O ₂ -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 NO_x 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>H</mml:mi><mml:mi>S</mml:mi><mml:msubsup><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow><mml:mrow><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math>/O₂ interaction) as key intermediates that combine with O₂/<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>N</mml:mi><mml:msubsup><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>-</mml:mo></mml:mrow></mml:msubsup></mml:math> 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.