Abstract
Abstract. Current atmospheric models do not include secondary organic aerosol (SOA) production from gas-phase reactions of polycyclic aromatic hydrocarbons (PAHs). Recent studies have shown that primary emissions undergo oxidation in the gas phase, leading to SOA formation. This opens the possibility that low-volatility gas-phase precursors are a potentially large source of SOA. In this work, SOA formation from gas-phase photooxidation of naphthalene, 1-methylnaphthalene (1-MN), 2-methylnaphthalene (2-MN), and 1,2-dimethylnaphthalene (1,2-DMN) is studied in the Caltech dual 28-m3 chambers. Under high-NOx conditions and aerosol mass loadings between 10 and 40 μg m−3, the SOA yields (mass of SOA per mass of hydrocarbon reacted) ranged from 0.19 to 0.30 for naphthalene, 0.19 to 0.39 for 1-MN, 0.26 to 0.45 for 2-MN, and constant at 0.31 for 1,2-DMN. Under low-NOx conditions, the SOA yields were measured to be 0.73, 0.68, and 0.58, for naphthalene, 1-MN, and 2-MN, respectively. The SOA was observed to be semivolatile under high-NOx conditions and essentially nonvolatile under low-NOx conditions, owing to the higher fraction of ring-retaining products formed under low-NOx conditions. When applying these measured yields to estimate SOA formation from primary emissions of diesel engines and wood burning, PAHs are estimated to yield 3–5 times more SOA than light aromatic compounds over photooxidation timescales of less than 12 h. PAHs can also account for up to 54% of the total SOA from oxidation of diesel emissions, representing a potentially large source of urban SOA.
Highlights
Organic aerosol (OA) accounts for a large fraction of urban particulate matter (Seinfeld and Pankow, 2003; Zhang et al, 2007), and has important implications for health, climate and visibility
OA has traditionally been classified into two categories: primary organic aerosol (POA), which is directly emitted as particulate matter, and secondary organic aerosol (SOA), which is formed from atmospheric oxidation of volatile organic compounds (VOCs)
The relative rates of these reactions depend on the concentration of NO2, which remained between 50 and 100 ppb in all high-NOx experiments, as measured by the GC/NO2-PAN analyzer. This level is typical of urban polluted conditions, and suggests that the branching ratios in these experiments are relevant for regions where polycyclic aromatic hydrocarbons (PAHs) are commonly emitted
Summary
Organic aerosol (OA) accounts for a large fraction of urban particulate matter (Seinfeld and Pankow, 2003; Zhang et al, 2007), and has important implications for health, climate and visibility. That closely resembles OA observed in field measurements (Weitkamp et al, 2007; Sage et al, 2008; Grieshop et al, 2009a) While some of these compounds, including long chain n-alkanes, polycyclic aromatic hydrocarbons (PAHs), and large olefins, exist exclusively in the gas phase, they have lower volatilities than traditional SOA precursors and are typically ignored in current atmospheric models. These compounds, which have saturation concentrations between 103 and 106 μg m−3, are termed intermediate volatility organic compounds (IVOCs) (Donahue et al, 2009). The SOA yields, the mass of SOA formed per mass of PAH reacted, are used to evaluate the importance of PAHs as effective SOA precursors
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