1 4.1 NITRIC ACID “RENOXIFICATION ” IN THE TROPOSPHERE: FROM A MODELING MYTH TO A LABORATORY REALITY

The partitioning of nitrogen oxides in the atmosphere, in particular the NOx/HNO3 ratio, has not historically been accurately reproduced by models. Although the model under-prediction of the NOx/HNO3 ratio has been improved by the new rate constants for the reactions of OH with NO2 and HNO3, the discrepancy between the field and modeled ratios has not been clearly resolved. Heterogeneous reactions of HNO3 with various atmospheric gaseous species, including NO, CO, CH4, and SO2, have been proposed to reconcile these ratios. We present laboratory studies in which the possibility of heterogeneous reactions between HNO3 adsorbed on silica surfaces and gaseous NO, CO, CH4, and SO2 were investigated. Two different silica surfaces, a porous glass plate and pressed silica (Cab-O-Sil®) pellets, were used as substrates for HNO3. Both the gas phase and the surface species were monitored with time using FTIR at 1 atm total pressure in N2 and 298 K. No reaction was observed between HNO3(ads) and CO, CH4 or SO2, from which upper limits for the reaction probabilities (γrxn) were derived: γCO and γSO2 ≤ 10-10, and γCH4 ≤ 10-12. The effect of Fe3+ and H2SO4 on the reaction with CO was also investigated, and no enhancement of the reaction was observed under either of these conditions. Nitric oxide does react with the undissociated form of HNO3 adsorbed on the silica pellets, but not the anion NO3-, to form NO2 and a water-nitric acid complex. A lower limit for the reaction probability of gaseous NO with HNO3 adsorbed on silica pellets was calculated: γNO = (6 ± 2) × 10-9 (2s). The atmospheric implications of HNO3 “renoxification ” on common surfaces will be discussed