Physicochemical Properties And Pollution Causes Of PM_2.5 During Heating Seasons In Beijing-Tianjin-Hebei And Its Surrounding Areas

Northern China plain has suffered from serious fine particular matter(PM_2.5,particles with an aerodynamic diameter of 2.5μm or less)pollution issue.High concentration of PM_2.5 can alter global climate by scattering and absorbing solar radiation,and act as cloud condensation nuclei（CCN）,influencing precipitation and the hydrologic cycle.PM_2.5,with characteristics of special particle size,solubility,morphology and species,is inclined to absorb toxic and harmful substances,which are easy to be inhaled into the human body,eventually resulting in a great threat to public health.Thus,mitigating PM_2.5 pollution is extremely urgent and necessary in China.In this study,comprehensive observation was carried out during four heating seasons from 2016 to 2020 in Beijing-Tianj in-Hebei and its surrounding areas.Main aims of this study mainly focused on changes of air quality,the temporal and spatial distribution of chemical composition in PM_2.5,and formation mechanism of secondary species in Beijing-Tianj in-Hebei and its surrounding areas,and further explored causes of PM_2.5 pollution events.（1）Compared with the 2016-2017 heating season,average values of daily average PM_2.5,PM₁₀,SO₂,NO₂ and CO concentrations in heating seasons of 2017-2020 decreased significantly,however,average values of daily average O₃ concentration increased by 23.5%,13.4%and 19.5%,respectively.The decreasing SO₂/NO₂ ratios in recent years indicate that SO₂ has achieved remarkable control.Compared with the 2016-2017 heating season,attainment of air quality standards for the remaining three heating seasons has increased.Correlation analysis shows that compared with the 2016-2017 heating season,in 2019-2020 heating season,the number of cities affected by SO₂ and NO₂ decreased and the effect of O₃ on PM_2.5 pollution has became prominent.（2）In 2016-2017 heating season,SO₄^2-is the dominant ion among water-soluble ions in Beijing,Tianjin,Langfang,Qinhuangdao,Zhengzhou,and Jinan,however,SO₄^2- is the dominant ion in Shijiazhuang and Tangshan.In Beijing,Tianjin,Langfang,Shijiazhuang,Tangshan,Qinhuangdao,and Zhengzhou,the formation of SOC（secondary organic carbon）has minor effect on the enhanced OC concentration,while POC（primary organic carbon）is responsible for the increase of OC concentrations.With the PM_2.5 pollution aggravating,SNA（secondary inorganic ions）made great contribution to PM_2.5 pollution.The photochemical reaction has greater effect on the formation of nitrate than that of sulfate,however,the effect of heterogeneous reaction on the formation of sulfate is greater than that of nitrate.In the Cross（2017） New Year’s haze,SNA was the dominant chemical component in PM_2.5 in eight cities,followed by OM（Organic matter）.The contribution of SNA to PM_2.5 was 64.2%,61.1%,and 56.4%,respectively,in Qinhuangdao,Zhengzhou,and Jinan.The PM_2.5 pollution was characterized by high motor vehicle and industrial emissions in northern cities（Beijing,Tianjin and Langfang）and the southern city（Zhengzhou）.The PM_2.5 pollution was characterized by high coal combustion emissions in the central city（Shijiazhuang）.Unfavorable meteorological conditions,local emission,secondary formation,and regional transport from polluted regions played important roles in PM_2.5 pollution.（3）Results shows that PM_2.5,SO₂,NO₂,O₃,and CO concentrations in 2017-2018 heating season decreased by 40.9%,46.0%,29.0%,11.0%,and 44.4%,respectively,compared with those in 2016-2017 heating season.Significant decreases were also observed for SO₄^2-（32.5%）,SO₄^2-（52.9%）,NH⁴⁺（56.0%）,Cl^-（64.6%）,and K⁺（68.2%）,on average.PM_2.5 pollution has changed from sulfate-driven to nitrate-driven.The decrease in SO₂ was more significant than NO₂ as a response to one reason of the larger decrease in SO₄^2- concentration.The formation of sulfate was dominated by heterogeneous reactions in two heating seasons.Low pH could facilitate more efficient conversion of SO₂ to sulfate.Photochemical reactions played a much more important role in the formation of nitrate in the second heating season,especially in the daytime.More nitrate was brought into Beijing when air masses coming from polluted regions in the southwest prevailed in 2017-2018 heating season.Thus,regional joint prevention and control are of great importance in the achievement of an effective reduction in PM_2.5 pollution in the future.（4）In fireworks episodes,according to results of PM_2.5 source apportionment,fireworks were the largest PM_2.5 source during fireworks periods in Qinghuangdao（QHD）,Tangshan（TS）,Shijiazhuang（SJZ）,and Zhengzhou（ZZ）,accounting for 68.9%,53.8%,61.1%,and 51.0% of PM_2.5,respectively.This result was similar to the result of the relative ratio method（68.6% in QHD,57.2% in TS,63.0%in SJZ,53.9%in ZZ）.Based on results of PM_2.5 source apportionment,it was found that the contribution of fireworks to PM_2.5 in Zhengzhou was significantly lower than that in QHD,TS,and SJZ due to strict control measures.During the COVID-19 outbreak,a substantial increase in the bulk coal and biomass consumption,leading to the decreased SO₄^2-/SO₄^2- and increased OC/EC,and the enhanced contribution of secondary pollution was found.The difference of the main chemical components in PM_2.5 in different cities was narrowed.The PM_2.5 pollution in five cities was affected the regional transport from their southern regions.