Kinetics study of heterogeneous reactions of O₃ and SO₂ with sea salt single droplets using micro-FTIR spectroscopy: Potential for formation of sulfate aerosol in atmospheric environment

The heterogeneous reactions of sea salt single droplets with the mixture of O₃ and SO₂ were studied in real time using microscopic Fourier transform infrared (micro-FTIR) spectrometer. Chemical conversion of SO₂ to sulfate and consumption of gaseous HCl occur on the surface of droplets in the presence of O₃. The sulfate formation rate and the uptake coefficient are obtained by quantitatively estimating the changes in absorbance area of the sulfate stretching band. In order to further establish a mechanistic framework, we observed the reaction kinetics versus ambient relative humidities (RHs) and droplet sizes. In the view of RH effect, sulfate formation rates are enhanced by about a factor of two on the MgCl₂ and ZnCl₂ single droplets with increasing RH ranges. High RH is favorable for the sulfate formation because water vapor can trap and activate more gas molecules on the interface of the single droplet. The values of uptake coefficient increase slightly with an increase in single droplet size for the two reaction systems, indicating that the effect of surface adsorption dominates the reactions. Considering the existence of combined pollution with high concentrations of trace gases and sea salt aerosols, as expected in coastal regions, the formation micro-mechanism of sulfate revealed in this work should be incorporated into air quality models to improve the prediction of sulfate concentrations.