Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol Episodes

Haze events in China megacities involve the rapid oxidation of SO₂ to sulfate aerosol. Given the weak photochemistry that takes place in these optically thick hazes, it has been hypothesized that SO₂ is mostly oxidized by NO₂ emissions in the bulk of pH > 5.5 aerosols. Because NO₂(g) dissolution in water is very slow and aerosols are more acidic, we decided to test such a hypothesis. Herein, we report that > 95% of NO₂(g) disproportionates [2NO₂(g) + H₂O­(l) = H⁺ + NO₃^–(aq) + HONO (R1)] upon hitting the surface of NaHSO₃ aqueous microjets for < 50 μs, thereby giving rise to strong NO₃^– (<i>m</i>/<i>z</i> 62) signals detected by online electrospray mass spectrometry, rather than oxidizing HSO₃^– (<i>m</i>/<i>z</i> 81) to HSO₄^– (<i>m</i>/<i>z</i> 97) in the relevant pH 3–6 range. Because NO₂(g) will be consumed via R1 on the surface of typical aerosols, the oxidation of S­(IV) may in fact be driven by the HONO/NO₂^– generated therein. S­(IV) heterogeneous oxidation rates are expected to primarily depend on the surface density and liquid water content of the aerosol, which are enhanced by fine aerosol and high humidity. Whether aerosol acidity affects the oxidation of S­(IV) by HONO/NO₂^– remains to be elucidated