Abstract
Abstract. A series of in situ measurements made by optical particle counters (OPCs) at Laramie, Wyoming, provides size-resolved stratospheric aerosol concentration data over the period 1971–2018. A subset of these data covering the period of 2008–2017 is 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 unimodal lognormal (UMLN) and gamma distribution models, with the minimum χ2 method employed to assess how well each ASD fits the measurements. The aerosol phase function (Pa(Θ)) for each ASD is calculated using Mie theory and is compared to the Pa(Θ) derived from the Community Aerosol and Radiation Model for Atmospheres (CARMA) sectional aerosol microphysics module. Comparing the χ2 values for the fits at altitudes of 20 and 25 km shows that the UMLN distribution better represents the OPC measurements; however, the gamma distribution fits the CARMA model results better than the UMLN model when the CARMA model results are subsetted into the OPC measurement bins. Comparing phase functions derived from the UMLN distribution fit to OPC data with gamma distributions fit to CARMA model results at the location of the OPC measurements shows a satisfying agreement (±5 %) within the scattering angle range of limb sounding satellites. This uncertainty is considerably larger if the CARMA data are fit with a UMLN.
Highlights
The presence of aerosol particles in the stratosphere has significant impact on atmospheric dynamics, atmospheric chemistry, and climate by altering the amount of radiation that reaches the Earth’s surface, as research over the past few decades has shown (Kremser et al, 2016; Ivy et al, 2017)
Comparisons of the extinction coefficients derived from optical particle counters (OPCs) measurements and Stratospheric Aerosol and Gas Experiment (SAGE) II occultation have shown differences that vary by more than 50 % for nonvolcanic periods (Kovilakam and Deshler, 2015); these differences have been largely eliminated after the calibration error identified by Kovilakam and Deshler (2015) was accounted for in the new method to derive uni/bimodal lognormal size distributions to fit OPC measurements (Deshler et al, 2019)
Measured limb scattered radiance is sensitive to the presence of stratospheric aerosols due to the long path the scattered solar photons have to travel through the aerosol layer to reach the sensor (Rieger et al, 2015; Loughman et al, 2018; Chen et al, 2018)
Summary
The presence of aerosol particles in the stratosphere has significant impact on atmospheric dynamics, atmospheric chemistry, and climate by altering the amount of radiation that reaches the Earth’s surface, as research over the past few decades has shown (Kremser et al, 2016; Ivy et al, 2017). Rieger et al (2018) have shown that when the radiance is simulated to include coarse mode particles in the atmosphere with an assumed AE, the differences between the lognormal parameters used in the simulation and the retrieval induce errors in the retrieved aerosol extinction as function of AE, which corresponds to 30 % for OSIRIS geometries and 50 % for SCIAMACHY geometries This is because the phase functions of the bimodal lognormal (BMLN) distributions vary more widely for a given AE, and this leads to a complicated relationship with the retrieved error. We conclude with a summary and recommendations on which distribution to choose depending on what kind of stratospheric aerosol measurements are available
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