Tuesday, 11 February 2003: 8:30 AM
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, is not 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, a discrepancy between the field and modeled ratios is still observed. 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 (grxn) were derived to be equal or smaller than 10-12 for SO2, and than 10-10 for CO and CH4. The effect of Fe3+ and H2SO4 on the reaction with CO was also investigated, and no reaction was observed under either of these conditions. Nitric oxide does react with the undissociated form of HNO3 adsorbed on the silica pellets, and 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 to be equal or greater than (6 ± 2) x 10-9 (2s) for NO. The atmospheric implications of NO3 “renoxification” on common silica surfaces will be discussed.