上世纪初以来，人类活动导致地表大气活性氮浓度和沉降水平快速持续增加，已经成为全球最突出的环境问题之一。五十年代，氮同位素（δ15N）技术开始用于地表不同介质氮源示踪（Hoering, 1955），随即也开始了大气氮污染物的来源研究（Hoering, 1957），至今已在东亚、欧洲和北美三大高氮排放区对雨水和颗粒物开展了很多观测。然而，由于缺少约束大气氮转化过程原位同位素效应的方法，阻碍了同位素对大气氮源的准确区分。
近年，天津大学地科院刘学炎、宋韦等将其在氮素生物地球化学方面的研究方法和思路（Liu et al., 2013a, 2013b, 2018; Hu et al., 2019）应用到大气无机氮来源和过程机制的研究中，提出了运用多源同位素质量平衡模型分析大气降水和颗粒物氮源的方案（Wang et al., 2016, 2017; Zheng et al., 2018），初步评价了非化石源氮氧化物和化石源氨排放对城市氮沉降的贡献（Liu et al., 2017; Song et al., 2019）。
最近，他们发展了△17O定量区分大气颗粒物硝酸根生成途径的新方法，揭示了人为污染对主要氧化过程贡献的影响（Wang et al., 2019），并提高了大气硝酸根生成途径贡献及其同位素分馏效应的计算（Song et al., 2020）。同时，他们整合陆地环境大气主要无机硝态氮种（氮氧化物、气态硝酸、颗粒态硝酸根和降水硝酸根）的氮同位素观测，约束了自然大气环境氮氧化物转化和沉降过程的同位素效应，推动了大气硝态氮来源定量示踪方案的发展（Liu et al., 2020）。
上述研究扩展了氮同位素地球化学示踪研究的方法学和机理前沿，并有助于区域空气氮污染减排和效应评估。感谢国家自然科学重点和优青项目、国家重点研发大气专项课题等的资助。
References:
1. Hoering T. (1955) Variations in nitrogen-15 in naturally occurring substances. Science, 122: 1233. https://science.sciencemag.org/content/122/3182/1233
2. Hoering T. (1957) The isotopic composition of the ammonia and the nitrate ion in rain. Geochimica et Cosmochimica Acta, 12: 97–102. https://www.sciencedirect.com/science/article/abs/pii/0016703757900212
3. Liu XY*, Yin YM, Song W. (2020) Nitrogen isotope differences between major atmospheric NOy species: Implications for transformation and deposition processes. Environmental Science & Technology Letters, DOI: 10.1021/acs.estlett.0c00105. https://pubs.acs.org/doi/10.1021/acs.estlett.0c00105
4. Liu XY*, Xiao HW, Xiao HY*, Song W, Sun XC, Zheng XD, Liu CQ, Keisuke Koba. (2017) Stable isotope analyses of precipitation nitrogen sources in Guiyang, southwestern China. Environmental Pollution, 230: 486–494. https://www.sciencedirect.com/science/article/pii/S0269749116324708
5. Liu XY*, Koba K*, Koyama L, Hobbie SE, Weiss MS, Inagaki Y, Shaver GR, Giblin AE, Hobara S, Nadelhoffer KJ, Sommerkorn M, Rastetter EB, Kling GW, Laundre JA, Yano Y, Makabe A, Yano M, Liu CQ. (2018) Nitrate is an important nitrogen source for arctic tundra plants. PNAS, 115 (13): 3398–3403. https://www.pnas.org/content/115/13/3398
6. Liu XY, Koba K*, Makabe A, Li XD, Yoh M, Liu CQ. (2013a) Ammonium first: natural mosses prefer atmospheric ammonium but vary utilization of dissolved organic nitrogen depending on habitat and nitrogen deposition. New Phytologist, 199 (2): 407–419. https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.12284
7. Liu XY, Koba K*, Takebayashi Y, Liu CQ, Fang YT, Yoh M. (2013b). Dual N and O isotopes of nitrate in natural plants: first insights into individual variability and organ-specific pattern. Biogeochemistry, 114: 399–411. https://link.springer.com/article/10.1007/s10533-012-9721-4
8. Hu CC, Lei YB, Tan YH, Sun XC, Xu H, Liu CQ, Liu XY*. (2019) Plant nitrogen and phosphorus utilization under invasive pressure in a montane ecosystem of tropical China. Journal of Ecology, 107: 372–386. https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.13008
9. Wang YL, Song W, Yang W, Sun XC, Dong YP, Hu CC, Tong YD, Wang XM, Liu CQ, Bai ZP*, Liu XY*. (2019) Influences of atmospheric pollution on the contributions of major oxidation pathways to PM2.5 nitrate formation in Beijing. Journal of Geophysical Research-Atmospheres, 124. doi.org/10.1029/ 2019JD030284. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JD030284
10. Zheng XD, Liu XY*, Song W, Sun XC, Liu CQ. (2018) Nitrogen isotope variations of ammonium across rain events: Implications for different scavenging between ammonia and particulate ammonium. Environmental Pollution, 239: 392–398. https://www.sciencedirect.com/science/article/pii/S0269749117346791
11. Song W, Liu XY*, Wang YL, Tong YD, Bai ZP, Liu CQ. (2020) Nitrogen isotope differences between atmospheric nitrate and corresponding nitrogen oxides: A new constraint using oxygen isotopes. Science of the Total Environment, 701, 134515. https://www.sciencedirect.com/science/article/pii/S0048969719345061
12. Song W, Wang YL, Yang W, Sun XC, Dong YP, Hu CC, Tong YD, Wang XM, Liu CQ, Bai ZP*, Liu XY*. (2019) Isotopic evaluation on relative contributions of major NOx sources to nitrate of PM2.5 in Beijing. Environmental Pollution, 248, 183–190. https://www.sciencedirect.com/science/article/pii/S0269749118324576
13. Wang YL, Liu XY*, Song W, Yang W, Han B, Dou XY, Zhao XD, Song ZL, Liu CQ, Bai ZP*. (2017) Source appointment of nitrogen in PM2.5 based on bulk δ15N signatures and a Bayesian isotope mixing model. Tellus B: Chemical and Physical Meteorology, 69:1, 1299672, DOI: 10.1080/16000889.2017.1299672. https://www.tandfonline.com/doi/full/10.1080/16000889.2017.1299672