Retrieval of aerosol size distributions from in situ particle counter measurements accounting for instrument counting efficiency, and comparisons with satellite measurements of extinction and estimates of aerosol surface area.

Abstract:

The method to derive aerosol size distributions from in situ stratospheric measurements above Laramie, Wyoming, (41˚N, 106˚W) is modified to include in the size distribution retrieval an explicit cumulative distribution function (CDF) to describe the channel dependent instrument counting efficiency. This is motivated by the discovery [Kovilakam and Deshler, 2015] of an error in the calibration method applied to the optical particle counter (OPC40) developed in the late 1980s and used above Laramie from 1991-2012. The CDF employed is the integral of the normal distribution representing the pulse height distribution characteristic of the measurements of an aerosol channel. The integral of such a distribution is the aerosol counting efficiency. Results using the CDF are compared to previous derivations of aerosol size distributions both before [Deshler et al., 2003] and after Kovilakam and Deshler’s correction of the instrument number concentration for the OPC40 calibration error. The CDF method is found, without any tuning parameter, to reproduce or improve upon the Kovilakam and Deshler results, thus accounting for the calibration error without any external comparisons other than the laboratory determined counting efficiency at each aerosol channel. The method proposed applies equally well to other aerosol instruments used at Wyoming from 1971-1990 and beginning in 2006. Moments of the new aerosol size distributions compare well with aerosol extinctions measured by SAGE II and HALOE in the volcanic period 1991-1996 and non-volcanic period after 1996. SAGE II and HALOE estimates of aerosol surface area are found to be generally in good agreement with those derived using the new CDF method.