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Simultaneous retrieval of effective refractive index and density from size distribution and light-scattering data: weakly absorbing aerosol

Simultaneous retrieval of effective refractive index and density from size distribution and light-scattering data: weakly absorbing aerosolAtmospheric Measurement Techniques, 7, 3247-3261, 2014Author(s): E. Kassianov, J. Barnard, M. Pekour, L. K. Berg, J. Shilling, C. Flynn, F. Mei, and A. JeffersonWe propose here a novel approach for retrieving in parallel the effective
density and real refractive index of weakly absorbing aerosol from optical
and size distribution measurements. Here we define "weakly absorbing" as
aerosol single-scattering albedos that exceed 0.95 at 0.5 μm. The
required optical measurements are the scattering coefficient and the
hemispheric backscatter fraction, obtained in this work from an integrating
nephelometer. The required size spectra come from mobility and aerodynamic
particle size spectrometers commonly referred to as a scanning mobility particle
sizer and an aerodynamic particle sizer. The performance of this approach is
first evaluated using a sensitivity study with synthetically generated but
measurement-related inputs. The sensitivity study reveals that the proposed
approach is robust to random noise; additionally the uncertainties of the
retrieval are almost linearly proportional to the measurement errors, and
these uncertainties are smaller for the real refractive index than for the
effective density. Next, actual measurements are used to evaluate our
approach. These measurements include the optical, microphysical, and chemical
properties of weakly absorbing aerosol which are representative of a variety
of coastal summertime conditions observed during the Two-Column Aerosol
Project (TCAP; http://campaign.arm.gov/tcap/). The evaluation includes
calculating the root mean square error (RMSE) between the aerosol
characteristics retrieved by our approach, and the same quantities calculated
using the conventional volume mixing rule for chemical constituents. For dry
conditions (defined in this work as relative humidity less than 55%) and
sub-micron particles, a very good (RMSE ~ 3%) and reasonable
(RMSE ~ 28%) agreement is obtained for the retrieved real
refractive index (1.49 ± 0.02) and effective density
(1.68 ± 0.21), respectively. Our approach permits discrimination
between the retrieved aerosol characteristics of sub-micron and sub-10-micron
particles. The evaluation results also reveal that the retrieved density and
refractive index tend to decrease with an increase of the relative humidity.

Atmospheric Measurement Techniques 2014/10/01 - 23:45 Czytaj