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

Abstract

Abstract. The oxygen isotopic composition (Δ17O) of atmospheric nitrate is a function of the relative abundance of atmospheric oxidants (O3, HOx=OH +HO2+RO2) and the formation pathway of nitrate from its precursor NOx (=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 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 NOx 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 6‰ in the tropics to 41‰ in the polar-regions. On the global scale, O3 is the dominant oxidant (81% annual-mean) during NOx cycling reactions. 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%). Model discrepancies are largest in the polar spring and summer, most likely due to the lack of reactive halogen chemistry in the model. The influence of organic nitrates on observations of nitrate Δ17O needs to be determined, especially for observations in summertime and tropical forested regions where organic nitrates can contribute up to 80% of the total NOy (organic plus inorganic nitrate) budget.