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Aerosol optical properties in the southeastern United States in summer – Part...

Brock, C., N. L. Wagner, B. E. Anderson, A. R. Attwood, A. Beyersdorf, Campuzano Jost, A. G. Carlton, D. A. Day, G. S. Diskin, T. Gordon, J. Jimenez-Palacios, D. A. Lack, J. Liao, M. Markovic, A. M. Middlebrook, N. L. Ng, A. Perring, M. S. Richardson, J. Schwarz, R. A. Washenfelder, A. Welti, L. Xu, L. D. Ziemba, and D. Murphy (2016), Aerosol optical properties in the southeastern United States in summer – Part 1: Hygroscopic growth, Atmos. Chem. Phys., 16, 4987-5007, doi:10.5194/acp-16-4987-2016.

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 κ-Köhler 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 ∼ 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 f (RH) values of 1.43 ± 0.67 at 70 % RH and 2.28 ± 1.05 at 90 % RH. The subsaturated κ-Köhler hygroscopicity parameter κ 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 κ = 0.05. This subsaturated κ 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.

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Tropospheric Composition Program (TCP)