Aerosol optical and hygroscopic properties during TexAQS-GoMACCS 2006 and their impact on aerosol direct radiative forcing

In situ measurements of aerosol optical and hygroscopic properties were made on board the National Oceanic and Atmospheric Administration R/V Ronald H. Brown during the Texas Air Quality Study-Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS). The aerosol light extinction coefficient (sigma(ep)) was measured at 355, 532, and 1064 nm at 25%, 60%, and 85% relative humidity (RII) for both sub-1- and sub-10-eta m-diameter particles with a cavity ring-down aerosol extinction spectrometer. The 532-nm sigma(ep) was coupled with the 532-nm light absorption coefficient (sigma(ap)) measured with a photoacoustic absorption spectrometer to calculate the aerosol single scattering albedo (omega) with absolute uncertainty < 0.01. The sigma(ep) dependence on RII was expressed in terms of gamma (gamma). The sampled aerosols covered a broad spectrum of gamma and omega values; aerosols from traffic emissions were hydrophobic and highly light-absorbing with gamma similar to 0.4 and omega similar to 0.6, whereas the regional aerosols exhibited variable values of both g and w. Aerosols with the highest sulfate content also had the highest gamma and omega values (>0.65 and >0.9, respectively). The optical data were used to estimate local, top of atmosphere aerosol-induced climate forcing (Delta F-R). The Delta F-R calculations were performed using both omega values measured at 25% RII and omega values converted to ambient RII. The calculated ambient Delta F-R ranged from -7 to -40 W/m(2) with absolute uncertainty between 0.7 and 2.5 W/m(2). The results show that including aerosol hygroscopic properties in climate calculations is critical for improving estimates of aerosol forcing on climate.