Atmospheric field measurements have revealed rapid sulfate formation during severe haze episodes in China, with fine particulate matter (PM) consisting of a multi-component mixture that is dominated by organic species. Several recent studies using thermodynamic models estimated particle acidity and aqueous sulfate production by treating the PM exclusively as a mixture of inorganic salts dominated by ammonium sulfate and neglecting the effects of organic compounds. Noticeably, the estimated pH and sulfate formation rate during pollution periods in China are highly conflicting among these studies. Here we show distinct sulfate formation for organic seed particles from that of ammonium sulfate seed particles, when the particles are exposed to vapors of SO2, NO2, and NH3 at high relative humidity (RH) in laboratory experiments. Oxalic acid particles, which are representative of organic dominant particles in China, exhibit significant hygroscopic growth and sulfate production because of a lower acidity, while ammonium sulfate particles show little sulfate production, attributable to a higher acidity. Our results reveal that a particle mixture of inorganic salts adopted by the previous studies does not represent a suitable model system and that the acidity and aqueous sulfate formation cannot be reliably inferred without accounting for the effects of multi-aerosol compositions during severe haze events in China. We conclude that organics-dominated aerosols are weakly acidic to efficiently promote SO2 oxidation by NO2 with NH3 neutralization under polluted conditions in China.

Particle acidity and sulfate production during severe haze in China

256th National Meeting (2018)

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