Photolysis of Nitroaromatic Compounds under Sunlight: A Possible Daytime Photochemical Source of Nitrous Acid?

Abstract

Nitrous acid (HONO) significantly contributes to OH radicals in the atmosphere, whereas the HONO sources need to be urgently further explored. The photolysis of solid nitroaromatic compounds (Ar-NO2), including nitrophenols (NPs) and nitro-polycyclic aromatic hydrocarbons (NPAHs), was investigated in a cylindrical reactor coupled to a NOx analyzer at the ambient pressure. The formation of HONO and NOx (NOx = NO2 + NO) by the photolysis of five NPs and four NPAHs was quantified. HONO and NO formations from Ar-NO2 were obviously enhanced under irradiation. For NPs, the Ar-NO2 photolysis frequency for HONO (JAr-NO2→HONO) and NO (JAr-NO2→NO) formations varied in the range of (0.34–4.16) × 10–7 and (0.38–3.21) × 10–7 s–1, respectively. For NPAHs, JAr-NO2→HONO and JAr-NO2→NO were measured to be (0.02–2.01) × 10–5 and (0.001–1.59) × 10–4 s–1, respectively. The photolysis of Ar-NO2 in aerosols was estimated to establish a HONO source strength lower than 17 ppt·h–1, suggesting that the photolysis of Ar-NO2 may not well explain daytime HONO source strength in the atmosphere.