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Optical measurements of aerosol size distributions in Great Smoky Mountains National Park: particle hygroscopicity and its impact on visibility




Kreidenweis, Sonia M., author
Ames, Rodger B., author

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Aerosol size distributions were measured during the 1995 Southeastern Aerosol and Visibility Study (SEAVS) in Great Smoky Mountains National Park using a PMS ASASP-X optical aerosol spectrometer. Ambient aerosol was conditioned in a relative humidity (RH) controlled inlet before sampling. 130 dry (RH ~ 15%) and 112 humidified aerosol size distributions, plus 24 distributions at ambient RH, were recorded during daylight hours for aerosol in the size range 0.1 < Dp <2.5 µ. Particle light scattering from the ASASP-X was inverted to particle sizes using Mie theory and applying a refractive index of either 1.530-0i or 1.501-0i for dry conditions, depending on the ambient aerosol chemical composition. A dry aerosol volume concentration time line from this work, when compared with a similar time line of aerosol mass concentration from IMPROVE samplers, indicates the ASASP-X provided a reliable representation of temporal trends in the ambient aerosol loading. The median dry aerosol geometric mass mean diameter measured during SEAVS was 0.28 µm, with a range from 0.24 to 0.38 µm, and median geometric standard deviation of 1.64. Sequential dry and humidified aerosol size distributions were corrected for refractive index dependence on RH and used to derive ambient aerosol hygroscopicity as a function of RH. This work demonstrates that experimentally derived water absorption is equivalent to or less than predicted by theory, assuming ambient aerosol water uptake is dictated by ionic compounds that have a chemical composition consistent with the particle fine mass measured during SEAVS. In this work, special consideration is given to the uncertainty in derived aerosol water contents and the degree to which this uncertainty propagates to estimates of light scattering. An ultimate goal of this project is to augment visibility and radiative transfer models through a better understanding of how RH affects the ambient aerosol size distribution in the southeastern U.S.


National Park Service.

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southeastern aerosol and visibility study (SEAVS)
Aerosols -- Measurements
Air -- Pollution -- Great Smoky Mountains National Park (N.C. and Tenn.)


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