Genetic Algorithm Retrieval of Real Refractive Index from Aerosol Distributions that are not Lognormal

Abstract

Size distributions of real-world aerosols typically deviate substantially from log-normal or other simple mathematical descriptions. The effect of assuming a lognormal shape for distributions that are not well described by this function on the genetic algorithm (GA) retrieval of the real refractive index (m r ) from scattering properties is investigated. Tests using several laboratory-generated ammonium sulfate aerosols with relatively broad bimodal distributions and with synthetic distributions generated by combining two log-normal distributions are examined. Scattering of the ammonium sulfate aerosols was measured with a dual polarization polar nephelometer. Analysis of the GA retrieval with the assumption that the distribution is lognormal for both the ammonium sulfate aerosol and synthetic distorted distributions show that the retrieved m r is within 0.015 of the expected value in all cases tested. The retrieved values for both the mean and the standard deviation of the aerosol size distribution become more inaccurate as the level of distortion increases. In our experiments, the GA determined mean aerosol diameter is always larger than the measured mean because in all of the distorted distributions there are significantly more large particles than the lognormal assumption. The intensity of particle scattering is generally a function of the square of the particle diameter hence as the distortion increases, the larger particles contribute more to the overall scattering on which the GA retrieval is based. We show that assuming a distribution is lognormal could potentially introduce errors in calculating radiative forcing by at least 12%. The total scattering coefficient measured by integrating nephelometers can be off by a factor of 2 for distortion levels similar to those seen in some of our experiments.

Retrievals of Aerosol Optical and Microphysical Properties from Imaging Polar Nephelometer Scattering Measurements

“…Retrievals of the monodisperse PSL spheres produced real refractive index values that were within the range of existing measurements available in the literature at all three wavelengths (Bateman et al, 1959;Ma et al, 2003;Sultanova et al, 2003;Jones et al, 2013). The spectral dependence of the retrieved values, as well as the three most recently reported Cauchy's equation parameterizations of PSL refractive index can be found in Fig.…”

“…9. The retrieved imaginary part of the refractive index for these spheres was of the order of 10 −3 for all three wavelengths, slightly higher than the values of around 4 × 10 −4 that have been reported by more sensitive techniques (Bateman et al, 1959;Ma et al, 2003). Figure 10 shows the spectrally dependent distribution of the retrieved dry refractive indices for the 50 chosen SEAC 4 RS cases.…”

Real refractive indices of α‐ and β‐pinene and toluene secondary organic aerosols generated from ozonolysis and photo‐oxidation

“…The profiles of measured distributions with Δ dist < 0.07 do not seem distorted at first glance, while Δ dist values > 0.2 can have distinct dual mode features. Barkey et al [2010] show theoretically and experimentally that the retrieved m r is accurate to within ±0.014 of the expected m r when Δ dist is less than 0.585. The average Δ dist for the aerosols seen in these experiments is about 0.2, with a range of 0.04 to 0.4.…”

“…The mean falls within ±10% in 77% of cases. This is expected as the scattering from a nonlognormal distribution is not the same as that from a lognormal distribution as discussed by Barkey et al [2010].…”

Retrievals of aerosol optical and microphysical properties from Imaging Polar Nephelometer scattering measurements

“…Intensity and polarization measurements of ambient aerosols made by the Tohoku University single wavelength polar nephelometer in Sendai, Japan have been inverted to obtain complex refractive index and number concentrations in six log-spaced size bins (Tanaka et al, 1983;Zhao, 1999). There have also been attempts to retrieve only the refractive index, while constraining the model's size distribution with a traditional particle sizer (Barkey et al, 2007(Barkey et al, , 2010. The converse approach was reported by Lienert et al (2003), who took polarized measurements of sea spray and determined PSD by assuming a refractive index value expected for sodium chloride particles at the ambient relative humidity.…”