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Atmospheric particles can undergo nucleation, coagulation, chemical-aging, dissolution-precipitation or other atmospheric processes, resulting in complex multicomponent aerosols. The coexisting species have potentially important consequences in the heterogeneous reactions of multicomponent aerosol particles with polluted gases, which are still poorly understood. The effect of coexisting Na 2 SO 4 on heterogeneous uptake of NO 2 on CaCO 3 particles is investigated in a broad RH range. The combination of DRIFTS, Raman, SEM and IC provides qualitative and quantitative information about the formation of nitrate and other surface species. Ca(NO 3 ) 2 and NaNO 3 are generated on mixed CaCO 3 -Na 2 SO 4 particles under dry condition. Both the amount of NO 3 − formed and the NO 3 − formation rates for the mixtures can be predicted based on the linear addition of those for pure CaCO 3 and Na 2 SO 4 particles under dry condition. The further reaction of Ca(NO 3 ) 2 with Na 2 SO 4 could lead to the formation of crystal NaNO 3 and CaSO 4 ·0.5H 2 O at 30% RH. Coagulation between Ca 2 + and SO 4 2 − in surface adsorbed water is observed after part conversion of CaCO 3 to Ca(NO 3 ) 2 , resulting in the formation of CaSO 4 ·2H 2 O at 80% RH. The amount of NO 3 − formed on the mixtures is dramatically enhanced relative to the predictions at 30% and 80% RH. The findings presented here highlight the role of coexisting species in the heterogeneous reactions of trace gases with multicomponent aerosols due to the complexity of atmospheric particles. [ABSTRACT FROM AUTHOR]

Heterogeneous reactions of SO 2 and O 3 with CaCO 3 particles were investigated at a series of relative humidity (RH, 1% to 90%) and 298 K using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The uptake coefficients of SO 2 on CaCO 3 at different RHs were obtained for the first time. Our results proved that high RH could substantially promote the formation of sulfate, for which the highest concentration (80% RH and reaction time of 200 min) and highest formation rate in stable stage (85% RH) were 14 times and 43 times that at 1% RH, respectively. The surface products, increment of concentration and formation rate of sulfate changed with RH which were due to the surface adsorbed water (SAW) on the particles. SAW could increase the reactive sites on the particles and thus accelerate the conversion of SO 2 into sulfite, and sulfite could be oxidized rapidly. Liquid-like water layers formed on the particle surface could enhance the ion mobility and promote the aggregation of CaSO 4 hydrates, which could expose more reactive sites and result in additional adsorption of SO 2 . Piecewise equations of uptake coefficient with RH were given and could be referred by model simulation. The results are of importance in understanding the explosive growth of sulfate during severe haze episodes accompanied with high RH. [ABSTRACT FROM AUTHOR]