Experimental evidence for the efficient “dry deposition” of nitric acid on calcite

The reaction between gas-phase nitric acid and calcite powder has been studied in a low pressure flow reactor. It is found that nitric acid is adsorbed onto the solid samples with very high efficiency, and that the gas-phase products of the interaction are H2O and CO2. The uptake coefficient for the process, defined as the probability that a collision of a gas-phase species leads to irreversible uptake by the solid, is determined to be (10 ± 2) × 10−2. A second series of experiments conducted on calcite pellets reveals that the rate of initial uptake is independent of the sample “porosity”, i.e. the fraction of internal void. The implications of the finding are discussed in terms of the current understanding of the dry deposition of nitrogen oxides that may lead to the degradation of historical monuments and buildings.