51

Development of an aerosol microphysical module: Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS)Atmospheric Chemistry and Physics, 14, 10315-10331, 2014Author(s): H. Matsui, M. Koike, Y. Kondo, J. D. Fast, and M. TakigawaNumber concentrations, size distributions, and mixing states of aerosols are
essential parameters for accurate estimations of aerosol direct and indirect
effects. In this study, we develop an aerosol module, designated the Aerosol
Two-dimensional bin module for foRmation and Aging Simulation (ATRAS), that
can explicitly represent these parameters by considering new particle
formation (NPF), black carbon (BC) aging, and secondary organic aerosol
(SOA) processes. A two-dimensional bin representation is used for particles
with dry diameters from 40 nm to 10 μm to resolve both aerosol sizes
(12 bins) and BC mixing states (10 bins) for a total of 120 bins. The
particles with diameters between 1 and 40 nm are resolved using additional
eight size bins to calculate NPF. The ATRAS module is implemented in the WRF-Chem
model and applied to examine the sensitivity of simulated mass, number, size
distributions, and optical and radiative parameters of aerosols to NPF, BC
aging, and SOA processes over East Asia during the spring of 2009. The BC
absorption enhancement by coating materials is about 50% over East Asia
during the spring, and the contribution of SOA processes to the absorption
enhancement is estimated to be 10–20% over northern East Asia and 20–35% over southern East Asia. A clear north–south contrast is also
found between the impacts of NPF and SOA processes on cloud condensation
nuclei (CCN) concentrations: NPF increases CCN concentrations at higher
supersaturations (smaller particles) over northern East Asia, whereas SOA
increases CCN concentrations at lower supersaturations (larger particles)
over southern East Asia. The application of ATRAS in East Asia also shows
that the impact of each process on each optical and radiative parameter
depends strongly on the process and the parameter in question. The module
can be used in the future as a benchmark model to evaluate the accuracy of
simpler aerosol models and examine interactions between NPF, BC aging, and
SOA processes under different meteorological conditions and emissions. 2014/10/03 - 11:51

A molecular-level approach for characterizing water-insoluble components of ambient organic aerosol particulates using ultrahigh-resolution mass spectrometryAtmospheric Chemistry and Physics, 14, 10299-10314, 2014Author(s): A. S. Willoughby, A. S. Wozniak, and P. G. HatcherThe chemical composition of organic aerosols in the atmosphere is strongly
influenced by human emissions. The effect these have on the environment,
human health, and climate change is determined by the molecular nature of
these chemical species. The complexity of organic aerosol samples limits the
ability to study the chemical composition, and therefore the associated
properties and the impacts they have. Many studies have addressed the water-soluble
fraction of organic aerosols and have had much success in identifying
specific molecular formulas for thousands of compounds present. However,
little attention is given to the water-insoluble portion, which can contain
most of the fossil material that is emitted through human activity. Here we
compare the organic aerosols present in water extracts and organic solvent
extracts (pyridine and acetonitrile) of an ambient aerosol sample collected
in a rural location that is impacted by natural and anthropogenic emission
sources. A semiquantitative method was developed using proton nuclear
magnetic resonance spectroscopy to determine that the amount of organic
matter extracted by pyridine is comparable to that of water. Electrospray
ionization Fourier transform ion cyclotron resonance mass spectra show that
pyridine extracts a molecularly unique fraction of organic matter compared
to water or acetonitrile, which extract chemically similar organic matter
components. The molecular formulas unique to pyridine were less polar, more
aliphatic, and reveal formulas containing sulfur to be an important
component of insoluble aerosol organic matter. 2014/10/03 - 11:51

Linking climate and air quality over Europe: effects of meteorology on PM2.5 concentrationsAtmospheric Chemistry and Physics, 14, 10283-10298, 2014Author(s): A. G. Megaritis, C. Fountoukis, P. E. Charalampidis, H. A. C. Denier van der Gon, C. Pilinis, and S. N. PandisThe effects of various meteorological parameters such as temperature, wind
speed, absolute humidity, precipitation and mixing height on PM2.5
concentrations over Europe were examined using a three-dimensional chemical
transport model, PMCAMx-2008. Our simulations covered three periods,
representative of different seasons (summer, winter, and fall). PM2.5
appears to be more sensitive to temperature changes compared to the other
meteorological parameters in all seasons.

PM2.5 generally decreases as temperature increases, although the
predicted changes vary significantly in space and time, ranging from
−700 ng m−3 K−1 (−8% K−1) to
300 ng m−3 K−1 (7% K−1). The predicted decreases of
PM2.5 are mainly due to evaporation of ammonium nitrate, while the
higher biogenic emissions and the accelerated gas-phase reaction rates
increase the production of organic aerosol (OA) and sulfate, having the
opposite effect on PM2.5. The predicted responses of PM2.5 to
absolute humidity are also quite variable, ranging from
−130 ng m−3 %−1 (−1.6% %−1)
to 160 ng m−3 %−1 (1.6% %−1)
dominated mainly by changes in inorganic PM2.5 species. An increase in
absolute humidity favors the partitioning of nitrate to the aerosol phase and
increases the average PM2.5 during summer and fall. Decreases in sulfate
and sea salt levels govern the average PM2.5 response to humidity during
winter. A decrease of wind speed (keeping the emissions constant) increases
all PM2.5 species (on average 40 ng m−3 %−1) due to
changes in dispersion and dry deposition. The wind speed effects on sea salt
emissions are significant for PM2.5 concentrations over water and in
coastal areas. Increases in precipitation have a negative effect on
PM2.5 (decreases up to 110 ng m−3 %−1) in all periods
due to increases in wet deposition of PM2.5 species and their gas
precursors. Changes in mixing height have the smallest effects (up to
35 ng m−3 %−1) on PM2.5 .

Regarding the relative importance of each of the meteorological parameters in
a changed future climate, the projected changes in precipitation are expected
to have the largest impact on PM2.5 levels during all periods (changes
up to 2 μg m−3 in the fall). The expected effects in future
PM2.5 levels due to wind speed changes are similar in all seasons and
quite close to those resulting from future precipitation changes (up to
1.4 μg m−3). The expected increases in absolute humidity in
the future can lead to large changes in PM2.5 levels (increases up to
2 μg m−3) mainly in the fall due to changes in particulate
nitrate levels. Despite the high sensitivity of PM2.5 levels to
temperature, the small expected increases of temperature in the future will
lead to modest PM2.5 changes and will not dominate the overall change. 2014/10/03 - 11:51

Size-resolved cloud condensation nuclei (CCN) activity and closure analysis at the HKUST Supersite in Hong KongAtmospheric Chemistry and Physics, 14, 10267-10282, 2014Author(s): J. W. Meng, M. C. Yeung, Y. J. Li, B. Y. L. Lee, and C. K. ChanThe cloud condensation nuclei (CCN) properties of atmospheric aerosols were
measured on 1–30 May 2011 at the HKUST (Hong Kong University of Science and
Technology) Supersite, a coastal site in Hong Kong. Size-resolved CCN
activation curves, the ratio of number concentration of CCN
(NCCN) to aerosol concentration (NCN) as a function
of particle size, were obtained at supersaturation (SS) = 0.15, 0.35,
0.50, and 0.70% using a DMT (Droplet Measurement Technologies) CCN counter
(CCNc) and a TSI scanning mobility particle sizer (SMPS). The mean bulk
size-integrated NCCN ranged from ~500 cm−3 at
SS = 0.15% to ~2100 cm−3 at SS = 0.70%, and
the mean bulk NCCN / NCN ratio ranged from 0.16
at SS = 0.15% to 0.65 at SS = 0.70%. The average critical
mobility diameters (D50) at SS = 0.15, 0.35, 0.50, and 0.70%
were 116, 67, 56, and 46 nm, respectively. The corresponding average
hygroscopic parameters (κCCN) were 0.39, 0.36, 0.31, and
0.28. The decrease in κCCN can be attributed to the increase
in organic to inorganic volume ratio as particle size decreases, as measured
by an Aerodyne high resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS). The κCCN correlates reasonably well with
κAMS_SR based on size-resolved AMS measurements:
κAMS_SR = κorg × forg
+ κinorg × finorg, where
forg and finorg are the organic and inorganic volume
fractions, respectively, κorg = 0.1 and
κinorg = 0.6, with a R2 of 0.51.

