Aerosol characterization studies at Great Smoky Mountains National Park, summer 2006

Abstract

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 PM2.5 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 NH4+/SO4= 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 μm 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.