Abstract
Abstract. The species and chemistry responsible for secondary organic aerosol (SOA) formation remain highly uncertain. Laboratory studies of the oxidation of individual, high-flux SOA precursors do not lead to particles with mass spectra (MS) matching those of ambient aged organic material. Additionally, the complexity of real organic particles challenges efforts to identify their chemical origins. We have previously hypothesized that SOA can form from the atmospheric oxidation of a large suite of precursors with varying vapor pressures. Here, we support this hypothesis by using an aerosol mass spectrometer to track the chemical evolution of diesel exhaust as it is photochemically oxidized in an environmental chamber. With explicit knowledge of the condensed-phase MS of the primary emissions from our engine, we are able to decompose each recorded MS into contributing primary and secondary spectra throughout the experiment. We find that the SOA becomes increasingly oxidized as a function of time, quickly approaching a final MS that closely resembles that of ambient aged organic particulate matter. This observation is consistent with our hypothesis of an evolving suite of SOA precursors. Low vapor pressure, semi-volatile organic emissions can form condensable products with even a single generation of oxidation, resulting in an early-arising, relatively less-oxidized SOA. Continued gas-phase oxidation can form highly oxidized SOA in surprisingly young air masses via reaction mechanisms that can add multiple oxygen atoms per generation and result in products with sustained or increased reactivity toward OH.
Highlights
Based on available emissions profiles and laboratorygenerated yield curves, secondary organic aerosol (SOA) has been estimated to comprise around 60% of the global burden of organic aerosol (OA) (Kanakidou et al, 2005)
High mass loadings of oxidized organic aerosol (OOA) in photochemically young air masses in Mexico City indicate that SOA formation is rapid enough that SOA can be the dominant component of OA there, even locally, at certain times of day (Volkamer et al, 2006)
We find that reducing the amount of MSPOA subtracted from MSt to meet this criterion results in only a slightly less oxidized MSresidual; the change in f57 is on the order of 1%
Summary
Based on available emissions profiles and laboratorygenerated yield curves, secondary organic aerosol (SOA) has been estimated to comprise around 60% of the global burden of organic aerosol (OA) (Kanakidou et al, 2005). A recent global mass-balance calculation for the removal of volatile organic compounds (VOCs) from the atmosphere suggests that this estimate underpredicts SOA production rates by as much as an order of magnitude (Goldstein and Galbally, 2007). High mass loadings of oxidized organic aerosol (OOA) in photochemically young air masses in Mexico City indicate that SOA formation is rapid enough that SOA can be the dominant component of OA there, even locally, at certain times of day (Volkamer et al, 2006). The oxidation chemistry of the high-flux VOCs that have traditionally been included in models as SOA precursors cannot explain these observations. Nor do chamber studies of the SOA resulting from these precursor produce particulate matter with mass spectra (MS) matching those of ambient OOA (Alfarra et al, 2006; de Gouw et al, 2005)
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