Laboratory studies of potential mechanisms of renoxification of tropospheric nitric acid.
Laboratory studies of the heterogeneous reactions between HNO3 in thin water films on silica surfaces and gaseous NO, CO, CH4, and SO2, proposed as potential "renoxification" mechanisms in the atmosphere, are reported. Transmission FTIR was used to monitor reactants and products on the silica surface and in the gas phase as a function of time. No reaction of CO, CH4, or SO2 was observed; upper limits to the reaction probabilities (gamma(rxn)) are < or = 10(-10) for CO and SO2 and < or = 10(-12) for CH4. However, the reaction of HNO3 with NO does occur with a lower limit for the reaction probability of gammaNO > or = (6 +/- 2) x 10(-9) (2s). The experimental evidence shows that the chemistry is insensitive to whether the substrate is pure silica or borosilicate glass. Nitric acid in its molecular form, and not the nitrate anion form, was shown to be the reactive species, and NH4NO3 was shown not to react with NO. The HNO3-NO reaction could be a significant means of renoxification of nitric acid on the surfaces of buildings and soils in the boundary layer of polluted urban atmospheres. This chemistry may help to resolve some discrepancies between model-predicted ozone and field observations in polluted urban atmospheres.