Hygroscopic properties of an organic-laden aerosol

Observation and appreciation of scenic landscape features, airport runway visibility, and the earth's radiation balance are all dependent on the radiative properties of the atmosphere, which in turn are dependent on the scattering and absorption characteristics of ambient aerosols. Atmospheric scattering and, to a lesser degree, absorption characteristics are highly dependent on the amount of water vapor absorbed by aerosols under ambient relative humidity (RH) conditions. Water vapor absorptive properties of inorganic aerosols have been extensively measured and modeled; however, hygroscopic properties of organic aerosols are less understood, especially as they occur in the ambient atmosphere. Therefore, an aerosol characterization study was conceived and implemented at Yosemite National Park, which is highly impacted by carbonaceous aerosols. The overall objective of the study was to characterize the physical, chemical, and optical properties of a carbon-dominated aerosol, including the ratio of total organic matter weight to organic carbon, organic mass scattering efficiencies, and the hygroscopic characteristics of a carbon-laden ambient aerosol. The study was conducted during July, August, and the first week of September at Turtleback Dome on the south rim of the entrance to Yosemite Valley. The ratio of the scattering coefficient at some RH divided by the scattering coefficient at some minimum RH (f(RH) = b(scat)(RH)/b(scat)(RHmin)) was measured over a wide range of RHs. f(85 < RH < 90) decreased from about 2.0 to < 1.2 as the organic carbon mass (OMC/(NH4)(2)(SO4)) ratio increased from a low of 0.57 to 11.15, implying that the f(RH) associated with organics is small, possibly on the order of 1.1 or less. Furthermore, modeling f(RH) as a function of RH suggested that ambient organic mass aerosols may be weakly hygroscopic with an f(RH) at RH = 85-90% of about 1.1. (c) 2005 Elsevier Ltd. All rights reserved.