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

摘要

Aircraft observations of meteorological, trace gas, and aerosol propertieswere made during May–September 2013 in the southeastern United States (US)under fair-weather, afternoon conditions with well-defined planetary boundarylayer structure. Optical extinction at 532 nm was directly measured atrelative humidities (RHs) of  ∼  15,  ∼  70, and  ∼  90 % andcompared with extinction calculated from measurements of aerosol compositionand size distribution using the -Köhler approximation forhygroscopic growth. The calculated enhancement in hydrated aerosol extinctionwith relative humidity, (RH), calculated by this method agreed well withthe observed (RH) at  ∼  90 % RH. The dominance of organicaerosol, which comprised 65 ± 10 % of particulate matter withaerodynamic diameter  <  1 µm in the planetary boundary layer,resulted in relatively low (RH) values of 1.43 ± 0.67 at70 % RH and 2.28 ± 1.05 at 90 % RH. The subsaturated -Köhler hygroscopicity parameter for the organic fraction ofthe aerosol must have been  <  0.10 to be consistent with 75 % of theobservations within uncertainties, with a best estimate of   =  0.05. This subsaturated value for the organic aerosol inthe southeastern US is broadly consistent with field studies in ruralenvironments. A new, physically based, single-parameter representation wasdeveloped that better described (RH) than did the widely used gammapower-law approximation.