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 (Delta O-17) is quantified as Delta O-17=delta O-17 similar to 0.52x delta O-18 and it represents the enrichment in O-17 relative to O-18 over the expected relationship (delta O-17 similar to 0.52x delta O-18) in mass dependent fractionation processes. The Delta O-17 values in atmospheric nitrate (Delta(NO3-)-N-17) depend on the mixing of oxygen sources (e.g. O-3, H2O and O-2) when NOx is photochemically converted into nitrate. In that case, the observation of Delta(NO3-)-N-17 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 (O-16, O-17, O-18) measurement of atmospheric nitrate are compared: (1) The precision of the pyrolysis technique is high (+/- 0.3 parts per thousand), but this method requires relatively large amount of nitrate (>50 mu mol 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 Delta O-17 of +/- 0.6 parts per thousand. (3) The reduction-azide technique also has the advantage of high precision (+/- 0.2 parts per thousand), 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 Delta(NO3-)-N-17 are compared. Delta(NO3-)-N-17 tends to be higher in high latitudes than in mid-latitudes, whereas Delta(NO3-)-N-17 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 O-3 or HO2/RO2 in NOx cycle, and then oxidized into nitrate via NO2+center dot 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 center dot OH have lower Delta(17) values, and higher Delta(NO3-)-N-17 normally suggests more O-3 oxidation. Fourthly, the Delta O-17 values of oxygen sources that contribute to nitrate are introduced. Among all the oxygen sources (O-3, center dot OH, HO2/RO2, O-2 and tropospheric water), only the Delta O-17 of ozone is exhibited with high value (25 parts per thousand-37 parts per thousand), other compounds are considered to have Delta O-17 similar to 0 parts per thousand. Therefore, the Delta(NO3-)-N-17 from each pathway can be presented based on these values. Next, the photochemical box model and other atmospheric chemical models simulating the Delta(NO3-)-N-17 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 NOx and VOCs. Finally, the future directions and the application of the researches on Delta(NO3-)-N-17 in the field of atmospheric chemistry are proposed: (1) The target areas of Delta(NO3-)-N-17 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 Delta(NO3-)-N-17 (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).