Effect of aerosol humidification on the column aerosol optical thickness over the Atmospheric Radiation Measurement Southern Great Plains site

[1] This study investigates the aerosol humidification effect (AHE) using 70 profiles of the aerosol scattering coefficients at high ( similar to 80%) and low ( similar to 40%) relative humidity ( RH) levels and absorption coefficient at a low RH level obtained by a light aircraft (Cessna C- 172N) over the Southern Great Plains (SGP) site from April 2003 to June 2004. The column aerosol humidification factor, R( RH), defined as the ratio of the aerosol optical thickness (AOT) at the ambient RH to that at RH = 40% throughout the column rarely exceeded 1.3 ( mean, 1.09 +/- 0.12) over the SGP site. However, for an atmospheric column of a constant RH = 85%, R( RH) is greater than 1.5 for the majority of cases ( mean, 1.57 +/- 0.28). R( RH) was fitted to a function of column RH based on this unique aerosol data set. Several methods were proposed to estimate R( RH) for use when direct measurements of R( RH) are not available. It was found that the relationship between R( RH) and aerosol extinction coefficient weighted column-mean RH works best. Performance of other methods depends on the measurements available. Sensitivity of R( RH) to a very humid ( RH = 99%) layer with varying thickness values (0.1 - 0.3 km) is examined. The results indicate that the AHE on the AOT over the SGP site is not likely to exceed 50% on the average. The methods and results of this study may be utilized with caution to remove the AHE from the AOT retrieved from satellite or automated Sun photometer measurements, which will be useful for studies on aerosol indirect effect or quantifying cloud contamination in aerosol retrievals.