Quantifying atmospheric nitrate formation pathways based on a global model of the oxygen isotopic composition (Δ<sup>17</sup>O) of atmospheric nitrate
The oxygen isotopic composition (117O) of at- mospheric nitrate is a function of the relative abundance of atmospheric oxidants (O3,ROx=OH+HO2+RO2) and the formation pathway of nitrate from its precursor NOx (=NO+NO2). Coupled observations and modeling of ni- trate 117O can be used to quantify the relative importance of chemical formation pathways leading to nitrate forma- tion and reduce uncertainties in the budget of reactive ni- trogen chemistry in the atmosphere. We present the first global model of atmospheric nitrate117O and compare with available observations. The largest uncertainty for calcula- tions of nitrate 117O is the unconstrained variability in the 117Ovalue of tropospheric ozone. The model shows the best agreement with a global compilation of observations when assuming a 117O value of tropospheric ozone equal to 35‰ and preferential oxidation of NOx by the terminal oxygen atoms of ozone. Calculated values of annual-mean nitrate 117O in the lowest model layer (0–200m above the surface) vary from7‰in the tropics to 41‰ in the polar-regions. The global, annual-mean tropospheric inorganic nitrate burden is dominated by nitrate formation via NO2+OH (76%), fol- lowed by N2O5 hydrolysis (18%) and NO3+DMS/HC (4%). Calculated nitrate 117O is sensitive to the relative impor- tance of each nitrate formation pathway, suggesting that ob- servations of nitrate117O can be used to quantify the impor- tance of individual reactions (e.g. N2O5 hydrolysis) leading to nitrate formation if the 117O value of ozone is known.