Influence of bulk-phase acidity, organic fraction, and dissolved oxygen on the photosensitized renoxification of nitrate in NaNO/humic acid mixtures.

Nitrate renoxification significantly influences atmospheric nitrogen cycling and global OH budgets. Although numerous nitrite acid (HONO) formation pathways from nitrate photolysis have been widely reported, the influence of various environmental factors and aerosol properties on reactive nitrogen production remains largely unclear. In this work, we employed NaNO/humic acid (HA) as a model nitrate photosensitization system to investigate the crucial roles of aerosol acidity, organic fraction, and dissolved oxygen in the production of HONO, NO, and NO. The presence of HA at 10 mg/L resulted in a remarkable increase in HONO production rates by approximately 2-3 times and NO concentration by 3-6 times across a pH range of 5.2 to 2.0. Meanwhile, the molar fraction of gaseous HONO in total N(III) production increased from 4 % to 69 % as bulk-phase pH decreased from 5.2 to 2.0. The higher organic fraction (i.e., 20 and 50 mg/L HA concentration) instead inhibited HONO and NO release. The presence of dissolved oxygen was found to be adverse for reactive nitrogen production. This suggests that the HA photosensitizer promoted the secondary conversion of NO to HONO mainly via reduced ketyl radical intermediates, while superoxide radical formation might exert a negative effect. Our findings provide comprehensive insights into reactive nitrogen production from photosensitized nitrate photolysis mediated by various external and internal factors, potentially accounting for discrepancies between field observations and model simulations.