A laboratory study of the heterogeneous reaction of nitric acid on calciumcarbonate particles

It has been postulated that the reaction of nitric acid with calcium carbon ate, namely, CaCO3(s)+ 2HNO(3)(g) -> Ca(NO3)(2)(s) + CO2(g)+ H2O(g), plays an important role in the atmosphere. In this study, transmission FTIR spect roscopy, diffuse reflectance UV-visible spectroscopy, transmission electron microscopy and a Knudsen cell reactor coupled to a quadrupole mass spectro meter have been used to investigate the heterogeneous reactivity of HNO3 on CaCO3 at 295 K as a function of relative humidity. Transmission FTIR spect roscopy was used to probe both gas-phase and adsorbed products and showed t hat the reaction of HNO3 and CaCO3 is limited to the surface of the CaCO3 p article in the absence of adsorbed water. However, in the presence of water vapor, the reaction is greatly enhanced and is not limited to the surface of the particle producing both solid calcium nitrate and gaseous carbon dio xide. The enhanced reactivity of the particles is attributed to the presenc e of a layer of adsorbed water on the particle surface. The amount of adsor bed water on the particle surface is strongly dependent on the extent of th e reaction. This can be understood in terms of the increased hydrophilicity of calcium nitrate as compared to calcium carbonate, Data from experiments using a mass-calibrated Knudsen cell reactor showed the stoichiometry for the reaction determined from gas-phase species deviated from that expected from the balanced equation. Water adsorption on the particle surface and ga ses dissolved into the water layer appear to be the cause of this discrepan cy. The measured uptake coefficient accounting for the SET area of the samp le is determined to be 2.5 + 0.1 x 10(-4) for HNO3 on CaCO3 under dry condi tions and is found to increase in the presence of water vapor. Atmospheric implications of the results presented here are discussed.