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含碳气溶胶是大气气溶胶中的重要含碳组分,主要由元素碳(elemental carbon,EC)和有机碳(organic carbon,OC)组成[1]。元素碳EC在文献中也称为黑碳(black carbon,BC)或石墨碳[2],它具有较好的化学稳定性和良好的光吸收性,可以作为人为一次排放源的标识物质[3],主要是由化石燃料和生物质的不完全燃烧产生[4]。OC则是一种具有光散射性的混合气溶胶,来源既包括由排放源直接排放的一次有机碳(primary organic carbon,POC)又包括一些大气中气态前体物(如VOCs等)经过光化学反应、二次凝结凝聚及吸湿增长后生成的二次有机碳气溶胶(secondary carbon,SOC)[5-7]。研究表明,含碳气溶胶已对大气污染、气候的变化和人类身体健康产生非常多的影响[8-12],因此对含碳气溶胶的研究对大气污染和环境治理都有着重要意义。
长三角地区是中国经济最发达的地区之一,同时也是中国和世界PM2.5污染最为严重的地区之一[13]。近年来,在长三角地区对含碳气溶胶也展开了非常多的研究。吴梦龙等[14]分析了2011年南京市区和南京北郊工业区全年PM2.1中OC与EC的含量,发现工业区污染更加严重,OC、EC的含量和OC与EC的相关性更高的原因都是春冬两季燃煤量增加和气候原因导致。Wang等[15]则分析了花鸟岛中PM2.5和总悬浮颗粒物中的OC、EC和正构烷烃,发现OC、EC和正构烷烃主要集中于PM2.5中。张懿华等[16]分析上海城区中OC与EC的来源发现,OC与EC的主要来源是机动车尾气、燃煤排放、道路扬尘和生物质燃烧。同时PM2.5引起的灰霾和以O3为主要污染物的光化学污染形成的复合型污染,是目前中国面临的主要大气污染问题,尤其是在长三角地区、京津冀地区、珠三角地区以及西南四川盆地更为严重[17]。O3的强氧化性会促进大气化学反应的生成,从而产生更多的二次气溶胶,而气溶胶对O3的质量浓度也有影响。Feng等[18]对夏季西安及其周边地区一次高O3和PM2.5个例过程研究认为,高质量浓度的气溶胶显著降低O3的光解率并且使其质量浓度降低超过50.0 μg·m−3。蔡彦枫等[19]对个例中大气颗粒物影响近地面O3的过程进行模拟,发现大气颗粒物质量浓度的升高使得气溶胶光学厚度增加,造成O3净生成率下降。说明在大气复合型污染中,PM2.5与O3之间的相互作用十分复杂,气溶胶与O3之间的变化特征和相互关系需要进一步的研究分析。目前基于在线仪器长时间序列对南京地区OC、EC研究开展较少。
为进一步了解南京城区含碳气溶胶污染的变化趋势及季节分布特征等,本研究基于在线监测设备TCA08型总碳分析仪和AE-33型Aethalometer,初步分析了南京2019年4月19日—6月27日PM2.5中含碳气溶胶季节、日变化特征、对比分析了3次O3污染过程中的含碳气溶胶变化特征,以期为有效控制及治理南京含碳气溶胶污染提供基础资料和科学依据。
南京春夏季大气PM2.5中含碳气溶胶污染特征
Study on the pollution characteristics of carbon-containing aerosols in atmospheric PM2.5 in Nanjing in spring and summer
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摘要: 使用TCA08型总碳分析仪观测了南京市2019年4月19日—6月27日大气气溶胶中有机碳(OC)与元素碳(EC)的含量,结合空气质量数据(PM2.5、PM10、SO2、NO2、CO和O3)和气象数据,分析了春夏季含碳气溶胶的污染特征及其影响因素。结果表明,观测期间南京市不同污染物和含碳气溶胶质量浓度春夏季节性差异明显。O3、CO、EC和POC在夏季平均质量浓度分别为109.5、0.74、3.1 、2.1 μg·m−3,分别比春季高了32.1%、4.8%、38.0%和30.0%;而SO2、NO2、PM2.5、PM10、OC和SOC的质量浓度在春季的平均质量浓度分别为12.0、37.2、31.4、65.4、6.2 、4.6 μg·m-3,分别比夏季高了23.3%、24.7%、7.1%、22.8%、26.5%和56.5%。不同空气质量等级下不同碳气溶胶的日变化特征不同。OC和SOC日变化在空气质量为优时日变化平缓,空气质量为良时均为单峰型分布,在轻度污染时均为三峰型分布。EC和POC在空气质量为优时日变化均为双峰型分布,在良和轻度污染下日变化均为单峰型分布。3次污染过程中O3与SOC的相关性不同,相关系数分别为﹣0.64、﹣0.66和0.63。3次O3污染过程中含碳气溶胶日变化特征不同。OC和SOC在过程一和过程二中日变化均表现出夜间高白天低的特征;而过程三中昼夜浓度差异不显著。过程一和过程二中EC和POC均呈单峰型分布,峰值均出现在4:00—6:00,但过程三中却均呈显著的双峰型分布,峰值均分别出现在5:00—7:00和21:00—24:00。Abstract: The TCA08 total carbon analyzer was used to observe the content of organic carbon (OC) and elemental carbon (EC) in atmospheric aerosols in Nanjing from April 19 to June 27, 2019, combined with air quality data (PM2.5, PM10, SO2, NO2, CO and O3) and meteorological data, and analyzed the pollution characteristics and influencing factors of carbon-containing aerosols in spring and summer. The results show that the different pollutants and carbon aerosol mass concentrations in Nanjing during the observation period have obvious seasonal differences in spring and summer. The average mass concentrations of O3, CO, EC and POC in summer are 109.5, 0.74, 3.1, 2.1 μg·m−3, respectively, which are 32.1%, 4.8%, 38.0% and 30.0% higher in summer than in spring; while the average mass concentrations of SO2, NO2, PM2.5, PM10, OC and SOC in spring are 12.0, 37.2, 31.4, 65.4, 6.2 , 4.6 μg·m-3 are respectively 23.3%, 24.7%, 7.1%, 22.8%, 26.5% and 56.5% higher than summer. The diurnal variations characteristics of different carbon aerosols under different air quality levels are different.The diurnal changes of OC and SOC change smoothly when the air quality is excellent. When the air quality is good, they all have a unimodal distribution, and when the air quality is light pollution, they all have a trimodal distribution. The daily changes of EC and POC are both bimodal distributions when the air quality is excellent, and the daily changes are all unimodal distributions when the air quality is good and light pollution.The correlation between O3 and SOC during the three pollution processes was different, and the correlation coefficients were -0.64, -0.66 and 0.63, respectively. The daily variation characteristics of carbon-containing aerosols during the three O3 pollution processes were different. The diurnal changes of OC and SOC in process one and process two showed the characteristics of high at night and low during the day; while the difference in concentration between day and night in process three was not significant. Both EC and POC in process one and process two showed a unimodal distribution, with peaks appearing at 4:00—6:00, but in process three ,they showed significant double peak distribution, and the peaks appeared in 5:00—7:00 and 21:00—24:00, respectively.
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Key words:
- Nanjing /
- organic carbon( OC) /
- elemental carbon( EC) /
- O3 /
- diurnal variation of mass concentration
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