2015—2021年南京市大气污染特征及污染个例研究

Analysis of Air Pollution Characteristics and Associated Compound Air Pollution Case in Nanjing from 2015 to 2021

探究细颗粒物(PM_(2.5))和臭氧(O_(3))污染的时间变化特征,阐明PM_(2.5)和O_(3)复合污染过程中不同阶段环境空气污染物及气溶胶粒径分布的详细演变过程,对南京及长三角地区的大气污染防治具有重要指导意义.本文使用2015—2021年南京市环境空气污染物小时浓度数据,分析了该地区多年大气污染演变过程,并选取2015年10月12—17日时间段作为复合污染典型个例,对其生消过程和内在机理进行了详细分析.结果表明:(1)2015—2021年南京市各种大气污染物的变化特征具有明显差异.PM_(2.5)、PM_(10)和SO_(2)浓度的年下降率分别为8.9%、6.2%和15.4%,O_(3)浓度变化较小.CO浓度在2016年达峰后以每年7.6%的速率下降.NO_(2)浓度在2015—2019年呈增加趋势.(2)2015—2021年污染特征发生较大变化,由PM_(2.5)为主导变为由O_(3)为主导的大气复合污染.PM_(2.5)污染频次和强度均显著降低,O_(3)污染频次和强度均未发生显著变化.(3)污染过程中不同阶段的气溶胶数浓度谱均呈双峰型,但峰值粒径和浓度有较大差异.PM_(2.5)污染阶段,颗粒物峰值粒径位于30 nm和100 nm处,且峰值粒径数浓度最低.数浓度谱中爱根核模态的峰值粒径在NO_(2)污染阶段向小粒径段偏移,位于22~26 nm之间,而在PM_(2.5)和O_(3)复合污染阶段,其峰值粒径则向大粒径段偏移,位于30 nm处;积聚模态的峰值粒径没有变化,但峰值数浓度和峰宽均增大,尤其是>100 nm粒径的数浓度明显高于污染前.O_(3)污染阶段,数浓度谱峰值粒径没有变化,但峰值浓度显著增加,尤其是爱根核模态的峰值浓度是污染前的3~5倍.在污染过程的不同阶段内,环境空气污染物和各模态气溶胶数浓度呈现不同的日变化特征.研究显示,长三角地区城市O_(3)污染和PM_(2.5)污染存在密切的内在联系,复合污染防控和治理需要重点关注本地污染物变化特征和排放源影响,尤其是影响O_(3)生成的前驱污染物质.

The Yangtze River Delta(YRD)urban agglomeration is the most economically developed and urbanized area in China,and it is an important intersection between the‘Belt and Road Initiative’and the Yangtze River Economic Belt.Nanjing is a megacity in the YRD region and an important central city in the eastern region.The rapid development of urbanization has an obvious regional radiation effect.The industry,traffic and population of Nanjing and surrounding areas have shown rapid growing pollution problems.Studies on the spatialtemporal characteristics of air pollution have been extensively carried out,but research remains scarce in long-term systematic tracking investigation on the relationship between air pollutants and meteorological variables,especially in China′s urban sites(e.g.Nanjing)and recent comprehensive environmental pollution control.Here,we used hourly concentrations of six air pollutants and meteorological data in Nanjing between 2015 and 2021 and revealed the characteristics of PM_(2.5)and ozone(O_(3))pollution and their interactions with meteorological conditions.In addition,the pollution types of megacities were mostly compound air pollution.Based on aerosol size distribution data(10 nm-10μm),we selected a typical compound air pollution episode(October 12^(th) to 17^(th),2015)in Nanjing to further clarify the local pollution characteristics.The results showed that the characteristics of six ambient air pollutants differed from one another.The concentrations of PM_(2.5),PM_(10),and SO_(2)decreased significantly,with an average annual decline rate of 8.9%,6.2%and 15.4%,respectively.In contrast,NO_(2)showed an upward trend from 2015 to 2019.CO peaked in 2016 and then declined at a rate of 7.6%per year.The concentration of O_(3)showed a small variation during 2015-2021.Overall,the type of air pollution in Nanjing underwent complex changes,shifting from PM_(2.5)dominance to O_(3)dominance.The frequency and intensity of PM_(2.5)dominated pollution decreased notably,while the frequency and intensity of O_(3)dominated pollution did not change significantly.At different air pollution evolution stages,the number concentration spectra of aerosols were bimodal while the peak particle size and concentration were different.The peak aerosol concentrations were the lowest in the PM_(2.5)pollution stage,with peak values of 30 nm and 100 nm,respectively.The aerosol size distribution peaks in nucleation mode shifted to smaller size pattern in the NO_(2)pollution stage,and the value was 22-26 nm.In PM_(2.5)and O_(3)combined pollution stage,the first peak of aerosol distribution in nucleation mode shifted to a larger size pattern around 30 nm,which differed from that in accumulation mode,and the second peak concentration increased and peak width became wider,especially for the size>100 nm.In the O_(3)dominated pollution stage,the peak aerosol size remained unchanged,but the peak concentration increased significantly,especially the nucleation mode aerosol,which was 3-5 times higher than the counterpart before the pollution.In all different pollution stages,ambient air pollutants and aerosol number concentrations in different modes showed obvious diurnal variations.The coordinated prevention and control of PM_(2.5)and O_(3)pollution in Nanjing City needs to focus on the characteristics of local pollutants and the impact of emission sources,especially the variation and emission sources of precursors affecting O_(3)pollution.