A comparison of lognormal and gamma size distributions for characterizing the stratospheric aerosol phase function from OPC measurements

Abstract

A series of in situ measurements made by optical particle counters (OPC) at Laramie, Wyoming provides size-resolved stratospheric aerosol concentration data for the period of 2008-2017. These data are analyzed in this study for the purpose of assessing the sensitivity of the stratospheric aerosol phase function to the aerosol size distribution (ASD) model used to fit the measurements. The two unimodal ASD models investigated are the uni-modal lognormal (UMLN) and gamma distribution 5 models, with the minimum χ 2 method employed to assess how well each ASD fits the measurements. The aerosol phase function (APF) for each ASD is calculated using Mie theory, and is compared to the APF derived from the Community Aerosol and Radiation Model for Atmospheres (CARMA) sectional aerosol microphysics module. Comparing the χ 2 values for the fits at altitudes 20 km and 25 km shows that the UMLN distribution better represents the OPC measurements. The importance of data at aerosol radius below 0.1 µm is also demonstrated: When these data are not available from OPC measurements, the 10 gamma distribution provides a more stable derived APF. The gamma distribution also fits the CARMA model results better than the UMLN model, when the CARMA model results are binned to mimic the OPC measurement bins (and therefore measurements between 0.05 and 0.1 µm are excluded).