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

Abstract

Haze events in China megacities involve the rapid oxidation of SO_2 to sulfate aerosol. Given the weak photochemistry that takes place in these optically thick hazes, it has been hypothesized that SO_2 is mostly oxidized by NO_2 emissions in the bulk of pH > 5.5 aerosols. Because NO_2(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 NO_2(g) disproportionates [2NO_2(g) + H_2O(l) = H+ + NO_3–(aq) + HONO (R1)] upon hitting the surface of NaHSO_3 aqueous microjets for < 50 μs, thereby giving rise to strong NO_3– (m/z 62) signals detected by online electrospray mass spectrometry, rather than oxidizing HSO_3– (m/z 81) to HSO_4– (m/z 97) in the relevant pH 3–6 range. Because NO_2(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/NO_2– 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/NO_2– remains to be elucidated.

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