广州市城区站点冬季PM2.5时间变化及碳氮同位素组成特征

Temporal Variation of PM_(2.5) and Its Carbon and Nitrogen Isotopic Composition at an Urban Site in Guangzhou in Winter

于2018年冬季在广州城区磨碟沙站点开展细颗粒物(PM_(2.5))样品采集,并获得PM_(2.5)中水溶性离子、含碳组分、稳定碳氮同位素的组成及时间变化特征,重点讨论了PM_(2.5)浓度升高时段的化学组成特征变化,进而利用稳定碳氮同位素变化特征探究了主要污染来源。结果表明:采样期间,研究站点PM_(2.5)平均质量浓度为22.1μg/m^(3),共出现两个PM_(2.5)浓度水平升高时段,所对应的平均质量浓度分别达46.0μg/m^(3)和63.0μg/m^(3)。风速降低、温度升高等不利气象条件是导致上述时段PM_(2.5)浓度上升的重要原因。在上述时段,伴随着PM_(2.5)浓度的升高,NO_(3)^(-)和NH_(4)^(+)浓度均出现显著升高,NO_(3)^(-)与SO_(4)^(2-)的摩尔质量比(NO_(3)^(-)/SO_(4)^(2-))同时出现高值,表明当地PM_(2.5)浓度主要受到机动车排放的影响。研究发现,伴随着PM_(2.5)浓度的升高,稳定氮同位素比值(δ15 N)显著升高,而稳定碳同位素比值(δ^(13) C)则出现降低。δ^(13) C与NH_(4)^(+)、NO_(3)^(-)、NO_(3)^(-)/SO_(4)^(2-)之间均呈现显著的负相关关系,说明机动车排放对δ^(13) C有重要影响。硝酸盐氮(NO_(3)^(-)-N)和铵盐氮(NH_(4)^(+)-N)表现出与δ15 N相似的变化趋势。其中,NH_(4)^(+)对总氮(TN)的贡献为46%,高于NO_(3)^(-)对TN的贡献(36%)。在PM_(2.5)浓度升高时段,NH_(4)^(+)/SO_(4)^(2-)表现出高值,NO_(3)^(-)也出现浓度高值,表明PM_(2.5)中富含铵盐,并进一步证明机动车排放对PM_(2.5)污染贡献较大。

In this study,fine particulate matter(PM_(2.5))samples were collected at an urban site(Modiesha)in Guangzhou during the winter of 2018.The composition and variation characteristics of water-soluble ions,carbonaceous components and stable carbon and nitrogen isotopic compositions in PM_(2.5) were obtained.The characteristics of chemical components in PM_(2.5) during the period of PM_(2.5) enhancement were discussed,and then the pollution sources were explored.The results showed that the average mass concentration of PM_(2.5) during the sampling period was 22.1μg/m^(3).Two periods with PM_(2.5) enhancements were observed with the average PM_(2.5) mass concentrations of 46.0μg/m^(3) and 63.0μg/m^(3),respectively.Unfavorable weather conditions such as wind speed decrease and temperature increase were suggested as the main factors affecting the increase of PM_(2.5) concentration.NO_(3)^(-) and NH_(4)^(+) concentrations increased significantly during these two periods with PM_(2.5) enhancement.The ratio of NO_(3)^(-)/SO_(4)^(2-) was relatively high,indicating that the increase of PM_(2.5) was possibly affected by motor vehicle emissions.When the concentration of PM_(2.5) increased,δ15 N increased significantly butδ^(13) C decreased.There were significant negative correlations betweenδ^(13) C and NH_(4)^(+),NO_(3)^(-) and NO_(3)^(-)/SO_(4)^(2-),indicating the important impact of vehicle emissions.δ15 N and NO_(3)^(-)-N and NH_(4)^(+)-N showed quite similar trends during the mentioned periods.The contribution of NH_(4)^(+)(46%)to total nitrogen(TN)was higher than that of NO_(3)^(-)(36%).High ratios of NH_(4)^(+)/SO_(4)^(2-) during the periods with PM_(2.5) enhancement was observed,and NO_(3)^(-) also showed high concentration.It showed that the observed PM_(2.5) aerosol was rich in sufficient ammonium salt.In summary,it further proved that motor vehicle emission was the major contributor to PM_(2.5) enhancement.