Impact of aerosol size distribution on extinction and spectral dependence of radiances measured by the OMPS Limb profiler instrument

Abstract. The Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP) has been flying on the Suomi NPP satellite since Oct 2011. It is designed to produce ozone and aerosol vertical profiles at ~ 2 km vertical resolution over the entire sunlit globe. The current operational (V1) aerosol extinction retrieval algorithm assumes a bimodal lognormal aerosol size distribution (ASD) whose parameters were derived from in situ data taken from an aircraft. In this paper we discuss the impact on the retrieval of using an ASD derived by the Community Aerosol and Radiation Model for Atmospheres (CARMA). We find that the impact of ASD on the retrieved extinctions varies strongly with the underlying reflectivity of the scene, and the functional form of this variation is very different at different scattering angles. We also evaluate how well the two ASDs perform in explaining the spectral dependence of Aerosol Scattering Index (ASI); a dimensionless quantity that we derive from the measured radiances by subtracting out the Rayleigh contribution. ASI is easier to interpret than radiances themselves and serves as our measurement vector. The results show that even though the two ASDs produce very different aerosol scattering phase function values at small and large scattering angles, the effect of the ASD on the spectral dependence of ASI is significant only at small angles. This implies that while OMPS/LP measurements have some information to evaluate the ASDs, they are most effective only at small scattering angles, which for LP measurement geometry occur only in the northern hemisphere. Our analysis suggests that overall CARMA ASD does a better job in explaining the spectral dependence of measured ASI than the ASD used in the operational V1 algorithm.