Atmospheric Oxidation of NH3, HNO3 and NH3···HNO3 by OH, NH2, and NO3 Radicals. The Effect of Water Vapor.

Atmospheric ammonia, in both particulate and gaseous forms, has major ecological, health, and economic impacts, making it essential to understand its chemical processes. The reactions of ammonia and ammonia complexed with nitric acid with hydroxyl radical and the oxidation of nitric acid by amidogen radical and ammonia by nitrate radical, both taking into account the effect of water vapor, have been investigated using quantum mechanical (QCISD and CCSD(T)) calculations with the 6-311+G(2df,2p), aug-cc-pVTZ, and aug-cc-pVQZ and extrapolation to the CBS basis sets. From a mechanistic point of view, the reaction of NH3 + OH follows a conventional hydrogen transfer mechanism, but for the rest of reactions considered, the proton coupled electron transfer mechanism plays a key role. For the reaction of ammonia with hydroxyl radical we have computed a rate constant of 1.24 × 10-13 cm3·molecule-1·s-1 at 298 K, and the effect of water vapor is negligible. The calculated rate constant for the HNO3···NH3 + OH reaction is 6.50 × 10-16 cm3·molecule-1·s-1 at 298 K. and our results show that both HNO3 and NH3 moieties can be oxidized. The effect of water vapor on the oxidation of nitric acid by an amidogen radical is significant. We have computed a rate constant of 1.98 × 10-13 cm3·molecule-1·s-1, at 298 K and 100% of RH for the whole HNO3 + NH2 + H2O reaction, which is an 11% greater than the calculated value for the naked reaction. For the oxidation of ammonia by a nitrate radical, the effect of water vapor is huge. The calculated rate constant at 298 K and 100% of RH is 16 × 10-15 cm3·molecule-1·s-1 for the whole NH3 + NO3 + H2O reaction, that is, 751% greater than the value of the naked reaction at 298 K.