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

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 (J_{Ar‑NO2→HONO}) and NO (J_{Ar‑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, J_{Ar‑NO2→HONO} and J_{Ar‑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.