Observation-Based Diagnostics of Reactive Nitrogen Recycling through HONO Heterogenous Production: Divergent...

Understanding of nitrous acid (HONO) production is crucial to photochemical studies, especially in polluted environments like eastern China. In-situ measurements of gaseous and particulate compositions were conducted at a rural coastal site during the 2018 spring Ozone Photochemistry and Export from China Experiment (OPECE). This data set was applied to investigate the recycling of reactive nitrogen through daytime heterogeneous HONO production. Although HONO levels increase during agricultural burning, analysis of the observation data does not indicate more efficient HONO production by agricultural burning aerosols than other anthropogenic aerosols. Box and 1-D modeling analyses reveal the intrinsic relationships between nitrogen dioxide (NO₂), particulate nitrate (pNO₃), and nitric acid (HNO₃), resulting in comparable agreement between observed and simulated HONO concentrations with any one of the three heterogeneous HONO production mechanisms, photosensitized NO₂ conversion on aerosols, photolysis of pNO₃, and conversion from HNO₃. This finding underscores the uncertainties in the mechanistic understanding and quantitative parametrizations of daytime heterogeneous HONO production pathways. Furthermore, the implications for reactive nitrogen recycling, ozone (O₃) production, and O₃ control strategies vary greatly depending on the HONO production mechanism. On a regional scale, the conversion of HONO from pNO₃ can drastically enhance O₃ production, while the conversion from NO₂ can reduce O₃ sensitivity to NOx changes in polluted eastern China.