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 ( σ ep ) was measured at 355, 532, and 1064 nm at 25%, 60%, and 85% relative humidity (RH) for both sub‐1‐ and sub‐10‐ μ m‐diameter particles with a cavity ring–down aerosol extinction spectrometer. The 532‐nm σ ep was coupled with the 532‐nm light absorption coefficient ( σ ap ) measured with a photoacoustic absorption spectrometer to calculate the aerosol single scattering albedo ( ω ) with absolute uncertainty <0.01. The σ ep dependence on RH was expressed in terms of gamma ( γ ). The sampled aerosols covered a broad spectrum of γ and ω values; aerosols from traffic emissions were hydrophobic and highly light‐absorbing with γ ∼ 0.4 and ω ∼ 0.6, whereas the regional aerosols exhibited variable values of both γ and ω . Aerosols with the highest sulfate content also had the highest γ and ω values (>0.65 and >0.9, respectively). The optical data were used to estimate local, top of atmosphere aerosol‐induced climate forcing (Δ F R ). The Δ F R calculations were performed using both ω values measured at 25% RH and ω values converted to ambient RH. The calculated ambient Δ 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.