Quantifying atmospheric nitrate formation pathways based on a global model of the oxygen isotopic composition (Δ¹⁷O) of atmospheric nitrate

Abstract. The oxygen isotopic composition (Δ¹⁷O) of atmospheric nitrate is a function of the relative abundance of atmospheric oxidants (O₃, RO_x=OH+HO₂+RO₂) and the formation pathway of nitrate from its precursor NO_x (=NO+NO₂). Coupled observations and modeling of nitrate Δ¹⁷O 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 Δ¹⁷O and compare with available observations. The largest uncertainty for calculations of nitrate Δ¹⁷O is the unconstrained variability in the Δ¹⁷O value of tropospheric ozone. The model shows the best agreement with a global compilation of observations when assuming a Δ¹⁷O value of tropospheric ozone equal to 35‰ and preferential oxidation of NO_x by the terminal oxygen atoms of ozone. Calculated values of annual-mean nitrate Δ¹⁷O in the lowest model layer (0–200 m above the surface) vary from 7‰ 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%), followed by N₂O₅ hydrolysis (18%) and NO₃+DMS/HC (4%). Calculated nitrate Δ¹⁷O is sensitive to the relative importance of each nitrate formation pathway, suggesting that observations of nitrate Δ¹⁷O can be used to quantify the importance of individual reactions (e.g. N₂O₅ hydrolysis) leading to nitrate formation if the Δ¹⁷O value of ozone is known.