Aerosol optical properties in the southeastern United States in summer - Part 1: Hygroscopic growth Atmospheric Chemistry and Physics

Aircraft observations of meteorological, trace gas, and aerosol properties were made during May-September 2013 in the southeastern United States (US) under fair-weather, afternoon conditions with well-defined planetary boundary layer structure. Optical extinction at 532aEuro-nm was directly measured at relative humidities (RHs) of aEuro-15, aEuro-70, and aEuro-90aEuro-% and compared with extinction calculated from measurements of aerosol composition and size distribution using the kappa-Kohler approximation for hygroscopic growth. The calculated enhancement in hydrated aerosol extinction with relative humidity, f(RH), calculated by this method agreed well with the observed f(RH) at aEuro-90aEuro-% RH. The dominance of organic aerosol, which comprised 65aEuro-+/- aEuro-10aEuro-% of particulate matter with aerodynamic diameter < aEuro-1aEuro-A mu m in the planetary boundary layer, resulted in relatively low f(RH) values of 1.43aEuro-+/- aEuro-0.67 at 70aEuro-% RH and 2.28aEuro-+/- aEuro-1.05 at 90aEuro-% RH. The subsaturated kappa-Kohler hygroscopicity parameter kappa for the organic fraction of the aerosol must have been < aEuro-0.10 to be consistent with 75aEuro-% of the observations within uncertainties, with a best estimate of kappa aEuro- = aEuro-0.05. This subsaturated kappa value for the organic aerosol in the southeastern US is broadly consistent with field studies in rural environments. A new, physically based, single-parameter representation was developed that better described f(RH) than did the widely used gamma power-law approximation.