Global Modelling of Organic Nitrates: Contribution of Isoprene

The oxidation of isoprene by OH and NO3 has a large but uncertain impact on the budget of NOx (and hence O3) through the formation and photochemical transformation of organic nitrates. Recent laboratory work have put strong constraints on the chemical lifetimes of the primary hydroxynitrates formed from isoprene+OH. We show that the carbonyl nitrates formed in the subsequent oxidation of the primary nitrates by OH photolyze much more more rapidly than previously believed, due to absorption cross section enhancement effects and a quantum yield likely of the order of unity. New recommendations for the photolysis parameters are provided, and the resulting photorates are validated using the temporal profiles of different compounds abserved in the isoprene oxidation laboratory experiment of Paulot et al. (209).

Next, a detailed isoprene oxidation mechanism based on the recently published Leuven Isoprene Mechanism version 1 (LIM1) (Peeters et al., 2014) and on the Master Chemical Mechanism (MCMv32) has been developed. The impact of the fast photolysis of carbonyl nitrates is investigated by box model simulations. A reduced version of this explicit mechanism is constructed, which is shown to provide essentially the same results as the explicit mechanism for key compounds. The reduced mechanism is incorporated in a global chemistry-transport model, IMAGES (Stavrakou et al., 2013). We investigate the budget of organic nitrates, considering contributions from isoprene, monoterpenes, small alkenes and other precursors. The model predictions are tested against aircraft campaigns such as INTEX-A and ARCTAS. The predicted organic nitrate concentrations are in very reasonable agreement with the data in the boundary layer over isoprene-rich areas. Examination of the discrepancies in other areas (boreal forests, N-E United States, also the free troposphere) provide clues regarding missing pathways in the model.