Atmos. Chem. Phys. Discuss., 9, 7115-7154, 2009
www.atmos-chem-phys-discuss.net/9/7115/2009/
doi:10.5194/acpd-9-7115-2009
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Kinetics and mechanisms of heterogeneous reaction of NO2 on CaCO3 surfaces under dry and wet conditions
H. J. Li, T. Zhu, D. F. Zhao, Z. F. Zhang, and Z. M. Chen
State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

Abstract. Calcium nitrate (Ca(NO3)2) was observed in mineral dust and could change the hygroscopic and optical properties of mineral dust significantly due to its strong water solubility. The reaction of calcium carbonate (CaCO3) with nitric acid (HNO3) is believed the main reason for the observed Ca(NO3)2 in the mineral dust. In the atmosphere, the concentration of nitrogen dioxide (NO2) is orders of magnitude higher than that of HNO3; however, little is known about the reaction of NO2 with CaCO3. In this study, the heterogeneous reaction of NO2 on the surface of CaCO3 particles was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) under wet and dry conditions. Nitrate formation was observed in both conditions, and nitrite was observed under wet conditions, indicating the reaction of NO2 on the CaCO3 surface produced nitrate and probably nitrous acid (HONO). Relative humidity (RH) influenced both the initial uptake coefficient and the reaction mechanism. With RH<52%, surface –OH was formed through dissociation of the surface adsorbed water via oxygen vacancy, thus determining the reaction order. With RH>52%, a monolayer of water formed on the surface of the CaCO3 particles, which reacted with NO2 as a first order reaction, forming HNO3 and HONO. The initial uptake coefficient γ0 was determined to be (1.66±0.38)×10−7 under dry conditions and up to (0.84±0.44)×10−6 under wet conditions. Considering that NO2 concentrations in the atmosphere are orders of magnitude higher than those of HNO3, the reaction of NO2 on CaCO3 particle should have similar importance as that of HNO3 in the atmosphere and could also be an important source of HONO in the atmosphere.

Citation: Li, H. J., Zhu, T., Zhao, D. F., Zhang, Z. F., and Chen, Z. M.: Kinetics and mechanisms of heterogeneous reaction of NO2 on CaCO3 surfaces under dry and wet conditions, Atmos. Chem. Phys. Discuss., 9, 7115-7154, doi:10.5194/acpd-9-7115-2009, 2009.
 
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