Effect of nitrate particles on the formation of nitro-polycyclic aromatic hydrocarbons generated by the photooxidation of naphthalene.

Nitrate is an important inorganic aerosol component that can affect the formation and optical properties of nitro-polycyclic aromatic hydrocarbons (NPAHs) present in secondary organic aerosols (SOAs). Sodium nitrate (NaNO3) was selected as a representative nitrate, and the atmospheric chemical process for naphthalene photooxidation to generate SOA in the presence of NaNO3 particles was investigated using a smog chamber system. NPAH components were measured online using an aerosol laser time-of-flight mass spectrometer (ALTOFMS) and offline using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), electrospray ionization mass spectrometry (ESI-MS) and quantum chemical calculations. The optical parameter of the averaged mass absorption coefficient (〈MAC〉) of SOA was detected using a UV-visible absorption spectrometer and an organic carbon analyzer. The experimental results demonstrated that NaNO3 particles promote the formation of SOA and generation of NPAH components. Compared with the results of previous experiments on naphthalene photooxidation, 2-nitronaphthalene, 2,4-dinitronaphthalene, 2-nitro-1-naphthol, 2,4-dinitro-1-naphthol, 1,3-dihydroxy-2-nitro-naphthalene and 1,3-dihydroxy-2,4-dinitro-naphthalene were determined as the main components of SOA in the presence of NaNO3 particles. The reaction energy and Gibbs free energy change for the formation of 2-nitronaphthalene were the largest among all the reactions. 2-Nitronaphthalene was the main NPAH product. Naphthalene and its gaseous photooxidation products condensed and reacted on NaNO3 particles' surface to yield NPAHs with intense light absorbability, resulting in a significant increase in the 〈MAC〉 of SOA. The 〈MAC〉 of naphthalene SOA steadily increased with increasing NaNO3 particles' concentration. The 〈MAC〉 for naphthalene SOA in the presence of 100 µg m-3 NaNO3 particles was 645 cm2 g-1, slightly less than the 〈MAC〉 of organic aerosols from biomass burning (700 cm2 g-1). This study provided the experimental basis for source identification and light absorption characteristic study of NPAH components in atmospheric SOA in the presence of NaNO3 particles.