Quantifying atmospheric nitrate formation pathways based on a global model of the oxygen isotopic composition (Delta;17O) of atmospheric nitrate

摘要

The oxygen isotopic composition (ΔO) of atmospheric nitrateis a function of the relative abundance of atmospheric oxidants (O,RO=OH+HO+RO) and the formation pathway of nitratefrom its precursor NO (=NO+NO). Coupled observations andmodeling of nitrate ΔO can be used to quantify the relativeimportance of chemical formation pathways leading to nitrate formation andreduce uncertainties in the budget of reactive nitrogen chemistry in theatmosphere. We present the first global model of atmospheric nitrate ΔO and compare with available observations. The largest uncertaintyfor calculations of nitrate ΔO is the unconstrainedvariability in the ΔO value of tropospheric ozone. The modelshows the best agreement with a global compilation of observations whenassuming a ΔO value of tropospheric ozone equal to 35‰ andpreferential oxidation of NO by the terminal oxygen atoms of ozone.Calculated values of annual-mean nitrate ΔO in the lowestmodel layer (0–200 m above the surface) vary from 7‰ in the tropics to41‰in the polar-regions. The global, annual-mean tropospheric inorganic nitrateburden is dominated by nitrate formation via NO+OH (76%),followed by NO hydrolysis (18%) and NO+DMS/HC(4%). Calculated nitrate ΔO is sensitive to the relativeimportance of each nitrate formation pathway, suggesting that observationsof nitrate ΔO can be used to quantify the importance ofindividual reactions (e.g. NO hydrolysis) leading to nitrateformation if the ΔO value of ozone is known.