ALBERT

Description: [1]  A combined field and laboratory study was conducted to improve our understanding of the chemical and hygroscopic properties of organic compounds in aerosols sampled in the background continental atmosphere. PM 2.5 (particles with aerodynamic diameters smaller than 2.5 µm) aerosols were collected from 24 June to 28 July, 2010 at Storm Peak Laboratory in the Park Range of northwestern Colorado. New particle formation was frequent at Storm Peak Laboratory during this campaign, and the samples were not influenced by regional dust storms. Filter samples were analyzed for organic carbon (OC) and elemental carbon (EC), water soluble OC (WSOC), major inorganic ions, and detailed organic speciation. WSOC was isolated from inorganic ions using solid phase absorbents. Hygroscopic growth factors and cloud condensation nucleus (CCN) activity of the WSOC were measured in the laboratory. Organic compounds comprised the majority (average of 64% with a standard deviation of 9%) of the mass of measured species and WSOC accounted for an average of 89% (with a standard deviation of 21%) of OC mass. Daily samples were composited according to back-trajectories. On average, organic acids, sugars, and sugar alcohols accounted for 12.5 ± 6.2% (average ± standard deviation) of WSOC. Based on the composition of these compounds and that of high molecular weight compounds identified using ultra high resolution mass spectrometry, the organic mass to organic carbon ratio of the WSOC is estimated to be 2.04. The average hygroscopic growth factors at RH = 80% (GF 80 ) were 1.10 ± 0.03 for particles derived from isolated WSOC and 1.27 ± 0.03 for particles derived from the total water-soluble material (WSM). CCN activity followed a similar pattern. The critical diameters at a super-saturation of 0.35% were 0.072 ± 0.009 and 0.094 ± 0.006 µm for particles derived from WSM and isolated WSOC, respectively. These growth factor results compare favorably with estimates from thermodynamic models, which explicitly relate the water activity (RH) to concentration for the total soluble material identified in this study.