In closure analysis, NCCN was estimated by integrating the
measured size-resolved NCN for particles larger than D50
derived from κ assuming internal mixing state. Estimates using
κAMS_SR show that the measured and predicted
NCCN were generally within 10% of each other at all four SS.
The deviation increased to 26% when κAMS was calculated
from bulk PM1 AMS measurements of particles because PM1 was dominated
by particles of 200 to 500 nm in diameter, which had a larger inorganic
fraction than those of D50 (particle diameter < 200 nm). A
constant κ = 0.33 (the average value of κAMS_SR
over the course of campaign) was found to give an NCCN prediction
within 12% of the actual measured values. We also compared
NCCN estimates based on the measured average D50 and the
average size-resolved CCN activation ratio to examine the relative importance
of hygroscopicity and mixing state. NCCN appears to be relatively
more sensitive to the mixing state and hygroscopicity at a high
SS = 0.70% and a low SS = 0.15%, respectively. 2014/09/28 - 13:45

Submicron aerosols at thirteen diversified sites in China: size distribution, new particle formation and corresponding contribution to cloud condensation nuclei productionAtmospheric Chemistry and Physics, 14, 10249-10265, 2014Author(s): J. F. Peng, M. Hu, Z. B. Wang, X. F. Huang, P. Kumar, Z. J. Wu, S. Guo, D. L. Yue, D. J. Shang, Z. Zheng, and L. Y. HeUnderstanding the particle number size distributions in diversified
atmospheric environments is important in order to design mitigation
strategies related to submicron particles and their effects on regional air
quality, haze and human health. In this study, we conducted 15 different
field measurement campaigns between 2007 and 2011 at 13 individual sites in
China, including five urban sites, four regional
sites, three coastal/background sites
and one ship cruise measurement along eastern coastline of China. Size
resolved particles were measured in the 15–600 nm size range. The median
particle number concentrations (PNCs) were found to vary in the range of
1.1−2.2 × 104 cm−3 at urban sites,
0.8−1.5 × 104 cm−3 at regional sites,
0.4−0.6 × 104 cm−3 at coastal/background sites, and
0.5 × 104 cm−3 during cruise measurement. Peak diameters
at each of these sites varied greatly from 24 to 115 nm. Particles in the
15–25 nm (nucleation mode), 25–100 nm (Aitken mode) and 100–600 nm
(accumulation mode) range showed different characteristics at each sites,
indicating the features of primary emissions and secondary formation in these
diversified atmospheric environments. Diurnal variations show a build-up of
accumulation mode particles belt at regional sites, suggesting the
contribution of regional secondary aerosol pollution. Frequencies of new
particle formation (NPF) events were much higher at urban and regional sites
than at coastal sites and during cruise measurement. The average growth rates (GRs)
of nucleation mode particles were 8.0–10.9 nm h−1 at urban sites,
7.4–13.6 nm h−1 at regional sites and 2.8–7.5 nm h−1 at
coastal sites and during cruise measurement. The high gaseous precursors and strong
oxidation at urban and regional sites not only favored the formation of
particles, but also accelerated the growth rate of the nucleation mode
particles. No significant difference in condensation sink (CS) during NPF
days were observed among different site types, suggesting that the NPF events
in background areas were more influenced by the pollutant transport. In
addition, average contributions of NPF events to potential cloud condensation
nuclei (CCN) at 0.2% super-saturation in the afternoon of all sampling
days were calculated as 11% and 6% at urban sites and regional sites,
respectively. On the other hand, NPF events at coastal sites and during cruise
measurement had little impact on potential production of CCN. This study
provides a large data set of particle size distribution in diversified
atmosphere of China, improving our general understanding of emission,
secondary formation, new particle formation and corresponding CCN activity of
submicron aerosols in Chinese environments. 2014/09/28 - 13:45

Modeling analysis of the seasonal characteristics of haze formation in BeijingAtmospheric Chemistry and Physics, 14, 10231-10248, 2014Author(s): X. Han, M. Zhang, J. Gao, S. Wang, and F. ChaiThe air quality modeling system RAMS-CMAQ (Regional
Atmospheric Modeling System–Community Multiscale Air Quality), coupled with
an aerosol optical property scheme, was applied to simulate the
meteorological field, major aerosol components (sulfate, nitrate, ammonium,
black carbon, organic carbon, dust, and sea salt), and surface visibility
over the North China Plain (NCP) in 2011. The modeled results in February
and July 2011 were selected and analyzed to obtain an in-depth understanding
of the haze formation mechanism in Beijing for different seasons. The
simulation results showed that the visibility was below 10 km for most
regions of the NCP, and dropped to less than 5 km over the megacities of
Beijing and Tianjin, the whole of Hebei Province, and the northwest part of
Shandong Province during pollution episodes in February and July. The heavy
mass concentration of PM2.5 ranged from 120 to 300 μg m−3 and was concentrated in the areas with low visibility. The
haze formation mechanism in Beijing in winter was different from that in
summer. The mass concentration of PM2.5 was higher, and the components
more complicated, in winter. While the mass concentration of PM2.5 in
summer was lower than that in winter, the mass concentrations of hygroscopic
inorganic salts were comparable with those in winter, and the relative
humidity was, as expected, higher. Therefore, the water uptake of
hygroscopic aerosols played a key role in summer. Moreover, the analysis
showed that the influence of the PM2.5 mass burden on visibility was
very weak when its value was larger than 100 μg m−3. Only when the
mass burden of PM2.5 decreased to a certain threshold interval did the
visibility increase rapidly. This indicates that, when emission reduction
measures are taken to control haze occurrence, the mass burden of PM2.5
must be cut to below this threshold interval. The relationship between the
threshold of haze occurrence and the relative humidity in Beijing was fitted
by an exponential function, and the resulting fitting curves could provide a
new theoretical basis to understand and control haze formation in Beijing. 2014/09/28 - 13:45

Horizontal distributions of aerosol constituents and their mixing states in Antarctica during the JASE traverseAtmospheric Chemistry and Physics, 14, 10211-10230, 2014Author(s): K. Hara, F. Nakazawa, S. Fujita, K. Fukui, H. Enomoto, and S. SugiyamaMeasurements of aerosol number concentrations and direct aerosol sampling
were conducted on continental Antarctica during the traverse of the
Japanese–Swedish joint Antarctic expedition (JASE) from 14 November 2007
until 24 January 2008. Aerosol concentrations in background conditions
decreased gradually with latitude in inland regions during the traverse. The
lowest aerosol number concentrations were 160 L−1 in Dp
> 0.3 μm, and 0.5 L−1 in Dp 2 μm.
In contrast, aerosol concentrations reached 3278 L−1 in Dp > 0.3 μm, and 215 L−1 in Dp > 2 μm
under strong wind conditions. The estimated aerosol mass concentrations were
0.04–5.7 μg m−3. Single particle analysis of aerosol particles
collected during the JASE traverse was conducted using a scanning electron
microscope equipped with an energy dispersive x ray spectrometer. Major
aerosol constituents were sulfates in fine mode, and sulfate, sea salts,
modified sea salts, and fractionated sea salts in coarse mode. K-rich
sulfates, Mg-rich sulfate, Ca-rich sulfates, and minerals were identified as
minor aerosol constituents. Horizontal features of Cl / Na ratios imply that
sea-salt modification (i.e. Cl loss) occurred on the Antarctic continent
during the summer. Most sea-salt particles in the continental region near
the coast were modified with acidic sulfur species such as H2SO4
and CH3SO3H. By contrast, acidic species other than the acidic
sulfur species (likely HNO3) contributed markedly to sea-salt
modification in inland areas during the traverse. Mg-rich sea-salt particles
and Mg-free sea-salt particles were present in coarse and fine modes from
the coast to inland areas. These sea-salt particles might be associated with
sea-salt fractionation on the snow surface of continental Antarctica. 2014/09/28 - 13:45

