Significant Changes of δ15N-NOx in Diesel Vehicle Emissions with Emission Standards and Cumulative Mileage: Insight into Isotope-Based NOx Source Analysis
{"title":"Significant Changes of δ15N-NOx in Diesel Vehicle Emissions with Emission Standards and Cumulative Mileage: Insight into Isotope-Based NOx Source Analysis","authors":"Weijie Huang, Xingnan Ye*, Xiaofei Wang, Cheng Huang, Bingyue Fu, Yinghui Yao, Yuanqiao Zhou and Jianmin Chen, ","doi":"10.1021/acsestair.4c0028910.1021/acsestair.4c00289","DOIUrl":null,"url":null,"abstract":"<p >Nitrogen isotope (δ<sup>15</sup>N) analysis is a robust method for tracing atmospheric NO<sub><i>x</i></sub> and nitrate. However, the isotopic signatures of NO<sub><i>x</i></sub> emissions from heavy-duty diesel vehicles (HDDVs) remain poorly characterized. In this study, we conducted on-site tests on 18 on-road HDDVs with different emission standards in Shanghai under the condition of China World Transient Vehicle Cycle for trucks (C-WTVC). The average δ<sup>15</sup>N value under C-WTVC (−5.71 ± 2.60‰) was significantly higher than that in idle mode (−13.85 ± 1.97‰), which is attributed to the lower temperatures in the combustion chamber due to the lower engine output in idle mode, favoring the breakage of the lighter <sup>14</sup>N<sup>14</sup>N molecules. The δ<sup>15</sup>N-NO<sub><i>x</i></sub> values under C-WTVC were significantly lower than those in driving mode in the United States, suggesting that δ<sup>15</sup>N values may be localized in isotope-based NO<sub><i>x</i></sub> source analysis. NO<sub><i>x</i></sub> emissions were reduced considerably with the upgrade of emission standards while δ<sup>15</sup>N-NO<sub><i>x</i></sub> levels showed an opposite trend. The NH<sub>3</sub>-selective catalytic reduction (SCR) reduced automobile NO<sub><i>x</i></sub> emissions by more than 50% and significantly enriched <sup>15</sup>N relative to diesel-generated NO<sub><i>x</i></sub>. Indicated by the correlations between NO<sub><i>x</i></sub> emissions, δ<sup>15</sup>N-NO<sub><i>x</i></sub>, and cumulative mileage, the SCR de-NO<sub><i>x</i></sub> efficiency decreased linearly with the increase of cumulative mileage. Our results suggest that the routine maintenance and mandatory inspection of aged SCR systems should be strengthened. The distribution of provincial δ<sup>15</sup>N-NO<sub><i>x</i></sub> values of diesel vehicle emissions was established based on a weighted average model. The contribution of vehicle NO<sub><i>x</i></sub> emissions in typical megacities in China has been revisited by upgrading the δ<sup>15</sup>N-NO<sub><i>x</i></sub> values of diesel vehicles, demonstrating that the application of regional δ<sup>15</sup>N-NO<sub><i>x</i></sub> values of diesel vehicles can narrow the gap between isotope-based analysis and emission inventory. This study will enrich the NO<sub><i>x</i></sub> isotopic fingerprint database in China and minimize the uncertainty in isotope-based NO<sub><i>x</i></sub> source apportionment, helping to formulate air pollution prevention and control strategies.</p>","PeriodicalId":100014,"journal":{"name":"ACS ES&T Air","volume":"2 3","pages":"396–405 396–405"},"PeriodicalIF":0.0000,"publicationDate":"2025-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"ACS ES&T Air","FirstCategoryId":"1085","ListUrlMain":"https://pubs.acs.org/doi/10.1021/acsestair.4c00289","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
引用次数: 0
Abstract
Nitrogen isotope (δ15N) analysis is a robust method for tracing atmospheric NOx and nitrate. However, the isotopic signatures of NOx emissions from heavy-duty diesel vehicles (HDDVs) remain poorly characterized. In this study, we conducted on-site tests on 18 on-road HDDVs with different emission standards in Shanghai under the condition of China World Transient Vehicle Cycle for trucks (C-WTVC). The average δ15N value under C-WTVC (−5.71 ± 2.60‰) was significantly higher than that in idle mode (−13.85 ± 1.97‰), which is attributed to the lower temperatures in the combustion chamber due to the lower engine output in idle mode, favoring the breakage of the lighter 14N14N molecules. The δ15N-NOx values under C-WTVC were significantly lower than those in driving mode in the United States, suggesting that δ15N values may be localized in isotope-based NOx source analysis. NOx emissions were reduced considerably with the upgrade of emission standards while δ15N-NOx levels showed an opposite trend. The NH3-selective catalytic reduction (SCR) reduced automobile NOx emissions by more than 50% and significantly enriched 15N relative to diesel-generated NOx. Indicated by the correlations between NOx emissions, δ15N-NOx, and cumulative mileage, the SCR de-NOx efficiency decreased linearly with the increase of cumulative mileage. Our results suggest that the routine maintenance and mandatory inspection of aged SCR systems should be strengthened. The distribution of provincial δ15N-NOx values of diesel vehicle emissions was established based on a weighted average model. The contribution of vehicle NOx emissions in typical megacities in China has been revisited by upgrading the δ15N-NOx values of diesel vehicles, demonstrating that the application of regional δ15N-NOx values of diesel vehicles can narrow the gap between isotope-based analysis and emission inventory. This study will enrich the NOx isotopic fingerprint database in China and minimize the uncertainty in isotope-based NOx source apportionment, helping to formulate air pollution prevention and control strategies.