The oxygen isotopic composition (D¹⁷O) of atmospheric nitrate is a function of the relative abundance of atmospheric oxidants (O₃, HO_x = OH + HO₂ + RO₂) and the formation pathway of nitrate from its precursor NO_x (= NO + NO₂).  Coupled observations and modeling of nitrate D¹⁷O 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 D¹⁷O and compare with available observations.  The largest uncertainty for calculations of nitrate D¹⁷O is the unconstrained variability in the D¹⁷O value of tropospheric ozone.  The model shows the best agreement with a global compilation of observations when assuming a D¹⁷O 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 D¹⁷O 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 D¹⁷O is sensitive to the relative importance of each nitrate formation pathway, suggesting that observations of nitrate D¹⁷O can be used to quantify the importance of individual reactions (e.g. N₂O₅ hydrolysis) leading to nitrate formation if the D¹⁷O value of ozone is known.

Figure 4. Annual-mean fractional importance of each nitrate production pathway leading to total inorganic nitrate at the surface in the model:  NO₂ + OH (top left), N₂O₅ hydrolysis (top right), NO₃ + DMS/HC (bottom left), and stratospheric denitrification (bottom right).

Meredith G. Hastings, Brown University

Jordi Dachs, Instituto de Investigaciones Químicas y Ambientales de Barcelona/Consejo Superior de Investigaciones Científicas