Unraveling a New Chemical Mechanism of Missing Sulfate Formation in Aerosol Haze: Gaseous NO₂ with Aqueous HSO₃ ^-/SO₃ ^2-

A source of missing sulfate production associated with high-level fine-particle pollution in the megacities of China is believed to stem from the oxidation of a notable fraction of sulfur dioxide (SO₂) by nitrogen dioxide (NO₂) in aqueous aerosol environments, suggesting that an unknown reaction pathway exists for aqueous sulfur oxidation. At weakly acidic aerosols, the dissolved SO₂ mainly exists in the form of HSO₃ ^-, whereas at neutral aerosols, SO₃ ^2- becomes the main form. Herein, by using both ab initio molecular metadynamics simulations and high-level quantum mechanical calculations, we show a hitherto unreported chemical mechanism for the formation of sulfate through the reaction between HSO₃ ^-/SO₃ ^2- anions at the surface/in the interior of a water nanodroplet and gas-phase NO₂ molecules. For weakly acidic aerosols, contrary to the conventional high-barrier electron-transfer pathway in the gas phase, HSO₃ ^- at the water nanodroplet surface can transfer an electron to NO₂ with a low free-energy barrier of 4.7 kcal/mol through a water bridge. For neutral aerosols, the electron-transfer pathway between SO₃ ^2- in the interior of the water nanodroplet and NO₂ needs to overcome a lower free-energy barrier of 3.6 kcal/mol to form SO₃ ^-, with the assistance of the hydrogen-bonding network of water molecules. This new reaction pathway for the sulfate formation from HSO₃ ^-/SO₃ ^2- via water nanodroplets and gaseous NO₂ provides a new perspective on the growth of haze particles from pre-existing aqueous aerosols and suggests that new control strategies are needed to address haze pollution.