Vertical Differences in NO ₂ -to-HONO Heterogeneous Conversion and HONO-Driven OH Production over Inland, Coastal, and Island Regions

Photolysis of nitrous acid (HONO) is a key source of hydroxyl radicals (OH), but the vertical distribution of its generation mechanism and photolysis effects across different land-sea locations remains unclear. This study used ground-based hyperspectral technology to conduct long-term vertical observations in typical inland, coastal, and island regions of China. Results show that, near the surface, the HONO concentration increases from inland to island regions, while NO₂ decreases. The HONO/NO₂ ratio rises by 424% from inland to island. Inland NO₂ heterogeneous reaction rates exhibit a much lower standard deviation (9.96%) compared to two marine sites, indicating higher spatiotemporal stability. A temperature "turning-point effect" is observed, strengthening as the land-sea relationship shifts from inland to island. At low altitudes, the HONO/NO₂ ratio across the three sites shows a shift with the aerosol concentration and humidity. At low humidity, the ratio is higher at low aerosol concentrations, while this trend reverses as the humidity increases. The inflection point of this change shifts from land (RH = 70%) to coastal (RH = 63%) and island (RH = 56%) regions. With the land-coastal-island transition, the daily generation rate of OH (P(OH)_HONO) increases from 0.67 to 0.76 ppb·h^-1 at low altitudes but decreases from 0.09 to 0.05 ppb·h^-1 above 500 m.