# Significant but Overlooked: Atmospheric HONO Formation from Surface Ammonium Oxidation with Superoxide Radicals

## TL;DR

This study reveals a new pathway for HONO formation in humid conditions, involving ammonium and superoxide radicals.

## Contribution

The paper identifies a previously overlooked HONO formation mechanism involving ammonium oxidation with superoxide radicals under high humidity.

## Key findings

- Ammonium on mineral dust surfaces generates HONO more efficiently under high humidity.
- Superoxide radicals drive the conversion of ammonium to HONO, bypassing the nitrate pathway.
- High humidity inhibits nitrate's role in HONO formation but enhances ammonium's contribution.

## Abstract

Resolving the sources of HONO formation is an indispensable aspect in understanding the enhancement of atmospheric oxidation. However, the contributing sources of high HONO formation rate remain unclear during humid haze episodes. The photochemical conversion of surface nitrate (NO3−), considered as the dominant contributor to the daytime HONO generation, exhibits severe constraint under high relative humidity (RH) conditions. Unexpectedly, ammonium (NH4+) on the surface of photoactive mineral dust shows a gradual acceleration of HONO generation with increasing RH under simulated solar irradiation, especially at high RH. This reversed observation stems from a change in the photochemical pathway for the HONO formation from NO3− and NH4+. The photochemical conversion of surface NO3− is determined by photogenerated electrons (NO3−→NO2→NO2−→HONO), while the superoxide radical (∙O2−) generated during photochemical reaction drives the surface NH4+ to directly form HONO with the pathway (NH4+∙+∙O2−→NO2− + H2O→HONO). Under high RH conditions, oxygen molecules (O2) have greatly better access to photogenerated electrons than NO2, resulting in an interruption of the procedure from NO2 to NO2− during NO3− conversion. Therefore, the favorably generated ∙O2− fuels the photochemical conversion of surface NH4+ while inhibiting the conversion of NO3− to diurnal HONO formation. This work highlights the overlooked contribution of HONO formation from NH4+, especially under high RH conditions, and advances the understanding of a renewed role for O2 in atmospheric chemical processes.