The heterogeneous formation of N2O over bulk condensed phases in the presence of SO2 at high humidities

In view of the uncertainty of the origin of the secular increase of N2O, we studied heterogeneous processes that contribute to formation of N2O in an environment that comes as close as possible to exhaust conditions contg. NO and SO2, among other constituents. The N2O formation was followed using electron capture gas chromatog. (ECD-GC). The other reactants and intermediates (SO2, NO, NO2 and HONO) were monitored using gas phase UV-VIS absorption spectroscopy. Expts. were conducted at 298 and 368 K as well as at dry and high humidity (approaching 100% rh) conditions. There is a significant heterogeneous rate of N2O formation at conditions that mimic an exhaust plume from combustion processes. The simultaneous presence of NO, SO2, O2 in the gas phase and condensed phase water, either in the bulk liq. or adsorbed state has been confirmed to be necessary for the prodn. of significant levels of N2O. The stoichiometry of the overall reaction is: 2 NO + SO2 + H2O -> N2O + H2SO4. The max. rate of N2O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its vol. A significant rate of N2O formation at 368 K at 100% rh was also obsd. in the absence of a bulk substrate. The diffusion of both gas and liq. phase reactants is not rate limiting as the reaction kinetics is dominated by the rate of N2O formation under the exptl. conditions used in this work. The simultaneous presence of high humidity (90-100% r.h, at 368 K) and bulk condensed phase results in the max. rate and final yield of N2O approaching 60% and 100% conversion after one hour in the presence of amorphous carbon and fly-ash, resp.