O2-Mediated Synergistic Oxidation of Aqueous Bisulfite and Nitrite: A Key Pathway for Sulfate and Nitrate Formation in Atmospheric Sulfur-Nitrogen Cycles

The coupled chemical transformation of NOx and SO2 plays a pivotal role in the atmospheric sulfur and nitrogen cycles. While these pollutants show negligible gas-phase interrelationship, they exhibit strong synergistic interactions in multiphase aerosol chemistry, yet their underlying reaction mechanisms remain elusive. We demonstrate that O2 serves as the key mediator enabling efficient coupling between aqueous-phase bisulfite (S(IV)) and nitrite (N(III)) through a previously overlooked reaction pathway at environmentally relevant pH (3.0–6.0). Under isolated conditions at pH 4.4, S(IV) undergoes sluggish O2-driven oxidation (4% sulfate yield), whereas N(III) remains oxidation-resistant. In the absence of O2, S(IV)-N(III) redox reactions yield limited sulfate (4%) and N2O, which is consistent with traditional mechanism cognition. Strikingly, the presence of O2 dramatically enhances co-oxidation, simultaneously producing sulfate (43%) and nitrate (36%). Theoretical calculations identify •HSO3 radicals (from HSO3‐/O2 interaction) as key intermediates that combine with O2/NO2‐ to form transient complexes ([•SO5···ONOH]− and [SO3-NO2···•HO2]−), which ultimately decompose into sulfate and nitrate. Observations in Beijing highlight the dominant role of the S(IV)-N(III) synergistic oxidation mechanism in sulfate formation, surpassing the conventional NO2- and O3-driven pathways. This breakthrough enhances our knowledge of atmospheric chemistry, particularly in the coupled cycling of sulfur and nitrogen, including sulfate/nitrate generation and HONO removal dynamics.