We investigated the heterogeneous processes that contribute towards the formation of N 2 O in an environment that comes as closely as possible to exhaust conditions containing NO and SO 2 among other constituents. The simultaneous presence of NO, SO 2 , O 2 , and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N 2 O. The maximum rate of N 2 O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO 2 or HONO significantly increases the rate constant for N 2 O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N 2 O formation for the three different nitrogen reactants reveal the following series of increasing reactivity:
indicating the probable se-NO Ͻ NO Ͻ HONO, 2 quential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N 2 O are largest at of a H 2 SO 4 /H 2 O solution. The pH ϭ 0 results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N 2 O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O 2 to NO 2 mainly in the gas phase, with the presence of both H 2 O and active surfaces significantly accelerating NO 2 production. Subsequently, NO 2 reacts with excess NO to obtain HONO which reacts with S(IV) to result in N 2 O and H 2 SO 4 through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid.