In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 1. Aerosol optical properties

Aerosol optical properties were measured over the Southern Great Plains (SGP) cloud and radiation test bed site using a light aircraft (Cessna C-172N). The aircraft flew level legs at altitudes between 500 m and 3500 m several times per week over the course of 2 years in order to obtain a statistically representative data set of in situ aerosol vertical profiles. Instrumentation on the aircraft was similar to that at the surface SGP site so that measurements at the surface and aloft could easily be compared. Measured parameters included total light scattering, backscattering, and absorption, while calculated parameters included single-scattering albedo, backscatter fraction, and Angstrom exponent. Statistical plots of aerosol optical properties and their variation in the lower column (0-4000 m) showed that over the course of the 2 years studied, albedo, backscatter fraction, and Angstrom exponent were fairly invariant with altitude (at least up to 1800 m). Despite the vertical consistency the correlation between column average and surface values for single-scattering albedo, backscatter fraction, and Angstrom exponent tended to be quite low, with R-2 ranging from 0.3 to 0.6 and linear regression slopes ranging from 0.2 to 0.6. These results suggest that long-term surface aerosol measurements capture the column aerosol properties but may not be as representative of day-to-day variations in the column. Comparison of aerosol optical depth (AOD) calculated from the vertical profiles with other measurements of AOD made at SGP (i.e., by the Cimel Sun photometer and the multifilter rotating shadowband radiometer (MFRSR)) showed fair correlation (R(2)similar to0.7 Cimel, R(2)similar to0.8 MFRSR), although the aircraft AODs tend to have a consistent offset of -0.04, even after incorporating a correction for supermicrometer aerosol and stratospheric aerosol.