Quantifying atmospheric nitrate formation pathways based on a global model of the oxygen isotopic composition (δ17O) of atmospheric nitrate

The oxygen isotopic composition (delta O-17) of atmospheric nitrate is a function of the relative abundance of atmospheric oxidants (O-3, ROx=OH+HO2+RO2) and the formation pathway of nitrate from its precursor NOx (=NO+NO2). Coupled observations and modeling of nitrate delta O-17 can be used to quantify the relative importance of chemical formation pathways leading to nitrate formation and reduce uncertainties in the budget of reactive nitrogen chemistry in the atmosphere. We present the first global model of atmospheric nitrate delta O-17 and compare with available observations. The largest uncertainty for calculations of nitrate delta O-17 is the unconstrained variability in the delta O-17 value of tropospheric ozone. The model shows the best agreement with a global compilation of observations when assuming a delta O-17 value of tropospheric ozone equal to 35 parts per thousand and preferential oxidation of NOx by the terminal oxygen atoms of ozone. Calculated values of annual-mean nitrate delta O-17 in the lowest model layer (0-200 m above the surface) vary from 7 parts per thousand in the tropics to 41 parts per thousand in the polar-regions. The global, annual-mean tropospheric inorganic nitrate burden is dominated by nitrate formation via NO2+OH (76%), followed by N2O5 hydrolysis (18%) and NO3+DMS/HC (4%). Calculated nitrate delta O-17 is sensitive to the relative importance of each nitrate formation pathway, suggesting that observations of nitrate delta O-17 can be used to quantify the importance of individual reactions (e.g. N2O5 hydrolysis) leading to nitrate formation if the delta O-17 value of ozone is known.