Abstract
Abstract. 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.
Highlights
Particles in the atmosphere scatter and absorb solar radiation
The aerosol mass spectrometers (AMS) used during Southeastern Nexus of Air Quality and Climate (SENEX) employed a compact time-of-flight mass spectrometer (CToF), while that used during SEAC4RS employed a highresolution time-of-flight mass spectrometer with greater resolving power (HR-ToF; DeCarlo et al, 2006)
We describe the observed aerosol properties and their vertical structure, explore the relationship between particle composition and hygroscopicity, place constraints on the hygroscopicity of the organic aerosol (OA) component of the submicron aerosol, and present a new parameterization to describe the f (RH) curve
Summary
Particles in the atmosphere scatter and absorb solar radiation. Atmospheric aerosol extinction (i.e., scattering + absorption) reduces visibility and usually cools the earth, especially over dark surfaces such as oceans and forests. Brock et al.: Aerosol optical properties in the southeastern US second largest contributor to total uncertainty in climate forcing (IPCC, 2013; Bond et al, 2013)
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