Kinetics and mechanisms of heterogeneous reaction of NO2 on CaCO3 surfaces under dry and wet conditions

Other literature type English OPEN
Li, H. J. ; Zhu, T. ; Zhao, D. F. ; Zhang, Z. F. ; Chen, Z. M. (2010)

With increasing NO<sub>2</sub> concentration in the troposphere, the importance of NO<sub>2</sub> reaction with mineral dust in the atmosphere needs to be evaluated. Until now, little is known about the reaction of NO<sub>2</sub> with CaCO<sub>3</sub>. In this study, the heterogeneous reaction of NO<sub>2</sub> on the surface of CaCO<sub>3</sub> particles was investigated at 296 K and NO<sub>2</sub> concentrations between 4.58&times;10<sup>15</sup> molecules cm<sup>&minus;3</sup> to 1.68&times;10<sup>16</sup> molecules cm<sup>&minus;3</sup>, 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 under both conditions, while nitrite was observed under wet conditions, indicating the reaction of NO<sub>2</sub> on the CaCO<sub>3</sub> surface produced nitrate and probably nitrous acid (HONO). Relative humidity (RH) influences both the initial uptake coefficient and the reaction mechanism. At low RH, surface &minus;OH is formed through dissociation of the surface adsorbed water via oxygen vacancy, thus determining the reaction order. As RH increases, water starts to condense on the surface and the gas-liquid reaction of NO<sub>2</sub> with the condensed water begins. With high enough RH (&gt;52% in our experiment), the gas-liquid reaction of NO<sub>2</sub> with condensed water becomes dominant, forming HNO<sub>3</sub> and HONO. The initial uptake coefficient &gamma;<sub>0</sub> was determined to be (4.25&plusmn;1.18)&times;10<sup>&minus;9</sup> under dry conditions and up to (6.56&plusmn;0.34)&times;10<sup>&minus;8</sup> under wet conditions. These results suggest that the reaction of NO<sub>2</sub> on CaCO<sub>3</sub> particle is unable to compete with that of HNO<sub>3</sub> in the atmosphere. Further studies at lower NO<sub>2</sub> concentrations and with a more accurate assessment of the surface area for calculating the uptake coefficient of the reaction of NO<sub>2</sub> on CaCO<sub>3</sub> particle and to examine its importance as a source of HONO in the atmosphere are needed.
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