On the impact of the temporal variability of the collisional quenching process on the mesospheric OH emission layer: a study based on SD-WACCM4 and SABERAtmospheric Chemistry and Physics, 14, 10193-10210, 2014Author(s): S. Kowalewski, C. von Savigny, M. Palm, I. C. McDade, and J. NotholtThe mesospheric OH Meinel emissions are subject of many theoretical and
observational studies devoted to this part of the atmosphere. Depending on
the initial vibrational level of excitation the altitude of the considered OH
Meinel emission is systematically shifted, which has important implications
for the intercomparison of different studies considering different transition
bands. Previous model studies suggest that these vertical shifts are
essentially caused by the process of collisional quenching with atomic
oxygen. Following this hypothesis, a recent study found experimental evidence
of a coherent seasonality at tropical latitudes between vertical shifts of
different OH Meinel bands and changes in atomic oxygen concentrations.
Despite the consistent finding of the above mentioned hypothesis, it cannot
be excluded that the actual temporal variability of the vertical shifts
between different OH Meinel bands may in addition be controlled or even
dominated by other processes. It remains an open question whether the
observed temporal evolution is indeed mainly controlled by the modulation of
the collisional quenching process with atomic oxygen. By means of
a sensitivity study which employs a quenching model to simulations made with
the SD-WACCM4 chemistry climate model, we aim at assessing this question.
From this study we find that the observed seasonality of vertical OH Meinel
shifts is only partially controlled by temporal changes in atomic oxygen
concentrations, while molecular oxygen has another noticeable impact on the
vertical OH Meinel shifts. This in particular becomes evident for the diurnal
variability of vertical OH Meinel shifts, which reveal only a poor
correlation with the atomic oxygen species. Furthermore, changes in the
H + O3 source gases provide another mechanism that can
potentially affect the diurnal variability in addition. By comparison with
limb radiance observations from the SABER/TIMED satellite this provides an
explanation for the less evident diurnal response between changes in
O concentrations and vertical OH Meinel shifts. On the other hand, at
seasonal timescales the coherency between both quantities is again evident in
SABER/TIMED but less pronounced compared to our model simulations. 2014/09/25 - 13:46

Spatial distributions and seasonal cycles of aerosol climate effects in India seen in a global climate–aerosol modelAtmospheric Chemistry and Physics, 14, 10177-10192, 2014Author(s): S. V. Henriksson, J.-P. Pietikäinen, A.-P. Hyvärinen, P. Räisänen, K. Kupiainen, J. Tonttila, R. Hooda, H. Lihavainen, D. O'Donnell, L. Backman, Z. Klimont, and A. LaaksonenClimate–aerosol interactions in India are studied
by employing the global climate–aerosol model ECHAM5-HAM and the GAINS
inventory for anthropogenic aerosol emissions. Model validation is done for
black carbon surface concentrations in Mukteshwar and for features of the
monsoon circulation. Seasonal cycles and spatial distributions of radiative
forcing and the temperature and rainfall responses are presented for
different model setups. While total aerosol radiative forcing is strongest in
the summer, anthropogenic forcing is considerably stronger in winter than in
summer. Local seasonal temperature anomalies caused by aerosols are mostly
negative with some exceptions, e.g., parts of northern India in March–May.
Rainfall increases due to the elevated heat pump (EHP) mechanism and
decreases due to solar dimming mechanisms (SDMs) and the relative strengths
of these effects during different seasons and for different model setups are
studied. Aerosol light absorption does increase rainfall in northern India,
but effects due to solar dimming and circulation work to cancel the increase.
The total aerosol effect on rainfall is negative for northern India in the
months of June–August, but during March–May the effect is positive for most
model setups. These differences between responses in different seasons might
help converge the ongoing debate on the EHPs and SDMs. Due to the complexity
of the problem and known or potential sources for error and bias, the results
should be interpreted cautiously as they are completely dependent on how
realistic the model is. Aerosol–rainfall correlations and anticorrelations
are shown not to be a reliable sole argument for deducing causality. 2014/09/25 - 13:46

Intercontinental transport and deposition patterns of atmospheric mercury from anthropogenic emissionsAtmospheric Chemistry and Physics, 14, 10163-10176, 2014Author(s): L. Chen, H. H. Wang, J. F. Liu, Y. D. Tong, L. B. Ou, W. Zhang, D. Hu, C. Chen, and X. J. WangGlobal policies that regulate anthropogenic mercury emissions to the
environment require quantitative and comprehensive source–receptor
relationships for mercury emissions, transport and deposition among major
continental regions. In this study, we use the GEOS-Chem global chemical transport model to establish
source–receptor relationships among 11 major continental regions
worldwide. Source–receptor relationships for surface mercury concentrations
(SMC) show that some regions (e.g., East Asia, the Indian subcontinent, and
Europe) should be responsible for their local surface Hg(II) and Hg(P)
concentrations due to near-field transport and deposition contributions
from their local anthropogenic emissions (up to 64 and 71% for
Hg(II) and Hg(P), respectively, over East Asia). We define the region of primary
influence (RPI) and the region of secondary influence (RSI) to establish
intercontinental influence patterns. Results indicate that East Asia is the
SMC RPI for almost all other regions, while Europe, Russia, and the Indian
subcontinent also make some contributions to SMC over some receptor regions
because they are dominant RSI source regions. Source–receptor relationships
for mercury deposition show that approximately 16 and 17% of dry
and wet deposition, respectively, over North America originate from East
Asia, indicating that transpacific transport of East Asian emissions is the
major foreign source of mercury deposition in North America. Europe,
Southeast Asia, and the Indian subcontinent are also important mercury
deposition sources for some receptor regions because they are the dominant RSIs.
We also quantify seasonal variation on mercury deposition contributions over
other regions from East Asia. Results show that mercury deposition
(including dry and wet) contributions from East Asia over the Northern
Hemisphere receptor regions (e.g., North America, Europe, Russia, the Middle
East, and Middle Asia) vary seasonally, with the maximum values in summer and
minimum values in winter. The opposite seasonal pattern occurs on mercury
dry deposition contributions over Southeast Asia and the Indian
subcontinent. 2014/09/25 - 13:46

Chemical mass balance of 300 °C non-volatile particles at the tropospheric research site Melpitz, GermanyAtmospheric Chemistry and Physics, 14, 10145-10162, 2014Author(s): L. Poulain, W. Birmili, F. Canonaco, M. Crippa, Z. J. Wu, S. Nordmann, G. Spindler, A. S. H. Prévôt, A. Wiedensohler, and H. HerrmannIn the fine-particle mode (aerodynamic diameter < 1 μm) non-volatile material has been associated with black carbon (BC) and
low-volatile organics and, to a lesser extent, with sea salt and mineral
dust. This work analyzes non-volatile particles at the tropospheric research
station Melpitz (Germany), combining experimental methods such as a mobility
particle-size spectrometer (3–800 nm), a thermodenuder operating at
300 °C, a multi-angle absorption photometer (MAAP), and an aerosol
mass spectrometer (AMS). The data were collected during two atmospheric
field experiments in May–June 2008 as well as February–March 2009. As a
basic result, we detected average non-volatile particle–volume fractions of
11 ± 3% (2008) and 17 ± 8% (2009). In both periods, BC was
in close linear correlation with the non-volatile fraction, but not
sufficient to quantitatively explain the non-volatile particle mass
concentration. Based on the assumption that BC is not altered by the heating
process, the non-volatile particle mass fraction could be explained by the
sum of black carbon (47% in summer, 59% in winter) and a non-volatile
organic contribution estimated as part of the low-volatility
oxygenated organic aerosol (LV-OOA) (53% in summer, 41% in winter);
the latter was identified from AMS data by factor analysis. Our results
suggest that LV-OOA was more volatile in summer (May–June 2008) than in
winter (February–March 2009) which was linked to a difference in oxidation
levels (lower in summer). Although carbonaceous compounds dominated the
sub-μm non-volatile particle mass fraction most of the time, a
cross-sensitivity to partially volatile aerosol particles of maritime origin
could be seen. These marine particles could be distinguished, however from
the carbonaceous particles by a characteristic particle volume–size
distribution. The paper discusses the uncertainty of the volatility
measurements and outlines the possible merits of volatility analysis as part
of continuous atmospheric aerosol measurements. 2014/09/25 - 13:46

Carbon balance of China constrained by CONTRAIL aircraft CO2 measurementsAtmospheric Chemistry and Physics, 14, 10133-10144, 2014Author(s): F. Jiang, H. M. Wang, J. M. Chen, T. Machida, L. X. Zhou, W. M. Ju, H. Matsueda, and Y. SawaTerrestrial carbon dioxide (CO2) flux estimates in China using
atmospheric inversion method are beset with considerable uncertainties
because very few atmospheric CO2 concentration measurements are
available. In order to improve these estimates, nested atmospheric CO2 inversion during 2002–2008 is performed in this study using passenger
aircraft-based CO2 measurements over Eurasia from the Comprehensive
Observation Network for Trace gases by Airliner (CONTRAIL) project. The
inversion system includes 43 regions with a focus on China, and is based on
the Bayesian synthesis approach and the TM5 transport model. The terrestrial
ecosystem carbon flux modeled by the Boreal Ecosystems Productivity Simulator (BEPS) model and the ocean exchange
simulated by the OPA-PISCES-T model are considered as the prior fluxes. The
impacts of CONTRAIL CO2 data on inverted China terrestrial carbon
fluxes are quantified, the improvement of the inverted fluxes after adding
CONTRAIL CO2 data are rationed against climate factors and evaluated
by comparing the simulated atmospheric CO2 concentrations with three
independent surface CO2 measurements in China. Results show that with
the addition of CONTRAIL CO2 data, the inverted carbon sink in China
increases while those in South and Southeast Asia decrease. Meanwhile, the
posterior uncertainties over these regions are all reduced (2–12%). CONTRAIL CO2 data also have a large effect on the
inter-annual variation of carbon sinks in China, leading to a better
correlation between the carbon sink and the annual mean climate factors.
Evaluations against the CO2 measurements at three sites in China also
show that the CONTRAIL CO2 measurements may have improved the inversion
results. 2014/09/25 - 13:46

