Role of Heterogeneous Reactions in the Atmospheric Oxidizing Capacity in Island Environments

The source of nitrous acid (HONO) and its importance in island or marine environments are poorly understood. Herein, based on comprehensive field measurements at a hilltop on Corsica Island, we find an inverse diel variation of HONO with higher concentrations during daytime. Night-time HONO budget analysis indicates significant HONO formation during air mass transport along the hillside. In the daytime, although photosensitized NO₂ uptake on the ground and NO + OH make considerable contributions (26% and 5%, respectively), a large part of HONO formation (67%, 320 pptv h^-1) still cannot be explained with state-of-the-art parametrization. Nevertheless, photosensitized heterogeneous NO₂ reactions are likely to account for the missing source, due to underestimation of the source by typical parametrizations at low NO₂ levels. Furthermore, we demonstrate a significant role of HONO formation as a OH primary source at this island site, with a OH production rate exceeding one-fourth of that of O₃ photolysis. Our findings underscore a potential role of heterogeneous surface reactions in the oxidizing capacity of the island environments.