Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2018-385
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
08 May 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Nitrogen isotope fractionation during gas-particle conversion of NOx to NO3 in the atmosphere – implications for isotope-based NOx source apportionment
Yunhua Chang1, Yanlin Zhang1, Chongguo Tian2, Shichun Zhang3, Xiaoyan Ma4, Fang Cao1, Xiaoyan Liu1, Wenqi Zhang1, Thomas Kuhn5, and Moritz F. Lehmann5 1Yale-NUIST Center on Atmospheric Environment, Nanjing University of Information Science and Technology, Nanjing 10044, China
2Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China
3Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, 4888 Shengbei Road, Changchun 130102, China
4Key Laboratory for Aerosol Cloud-Precipitation of China Meteorological Administration, Earth System Modeling Center, Nanjing University of Information Science and Technology, Nanjing 10044, China
5Aquatic and Isotope Biogeochemistry, Department of Environmental Sciences, University of Basel, Basel 4056, Switzerland
Abstract. Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate (pNO3), the end product of the oxidation of NOx gases (= NO + NO2) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO3 to constrain NOx source partitioning in the atmosphere requires the knowledge of the isotope fractionation during the reactions leading to NO3 formation. Here we determined the δ15N values of fresh pNO315N-pNO3) in PM2.5 at a rural site in Northern China, where atmospheric pNO3 can be attributed exclusively to biomass burning. The observed δ15N-pNO3 (12.17 ± 1.55 ‰; n = 8) was much higher than the N isotopic source signature of NOx from biomass burning (1.04 ± 4.13 ‰). The large difference between δ15N-pNO3 and δ15N-NOx (Δ(δ15N)) can be reconciled by the net N isotope effect (ԑN) associated with the gas-particle conversion from NOx to NO3. For the biomass-burning site, a mean ԑN (≈ Δ(δ15N)) of 10.99 ± 0.74 ‰ was assessed through a newly-developed computational quantum chemistry (CQC) module. ԑN depends on the relative importance of the two dominant N isotope exchange reactions involved (NO2 reaction with OH versus hydrolysis of dinitrogen pentoxide (N2O5) with H2O), and varies between regions, and on a diurnal basis. A second, slightly higher CQC-based mean value for ԑN (15.33 ± 4.90 ‰) was estimated for an urban site with intense traffic in Eastern China, and integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates for NOx at this site. Based on the δ15N values (10.93 ± 3.32 ‰, n = 43) of ambient pNO3 determined for the urban site, and considering the location-specific estimate for ԑN, our results reveal that the relative contribution of coal combustion and road traffic to urban NOx are 32 ± 11 % and 68 ± 11 %, respectively. This finding agrees well with a regional bottom-up emission inventory of NOx. Moreover, the variation pattern of OH contribution to ambient pNO3 formation calculated by the CQC module is consistent with that simulated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), further confirming the robustness of our estimates. Our investigations also show that, without the consideration of the N isotope effect during pNO3 formation, the observed δ15N-pNO3 at the study site would erroneously imply that NOx is derived almost entirely from coal combustion. Similarly, reanalysis of reported δ15N-NO3 data throughout China suggests that, nationwide, NOx emissions from coal combustion may be substantively overestimated (by > 30 %) when the N isotope fractionation during atmospheric pNO3 formation is neglected.
Citation: Chang, Y., Zhang, Y., Tian, C., Zhang, S., Ma, X., Cao, F., Liu, X., Zhang, W., Kuhn, T., and Lehmann, M. F.: Nitrogen isotope fractionation during gas-particle conversion of NOx to NO3 in the atmosphere – implications for isotope-based NOx source apportionment, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-385, in review, 2018.
Yunhua Chang et al.
Yunhua Chang et al.
Yunhua Chang et al.

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