Summertime tropospheric ozone assessment over the Mediterranean region using the thermal infrared IASI/MetOp sounder and the WRF-Chem modelAtmospheric Chemistry and Physics, 14, 10119-10131, 2014Author(s): S. Safieddine, A. Boynard, P.-F. Coheur, D. Hurtmans, G. Pfister, B. Quennehen, J. L. Thomas, J.-C. Raut, K. S. Law, Z. Klimont, J. Hadji-Lazaro, M. George, and C. ClerbauxOver the Mediterranean region, elevated tropospheric ozone (O3) values
are recorded, especially in summer. We use the thermal Infrared Atmospheric
Sounding Interferometer (IASI) and the Weather Research and Forecasting
Model with Chemistry (WRF-Chem) to understand and interpret the factors and
emission sources responsible for the high O3 concentrations observed in
the Mediterranean troposphere. Six years (2008–2013) of IASI data have been
analyzed and results show consistent maxima during summer, with an increase of up to
22% in the [0–8] km O3 column in the eastern part of the basin
compared to the middle of the basin. We focus on summer 2010 to investigate
the processes that contribute to these summer maxima. Using two modeled
O3 tracers (inflow to the model domain and local anthropogenic
emissions), we show that, between the surface and 2 km, O3 is mostly
formed from anthropogenic emissions, while above 4 km it is mostly transported
from outside the domain or from stratospheric origins. Evidence of
stratosphere-to-troposphere exchange (STE) events in the eastern part of the basin
is shown, and corresponds to a low water vapor mixing ratio and high
potential vorticity. 2014/09/25 - 13:46

Comparison of ice cloud properties simulated by the Community Atmosphere Model (CAM5) with in-situ observationsAtmospheric Chemistry and Physics, 14, 10103-10118, 2014Author(s): T. Eidhammer, H. Morrison, A. Bansemer, A. Gettelman, and A. J. HeymsfieldDetailed measurements of ice crystals in cirrus clouds were used to compare
with results from the Community Atmospheric Model Version 5 (CAM5) global
climate model. The observations are from two different field campaigns with
contrasting conditions: Atmospheric Radiation Measurements Spring Cloud
Intensive Operational Period in 2000 (ARM-IOP), which was characterized
primarily by midlatitude frontal clouds and cirrus, and Tropical Composition,
Cloud and Climate Coupling (TC4), which was dominated by anvil cirrus.
Results show that the model typically overestimates the slope parameter of
the exponential size distributions of cloud ice and snow, while the variation
with temperature (height) is comparable. The model also overestimates the
ice/snow number concentration (0th moment of the size distribution) and
underestimates higher moments (2nd through 5th), but compares well with
observations for the 1st moment. Overall the model shows better agreement
with observations for TC4 than for ARM-IOP in regards to the moments. The
mass-weighted terminal fall speed is lower in the model compared to
observations for both ARM-IOP and TC4, which is partly due to the
overestimation of the size distribution slope parameter. Sensitivity tests
with modification of the threshold size for cloud ice to snow autoconversion
(Dcs) do not show noticeable improvement in modeled moments,
slope parameter and mass weighed fall speed compared to observations. Further,
there is considerable sensitivity of the cloud radiative forcing to
Dcs, consistent with previous studies, but no value of
Dcs improves modeled cloud radiative forcing compared to
measurements. Since the autoconversion of cloud ice to snow using the
threshold size Dcs has little physical basis, future improvement
to combine cloud ice and snow into a single category, eliminating the need
for autoconversion, is suggested. 2014/09/25 - 13:46

Concentrations and fluxes of isoprene and oxygenated VOCs at a French Mediterranean oak forestAtmospheric Chemistry and Physics, 14, 10085-10102, 2014Author(s): C. Kalogridis, V. Gros, R. Sarda-Esteve, B. Langford, B. Loubet, B. Bonsang, N. Bonnaire, E. Nemitz, A.-C. Genard, C. Boissard, C. Fernandez, E. Ormeño, D. Baisnée, I. Reiter, and J. LathièreThe CANOPEE project aims to better understand the biosphere–atmosphere
exchanges of biogenic volatile organic compounds (BVOCs) in the case of
Mediterranean ecosystems and the impact of in-canopy processes on the
atmospheric chemical composition above the canopy. Based on an intensive
field campaign, the objective of our work was to determine the chemical
composition of the air inside a canopy as well as the net fluxes of reactive
species between the canopy and the boundary layer. Measurements were carried
out during spring 2012 at the field site of the Oak Observatory of the
Observatoire de Haute Provence (O3HP) located in the southeast of
France. The site is a forest ecosystem dominated by downy oak,
Quercus pubescens Willd., a typical Mediterranean species which
features large isoprene emission rates. Mixing ratios of isoprene, its
degradation products methylvinylketone (MVK) and methacrolein (MACR) and
several other oxygenated VOC (OxVOC) were measured above the
canopy using an online proton transfer reaction mass spectrometer (PTR-MS),
and fluxes were calculated by the disjunct eddy covariance approach. The
O3HP site was found to be a very significant source of isoprene
emissions, with daily maximum ambient concentrations ranging between
2–16 ppbv inside and 2–5 ppbv just above the top of the forest canopy.
Significant isoprene fluxes were observed only during daytime, following
diurnal cycles with midday net emission fluxes from the canopy ranging
between 2.0 and 9.7 mg m−2 h1. Net isoprene normalized flux (at
30 °C, 1000 μmol quanta m−2 s−1) was
estimated at 7.4 mg m−2 h−1. Evidence of direct emission of
methanol was also found exhibiting maximum daytime fluxes ranging between 0.2
and 0.6 mg m−2 h−1, whereas flux values for monoterpenes and
others OxVOC such as acetone and acetaldehyde were below the
detection limit.

The MVK+MACR-to-isoprene ratio provided useful information on the
oxidation of isoprene, and is in agreement with recent findings proposing
weak production yields of MVK and MACR, in remote forest regions where the
NOx concentrations are low. In-canopy chemical oxidation of isoprene was
found to be weak and did not seem to have a significant impact on isoprene
concentrations and fluxes above the canopy. 2014/09/25 - 13:46

Size distribution, mixing state and source apportionment of black carbon aerosol in London during wintertimeAtmospheric Chemistry and Physics, 14, 10061-10084, 2014Author(s): D. Liu, J. D. Allan, D. E. Young, H. Coe, D. Beddows, Z. L. Fleming, M. J. Flynn, M. W. Gallagher, R. M. Harrison, J. Lee, A. S. H. Prevot, J. W. Taylor, J. Yin, P. I. Williams, and P. ZotterBlack carbon aerosols (BC) at a London urban site were characterised in both
winter- and summertime 2012 during the Clean Air for London (ClearfLo)
project. Positive matrix factorisation (PMF) factors of organic aerosol mass
spectra measured by a high-resolution aerosol mass spectrometer (HR-AMS)
showed traffic-dominant sources in summer but in winter the influence of
additional non-traffic sources became more important, mainly from solid fuel
sources (SF). Measurements using a single particle soot photometer (SP2,
DMT), showed the traffic-dominant BC exhibited an almost uniform BC core
size (Dc) distribution with very thin coating thickness throughout the
detectable range of Dc. However, the size distribution of sf
(project average mass median Dc = 149 ± 22 nm in winter, and
120 ± 6 nm in summer) and BC coating thickness varied significantly in
winter. A novel methodology was developed to attribute the BC number
concentrations and mass abundances from traffic (BCtr) and from SF
(BCsf), by using a 2-D histogram of the particle optical properties as
a function of BC core size, as measured by the SP2. The BCtr and
BCsf showed distinctly different sf distributions and coating
thicknesses, with BCsf displaying larger Dc and larger coating
thickness compared to BCtr. BC particles from different sources were
also apportioned by applying a multiple linear regression between the total
BC mass and each AMS-PMF factor (BC–AMS–PMF method), and also attributed by
applying the absorption spectral dependence of carbonaceous aerosols to
7-wavelength Aethalometer measurements (Aethalometer method).

