Project at a Glance
Title: Improved characterization of primary and secondary carbonaceous particles
Principal Investigator / Author(s): Russell, Lynn
Contractor: UC San Diego
Contract Number: 09-328
Research Program Area: Atmospheric Processes
Topic Areas: Chemistry & Reactivity
Secondary organic aerosols (SOA), known to form in the atmosphere from oxidation of volatile organic compounds (VOCs) emitted by anthropogenic and biogenic sources, are a poorly understood but substantial component of atmospheric particles. In this report, we examined the chemical and physical properties of SOA in summer 2010 at Bakersfield, California, a site influenced by anthropogenic and terrestrial biogenic emissions.
This study is composed of the following sections: (i) collection and measurement of ambient fine particles during the intensive field campaign, (ii) factor analysis for source apportionment, (iii) identifying particle clusters using cluster analysis, and (iv) studying formation mechanisms of the SOA components. We measured chemical composition of submicron particle mass (PM1) and sub 2.5 micron particle mass (PM2.5) using a set of complementary, ensemble, and single particle analysis techniques, including Fourier transform infrared (FTIR) spectroscopy, Aerosol Mass Spectrometry (AMS), and scanning transmission X- ray microscopy with near edge absorption fine structure (STXM-NEXAFS). Positive matrix factorization (PMF) was used to extract major components that contributed to the organic mass. Cluster analysis applied to the ensemble infrared spectra, the single-particle mass, and NEXAFS spectra separated particles into several groups, each of which has characteristics of distinct sources or atmospheric processes. By showing the consistency of particle source types (from factor analysis) and clusters, we indicate that SOA accounts for a major fraction of fine particle mass. Based on the composition and diurnal cycle of the SOA components and their relationships with potential atmospheric oxidants, formation mechanisms of the SOA components were estimated.
We found that OM accounted for 56% of PM1 in summer at Bakersfield, California, with SOA components contributing 80% to 90% of OM from 15 May to 29 June 2010. SOA formed from alkane and aromatic compounds, the two major classes of vehicle-emitted hydrocarbons, accounted for 65% OM (72% SOA) in the summertime San Joaquin Valley. The alkane and aromatic SOA components were associated with 200- to 500-nm-accumulation-mode particles, likely from condensation of daytime photochemical products of VOCs. In contrast, biogenic SOA likely formed from condensation of secondary organic vapors, produced from NO3 radical oxidation reactions during nighttime hours, on 400- to 700-nm- sized primary particles, and accounted for less than 10% OM. Emissions from local petroleum operations contributed 13% to the OM. Vegetative detritus (10%) and cooking activities (7%) accounted for other small yet nonnegligible sources. While the mass spectra of several linearly-independent SOA components were nearly identical and external source markers were needed to separate them, each component had distinct infrared spectrum, likely associated with the source- specific VOCs from which they formed.
For questions regarding research reports, contact: Heather Choi at (916) 322-3893
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