2013~2020年北京大气PM_(2.5)和O_(3)污染演变态势与典型过程特征

The evolution trend and typical process characteristics of atmospheric PM_(2.5) and O_(3) pollution in Beijing from 2013 to 2020

细颗粒物(PM_(2.5))和臭氧(O_(3))是我国当前最受关注的两种大气污染物,影响空气质量并危害人体健康.本文以2013~2020年北京大气污染物浓度水平和气象数据为基础,揭示PM_(2.5)与O_(3)污染的逐年变化特征及典型污染过程的“发生-发展-消除”规律.研究表明,PM_(2.5)污染过程的发生频次、持续时间、峰值浓度均逐年下降;相比之下,O_(3)污染过程的年际变化趋势不明显,在历年的5~7月中大约1/3时间处于O_(3)污染过程;且在2018年,O_(3)超标天数首次超过PM_(2.5)超标天数,暗示O_(3)可能正在取代PM_(2.5)成为北京空气质量超标的主要污染物.通过剖析3次PM_(2.5)污染过程、3次O_(3)污染过程、3次PM_(2.5)与O_(3)双污染过程,发现PM_(2.5)爆发时和O_(3)爆发前对应着高氮氧化物(NO_(x))浓度.PM_(2.5)超标天多发生在冬季及其前后,且一个污染过程持续时间较长,PM_(2.5)浓度变化呈现横跨多天的山峰型或阶梯型;而O_(3)超标天多发生在夏季及其前后,O_(3)浓度呈现明显的日变化规律,即日间高夜间低;当O_(3)污染持续时间较长时,可能发展成O_(3)与PM_(2.5)双超标,此时两种污染物的浓度会在时间序列上呈现一定的峰谷交替现象,且与温湿度的昼夜变化规律较为吻合.本研究结果可为相关部门明晰与协同控制PM_(2.5)和O_(3)提供一定的理论依据,在现阶段大气污染防治已取得长足进展的基础之上,进一步提升空气质量.

Fine particulate matter(PM_(2.5))and tropospheric ozone(O_(3))are currently the two air pollutants of the greatest concern in China,affecting air quality and human health.Since 2013,Beijing has implemented multiple measures such as the“Air Pollution Prevention and Control Action Plan”and the“Three-Year Action Plan for Winning the Blue Sky Defense War”,which have significantly reduced primary emissions and improved air quality.This study focuses on the two major pollutants:PM_(2.5) and O_(3),which are tightly related to the secondary formation chemistry in ambient air.The statistical data on the number of days during the period from 2013 to 2020 on which the concentration of PM_(2.5) and O_(3) in Beijing exceeds the standard were obtained from the China National Environmental Monitoring Center,as well as the concentration data of PM_(2.5) and O_(3) from 2015 to 2020.The pollutant concentrations and meteorological parameters of the typical pollution process are the observation results of the typical urban site“PKUERS”located on top of the Science Building No.1 of Peking University campus in Haidian District,Beijing.In addition,the characteristics of the annual evolution of PM_(2.5) and O_(3) pollution,as well as the“generation-development-elimination”law of the typical pollution processes are revealed.Studies have shown that the frequency,duration,and peak concentration of the PM_(2.5) pollution process are all decreasing year by year.Compared with 155 d of PM_(2.5) exceeding the standard in 2013,there are only 35 d of PM_(2.5) exceeding the standard in 2020(a decrease of 77%),indicating that Beijing’s atmospheric PM_(2.5) control has made remarkable progress.However,in the months of October,November,January and February of 2020,there are still 6−8 d of PM_(2.5) exceedance every month,or 1.5−2 d of exceedance per week,which implies that PM_(2.5) pollution always exists in the cold season.In contrast,the trend of interannual change of O_(3) pollution process is not evident,about 1/3 of the time from May to July of each year is exposed to O_(3) pollution.In 2018,the number of days when O_(3) exceeded the standard surpassed PM_(2.5) for the first time,suggesting that O_(3) may gradually replace PM_(2.5) as the most important pollutant in Beijing.Moreover,this paper summarizes three PM_(2.5) pollution processes,three O_(3) pollution processes,and three PM_(2.5) and O_(3) dual pollution processes.It is shown that high NO_(x) concentrations are often associated with the occurrence of PM_(2.5) and O_(3).PM_(2.5) episodes mainly occur in winter,and a pollution process lasts for a long time,the change of PM_(2.5) concentration shows a peak or a step pattern;while O_(3) episodes mainly occur in summer,the O_(3) concentration shows an obvious daily variation,high concentration during the day and low concentration at night.When O_(3) pollution lasts for a long time,it can turn into a double episode of O_(3) and PM_(2.5) pollution.In this case,the concentration of both pollutants may show an alternating peakand-valley phenomenon,and this is consistent with the daily variation in temperature and relative humidity.In terms of dominant PM_(2.5) components,organic matter(including primary and secondary organic matter)accounted for the highest proportion of the three types of pollution processes,followed by nitrate and sulfate.The proportion of nitrate has increased over the years,while the proportion of sulfate has declined.In summary,this study could provide a theoretical basis for relevant departments to clarify and coordinate the control of PM_(2.5) and O_(3),and further improve the air quality based on the substantial progress made in air pollution prevention and control at the current stage.