Observational constraints on the chemistry of isoprene nitrates over the Eastern United States

The formation of organic nitrates during the oxidation of the biogenic hydrocarbon isoprene can strongly affect boundary layer concentrations of ozone and nitrogen oxides (NOx = NO + NO₂). We constrain uncertainties in the chemistry of these isoprene nitrates using chemical transport model simulations in conjunction with observations over the eastern United States from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign during summer 2004. The model best captures the observed boundary layer concentrations of organic nitrates and their correlation with ozone using a 4% yield of isoprene nitrate production from the reaction of isoprene hydroxyperoxy radicals with NO, a recycling of 40% NOx when isoprene nitrates react with OH and ozone, and a fast dry deposition rate of isoprene nitrates. Simulated boundary layer concentrations are only weakly sensitive to the rate of photochemical loss of the isoprene nitrates. An 8% yield of isoprene nitrates degrades agreement with the observations somewhat, but concentrations are still within 50% of observations and thus cannot be ruled out by this study. Our results indicate that complete recycling of NOx from the reactions of isoprene nitrates and slow rates of isoprene nitrate deposition are incompatible with the observations. We find that ∼50% of the isoprene nitrate production in the model occurs via reactions of isoprene (or its oxidation products) with the NO₃ radical, but note that the isoprene nitrate yield from this pathway is highly uncertain. Using recent estimates of rapid reaction rates with ozone, 20-24% of isoprene nitrates are lost via this pathway, implying that ozonolysis is an important loss process for isoprene nitrates. Isoprene nitrates are shown to have a major impact on the nitrogen oxide (NOx = NO + NO₂) budget in the summertime U.S. continental boundary layer, consuming 15-19% of the emitted NOx, of which 4-6% is recycled back to NOx and the remainder is exported as isoprene nitrates (2-3%) or deposited (8-10%). Our constraints on reaction rates, branching ratios, and deposition rates need to be confirmed through further laboratory and field measurements. The model systematically underestimates free tropospheric concentrations of organic nitrates, indicating a need for future investigation of the processes controlling the observed distribution.