Genetic Algorithm Inversion of Dual Polarization Polar Nephelometer Data to Determine Aerosol Refractive Index

Abstract

A polar nephelometer that measures the light scattered into a two dimensional plane by a stream of aerosols that intersect a 350 milliwatt diode laser beam with a wavelength of 670 nm is described. A half wave plate is used to orient the laser light both parallel and perpendicular to the measurement plane. This scattering information combined with simultaneously measured size distribution information is used to determine the real (m r ) refractive index of the aerosols using a Genetic Algorithm search method. Errors in the retrieved m r based on noise in the scattering measurement as well as uncertainties in the size distribution measurement are examined. The m r of polystyrene latex spheres and ammonium sulfate droplets are determined and match expectations within experimental uncertainties using Mie-Lorenz theory. The angular scattering properties of spherical secondary organic aerosols generated by oxidizing α-pinene do not follow this theory and calls into question the inferred m r .

Dependence of Real Refractive Indices on O:C, H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and α-Pinene

“…Real refractive indices were retrieved by combining data from a polar nephelometer and a scanning mobility particle sizer and applying a genetic algorithm retrieval scheme (Barkey et al [2007]; Kim et al [2010]). Details on the chamber, its physical parameters, aerosol generation, measurements to characterize the gas phase, and aerosol optical properties are described in the online supplementary information (SI).…”

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.…”

Assessment of direct radiative forcing due to secondary organic aerosol over China with a regional climate model

“…Variability should be added to SOA optical parameters within radiative transfer modelling (Kim et al, 2010;Cappa et al, 2011). Many works have been done to calculate optical parameters through model, for example, Barkey et al (2007) used a Genetic Algorithm search method to calculate the real part of SOA RI, and Redmond and Thompson (2011) established a structureproperty relationship for mid-visible RI of SOA.…”