Rethinking urban haze formation: Atmospheric sulfite conversion rate scales with aerosol surface area, not volume

Aerosols in the atmosphere are effective microreactors for aqueous reactions converting primary pollutants to secondary fine particulate matter (PM_2.5). These aqueous reactions are believed to scale with the volume of water, such as the water in deliquesced urban aerosols. Here, using single-particle Raman spectroscopy, we mapped the scaling law for the aqueous conversion rate of sulfite in microdroplets, a key reaction producing sulfate PM_2.5 and driving the formation of China's urban haze. We show that, in droplets below approximately 100 μm, this aqueous reaction scales not with water volume, but with droplet surface area, owing to the kinetic acceleration at the air-water interface. Therefore, when linearly extrapolating aqueous reaction rates with aerosol water volume, air-quality models may inaccurately predict PM_2.5 formation rates. These predictions are likely underpredictions, if the models adopt the kinetic parameters measured from laboratory aqueous systems, i.e., bulk solutions, with a surface-area-to-volume ratio much smaller than atmospheric aerosols.