AMS Measurements in National Parks of Aerosol Mass, Size and Composition, Comparison with Filter Samples and Correlation with Particle Hygroscopicity and Optical Extinction Properties

We report a comparison of results from aerosol studies at Great Smoky Mountain National Park (2006), Mt. Rainier National Park (2009) and Acadia National Park (2011), all class I visibility areas associated with IMPROVE (Interagency Monitoring of Protected Visual Environments) sites. This collaborative study was sponsored by the Electric Power Research Institute (EPRI) and was done with the cooperation of the National Park Service and the EPA. The atmospheric aerosol composition in these sites is influenced by a number of anthropogenic as well as biogenic sources, providing a rich environment for fundamental aerosol studies. The primary purpose of these studies was to add state-of-the-art aerosol instrumentation to the standard light extinction and aerosol measurements at the site, used to determine parameters for the IMPROVE light extinction reconstruction equation, adopted by the EPA to estimate light extinction from atmospheric aerosol concentrations and Rayleigh scattering. The combination of these diverse measurements also provides significant insight into fundamental aerosol properties such as aging and radiative forcing. New instrumentation included a quadrupole aerosol mass spectrometer (Aerodyne Q-AMS-Smoky Mountain Study), a high resolution aerosol time-of-flight mass spectrometer (Aerodyne HR-ToF-AMS - Mt. Rainier and Acadia studies) for real time measurements that directly address the relationship between sulfate, nitrate, and OC size and concentration, which is related to cloud and dry gas-to-particle conversion as air masses age during transport, the relationship between WSOC hygroscopic growth and oxygenated organic (OOA) composition, the OCM/OC ratio, and the chemical composition that determines the ambient hygroscopic state. The OCM/OC ratio and organic water uptake was addressed with high-volume and medium volume PM2.5 aerosol samples. Aerosols were collected daily on Teflon coated glass fiber filters (TGFF) in four high-volume PM2.5 samplers operated by DRI. These measurements provide the fraction of total OC that is water-soluble. The ambient hydration state was determined by the Don Collins group in the field with an ambient state TDMA (AS-TDMA). Aerosols are classified at ambient RH with an ambient diameter of Da. The sample is then dried to <20%, typically below the crystallization point of most aerosols. The sample is alternately left dry or humidified to 85-95% RH. The dried (path 1) or humidified (path 2) sample is then conditioned back to the original ambient RH, attaining a diameter D_f. Continuous measurement of the hydration state over the diurnal cycle will be used to assess the validity of the assumption that hygroscopic growth follows the metastable branch of the ammonium sulfate growth curve.