Quantifying atmospheric nitrate formation pathways based on a global model of the oxygen isotopic composition (Δ<sup>17</sup>O) of atmospheric nitrate

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.

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

“…It should also be noted that the Δ 17 O used for ozone has no impact on this estimation because all 17 O-excess transfer mechanisms scale identically with this parameter (15), thus conserving the Δ 17 O seasonality. Nevertheless, in this study, we used Δ 17 O(O 3 ) measured in the vicinity of the CVAO, applying our unique analytical approach (37), and thus did not adjust this variable to match the observed Δ 17 O, in contrast to previous studies (7,12,13,18 (Fig. 2), the CTM largely favored its daytime chemistry (i.e., NO 2 + OH), placing too much emphasis on the N 2 O 5 hydrolysis and not enough on the NO 3 pathways.…”

“…For also Table S2) shows the different chemistries used by the different models, with DMS (CH 3 SCH 3 ) being the main atmospheric sink of NO 3 (detailed information on the models and data reductions can be found in Materials and Methods and SI Text). (12,13,17,18). In the present study, a chemical box model (SSM) and a 3D model (CTM) are confronted with the CVAO observations, first with the observed concentrations and then with the stable oxygen isotope compositions.…”

“…The version of the CTM used in this study does not include halogen chemistry, such as the formation of atmospheric nitrate via BrNO 3 hydrolysis. Details of the CTM model can be found in the study by Alexander et al (18). We applied the same Δ 17 O uncertainty to the 3D CTM as calculated for the SSM.…”

“…Observational data would be useful for atmospheric chemistry modeling because these isotopic tracers provide constraints for chemical and physical models and constitute new markers for poorly constrained parameterizations (18). Furthermore, there is currently a lack of long-term observations of the oxidation capacity of the tropical MBL in NO x -poor environments (22).…”

NO<sub>x</sub> cycle and the tropospheric ozone isotope anomaly: an experimental investigation

“…It has been hypothesized that NO-O 2 -O 3 -NO 2 photochemical cycling in atmosphere results in NO x becoming enriched in the heavy oxygen isotopes through interactions with O 3 during intermediate oxidation steps (Michalski et al, 2003;Morin et al, 2008). Indeed, an initial NO x -O 3 isotopic equilibrium is the a priori assumption used in current models that predict atmospheric nitrate 17 O values (Michalski et al, 2003;Morin et al, 2008;Alexander et al, 2009). However, these equilibrium 17 O (or δ 18 O) values are based on assumptions about the isotopic composition of O 3 in the troposphere and isotope transfer mechanisms during NO x oxidation.…”