Understanding the impact of isoprene nitrates on ozone using recent advances in isoprene photooxidation chemistry

Isoprene photochemistry has large impacts on ozone, oxidized nitrogen, and secondary organic aerosol concentrations. Photooxidation of isoprene can produce unsaturated hydroxy nitrates, which serve as a major sink for NOx. Most air quality models do not accurately represent how the production efficiency and the fate of these isoprene nitrates affect the sensitivity of regional-scale ozone concentrations to changes in NOx and VOC emissions. Consequently, the chemistry of isoprene nitrates represents a major uncertainty in determining the response of ozone to future changes in biogenic emissions. A new isoprene photooxidation mechanism has been developed based on recent chamber experiments with chemical ionization mass spectrometry. The study provided new constraints on the yield of isoprene nitrates (12%) and the amount of NOx (~50%) released from the oxidation of these nitrates. Distinct differences were found in the behavior of δ- and β-hydroxy peroxy radical channels, with the δ branch accounting for about two thirds of the total isoprene nitrate yield. Other organic nitrates such as methylvinylketone nitrate and propanone nitrate were also observed and found to be much longer-lived, and therefore might play an important role in exporting NOy in remote areas. We incorporate these recent advancements into the SAPRC07T photochemical mechanism within the US EPA Community Multiscale Air Quality (CMAQ) model. The model predictions will be compared to observations of alkyl nitrates, peroxy nitrates, NO2, HCHO, isoprene, and other related species from the INTEX-NA campaign in summer 2004. Additional uncertainties include the dry deposition rates and reaction rates of isoprene nitrates with ozone, which have not been estimated in the chamber experiments. We conduct a series of sensitivity simulations and comparisons with observations to constrain these uncertainties. The updated SAPRC07 mechanism will be an essential tool for modeling future emission scenarios, so that the sensitivity of ozone to isoprene emissions changes can be better assessed.