Retrieval of aerosol size distribution moments from multiwavelength particulate extinction measurements

Abstract

Two methods for inferring aerosol size distribution moments (for example, particle area and volume) from multiwavelength particulate extinction measurements have been investigated. The first is an analytic, or eigenvalue, technique that approximates an appropriate moment‐weighting function by a linear combination of kernel functions (Mie extinction cross sections). It then uses the same linear combination of the measured particulate extinction coefficients to infer the size distribution moment. The second technique is a conversion ratio approach that uses the ratio of the particulate extinction measurements at two wavelengths to choose a model moment‐to‐extinction conversion ratio. For each method, expressions are derived that estimate the uncertainty in retrieved moments. The eigenvalue technique has been tested by application to synthetic particulate extinction measurements calculated at wavelengths corresponding to those of the satellite‐borne Stratospheric Aerosol and Gas Experiment (SAGE) II Sun photometer. The synthetic measurements were calculated from model stratospheric aerosol size distributions and from stratospheric aerosol size distributions measured by a wire impactor before and after the eruption of El Chichón. Moments have been derived for two‐ and three‐wavelength retrievals. In general, it was found that retrieved moments agreed well with the analytical model values, when random experimental uncertainties of a few percent or less were included in the synthetic extinction measurements. Typical retrieval errors were between 10 and 20%, with a few exceptions. When random measurement uncertainties with a rms mean value of 5% were included in the synthetic extinction coefficients, retrieval errors increased somewhat, but they rarely exceeded 40%. When applied to synthetic particulate extinction measurements that were calculated from the wire impactor‐measured stratospheric aerosol size distributions and which included 30% rms random experimental uncertainties, the eigenvalue retrieval technique successfully reproduced the large changes in particle volume, effective radius, and distribution width that followed the eruption of El Chichón. Both the eigenvalue and conversion ratio moment retrieval methods have been used to infer the third moment, or volume, of the aerosol size distribution from actual particulate extinction measurements taken by SAGE II during an April 1985 correlative measurement experiment in Brazil. Corresponding inferences are found to be in good agreement with each other.