Nitrite/Nitrous Acid Generation from the Reaction of Nitrate and Fe(II) Promoted by Photolysis of Iron-Organic Complexes

Gaseous nitrous acid (HONO) has the potential to greatly contribute to the atmospheric oxidation capacity. Increased attention has been paid to in-particle nitrite or nitrous acid, N(III), as one of the HONO sources. However, sources and formation mechanisms of N(III) remain uncertain. Here, we study a much less examined reaction of Fe(II) and nitrate as a source of N(III). The N(III) production was indirectly probed by its multiphase reaction with SO2 for sulfate production. Particles containing nitrate and Fe(III) were irradiated for generating Fe(II). Sulfate production was enhanced by the presence of UV and organic compounds likely because of the enhanced redox cycle between Fe(II) and Fe(III). Sulfate production rate increases with the concentration of iron-organic complexes in nitrate particles. Similarly, higher concentrations of iron-organic complexes yield higher nitrate decay rates. The estimated production rates of N(III) under simulated conditions in our study vary from 0.1 to 3.0 μg m-3 of air h-1. These values are comparable to HONO production rates of 0.2-1.6 ppbv h-1, which fall in the values reported in laboratory and field studies. The present study highlights a synergistic effect of the coexistence of iron-organic complexes and nitrate under irradiation as a source of N(III).