Fine particulate matter (PM_2.5) pollution was mainly caused by micron and submicron aerosol particles or micro-droplets generated by human activities. Still, the formation mechanism remains unclear, and corresponding effective solutions are lacking. The precise prevention and control of urban PM_2.5 pollution in China depends on our in-depth understanding of the formation mechanism of secondary aerosols. The current bottleneck and difficulty lie in the fact that aerosols' key physicochemical parameters (such as uptake coefficients) and their precursors still cannot be accurately measured. This will greatly limit our understanding of the evolution laws, such as the formation, rapid growth, and collision recombination of secondary aerosols. Therefore, to understand the formation mechanism of atmospheric particulate matter, it is necessary to accurately measure the key physicochemical parameters in the evolution process of fine particulate matter. Sulfate was an important component of PM_2.5 in the atmosphere of China. However, the current chemicalmodels underestimate the sulfate concentration in PM_2.5 haze pollution events. There were still many uncertainties regarding the surface oxidation process of SO₂ in aerosol droplets and the catalytic process of transition metal ions(TMI) in droplets, indicating that our understanding of the transformation mechanism of SO₂ in the atmosphere was still insufficient. Obtaining the kinetic data of sulfate secondary transformation, determining the rapid growth mechanism of secondary inorganic aerosols, and accurately determining the uptake reactionrate of trace gases in the atmosphere on the surface of particulate matter were the key parameters for quantitative analysis of atmospheric heterogeneous reactions. This study employed a laser confocal micro-Raman spectrometer to measure the kinetics of SO₂ oxidation catalyzed by Fe(Ⅲ) in ammonium chloride droplets. Based on the temporal variation of the spontaneous Raman signal of ${\text{SO}}_4^{2-}$, we established an accurate method for measuring the uptake coefficient of SO₂ in heterogeneous reactions with a single droplet. We investigated the reaction kinetics between trace SO₂ gas and a single droplet under external field conditions. This method validated the surface kinetic processes, and determined the reaction uptake coefficient.