Secondary Organic Aerosol Formation from High-NOx Photo-Oxidation of Low Volatility Precursors: n-Alkanes

Abstract

Smog chamber experiments were conducted to investigate secondary organic aerosol (SOA) formation from photo-oxidation of low-volatility precursors; n-alkanes were chosen as a model system. The experiments feature atmospherically relevant organic aerosol concentrations (C(OA)). Under high-NO(x) conditions SOA yields increased with increasing carbon number (lower volatility) for n-decane, n-dodecane, n-pentadecane, and n-heptadecane, reaching a yield of 0.51 for heptadecane at a C(OA) of 15.4 microg m(-3). As with other photo-oxidation systems, aerosol yield increased with UV intensity. Due to the log-linear relationship between n-alkane carbon number and vapor pressure as well as a relatively consistent product distribution it was possible to develop an empirical parametrization for SOA yields for n-alkanes between C(12) and C(17). This parametrization was implemented using the volatility basis set framework and is designed for use in chemical transport models. For C(OA) < 2 microg m(-3), the SOA mass spectrum, as measured with an aerosol mass spectrometer, had a large contribution from m/z 44, indicative of highly oxygenated products. At higher C(OA), the mass spectrum was dominated by m/z 30, indicative of organic nitrates. The data support the conclusion that lower volatility organic vapors are important SOA precursors.