Photolysis Mechanism of Solid Nitro Polycyclic Aromatic Compounds to Form Reactive Nitrogen Species and Environmental Implications

Nitroaromatics (NACs) are a component of particulate organic matter and can release HONO by photolysis. However, due to limited analysis of organic products on particle surfaces, the exact mechanism of NAC photolysis for HONO generation is unclear. In this study, HONO formation and surface products by the photolysis chemistry of ubiquitous solid nitro polycyclic aromatic compounds (NPACs) were explored. HONO and NO were dominant products in gaseous reactive nitrogen species released by the photolysis of solid NPACs, with the NPAC photolysis frequency for HONO (JN→HONO) and NO (JN→NO) in the range of (0.24–4.22) × 10–5 and (0.03–1.64) × 10–4 s–1, respectively. Denitrification organic products, such as polyphenols and ketones, were formed, indicating the loss of the −NO2 group during NPAC photolysis. H2O and O2 played important roles in the photolysis of NPACs to generate HONO, where H2O was capable of directly attacking the −NO2 group in NPACs, and hydroxyl radicals (•OH) produced under the H2O or O2 condition can react with NPACs or NO. The evolutions in the optical property and oxidation potential of NPACs via the photolysis were determined, which highly correlated with the formation of polyphenols and ketones.