A Study of the Extinction-to-Backscatter Ratio of Marine Aerosol during the Shoreline Environment Aerosol Study

Abstract

Ground-based aerosol optical measurements made at near-ambient relative humidity (RH) under clean marine sampling conditions are presented and compared to 1) almost identical optical measurements made at a polluted continental site and 2) optical properties calculated from measured size distributions and Mie theory. The use of Mie theory (which assumes homogeneous spheres) is justified based on the fact that the sea-salt aerosol was measured in a hydrated state. This study focuses on the extinction-to-backscatter ratio S, an optical property required to interpret remote measurements by elastic backscatter lidar. For clean marine conditions, S is found to be 25.4 ± 3.5 sr at 532 nm (central value ± 95% confidence uncertainty). Other optical properties reported include single-scattering albedo, wavelength dependence of scattering, fraction of scattering due to submicrometer particles, and hemispheric-backscatter fraction, as well as the extensive properties (e.g., scattering coefficient) upon which these intensive properties are based. In addition, correlation scale lengths are examined via the autocorrelation function. Except during deliberate drying experiments that lowered the measurement RH below 43%, S exhibited little variation with RH. A subtle but clearly detectable change in optical properties was observed at the onset of volcanically influenced sampling conditions.