Heterogeneous conversion of calcite aerosol by nitric acid

Abstract

The reaction of nitric acid with calcite aerosol at varying relative humidities has been studied under suspended particle conditions in an atmospheric reaction chamber using infrared absorption spectroscopy. The reactant concentration in the chamber, as well as the appearance of gas phase products and surface adsorbed species, was spectroscopically monitored before and after mixing with CaCO₃ (calcite) particles. The interaction with HNO₃ was found to lead to gas phase CO₂ evolution and increased water uptake due to heterogeneous conversion of the carbonate to particulate nitrate. The reaction was enhanced as the relative humidity of the system was increased, especially at relative humidities above the reported deliquescence point of particulate Ca(NO₃)₂. The measured reaction extent demonstrates that the total calcite particulate mass is available for reaction with HNO₃ and the conversion process is not limited to the particle surface. The spectroscopy of the surface formed nitrate suggests a highly concentrated solution environment with a significant degree of ion pairing. The implications of the HNO₃ loss and the formation of the particulate nitrate product for atmospheric chemistry are discussed.