Abstract
Abstract. The oxygen isotopic composition (Δ17O) of atmospheric 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 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 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 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.
Highlights
The formation and cycling of reactive nitrogen in the atmosphere has important implications for air quality, the oxidation capacity of the atmosphere, and atmospheric nitrate deposition
The photochemical cycling of NOx leads to the formation of tropospheric ozone (O3), a major air pollutant
Observations of the mean 17O value of tropospheric ozone ( 17O(O3)) at different locations range from 25–35‰ (Johnston and Thiemens, 1997; Krankowsky et al, 1995); the absolute variability is much larger (6–54‰) (Morin et al, 2007)
Summary
The formation and cycling of reactive nitrogen in the atmosphere has important implications for air quality, the oxidation capacity of the atmosphere, and atmospheric nitrate (nutrient) deposition. Observations of the mean 17O value of tropospheric ozone ( 17O(O3)) at different locations range from 25–35‰ (Johnston and Thiemens, 1997; Krankowsky et al, 1995); the absolute variability is much larger (6–54‰) (Morin et al, 2007) This large range in observed 17O(O3) is unexpected based on the pressure and temperature dependence of the isotopic enrichment measured in laboratory studies (Morton et al, 1990). Comparison with available observations sheds light on previous assumptions used in box model studies regarding the isotopic composition of ozone and the isotopic transfer function during NOx oxidation reactions, and provides a means to test and validate the model’s representation of reactive nitrogen chemistry
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