Producing aerosol size distributions consistent with optical particle counter measurements using space-based measurements of aerosol extinction coefficient

Abstract

Stratospheric aerosol has been observed by several long-lived observational systems. These include the University of Wyoming series of balloon-borne optical particle counters (OPCs) (1971-2020) and the Stratospheric Aerosol and Gas Experiment (SAGE) series of instruments, particularly SAGE II (1984-2005). Inferences of aerosol surface area density (SAD) and volume density are straightforward using data from OPCs. Conversely, many numerical methods to infer size distributions and SAD have been applied to SAGE II observations, but all are limited by the restricted number of independent wavelengths of the SAGE optical measurements. We have developed a new method that uses OPC observations to constrain SAGE II inferences of aerosol properties. We start by noting that, whatever the details of the underlying size distribution, the SAGE II-measured aerosol extinction coefficient ratio (525-1020 nm) must reflect the shape of the underlying aerosol size distribution for particles that dominate the extinction coefficient values (roughly radii from 0.1 to 0.5 μm). Since this extinction ratio can be easily calculated from OPC measurements, we use the OPC size distribution measurements, across a broad range of aerosol levels from background to highly volcanic, to compute the associated 525-1020 nm extinction coefficient ratios for each measurement. We then sort the OPC measurements by these ratios (across a range of roughly 1-6) into discrete ratio bins and derive mean bimodal log-normal size distributions for each bin using a particle swarm optimization. These fits can be applied to SAGE II observations without the need for further retrieval calculations, effectively producing an OPC-like product consisting of the six bimodal parameters for all SAGE II observations. This method successfully captures the median behavior of the OPC inferences of bulk parameters like aerosol surface area and volume densities, although we also observe a significant altitude dependence particularly in the lower stratosphere. In addition, there are occasional deviations of SAD from the fit behavior by as large as a factor of 10 for individual OPC measurements of SAD, primarily due to variations in small-radii particle number density (roughly smaller than about 0.15 μm). The presence of such particles is effectively invisible to extinction coefficient measurements such as those by SAGE II.