CHEMICAL CHARACTERIZATION OF THE WATER SOLUBLE FRACTION OF SIZE SEGREGATED ORGANIC AEROSOL AT TWO SITES IN NEW ENGLAND

Data from samples collected through the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) program were analyzed to determine the variation in organic chemical composition as a function of particle size. Specific marker compounds were targeted to assess the importance of primary organic aerosol in the fine (diameters < 2.5 um) and coarse (diameters between 2.5 and 10 um) size modes and to identify potential sources. Samples were taken at two sites: Appledore Island (AI), located approximately 10 km off the New Hampshire coast in the Atlantic Ocean, and Thompson Farm (TF), a rural site approximately 25 km northwest of AI. Size segregation was achieved using cascade impactors that utilize eight size partitions between 400 nm and 10 um in aerodynamic diameter and a filter stage sampling aerosol below 400 nm in aerodynamic diameter. Fine mode and coarse mode filter measurements were used to validate impactor performance. Samples were analyzed for organic carbon (OC) and elemental carbon (EC) using the thermo-optical transmission method with a Sunset Labs (Hillsborough, NC) OC/EC instrument. Based on this analysis, samples with increased OC concentrations in the sub-micron and super-micron size ranges were selected for further analyses. The water soluble organic carbon (WSOC) fraction was isolated and concentrations of C2-C5 dicarboxylic acids were determined by ion chromatography. A comparison of several extraction methods for the separation of water soluble and insoluble organic fractions was also performed. An ultrafiltration technique was used to assess the presence and size distribution of high molecular weight organic compounds, likely in the form of organic oligomers or polycyclic aromatic hydrocarbons. Sampling site differences and seasonal variations in the size distribution of WSOC composition are discussed and potential sources are identified. Effects of WSOC in the sub-micron and super-micron aerosol size ranges on cloud activation are also assessed.