SOA formation from naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene photooxidation

Abstract

The SOA yield and chemical characteristics of SOA formation from naphthalene and two methyl substituted naphthalenes, 1-methylnaphthalene and 2-methylnaphthalene, were studied for high NOx, low NOx, and ultra-low NOx conditions. The SOA yields are high compared to previous studies for all three PAHs precursors: 1-methylnaphthalene > 2-methylnaphthalene ~ naphthalene for all atmospheric conditions studied. The SOA yields range from 0.03 to 0.60 for naphthalene, 0.21-1.52 for 1-methylnaphthalene, and 0.34-0.55 for 2-methylnaphthalene under high NOx with HONO (initial PAH:NO ratio = 0.03-0.17) conditions. The SOA yield ranges from 0.04 to 0.31 for naphthalene, 0.14-0.72 for 1-methylnaphthalene, and 0.06-0.49 for 2-methylnaphthalene under low NOx (initial PAH:NO ratio = 0.54-2.20) conditions. SOA yields were substantially greater than 1.0 under H2O2 (ultra low NOx) and low NOx + H2O2 conditions for all three PAH precursors. The system reactivity influenced by OH radicals, NOx levels, initial PAH/NO ratios, NO2/NO ratios, and all impacted the SOA formation from the PAH precursors. Fractal-like SOA is observed for the methylnaphthalene isomers during high NOx photooxidation experiments, implying that researchers studying SOA formation from this precursor must carefully account for particle shape or effective density. A m/z 104 (C7H4O+,104.026) peak, consistent with SOA products phthalic acid from earlier studies, was observed as a potential marker of PAH oxidation during HR-ToF-AMS analysis.