Atmospheric sulfate aerosol formation enhanced by interfacial anions

Heterogeneous oxidation of SO₂ by NO₂ on aerosols has recently been found to be one of the major formation pathways of sulfate in the polluted troposphere, but the chemical mechanisms and kinetics remain uncertain. By combining lab experiments, theoretical chemistry calculations, and field measurements, here we show that the SO₂ oxidation by NO₂ is critically dependent on anions at the air-aerosol aqueous interface. The reaction rate of NO₂ with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mrow><mml:mi>HSO</mml:mi></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:math> (1.1 × 10⁸-1.6 × 10⁹ M^-1 s^-1) is more than four orders of magnitude larger than the traditionally held value for the bulk phase due to the abundant occurrence of chloride, nitrate, and carboxylic anions at the air-aqueous interface, which remarkably accelerates sulfate formation during China haze periods by enhancing the uptake of NO₂ through interfacial electrostatic attraction. Atmospheric models not accounting for this aerosol interfacial process likely produce major misrepresentations of tropospheric sulfate aerosols under polluted conditions.