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 F-Köhler hygroscopicity parameter F 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 F- Combining double low line -0.05. This subsaturated F 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.
CITATION STYLE
Brock, C. A., Wagner, N. L., Anderson, B. E., Attwood, A. R., Beyersdorf, A., Campuzano-Jost, P., … Murphy, D. M. (2016). Aerosol optical properties in the southeastern United States in summer - Part 1: Hygroscopic growth. Atmospheric Chemistry and Physics, 16(8), 4987–5007. https://doi.org/10.5194/acp-16-4987-2016
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