Abstract
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.
Highlights
The organic fraction of submicron particles has been shown to be significant in nearly every region of the troposphere: urban (Zhang et al, 2005), rural (Alfarra et al, 2004), marine (Middlebrook et al, 1998), and the free troposphere (Novakov et al, 1997)
Despite some small differences in the air mass regions we identified in this study, it is noteworthy that the variability of Organic mass (OM) concentrations and alkane and alcohol fractions of OM within each region is broader than the differences in their means
Single particle analysis using STXM-NEXAFS showed that a large variety of particles from a mixture of different sources and processes were responsible for the organic particle composition in this region
Summary
The organic fraction of submicron particles has been shown to be significant in nearly every region of the troposphere: urban (Zhang et al, 2005), rural (Alfarra et al, 2004), marine (Middlebrook et al, 1998), and the free troposphere (Novakov et al, 1997). Organic particles can enter the atmosphere directly though anthropogenic processes such as fossil fuel combustion and biomass burning or natural pathways such as wind-driven dust and soil suspension. Despite the recognition of the ubiquity of organic aerosol in the atmosphere, the organic composition is largely uncharacterized, in part due to the large number of chemicals involved (Lewis et al, 2000) and the analytical challenges associated with this heterogeneity (Turpin et al, 2001). Day et al.: Organic aerosol composition during INTEX-B metropolitan region, and the California Central Valley. The C130 payload included a large suite of aerosol, gas, and radiation instrumentation which is summarized in Singh et al (2009)
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