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

Abstract. With increasing NO₂ concentration in the troposphere, the importance of NO₂ reaction with mineral dust in the atmosphere needs to be evaluated. Until now, 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 at 296 K and NO₂ concentrations between 4.58×10¹⁵ molecules cm⁻³ to 1.68×10¹⁶ molecules cm⁻³, 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 NO₂ on the CaCO₃ 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 −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 NO₂ with the condensed water begins. With high enough RH (>52% in our experiment), the gas-liquid reaction of NO₂ with condensed water becomes dominant, forming HNO₃ and HONO. The initial uptake coefficient γ₀ was determined to be (4.25±1.18)×10⁻⁹ under dry conditions and up to (6.56±0.34)×10⁻⁸ under wet conditions. These results suggest that the reaction of NO₂ on CaCO₃ particle is unable to compete with that of HNO₃ in the atmosphere. Further studies at lower NO₂ concentrations and with a more accurate assessment of the surface area for calculating the uptake coefficient of the reaction of NO₂ on CaCO₃ particle and to examine its importance as a source of HONO in the atmosphere are needed.