Differences in NOy speciation predicted by three photochemical mechanisms

We describe the production and speciation of NOy in current chemical mechanisms using model simulations with three different chemical mechanisms run under three different chemical scenarios representative of a low-emission rural, a high-emission rural, and a heavily polluted urban scenario. In this study we examine predictions from the Carbon Bond 4 mechanism, the Regional Acid Deposition Model 2 mechanism, and an explicit mechanism used in global modeling. Several reactions are identified which influence the concentrations of important NOy species. The most important reaction under rural conditions is the oxidation of isoprene, which influences the production of PAN and organic nitrate. Differences among mechanisms cause large differences in the NOy concentrations, which propagate throughout the entire mechanism, affecting the chemistry of all species. The largest differences in NOy species among the mechanisms occur in the rural simulations, while predictions are the most similar under urban conditions, for which photochemical mechanisms have been studied and evaluated. The differences in NOy speciation cause uncertainties in the use of these mechanisms for predicting oxidant chemistry over long distances and multi-day episodes under non-urban conditions. Predictions from all mechanisms should be regarded with caution under rural conditions.