Air masses that originated from westerly (W), southeasterly (SE), and
easterly (E) sectors showed BCsf fractions that ranged from low to
high, and whose mass median Dc values were 137 ± 10 nm, 143 ± 11 nm
and 169 ± 29 nm, respectively. The corresponding bulk relative
coating thickness of BC (coated particle size/BC core – Dp/Dc) for
these same sectors was 1.28 ± 0.07, 1.45 ± 0.16 and 1.65 ± 0.19.
For W, SE and E air masses, the number fraction of BCsf ranged
from 6 ± 2% to 11 ± 5% to 18 ± 10%, respectively, but
importantly the larger BC core sizes lead to an increased fraction of
BCsf in terms of mass than number (for W, SE and E air masses, the
BCsf mass fractions ranged from 16 ± 6%, 24 ± 10% and
39 ± 14%, respectively). An increased fraction of non-BC particles
(particles that did not contain a BC core) was also observed when SF sources
were more significant. The BC mass attribution by the SP2 method agreed well
with the BC–AMS–PMF multiple linear regression method (BC–AMS–PMF : SP2 ratio
= 1.05, r2 = 0.80) over the entire experimental period. Good
agreement was found between BCsf attributed with the Aethalometer model
and the SP2. However, the assumed absorption Ångström exponent
(αwb) had to be changed according to the different air mass
sectors to yield the best comparison with the SP2. This could be due to
influences of fuel type or burn phase. 2014/09/25 - 13:46

Modeling regional aerosol and aerosol precursor variability over California and its sensitivity to emissions and long-range transport during the 2010 CalNex and CARES campaignsAtmospheric Chemistry and Physics, 14, 10013-10060, 2014Author(s): J. D. Fast, J. Allan, R. Bahreini, J. Craven, L. Emmons, R. Ferrare, P. L. Hayes, A. Hodzic, J. Holloway, C. Hostetler, J. L. Jimenez, H. Jonsson, S. Liu, Y. Liu, A. Metcalf, A. Middlebrook, J. Nowak, M. Pekour, A. Perring, L. Russell, A. Sedlacek, J. Seinfeld, A. Setyan, J. Shilling, M. Shrivastava, S. Springston, C. Song, R. Subramanian, J. W. Taylor, V. Vinoj, Q. Yang, R. A. Zaveri, and Q. ZhangThe performance of the Weather Research and Forecasting regional model with
chemistry (WRF-Chem) in simulating the spatial and temporal variations in
aerosol mass, composition, and size over California is quantified using the
extensive meteorological, trace gas, and aerosol measurements collected
during the California Nexus of Air Quality and Climate Experiment (CalNex)
and the Carbonaceous Aerosol and Radiative Effects Study (CARES) conducted
during May and June of 2010. The overall objective of the field campaigns
was to obtain data needed to better understand processes that affect both
climate and air quality, including emission assessments, transport and
chemical aging of aerosols, aerosol radiative effects. Simulations were
performed that examined the sensitivity of aerosol concentrations to
anthropogenic emissions and to long-range transport of aerosols into the
domain obtained from a global model. The configuration of WRF-Chem used in
this study is shown to reproduce the overall synoptic conditions,
thermally driven circulations, and boundary layer structure observed in
region that controls the transport and mixing of trace gases and aerosols.
Reducing the default emissions inventory by 50% led to an overall
improvement in many simulated trace gases and black carbon aerosol at most
sites and along most aircraft flight paths; however, simulated organic
aerosol was closer to observed when there were no adjustments to the primary
organic aerosol emissions. We found that sulfate was better simulated over
northern California whereas nitrate was better simulated over southern
California. While the overall spatial and temporal variability of aerosols
and their precursors were simulated reasonably well, we show cases where the
local transport of some aerosol plumes were either too slow or too fast,
which adversely affects the statistics quantifying the differences between
observed and simulated quantities. Comparisons with lidar and in situ
measurements indicate that long-range transport of aerosols from the global
model was likely too high in the free troposphere even though their
concentrations were relatively low. This bias led to an over-prediction in
aerosol optical depth by as much as a factor of 2 that offset the
under-predictions of boundary-layer extinction resulting primarily from
local emissions. Lowering the boundary conditions of aerosol concentrations
by 50% greatly reduced the bias in simulated aerosol optical depth for
all regions of California. This study shows that quantifying regional-scale
variations in aerosol radiative forcing and determining the relative role of
emissions from local and distant sources is challenging during `clean'
conditions and that a wide array of measurements are needed to ensure model
predictions are correct for the right reasons. In this regard, the combined
CalNex and CARES data sets are an ideal test bed that can be used to evaluate
aerosol models in great detail and develop improved treatments for aerosol
processes. 2014/09/25 - 13:46

Experimental determination of the temperature dependence of water activities for a selection of aqueous organic solutionsAtmospheric Chemistry and Physics, 14, 9993-10012, 2014Author(s): G. Ganbavale, C. Marcolli, U. K. Krieger, A. Zuend, G. Stratmann, and T. PeterThis work presents experimental data of the temperature
dependence of water activity in aqueous organic solutions relevant for
tropospheric conditions (200–273 K). Water activity (aw) at low
temperatures (T) is a crucial parameter for predicting homogeneous ice
nucleation. We investigated temperature-dependent water activities, ice
freezing and melting temperatures of solutions, and vapour pressures of a
selection of atmospherically relevant aqueous organic systems. To measure
aw over a wide composition range and with a focus on low
temperatures, we use various aw measurement techniques and
instruments: a dew point water activity meter, an electrodynamic balance
(EDB), differential scanning calorimetry (DSC), and a setup to measure the
total gas phase pressure at equilibrium over aqueous solutions. Water
activity measurements were performed for aqueous multicomponent and
multifunctional organic mixtures containing the functional groups typically
found in atmospheric organic aerosols, such as hydroxyl, carboxyl, ketone,
ether, ester, and aromatic groups. The aqueous organic systems studied at
several fixed compositions over a considerable temperature range differ
significantly in their temperature dependence. Aqueous organic systems of
1,4-butanediol and methoxyacetic acid show a moderate decrease in
aw with decreasing temperature. The aqueous M5 system (a
multicomponent system containing five different dicarboxylic acids) and
aqueous 2-(2-ethoxyethoxy)ethanol solutions both show a strong increase of
water activity with decreasing temperature at high solute concentrations for
T < 270 K and T < 260 K, respectively. These measurements show
that the temperature trend of aw can be reversed at low
temperatures and that linear extrapolations of high-temperature data may lead
to erroneous predictions. To avoid this, experimentally determined
aw at low temperature are needed to improve thermodynamic models
towards lower temperatures and for improved predictions of the ice nucleation
ability of organic–water systems. 2014/09/20 - 18:49

Source apportionment and seasonal variation of PM2.5 in a Sub-Saharan African city: Nairobi, KenyaAtmospheric Chemistry and Physics, 14, 9977-9991, 2014Author(s): S. M. Gaita, J. Boman, M. J. Gatari, J. B. C. Pettersson, and S. JanhällSources of airborne particulate matter and their seasonal variation in urban
areas in Sub-Saharan Africa are poorly understood due to lack of long-term
measurement data. In view of this, filter samples of airborne particulate
matter (particle diameter ≤2.5 μm, PM2.5) were collected
between May 2008 and April 2010 at two sites (urban background site and
suburban site) within the Nairobi metropolitan area. A total of 780 samples
were collected and analyzed for particulate mass, black carbon (BC) and
13 trace elements. The average PM2.5 concentration at the urban
background site was 21±9.5 μg m−3, whereas the
concentration at the suburban site was 13±7.3 μg m−3.
The daily PM2.5 concentrations exceeded 25 μg m−3 (the
World Health Organization 24 h guideline value) on 29% of the days at the
urban background site and 7% of the days at the suburban site. At both
sites, BC, Fe, S and Cl accounted for approximately 80% of all detected
elements. Positive matrix factorization analysis identified five source
factors that contribute to PM2.5 in Nairobi, namely traffic, mineral
dust, industry, combustion and a mixed factor (composed of biomass
burning, secondary aerosol and aged sea salt). Mineral dust and traffic
factors were related to approximately 74% of PM2.5.
The identified source factors exhibited seasonal variation, apart from
the traffic factor, which was prominently consistent throughout the sampling
period. Weekly variations
were observed in all factors, with weekdays having higher concentrations than
weekends. The results provide information that can be exploited for policy
formulation and mitigation strategies to control air pollution in Sub-Saharan
African cities. 2014/09/20 - 18:49

