[1] A study was conducted during summer 2006 at Great Smoky Mountains (GRSM) National Park (NP), TN, to address issues related to estimating aerosol light extinction in the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. The revised IMPROVE equation calculates PM 2.5 light scattering (Bsp) from ammonium sulfate, ammonium nitrate, organic carbon mass, and fine soil concentrations; dry scattering efficiencies; and factors that account for hygroscopic growth. Organics are assumed to be nonhygroscopic. The organic compound mass (OCM)/organic carbon (OC) ratio is assumed to be 1.8. Experiments involving in situ and laboratory measurements were conducted to address issues related to (1) concentration-varying scattering efficiencies; (2) aerosol hydration state; (3) the OCM/OC ratio; and (4) the organic hygroscopicity. Filter-based measurements indicated that sulfate was acidic, with an average NH 4 + /SO 4 = molar ratio of 1.16. Ambient State Hygroscopic Tandem Differential Mobility Analyzer measurements of the ambient hydration state rarely indicated deliquescence. The frequency of hysteresis ranged from 29% to 46% for 0.05 and 0.2 mm particles, respectively. There was a clear relationship between dry particle mean diameter and volume at GRSM, supporting the assumption that an increase in particle size during transport increases both the scattering efficiency and concentration. Water-soluble organic carbon (WSOC) was isolated from water extracts of high-volume filter samples using XAD solid-phase absorbents. The average ratios of OCM measured gravimetrically to OC measured by thermal optical reflectance in residues of isolated WSOC and dichloromethane (DCM) extracts were 2.4 ± 0.3 and 1.9 ± 0.2, respectively. Hygroscopic growth factors (GF) of aerosols generated from WSOC extracts averaged 1.10 ± 0.02, 1.13 ± 0.03, and 1.19 ± 0.04 at 80%, 85%, and 90% RH, respectively. These results indicate that, at GRSM during summer, at least some of the organic aerosol was hygroscopic.

Abstract. We present results from two field deployments of a unique tandem differential mobility analyzer (TDMA) configuration with two primary capabilities: identifying alternative stable or meta-stable ambient aerosol hydration states associated with hysteresis in aerosol hydration behavior and determining the actual Ambient hydration State (AS-TDMA). This data set is the first to fully classify the ambient hydration state of aerosols despite recognition that hydration state significantly impacts the roles of aerosols in climate, visibility and heterogeneous chemistry. The AS-TDMA was installed at a site in eastern Tennessee on the border of Great Smoky Mountains National Park for projects during the summer of 2006 and winter of [2007][2008]. During the summer, 12 % of the aerosols sampled in continuous AS-TDMA measurements were found to posses two possible hydration states under ambient conditions. In every case, the more hydrated of the possible states was occupied. The remaining 88 % did not posses multiple possible states. In continuous measurements during the winter, 49 % of the aerosols sampled possessed two possible ambient hydration states; the remainder possessed only one. Of those aerosols with multiple possible ambient hydration states, 65 % occupied the more hydrated state; 35 % occupied the less hydrated state. This seasonal contrast is supported by differences in the fine particulate (PM 2.5 ) composition and ambient RH as measured during the two study periods. In addition to seasonal summaries, this work includes case studies depicting the variation of hydration state with changing atmospheric conditions.

We present results from two field deployments of a unique tandem differential mobility analyzer (TDMA) configuration with two primary capabilities: identifying alternative stable or meta-stable ambient aerosol hydration states associated with hysteresis in aerosol hydration behavior and determining the actual Ambient hydration State (AS-TDMA). This data set is the first to fully classify the ambient hydration state of aerosols despite recognition that hydration state significantly impacts the roles of aerosols in climate, visibility and heterogeneous chemistry. The AS-TDMA was installed at a site in eastern Tennessee on the border of Great Smoky Mountains National Park for projects during the summer of 2006 and winter of 2007–2008. During the summer, 12 % of the aerosols sampled in continuous AS-TDMA measurements were found to posses two possible hydration states under ambient conditions. In every case, the more hydrated of the possible states was occupied. The remaining 88 % did not posses multiple possible states. In continuous measurements during the winter, 49 % of the aerosols sampled possessed two possible ambient hydration states; the remainder possessed only one. Of those aerosols with multiple possible ambient hydration states, 65 % occupied the more hydrated state; 35 % occupied the less hydrated state. This seasonal contrast is supported by differences in the fine particulate (PM_2.5) composition and ambient RH as measured during the two study periods. In addition to seasonal summaries, this work includes case studies depicting the variation of hydration state with changing atmospheric conditions

A field-deployable instrument has been developed that isolates from an ambient aerosol those particles that have critical supersaturations, S c , within a narrow, user-specified, range. This Differential Activation Separator (DAS) consists of two continuous flow diffusion chambers housed within a single enclosure. Particles are introduced into the upstream chamber referred to as the CCN remover (CCNR) near the centerline between a warm, water-soaked, plate and a cool, continuously circulated, water bath. Those particles that activate at the resulting peak supersaturation, S p , grow quickly and fall into the water bath. The remaining aerosol enters the second chamber referred to as the CCN separator (CCNS), which differs from the CCNR primarily in the use of a salt solution in the lower bath. The imposed temperature differential establishes an S p slightly higher than that maintained in the upstream chamber, while the presence of a salt solution at the lower boundary results in a subsaturated region in roughly the lower half of the chamber. Those particles having (S p ) CCNR < S c < (S p ) CCNS activate in this chamber and begin to fall due to gravitational settling. Before reaching the lower bath, the droplets evaporate in the subsaturated environment and continue to travel towards the chamber exit. The previously activated particles in the lower half of the chamber and the unactivated particles in the upper half are extracted in separate flows that are subsequently dried. Calibration of the DAS was achieved by measuring the size distribution of separated particles when a polydisperse ammonium sulfate aerosol was introduced.