Aerosol optical properties in the southeastern United States in summer – Part?1: Hygroscopic growth

摘要

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 532?nm was directly measured at relative humidities (RHs) of ?～??15, ?～??70, and ?～??90?% and compared with extinction calculated from measurements of aerosol composition and size distribution using the iκ/i-K?hler approximation for hygroscopic growth. The calculated enhancement in hydrated aerosol extinction with relative humidity, if/i(RH), calculated by this method agreed well with the observed if/i(RH) at ?～??90?% RH. The dominance of organic aerosol, which comprised 65?±?10?% of particulate matter with aerodynamic diameter ?&??1?μm in the planetary boundary layer, resulted in relatively low if/i(RH) values of 1.43?±?0.67 at 70?% RH and 2.28?±?1.05 at 90?% RH. The subsaturated iκ/i-K?hler hygroscopicity parameter iκ/i for the organic fraction of the aerosol must have been ?&??0.10 to be consistent with 75?% of the observations within uncertainties, with a best estimate of iκ/i??=??0.05. This subsaturated iκ/i 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 if/i(RH) than did the widely used gamma power-law approximation.