Organic composition of single and submicron particles in different regions of western North America and the eastern Pacific during INTEX-B 2006
Abstract. Single particles were collected from an aircraft platform as part of the Intercontinental Chemical Transport Experiment – Phase B (INTEX-B) conducted over the eastern Pacific and western North America. Single particle spectra were obtained using scanning transmission X-ray microscopy-near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). Bulk submicron particles were also collected and organic functional group and elemental concentrations were quantified with Fourier transform infrared (FTIR) spectroscopy and X-ray fluorescence (XRF). Single particle spectra were classified into metaclasses associated with different sources and atmospheric processing. Particles with spectra indicative of secondary organic aerosol production and combustion sources were found at several locations and a range of altitudes. At lower altitudes, particles with spectra resembling soil dust and biomass burning fingerprints were commonly observed. The types of particle spectra most commonly observed aloft were similar to those observed during a previous study over the northwestern Pacific, indicating that long-range transport may have been an important particle source. Single particle spectra provided evidence that condensation and surface-limited oxidation contributed to particle growth. Organic mass (OM) concentrations ranged from 1 to 7 μg m−3 and averaged 2.4–4.1 μg m−3. Alkane functional groups were the largest fraction of OM, averaging 1.9–2.1 μg m−3 or 50–76% of OM, followed by alcohol functional groups (0.35–0.39 μg m−3, 9–14%). Organic and elemental concentrations are compared within and among geographical air mass regions: "Pacific" free troposphere, "Continental" free troposphere, "Seattle" metropolitan region, and the California "Central Valley". OM concentrations were highest and most variable in the Central Valley (3.5±2 μg m−3). Oxygen-to-carbon ratios were relatively constant in the Central Valley but variable for the Continental air masses. Overall, the OM concentrations showed greater variability within air mass categories as compared to averages among them, suggesting sampled air mass regions included a variety of sources and processing of organic aerosol.
