Theoretical calculation of oxygen equilibrium isotope fractionation factors involving various NOy molecules, OH, and H2O and its implications for isotope variations in atmospheric nitrate
Abstract The oxygen stable isotope composition (d18O) of nitrogen oxides [NOx nitric oxide (NO) nitrogen dioxide (NO2) and their oxidation products (NOy NOx nitric acid (HNO3) particulate nitrate (p NO3 nitrate radical (NO3) dinitrogen pentoxide (N2O5) nitrous acid (HONO) have been shown to be a useful tool for inferring the proportion of NOx that is oxidized by ozone (O3) However, isotopic fractionation processes may have an influence on d18O of various NOy molecules and other atmospheric O bearing molecules pertinent to NOx oxidation chemistry. Here we have evaluated the impacts of O isotopic exchange involving NOy molecules, the hydroxyl radical OH) and water (H2O) using reduced partition function ratios (xb) calculated by hybrid density functional theory. Assuming atmospheric isotopic equilibrium is achieved between NO and NO2 during the daytime, and NO2, NO3, and N2O5 during the nighttime, d18O d15N compositions were predicted for the major atmospheric nitrate formation pathways using our calculated exchange fractionation factors and isotopic mass balance. Our equilibrium model predicts that various atmospheric nitrate formation pathways, including NO2 OH HNO3, N2O5 H2O surface 2HNO3, and NO3 R HNO3 R will yield distinctive d18O d15N compositions. Our calculated d18O d15N compositions match well with previous atmospheric nitrate measurements, and will potentially help better understand the role oxidation chemistry plays on the N and O isotopic composition of atmospheric nitrate.