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  • Item
    Open Access
    Aerosol hygroscopicity and visibility estimates in the Great Smoky Mountains National Park
    (Colorado State University. Libraries, 1997-06) Kreidenweis, Sonia M., author; Hand, Jenny L., author
    Summertime visibility in the National Parks in the Eastern United States is often very poor, due to high particulate mass concentrations and high relative humidities. As a part of the Southeastern Aerosol and Visibility Study (SEAVS) in the Great Smoky Mountains National Park during the summer of 1995, aerosol size distributions (Dp = 0.1-3 µm) were measured with an Active Scattering Aerosol Spectrometer (ASASP-X). A relative humidity (RH) controlled inlet allowed for both dry and humidified measurements. The objective of this experiment was to examine the aerosol size distribution and its variation with RH to characterize its effect on visibility in the region. The ASASP-X was calibrated with polystyrene latex spheres (PSL) (m = 1.588), however, the instrument response was sensitive to the refractive index of the measured particles, which was typically much lower than that of PSL. An inversion technique accounting for varying particle real refractive index was developed to invert ASASP-X data to particle size. Dry (RH < 15%) particle refractive indices were calculated using the partial molar refractive index method and 12-hour fine aerosol (<2.5 µm) chemical compositions from the National Park Service Interagency Monitoring of Protected Visual Environments (IMPROVE) filter samples. A study average dry refractive index of m = 1.49 ± 0.02 was determined. The dry aerosol number distributions inverted using the scaling method were fit with single mode lognormal curves, resulting in dry accumulation mode size parameters. A study average total volume concentration of 7 ± 5 µm3 cm-3 was determined, with a maximum value of 26 µm3 cm-3. The large variability was due to extremes in meteorological situations occurring during the study. The study average volume median diameter was 0.18 ± 0.03 µm, with an average geometric standard deviation of 1.45 ± 0.06. A newly-developed iteration method was used to determine wet refractive indices, wet accumulation mode volume concentrations and water mass concentrations as a function of relative humidity. Theoretical predictions of water mass concentrations were determined using a chemical equilibrium model assuming only ammonium and sulfate were hygroscopic. Comparisons of predicted and experimental water mass showed agreement within experimental uncertainties. To examine the effects of particles on visibility, particle light scattering coefficients, bsp, were calculated with derived size parameters, refractive index and Mie theory. Dry scattering agreed well with nephelometer measurements made at SEAVS, with an average bsp of 0.0406-km-1. Estimates of particle light scattering growth (b/b0) were determined from ratios of wet and dry light scattering coefficients, and also agreed with nephelometer results. The new inversion techniques were compared to earlier, simpler methods which ignored variations in aerosol chemical composition. The simpler method yielded smaller mean diameters, however, hygroscopicity estimates were comparable to those derived using daily varying chemical composition. This suggests that although the aerosol chemical composition is needed to determine aerosol size parameters, it may not be critical for deriving hygroscopicity (or other ratios of size parameters). This result may be specific to this study, as the variation in refractive index with RH assumed by previous models appears to be a good estimate for that observed during SEAVS.
  • Item
    Open Access
    Optical measurements of aerosol size distributions in Great Smoky Mountains National Park: particle hygroscopicity and its impact on visibility
    (Colorado State University. Libraries, 1996-08) Kreidenweis, Sonia M., author; Ames, Rodger B., author
    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.