Distinct Role of NH 3 in Enhancing Atmospheric Nitrous Acid Formation: Evidence From an Autumn Field Campaign in Beijing

Abstract Nitrous acid (HONO) is a key precursor of hydroxyl radicals (·OH), which plays a central role in atmospheric oxidation and aerosol formation. However, the mechanisms responsible for frequently elevated HONO levels, particularly the role of ammonia (NH 3 ) under complex atmospheric pollution conditions, remain under debate. Here, simultaneous measurements of HONO, NH 3 , and related atmospheric parameters were conducted in downtown Beijing during autumn 2021. A comprehensive HONO budget analysis combined with a residual based approach was applied to decouple the influence of relative humidity (RH) and isolate the contribution of NH 3 . The results indicate that known sources explain about 75% of the observed HONO, implying a substantial missing source (P unknown ) during the daytime and nighttime. Crucially, we provide observational evidence that NH 3 enhances HONO formation through distinct mechanisms both during nighttime and daytime. At night, HONO production efficiency after RH correction increased with NH 3 concentration, indicating that NH 3 independently facilitates heterogeneous NO 2 hydrolysis by neutralizing surface acidity. During the daytime, however, HONO production showed a pronounced contribution from particulate nitrate photolysis, with NH 3 indirectly promoting HONO via enhanced nitrate formation. Notably, this NH 3 driven enhancement of HONO production was profound even during relatively clean periods: elevated NH 3 levels (≥10 ppb) increased HONO derived ·OH production by 83%, significantly accelerating atmospheric oxidation cycles. These findings highlight NH 3 as a regulator of atmospheric oxidation capacity, suggesting that synergetic control of NH 3 is essential for mitigating surface ozone pollution given the remarkable role of HONO in ozone formation in megacities.