Deriving aerosol size distributions from the University of Wyoming optical particle counter measurements at SAGE II wavelengths

Stratospheric aerosol has been observed by several long-lived observational systems. These include the University of Wyoming balloon-borne optical particle counter (WOPC) (1971-present) and the Stratospheric Aerosol and Gas Experiment (SAGE) series of instruments and particularly SAGE II (1984-2005). WOPC reports unimodal and bimodal fits of its aerosol concentration measurements. Inferences of aerosol extinction coefficient are straightforward using SAGE II extinction kernels. In this project we have derived average shapes of bimodal lognormal aerosol size distributions (ASD) of WOPC measurements grouped in bins of 525 to 1020 nm extinction ratio. To do so we have utilized a particle swarm optimization algorithm to find best fits to the binned data. While this is successful the observations compared to the fit relationship shows a substantial spread. SAGE II measurements contain insufficient information about the density of particles much less than 0.1 μm to constrain the ASD beyond a point. We show the derived size distributions, discuss the fundamental limitations in attempting to infer ASD from SAGE II measurements, and the possible utility of offering the derived SAD/VD as a data product.