The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles

Heterogeneous reactions of SO2 and O-3 with CaCO3 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 SO2 on CaCO3 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 SO2 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 CaSO4 hydrates, which could expose more reactive sites and result in additional adsorption of SO2. Piece-wise 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. (C) 2018 Elsevier B.V. All rights reserved.