Photooxidation of naphthalene and 2-methylnaphthalene: acidity, humidity and seed aerosol effects on chemical mechanisms

In the atmosphere, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous compounds. They are mostly emitted into the troposphere by incomplete combustion processes, such as diesel exhaust, residential heating, and wood burning. PAHs may react with oxidants by either homogeneous or heterogeneous reactions to form nitro and/or oxy-PAHs known to be more carcinogenic and mutagenic than parent PAHs (IARC, 2010, 2013; Rosenkranz and Mermelstein, 1985). It has been recently shown that lighter PAHs such as naphthalene and methylnaphthalene are precursors of secondary organic aerosol (SOA) and could represent one of the missing sources of SOA in urban areas (Chan et al., 2009). The purpose of this study is to better understand the SOA formation from gaseous PAH oxidation under different experimental conditions. Photooxidation experiments were performed in the UNC outdoor smog chamber. Naphthalene and 2-methylnaphthalene were selected as the two most abundant PAHs emitted in the gas phase (Reisen and Arey, 2005). Experiments were performed in the presence of nitrogen oxides, varying the type of the seed aerosol (MgSO4 vs (NH4)2SO4), aerosol acidity and relative humidity inside the chamber, as these parameters are known to have an impact on SOA formation. A complete chemical characterization of the aerosol has been performed using three complementary high-resolution mass spectrometry techniques. The composition of the gas phase has been determined during the time course of the experiments using a chemical ionization time of flight mass spectrometer (ToF-CIMS) using iodide reagent ion chemistry and the composition of the condensed phase has been characterized using ultra-performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) and gas chromatography coupled to a high-resolution quadrupole time-of-flight mass spectrometry (GC-QTOFMS)...