氮氧同位素示踪北京冬季大气硝酸盐来源

Using δ^(15)N-NO_(3)^(-) and δ^(18)N-NO_(3)^(-) to Trace the Source of Atmospheric Nitrate in Beijing in Winter

硝酸盐是大气细颗粒物(PM_(2.5))主要的二次无机组分之一,探究颗粒物硝酸盐的形成机制及NOx排放源结构对治理PM_(2.5)污染和改善空气质量至关重要。为此,该研究收集并分析了北京市实施清洁空气行动计划前后(2013-2018年)冬季大气硝酸盐同位素组成特征。结果显示,北京市实施清洁空气行动期间,冬季大气PM_(2.5)中硝酸盐氮同位素(δ^(15)N-NO_(3)^(-))没有发生显著变化;δ^(15)N-NO_(3)^(-)平均值分别为+13.10‰±4.67‰(2013)、+12.65‰±4.08‰(2014)、+12.4‰±3.5‰(2017)、+11.54‰±4.22‰(2018)。通过剖析影响硝酸盐氮同位素的主要因素,研究认为该期间NOx排放源结构并没有显著变化。结合同位素组成和贝叶斯混合模型发现,2013-2018年期间机动车尾气对冬季NO_(x)的平均贡献值均保持在75%左右,表明机动车尾气可能是北京市冬季PM_(2.5)中硝酸盐的主要来源。由此可见,加强机动车尾气排放标准管理,是降低北京地区硝酸盐浓度和改善空气质量的有力途径。

Particulate nitrate has become the predominant secondary inorganic component in the atmospheric fine particulate matter(PM_(2.5))in the urban environment.Exploring the formation mechanism of particulate nitrate and the relative contribution of NOxemission sources are essential to cut off PM_(2.5)pollution and improve air quality.To this end,this study compiled and dissected the wintertime atmospheric nitrate isotopic compositions during the Beijing Clean Air Action Plan period(2013-2018).The results showed that there was no significant change in the nitrate nitrogen isotopes(δ^(15)N-NO_(3)^(-))of atmospheric PM_(2.5)in winter during this period;and the average values δ^(15)N-NO_(3)^(-)were+13.10‰±4.67‰(2013),+12.65‰±4.08‰(2014),+12.4‰±3.5‰(2017),+11.54‰±4.22‰(2018),respectively.By analyzing the main factors affecting nitrate nitrogen isotope,the study concluded that the structure of NO_(x) emission sources during this period did not change significantly.Combining isotopic composition and Bayesian mixture model,we found that the average contribution of vehicle exhaust to winter NOxfrom 2013 to 2018 remained at about 75%,indicating that vehicular exhaust may be the main source of nitrate in PM_(2.5)in winter of Beijing.Therefore,strengthening vehicle emission is an important way to reduce the concentration of nitrate and improve air quality in Beijing.