Advanced IMPROVE Studies
EMSL Project ID
36409
Abstract
The proposed collaboration with Dr. Lizabeth Alexander of PNNL will enable continuous measurement of size-resolved aerosol chemical composition using the EMSL high resolution time-of-flight Aerosol Mass Spectrometer (H-ToF AMS) at Mount Rainier National Park during August, 2009. This experiment is part of a long-term study of the "IMPROVE equation" which estimates particle light scattering from aerosol chemical composition. Similar studies were conducted at Great Smoky Mountains National Park (GRSM) during summer, 2006 and winter, 2008. The revised "IMPROVE" equation calculates PM2.5 light scattering from sulfate, nitrate, organic carbon mass (OCM), and soil concentrations, dry scattering efficiencies, and factors that account for hygroscopic growth. Organics are assumed to be non-hygroscopic. The organic mass to organic carbon (OCM/OC) ratio is assumed to be 1.8. The following aspects of the IMPROVE equation will be studied with ambient and laboratory measurements: 1) concentration-varying dry scattering efficiencies; 2) ambient hydration state (deliquescence, hysteresis, smooth growth; 3) the OCM/OC ratio; and 4) organic hygroscopicity. Concentration-varying dry scattering efficiencies are based on the assumption that as air masses age during transport to remote sites, gas-to-particle conversion, particularly in-cloud oxidation, increases particle size and bulk concentrations of sulfate, nitrate and possibly organics. The H-ToF AMS provides an efficient means (the ToF mass spectrometer provides a complete mass spectrum as a function of particle size) of confirming the relationship between particle size and concentration for various chemical species. The IMPROVE equation assumes that sulfate and nitrate hygroscopic growth follows the upper leg (hysteresis branch) of the ammonium sulfate growth curve. The ambient hygroscopic state depends on the particle composition and mixing state, particularly, the degree of neutralization of sulfate by ammonia. The ambient hygroscopic state will be measured with the Texas A&M Ambient State Hygroscopic Tandem Differential Mobility Analyzer (AS-HTDMA) as a function of particle size. Size-resolved chemical composition measured with the AMS will facilitate interpretation of the AS-HTDMA data by providing the ammonium to sulfate ratio and relative abundance of potentially insoluble organics. The OCM/OC ratio in part determines the contribution of organic carbon (OC) to light scattering. OCM in water extracts treated with XAD resins to remove inorganic ions and in dichloromethane extracts will be measured gravimetrically and OC will be measured by thermal optical reflectance. The H-ToF AMS provides an alternative approach for measuring the OCM/OC ratio as both OCM and OC can be derived from the organic fragments in the AMS mass spectrum. Hygroscopic growth factors (GFs) for isolated WSOC will be measured at Texas A&M with an HTDMA. WSOC GFs of aerosols at GRSM during summer averaged 1.10±0.02, 1.13±0.03, and 1.19±0.04 at 80, 85, and 90% RH, respectively. The AMS has been used to distinguish oxygenated organic aerosols (OOA) from hydrocarbon-like organic aerosols (HOA). Since OOA presumably contains more hydrophilic oxygenated functional groups (COOH, C=O, OH), higher growth factors should be associated with higher ratios of OOA to HOA. The EMSL H-ToF AMS will thus provide invaluable information which will help address all of the major scientific questions in this research.
Project Details
Project type
Exploratory Research
Start Date
2009-07-28
End Date
2010-08-01
Status
Closed
Released Data Link
Team
Principal Investigator