Large mixing ratios of atmospheric nitrous acid (HONO) at Concordia (East Antarctic Plateau) in summer: a strong source from surface snow?Atmospheric Chemistry and Physics, 14, 9963-9976, 2014Author(s): M. Legrand, S. Preunkert, M. Frey, Th. Bartels-Rausch, A. Kukui, M. D. King, J. Savarino, M. Kerbrat, and B. JourdainDuring the austral summer 2011/2012 atmospheric nitrous acid (HONO) was
investigated for the second time at the Concordia site (75°06' S,
123°33' E), located on the East Antarctic Plateau, by deploying a
long-path absorption photometer (LOPAP). Hourly mixing ratios of HONO
measured in December 2011/January 2012 (35 ± 5.0 pptv) were similar to
those measured in December 2010/January 2011 (30.4 ± 3.5 pptv). The
large value of the HONO mixing ratio at the remote Concordia site suggests a
local source of HONO in addition to weak production from oxidation of NO by
the OH radical. Laboratory experiments demonstrate that surface snow removed
from Concordia can produce gas-phase HONO at mixing ratios half that of
the NOx mixing ratio produced in the same experiment at typical
temperatures encountered at Concordia in summer. Using these lab data and
the emission flux of NOx from snow estimated from the vertical gradient
of atmospheric concentrations measured during the campaign, a mean diurnal
HONO snow emission ranging between 0.5 and 0.8 × 109 molecules
cm−2 s−1 is calculated. Model calculations indicate that, in addition to
around 1.2 pptv of HONO produced by the NO oxidation, these HONO snow
emissions can only explain 6.5 to 10.5 pptv of HONO in the atmosphere at
Concordia. To explain the difference between observed and simulated HONO
mixing ratios, tests were done both in the field and at lab to explore the
possibility that the presence of HNO4 had biased the measurements of
HONO. 2014/09/20 - 18:49

How stratospheric are deep stratospheric intrusions?Atmospheric Chemistry and Physics, 14, 9941-9961, 2014Author(s): T. Trickl, H. Vogelmann, H. Giehl, H.-E. Scheel, M. Sprenger, and A. StohlPreliminary attempts of quantifying the stratospheric ozone contribution in the observations at
the Zugspitze summit (2962 m a.s.l.) next to Garmisch-Partenkirchen in the German Alps
had yielded an approximate doubling of the stratospheric fraction of the Zugspitze ozone during
the time period 1978 to 2004. These investigations had been based on data filtering by using low
relative humidity (RH) and elevated 7Be as the criteria for selecting half-hour intervals of
ozone data representative of stratospheric intrusion air. To quantify the residual
stratospheric component in stratospherically influenced air masses, however, the mixing of
tropospheric air into the stratospheric intrusion layers must be taken into account. In fact, the
dewpoint mirror instrument at the Zugspitze summit station rarely registers RH values lower than 10% in stratospheric air intrusions. Since 2007 a programme of routine
lidar sounding of ozone, water vapour and aerosol has been conducted in the Garmisch-Partenkirchen
area. The lidar results demonstrate that the intrusion layers are drier by roughly one order of
magnitude than indicated in the in situ measurements. Even in thin layers RH values
clearly below 1% have frequently been observed. These thin, undiluted layers present an
important challenge for atmospheric modelling. Although the ozone values never reach values
typical of the lower-stratosphere it becomes, thus, obvious that, without strong wind shear
or convective processes, mixing of stratospheric and tropospheric air must be very slow in
most of the free troposphere. As a consequence, the analysis the Zugspitze data can be assumed to be more reliable
than anticipated. Finally, the concentrations of Zugspitze carbon monoxide rarely drop inside
intrusion layers and normally stay clearly above full stratospheric values. This indicates that
most of the CO, and thus the intrusion air mass, originates in the shallow "mixing layer" around
the thermal tropopause. The CO mixing ratio in these descending layers between 1990 and 2004
exhibits a slightly positive trend indicating some Asian influence on the lowermost stratosphere
in the high-latitude source region of most intrusions reaching the station. 2014/09/20 - 18:49

Aviation 2006 NOx-induced effects on atmospheric ozone and HOx in Community Earth System Model (CESM)Atmospheric Chemistry and Physics, 14, 9925-9939, 2014Author(s): A. Khodayari, S. Tilmes, S. C. Olsen, D. B. Phoenix, D. J. Wuebbles, J.-F. Lamarque, and C.-C. ChenThe interaction between atmospheric chemistry and ozone (O3) in the
upper troposphere–lower stratosphere (UTLS) presents a major uncertainty
in understanding the effects of aviation on climate. In this study, two
configurations of the atmospheric model from the Community Earth System
Model (CESM), Community Atmosphere Model with Chemistry,
Version 4 (CAM4) and Version 5 (CAM5), are used to evaluate the effects of aircraft
nitrogen oxide (NOx = NO + NO2) emissions on ozone and the background
chemistry in the UTLS. CAM4 and CAM5 simulations were both performed with
extensive tropospheric and stratospheric chemistry including 133 species and
330 photochemical reactions. CAM5 includes direct and indirect aerosol
effects on clouds using a modal aerosol module (MAM), whereby CAM4 uses a
bulk aerosol module, which can only simulate the direct effect. To examine
the accuracy of the aviation NOx-induced ozone distribution in the two
models, results from the CAM5 and CAM4 simulations are compared to
ozonesonde data. Aviation NOx emissions for 2006 were obtained from the AEDT
(Aviation Environmental Design Tool) global commercial aircraft emissions
inventory. Differences between simulated O3 concentrations and
ozonesonde measurements averaged at representative levels in the troposphere
and different regions are 13% in CAM5 and 18% in CAM4. Results show a
localized increase in aviation-induced O3 concentrations at aviation
cruise altitudes that stretches from 40° N to the North Pole. The
results indicate a greater and more disperse production of aviation
NOx-induced ozone in CAM5, with the annual tropospheric mean O3
perturbation of 1.2 ppb (2.4%) for CAM5 and 1.0 ppb (1.9%) for CAM4.
The annual mean O3 perturbation peaks at about 8.2 ppb (6.4%) and
8.8 ppb (5.2%) in CAM5 and CAM4, respectively. Aviation emissions also
result in increased hydroxyl radical (OH) concentrations and methane (CH4) loss rates,
reducing the tropospheric methane lifetime in CAM5 and CAM4 by 1.69 and
1.40%, respectively. Aviation NOx emissions are associated with an
instantaneous change in global mean short-term O3 radiative forcing
(RF) of 40.3 and 36.5 mWm−2 in CAM5 and CAM4, respectively. 2014/09/20 - 18:49

Corrigendum to "Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review" published in Atmos. Chem. Phys., 14, 9403–9450, 2014Atmospheric Chemistry and Physics, 14, 9923-9923, 2014Author(s): T. Vihma, R. Pirazzini, I. Fer, I. A. Renfrew, J. Sedlar, M. Tjernström, C. Lüpkes, T. Nygård, D. Notz, J. Weiss, D. Marsan, B. Cheng, G. Birnbaum, S. Gerland, D. Chechin, and J. C. GascardNo abstract available. 2014/09/20 - 18:49

Technical Note: The horizontal scale dependence of the cloud overlap parameter αAtmospheric Chemistry and Physics, 14, 9917-9922, 2014Author(s): I. Astin and L. Di GirolamoThe cloud overlap parameter α relates the combined cloud fraction between
two altitude levels in a grid box to the cloud fraction as derived under the
maximum and random overlap assumptions. In a number of published studies in
this and other journals, it is found that α tends to increase with an
increasing scale. In this Technical Note, we investigate this analytically by
considering what happens to α when two grid boxes are merged to give a
grid box with twice the area. Assuming that α depends only on scale, then
between any two fixed altitudes, there will be a linear relationship between
the values of α on the two scales. We illustrate this by finding the
relationship when cloud cover fractions are assumed to be uniformly
distributed, but with varying degrees of horizontal and vertical correlation.
Based on this, we conclude that α increases with scale if its value is
less than the vertical correlation coefficient in cloud fraction between the
two altitude levels. This occurs when the clouds are deeper than would be
expected at random (i.e. for exponentially distributed cloud depths). 2014/09/20 - 18:49

