氮氧同位素示踪湿沉降NO3−来源及氧化途径

Tracking the Origins and Oxidation Pathways of Nitrate in Wet Deposition Using Nitrogen and Oxygen Isotopes

    摘要
  • 摘要: 为探明焦作市大气湿沉降中硝态氮的污染水平，识别其来源及其形成过程，于2020年1月—2021年12月采集了焦作市大气湿沉降样品41个，测定并分析了TN、NH4+-N、NO3−-N浓度以及δ15N-NO3−、δ18O-NO3−值. 结果表明：①TN、NH4+-N、NO3−-N浓度范围分别为2.52~13.27、0.11~1.70、1.64~8.31 mg/L，焦作市湿沉降中氮的主要存在形态为NO3−-N，占比为52.11%~83.92%. ②δ18O-NO3−、δ15N-NO3−值的范围分别为54.9‰~93.9‰、−9.8‰~3.0‰，非雨季δ18O-NO3−值(62.5‰~93.3‰)更接近于N2O5氧化途径生成的δ18O-HNO3值(100.9‰~103.1‰)，雨季δ18O-NO3−值(53.2‰~73.0‰)更接近于OH氧化途径生成的δ18O-HNO3值(50.9‰~60.9‰)，说明非雨季大气湿沉降NO3−主要来自N2O5的氧化，而雨季主要来自OH的氧化. ③雨季，OH和N2O5氧化途径对湿沉降NO3−的贡献率分别为80.32%和19.67%，非雨季，两种氧化途径的贡献率分别为65.37%和34.62%；在考虑了同位素分馏情况下，基于贝叶斯模型识别了湿沉降NO3−的主要来源，即土壤排放、生物质燃烧、机动车尾气排放和煤炭燃烧，其贡献率平均值分别为31.70%±4.90%、28.30%±14.00%、22.80%±12.10%、17.20%±0.08%. 该研究结果将为降雨中硝酸盐的来源及形成途径提供证据.

     

    Abstract: To evaluate the nitrogen species and nitrate isotopic composition in wet deposition in Jiaozuo City, and quantitatively identify the sources of nitrate and its formation processes, a total of 41 wet deposition samples were collected in Jiaozuo City from January 2020 to December 2021. These samples were analyzed for various parameters, including total nitrogen (TN), ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), δ15N-NO3− and δ18O-NO3−. The results show that: (1) TN, NH4+-N and NO3−-N ranged from 2.52 mg/L to 13.27 mg/L, 0.11 mg/L to 1.70 mg/L,1.64 mg/L to 8.31 mg/L, respectively. Notably, NO3−-N accounted for a significant portion of TN, ranging from 52.11% to 83.92% of TN, highlighting that nitrate nitrogen was the predominant form of nitrogen. (2) δ18O-NO3− and δ15N-NO3− values ranged from 54.9‰ to 93.9‰ and −9.8‰ to 3.0‰, respectively. The δ18O-NO3− values in the non-rainy season samples (ranging from 62.5‰ to 93.3‰) closely matched calculated δ18O-HNO3 values derived from the N2O5 pathway (ranging from 100.9‰ to 103.1‰). Conversely, the δ18O-NO3− values in the rainy season samples (ranging from 53.2‰ to 73.0‰) closely aligned with the calculated δ18O-HNO3 values from the OH pathway (ranging from 50.9‰ to 60.9‰). This difference suggests that nitrate in wet deposition primarily originates from N2O5 pathway during the non-rainy season and shifts toward the OH pathway during the rainy season. (3) The fractional contributions of the N2O5 pathway averaged 65.37% during the non-rainy season and increased to 80.32% during the rainy season, while contribution of the OH pathway averaged 34.62% during the non-rainy season and decreased to 19.67% during the rainy season. Furthermore, a Bayesian mixing model was employed to calculate the contribution of four potential nitrate sources, which were identified soil emission, biomass combustion, vehicle emission, and coal combustion. The results indicated that these sources contributed approximately 31.70%±4.90%, 28.30%±14.00%, 22.80%±12.10%, 17.20%±0.08%, respectively, to the total nitrate in wet deposition. These findings provide valuable insights into the formation and sources of nitrate in wet deposition.

     

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