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Oxygen isotope anomaly (Δ17O) in atmospheric nitrate: A review

The nitrate is a significant component in atmospheric aerosol and has a great impact on the atmospheric chemistry, fine particulate formation, radiative balance and human health. The oxygen isotope anomaly (Δ17O) is quantified as Δ17O= δ 17O-0.52× δ 18O and it represents the enrichment in 17O relative to 18O over the expected relationship ( δ 17O~0.5× δ 18O) in mass dependent fractionation processes. The Δ17O values in atmospheric nitrate (Δ17NO3-) depend on the mixing of oxygen sources (e.g. O3, H2O and O2) when NO x is photochemically converted into nitrate. In that case, the observation of Δ17NO3- coupled with photochemical modeling can be used to quantify the relative contribution of each pathway. Here, the recent research progresses of oxygen isotope anomaly of atmospheric nitrate are reviewed. Firstly, the preparation of the triple oxygen isotope (16O, 17O, 18O) measurement of atmospheric nitrate are compared: (1) The precision of the pyrolysis technique is high (±0.3‰), but this method requires relatively large amount of nitrate (>50 μmol NO3-) and complicated preparations. (2) The size and purification limitations are largely overcomed by the denitrification method conducted with Pseudomonas aureofaciens bacteria. It is suitable for isotopic analysis of nanomolar amounts of nitrate with a precision in Δ17O of ±0.6‰. (3) The reduction-azide technique also has the advantage of high precision (±0.2‰), low detection limit and sample preparation. But the utility of toxic substances (cadmium and azide) are unavoidable in the reduction. Secondly, the global characterization of Δ17NO3- are compared. Δ17NO3- tends to be higher in high latitudes than in mid-latitudes, whereas Δ17NO3- in cold seasons are higher than that in warm seasons. Thirdly, the possible formation pathways of atmospheric nitrate are summarized. NO can be converted to NO2 by O3 or HO2/RO2 in NO x cycle, and then oxidized into nitrate via NO2+·OH, NO3+HC/DMS or N2O5 hydrolysis. Reactive halogen (e.g. BrO) and HNO4 photolysis can also play an important role in nitrate formation in polar regions. Nitrate oxidized by ·OH have lower Δ17O values, and higher Δ17NO3- normally suggests more O3 oxidation. Fourthly, the Δ17O values of oxygen sources that contribute to nitrate are introduced. Among all the oxygen sources (O3, ·OH, HO2/RO2, O2 and tropospheric water), only the Δ17O of ozone is exhibited with high value (25‰–37‰), other compounds are considered to have Δ17O ~0‰. Therefore, the Δ17NO3- from each pathway can be presented based on these values. Next, the photochemical box model and other atmospheric chemical models simulating the Δ17NO3- on regional and global scale are summarized. The photochemical box model is limited for not considering the transport of atmospheric nitrate from neighboring regions. The 3-D model is more advanced than the box model for including vertical and horizontal transport, and incorporating spatial variations in surface fluxes of important primary pollutants such as NO x and VOCs. Finally, the future directions and the application of the researches on Δ17NO3- in the field of atmospheric chemistry are proposed: (1) The target areas of Δ17NO3- observation should be expanded to cover more different regions such as polluted urban and rural areas. (2) More studies are in need for decreasing the uncertainties in the simulation of Δ17NO3-. (3) The nitrate formation mechanism under different atmospheric conditions in various regions still needs to be better understood. (4) The study of oxygen isotope anomaly should be coupled with other approaches in atmospheric research (e.g. air quality modeling and online observation of chemical composition in atmospheric aerosols).