Characterising terrestrial influences on Antarctic air masses using Radon-222 measurements at King George IslandAtmospheric Chemistry and Physics, 14, 9903-9916, 2014Author(s): S. D. Chambers, S.-B. Hong, A. G. Williams, J. Crawford, A. D. Griffiths, and S.-J. ParkWe report on one year of high-precision direct hourly radon observations at
King Sejong Station (King George Island) beginning in February 2013. Findings
are compared with historic and ongoing radon measurements from other
Antarctic sites. Monthly median concentrations reduced from 72 mBq m−3
in late-summer to 44 mBq m−3 in late winter and early spring. Monthly
10th percentiles, ranging from 29 to 49 mBq m−3, were typical of
oceanic baseline values. Diurnal cycles were rarely evident and local
influences were minor, consistent with regional radon flux estimates one
tenth of the global average for ice-free land. The predominant fetch region
for terrestrially influenced air masses was South America
(47–53° S), with minor influences also attributed to aged
Australian air masses and local sources. Plume dilution factors of 2.8–4.0
were estimated for the most terrestrially influenced (South American) air
masses, and a seasonal cycle in terrestrial influence on tropospheric air
descending at the pole was identified and characterised. 2014/09/20 - 18:49

Drivers of column-average CO2 variability at Southern Hemispheric Total Carbon Column Observing Network sitesAtmospheric Chemistry and Physics, 14, 9883-9901, 2014Author(s): N. M. Deutscher, V. Sherlock, S. E. Mikaloff Fletcher, D. W. T. Griffith, J. Notholt, R. Macatangay, B. J. Connor, J. Robinson, H. Shiona, V. A. Velazco, Y. Wang, P. O. Wennberg, and D. WunchWe investigate factors that drive the variability in total column CO2 at
the Total Carbon Column Observing Network sites in the Southern Hemisphere
using fluxes tagged by process and by source region from the CarbonTracker
analysed product as well as the Simple Biosphere model. We show that the
terrestrial biosphere is the largest driver of variability in the Southern
Hemisphere column CO2. However, it does not dominate in the same fashion
as in the Northern Hemisphere. Local- and hemispheric-scale biomass burning
can also play an important role, particularly at the tropical site, Darwin.
The magnitude of seasonal variability in the column-average dry-air mole
fraction of CO2, XCO2, is also much smaller in the
Southern Hemisphere and comparable in magnitude to the annual increase.
Comparison of measurements to the model simulations highlights that there is
some discrepancy between the two time series, especially in the early part of
the Darwin data record. We show that this mismatch is most likely due to
erroneously estimated local fluxes in the Australian tropical region, which
are associated with enhanced photosynthesis caused by early rainfall during
the tropical monsoon season. 2014/09/20 - 18:49

Lightning NOx, a key chemistry–climate interaction: impacts of future climate change and consequences for tropospheric oxidising capacityAtmospheric Chemistry and Physics, 14, 9871-9881, 2014Author(s): A. Banerjee, A. T. Archibald, A. C. Maycock, P. Telford, N. L. Abraham, X. Yang, P. Braesicke, and J. A. PyleLightning is one of the major natural sources of NOx in the atmosphere.
A suite of time slice experiments using a stratosphere-resolving
configuration of the Unified Model (UM), containing the United Kingdom
Chemistry and Aerosols sub-model (UKCA), has been performed to investigate
the impact of climate change on emissions of NOx from lightning
(LNOx) and to highlight its critical impacts on photochemical ozone
production and the oxidising capacity of the troposphere. Two Representative
Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) are explored.
LNOx is simulated to increase in a year-2100 climate by 33% (RCP4.5)
and 78% (RCP8.5), primarily as a result of increases in the depth of
convection. The total tropospheric chemical odd oxygen production
(P(Ox)) increases linearly with increases in total LNOx and
consequently, tropospheric ozone burdens of 29 ± 4 Tg(O3) (RCP4.5)
and 46 ± 4 Tg(O3) (RCP8.5) are calculated here. By prescribing a
uniform surface boundary concentration for methane in these simulations,
methane-driven feedbacks are essentially neglected. A simple estimate of the
contribution of the feedback reduces the increase in ozone burden to 24 and
33 Tg(O3), respectively. We thus show that, through changes in
LNOx, the effects of climate change counteract the simulated mitigation
of the ozone burden, which results from reductions in ozone precursor
emissions as part of air quality controls projected in the RCP scenarios.
Without the driver of increased LNOx, our simulations suggest that the
net effect of climate change would be to lower free tropospheric ozone.

In addition, we identify large climate-change-induced enhancements in the
concentration of the hydroxyl radical (OH) in the tropical upper troposphere
(UT), particularly over the Maritime Continent, primarily as a consequence
of greater LNOx. The OH enhancement in the tropics increases oxidation
of both methane (with feedbacks onto chemistry and climate) and very
short-lived substances (VSLS) (with implications for stratospheric ozone
depletion). We emphasise that it is important to improve our understanding
of LNOx in order to gain confidence in model projections of composition
change under future climate. 2014/09/20 - 18:49

Tropospheric ozone increases over the southern Africa region: bellwether for rapid growth in Southern Hemisphere pollution?Atmospheric Chemistry and Physics, 14, 9855-9869, 2014Author(s): A. M. Thompson, N. V. Balashov, J. C. Witte, J. G. R. Coetzee, V. Thouret, and F. PosnyIncreases in free-tropospheric (FT) ozone based on ozonesonde records from
the early 1990s through 2008 over two subtropical stations, Irene (near
Pretoria, South Africa) and Réunion (21° S, 55° E;
~2800 km NE of Irene in the Indian Ocean), have been reported. Over
Irene a large increase in the urban-influenced boundary layer (BL, 1.5–4 km)
was also observed during the 18-year period, equivalent to
30% decade−1. Here we show that the Irene BL trend is at least
partly due to a gradual change in the sonde launch times from early morning
to the midday period. The FT ozone profiles over Irene in 1990–2007 are
re-examined, filling in a 1995–1999 gap with ozone profiles taken during the
Measurements of Ozone by Airbus In-service Aircraft (MOZAIC) project over
nearby Johannesburg. A multivariate regression model that accounts for the
annual ozone cycle, El Niño–Southern Oscillation (ENSO) and possible
tropopause changes was applied to monthly averaged Irene data from 4 to
11 km and to 1992–2011 Réunion sonde data from 4 to 15 km.
Statistically significant trends appear predominantly in the middle and upper
troposphere (UT; 4–11 km over Irene, 4–15 km over Réunion) in winter
(June–August), with increases ~1 ppbv yr−1 over Irene and
~2 ppbv yr−1 over Réunion. These changes are equivalent to
~25 and 35–45% decade−1, respectively. Both stations also
display smaller positive trends in summer, with a 45% decade−1
ozone increase near the tropopause over Réunion in December. To explain
the ozone increases, we investigated a time series of dynamical markers,
e.g., potential vorticity (PV) at 330–350 K. PV affects UT ozone over Irene
in November–December but displays little relationship with ozone over
Réunion. A more likely reason for wintertime FT ozone increases over
Irene and Réunion appears to be long-range transport of growing pollution
in the Southern Hemisphere. The ozone increases are consistent with
trajectory origins of air parcels sampled by the sondes and with recent
NOx emissions trends estimated for Africa, South America and
Madagascar. For Réunion trajectories also point to sources from the
eastern Indian Ocean and Asia. 2014/09/20 - 18:49

Cloud droplet activity changes of soot aerosol upon smog chamber ageingAtmospheric Chemistry and Physics, 14, 9831-9854, 2014Author(s): C. Wittbom, A. C. Eriksson, J. Rissler, J. E. Carlsson, P. Roldin, E. Z. Nordin, P. T. Nilsson, E. Swietlicki, J. H. Pagels, and B. SvenningssonParticles containing soot, or black carbon, are generally considered to
contribute to global warming. However, large uncertainties remain in the net
climate forcing resulting from anthropogenic emissions of black carbon (BC),
to a large extent due to the fact that BC is co-emitted with gases and
primary particles, both organic and inorganic, and subject to atmospheric
ageing processes. In this study, diesel exhaust particles and particles from
a flame soot generator spiked with light aromatic secondary organic aerosol
(SOA) precursors were processed by UV radiation in a 6 m3 Teflon
chamber in the presence of NOx. The time-dependent changes of the soot
nanoparticle properties were characterised using a Cloud Condensation Nuclei
Counter, an Aerosol Particle Mass Analyzer and a Soot Particle Aerosol Mass
Spectrometer. The results show that freshly emitted soot particles do not
activate into cloud droplets at supersaturations ≤2%, i.e. the BC core coated with primary organic aerosol (POA) from the exhaust is
limited in hygroscopicity. Before the onset of UV radiation it is unlikely
that any substantial SOA formation is taking place. An immediate change in
cloud-activation properties occurs at the onset of UV exposure. This change
in hygroscopicity is likely attributed to SOA formed from intermediate
volatility organic compounds (IVOCs) in the diesel engine exhaust. The change
of cloud condensation nuclei (CCN) properties at the onset of UV radiation
implies that the lifetime of soot particles in the atmosphere is affected by
the access to sunlight, which differs between latitudes. The ageing of soot
particles progressively enhances their ability to act as cloud condensation
nuclei, due to changes in: (I) organic fraction of the particle, (II)
chemical properties of this fraction (e.g. primary or secondary organic
aerosol), (III) particle size, and (IV) particle morphology. Applying
κ-Köhler theory, using a κSOA value of 0.13
(derived from independent input parameters describing the organic material),
showed good agreement with cloud droplet activation measurements for
particles with a SOA mass fraction ≥0.12 (slightly aged particles). The
activation properties are enhanced with only a slight increase in organic
material coating the soot particles (SOA mass fraction < 0.12),
however not as much as predicted by Köhler theory. The discrepancy
between theory and experiments during the early stages of ageing might be
due to solubility limitations, unevenly distributed organic material or
hindering particle morphology.

The change in properties of soot nanoparticles upon photochemical processing
clearly increases their hygroscopicity, which affects their behaviour both
in the atmosphere and in the human respiratory system. 2014/09/20 - 18:49

To what extent could water isotopic measurements help us understand model biases in the water cycle over Western SiberiaAtmospheric Chemistry and Physics, 14, 9807-9830, 2014Author(s): V. Gryazin, C. Risi, J. Jouzel, N. Kurita, J. Worden, C. Frankenberg, V. Bastrikov, K. Gribanov, and O. StukovaWe evaluate the isotopic composition of water vapor and precipitation
simulated by the LMDZ (Laboratoire de Météorologie Dynamique-Zoom) GCM
(General Circulation Model) over Siberia using several data sets: TES
(Tropospheric Emission Spectrometer) and GOSAT (Greenhouse gases Observing
SATellite) satellite observations of tropospheric water vapor, GNIP (Global
Network for Isotopes in Precipitation) and SNIP (Siberian Network for
Isotopes in Precipitation) precipitation networks, and daily, in situ
measurements of water vapor and precipitation at the Kourovka site in Western
Siberia. LMDZ captures the spatial, seasonal and daily variations reasonably
well, but it underestimates humidity (q) in summer and overestimates
δD in the vapor and precipitation in all seasons. The performance of
LMDZ is put in the context of other isotopic models from the SWING2 (Stable
Water Intercomparison Group phase 2) models. There is significant spread
among models in the simulation of δD, and of the
δD-q relationship. This confirms that δD
brings additional information compared to q only. We specifically
investigate the added value of water isotopic measurements to interpret the
warm and dry bias featured by most GCMs over mid and high latitude continents
in summer. The analysis of the slopes in δD-q diagrams and
of processes controlling δD and q variations suggests that
the cause of the dry bias could be either a problem in the large-scale
advection transporting too much dry and warm air from the south, or too
strong boundary-layer mixing. However, δD-q diagrams using
the available data do not tell the full story. Additional measurements would
be needed, or a more sophisticated theoretical framework would need to be
developed. 2014/09/20 - 18:49

High-resolution mapping of vehicle emissions in China in 2008Atmospheric Chemistry and Physics, 14, 9787-9805, 2014Author(s): B. Zheng, H. Huo, Q. Zhang, Z. L. Yao, X. T. Wang, X. F. Yang, H. Liu, and K. B. HeThis study is the first in a series of papers that aim to develop
high-resolution emission databases for different anthropogenic sources in
China. Here we focus on on-road transportation. Because of the increasing
impact of on-road transportation on regional air quality, developing an
accurate and high-resolution vehicle emission inventory is important for
both the research community and air quality management. This work proposes a
new inventory methodology to improve the spatial and temporal accuracy and
resolution of vehicle emissions in China. We calculate, for the first time,
the monthly vehicle emissions for 2008 in 2364 counties (an administrative
unit one level lower than city) by developing a set of approaches to
estimate vehicle stock and monthly emission factors at county-level, and
technology distribution at provincial level. We then introduce allocation
weights for the vehicle kilometers traveled to assign the county-level
emissions onto 0.05° × 0.05° grids based on the
China Digital Road-network Map (CDRM). The new methodology overcomes the
common shortcomings of previous inventory methods, including neglecting the
geographical differences between key parameters and using surrogates that
are weakly related to vehicle activities to allocate vehicle emissions. The
new method has great advantages over previous methods in depicting the
spatial distribution characteristics of vehicle activities and emissions.
This work provides a better understanding of the spatial representation of
vehicle emissions in China and can benefit both air quality modeling and
management with improved spatial accuracy. 2014/09/20 - 18:49

Analysis of the diurnal development of a lake-valley circulation in the Alps based on airborne and surface measurementsAtmospheric Chemistry and Physics, 14, 9771-9786, 2014Author(s): L. Laiti, D. Zardi, M. de Franceschi, G. Rampanelli, and L. GiovanniniThis study investigates the thermal structures of the atmospheric
boundary layer (ABL) and the near-surface wind field associated with a
lake-valley circulation in the south-eastern Italian Alps – the so-called
Ora del Garda. Two flights of an equipped motorglider allowed for the exploration
of the diurnal evolution of this circulation, from the onset, on Lake Garda's
shoreline, throughout its development along the Sarca Valley and Lakes Valley (Valle dei Laghi), to
the outflow into the Adige Valley. At the same time, surface observations,
both from a targeted field campaign and from routinely operated weather
stations, supported the analysis of the development of the Ora del Garda at
the valley floor.

In particular, in the valleys typical ABL vertical structures, characterized
by rather shallow convective mixed layers (~ 500 m) and (deeper)
weakly stable layers above, up to the lateral crest height, are identified in
the late morning. In contrast, close to the lake the ABL is stably stratified
down to very low heights, as a consequence of the intense advection of colder
air associated with the Ora del Garda flow (up to 6 m s–1). The
combined analysis of surface and airborne observations (remapped over 3-D
high-resolution grids) suggests that the lake-breeze front propagating
up-valley from the shoreline in the late morning penetrates slightly later at
the eastern end of the valley inlet (delay: ~ 1 h), probably due to
the asymmetric radiative forcing caused by the N–S valley orientation. On
the other hand, in the early afternoon the Ora del Garda overflows through an
elevated gap, producing an anomalous, strong cross-valley wind
(5 m s–1) at the Adige Valley floor north of Trento, which overwhelms
the local up-valley wind. This feature is associated with a strong deepening
of the local mixed layer (from 400 to 1300 m). The potential temperature 3-D
field suggests that the intense turbulent mixing may be attributed to the
development of a downslope wind across the gap, followed by a hydraulic jump
downstream. 2014/09/20 - 18:49

Estimates of European emissions of methyl chloroform using a Bayesian inversion methodAtmospheric Chemistry and Physics, 14, 9755-9770, 2014Author(s): M. Maione, F. Graziosi, J. Arduini, F. Furlani, U. Giostra, D. R. Blake, P. Bonasoni, X. Fang, S. A. Montzka, S. J. O'Doherty, S. Reimann, A. Stohl, and M. K. VollmerMethyl chloroform (MCF) is a man-made chlorinated solvent contributing to
the destruction of stratospheric ozone and is controlled under the "Montreal
Protocol on Substances that Deplete the Ozone Layer" and its amendments, which called for its phase-out in 1996 in developed countries and 2015 in
developing countries. Long-term, high-frequency observations of MCF carried
out at three European sites show a constant decline in the background mixing
ratios of MCF. However, we observe persistent non-negligible mixing ratio
enhancements of MCF in pollution episodes, suggesting unexpectedly high
ongoing emissions in Europe. In order to identify the source regions and to
give an estimate of the magnitude of such emissions, we have used a Bayesian
inversion method and a point source analysis, based on high-frequency
long-term observations at the three European sites.

The inversion identified southeastern France (SEF) as a region with
enhanced MCF emissions. This estimate was confirmed by the point source
analysis. We performed this analysis using an 11-year data set, from
January 2002 to December 2012. Overall, emissions estimated for the European
study domain decreased nearly exponentially from 1.1 Gg yr−1 in 2002 to
0.32 Gg yr−1 in 2012, of which the estimated emissions from the SEF
region accounted for 0.49 Gg yr−1 in 2002 and 0.20 Gg yr−1 in
2012. The European estimates are a significant fraction of the total
semi-hemisphere (30–90° N) emissions, contributing a minimum of 9.8% in
2004 and a maximum of 33.7% in 2011, of which on average 50% are from
the SEF region. On the global scale, the SEF region is thus responsible for
a minimum of 2.6% (in 2003) and a maximum of 10.3% (in 2009) of the
global MCF emissions. 2014/09/20 - 18:49