The Extinction Characteristics Of Aerosol Compositions And Visibility Traceability Under Prevention And Control Of Air Pollution In The Beijing-Tianjin-Hebei Region

Aerosols（atmospheric particulate matters）are a variety of solid and liquid particles suspended in the atmosphere that not only affect weather and climate changes,but also affect air quality and endanger human health.Aerosols have very complex physical,chemical,optical characteristics,and significant spatial and temporal differences.The characteristics at different pollution levels are also significantly different.Since the implementation of the Air Pollution Prevention and Control Action Plan in 2013–2017 and the three-year plan of action for winning the war to protect blue skies in 2018–2021,the air quality in the Beijing–Tianjin–Hebei region has been rapidly improved and atmospheric particulate matter pollution has been systematically managed,with significant reduction in PM_2.5 concentration and noticeable improvement in atmospheric visibility.The systematic observation and study of the relationship between physical,chemical and optical properties of aerosols,tracing the main sources and contributions to atmospheric visibility,can provide a scientific basis for air pollution prevention and control.The paper presented a systematic experimental analysis of physical and chemical characteristics of aerosols(focusing on atmospheric particulate matters,PM_2.5)and atmospheric visibility in typical urban and non-urban areas（eight stations in urban,urban-rural,agricultural and ecological background）in the Beijing–Tianjin–Hebei region from 2016 to 2019.It provided a clear understanding of the dynamic distribution,spatial and temporal patterns,and the evolution trend of PM_2.5 in the region.We analyzed the PM_2.5 emission sources in different regions,quantified the contributions of anthropogenic and natural sources,and quantified the contributions of PM_2.5compositions and emission sources to aerosol extinction.The main results of the paper are as follows:The average PM_2.5 concentration in the Beijing–Tianjin–Hebei region decreased from 105.5μg m^–3 to 74.2μg m^–3 from 2016 to 2019,with urban areas showing a decreasing trend year by year.The visibility improved from 15.2 km to 18.7 km,with the best visibility in Beijing,and the visibility in Beijing,Tianjin and Shijiazhuang improved year by year.The concentration of carbonaceous aerosol in Beijing was 12μg m^–3,in Hebei was higher than Beijing by more than 5μg m^–3,and only 4μg m^–3 in the background area.The concentrations of water-soluble ions and elements also showed a similar spatial distribution.The concentration of secondary water-soluble inorganic ions（SIA）was 28μg m^–3 in Beijing,more than 30μg m^–3 in Hebei,and about21μg m^–3 in the background area.The secondary pollution was serious in the Beijing–Tianjin–Hebei region,with SIA concentrations accounting for about 42%–66%of the total component concentrations.In Beijing,NO₃^–（21%）was the largest contributing component to the concentration,with approximate contributions from SO₄^2–（18%）,organic matter（OM,17%）,and NH₄⁺（16%）.NO₃^–or OM was the largest contributing component in Hebei,with a significantly decreasing contribution from NH₄⁺.SO₄^2–（20%）was the largest contributing component in the agricultural area,Yucheng,and NO₃^–in the background area.Influenced by the establishment of the coal banning zone,PM_2.5 concentrations in non-background areas of the Beijing–Tianjin–Hebei region were significantly reduced by 26%–66%in winter 2017,with the largest decline in Beijing.SIA concentrations decreased by 47%–70%,and SO₄^2–decreased by 63%–74%.The decrease in urban areas was significantly greater than that in non-urban areas.Since the implementation of the Air Pollution Prevention and Control Action Plan and the three-year plan of action for winning the war to protect blue skies,the extinction characteristics of PM_2.5 compositions also changed significantly.From 2016 to 2019,the daytime PM_2.5 extinction coefficient decreased from 606 Mm^–1 to 288 Mm^–1.The daytime PM_2.5 extinction coefficient in Beijing was about 350 Mm^–1,which was significantly lower than that in Hebei(436–542 Mm^–1),and the background area was only about 200 Mm^–1.The largest contributing components of PM_2.5 extinction coefficient in the Beijing–Tianjin–Hebei region were ammonium sulfate（AS）and ammonium nitrate（AN）,which contributed 27%–41%and 24%–39%of the total extinction coefficient,respectively.OM was the next largest contributor,contributing12%–27%.The main contributing component of extinction coefficient was OM in winter,which contributed 27%–35%,and the maximum contributing component in summer was AS（39%–54%）.As a result of the establishment of the coal banning zone,there was a significant reduction in the PM_2.5 extinction coefficient in urban areas in winter 2017,with a decrease of 49%–66%,especially in AS,with a decrease of more than half（62%–84%）.It was also accompanied by a decrease in AN（47%–69%）.Similar to the changes of concentrations,the decrease in extinction coefficients was significantly greater in urban areas than in non-urban areas.Tracing the sources of PM_2.5 compositions and their contributions to atmospheric visibility in the Beijing–Tianjin–Hebei region revealed that,the main pollution sources were secondary emissions and coal combustion in urban areas of the Beijing–Tianjin–Hebei region,which contributed 45%–51%.The main sources in the urban-rural area were secondary emissions（31%）and biomass burning（24%）.The agricultural emissions（30%）and secondary emissions（25%）were found in the agricultural area.The background area was dominated by secondary emissions（41%）due to plant emissions and the long-distance transmission.The contribution of dust in heavy industrial areas increased significantly and could be as high as 41%.The largest contributing source of extinction in urban areas was secondary emissions（35%–49%）,followed by coal combustion（12%–29%）.Due to the establishment of the coal banning zone,the contribution of extinction of coal combustion and secondary emissions decreased in the Beijing–Tianjin–Hebei region.The contribution of coal combustion and secondary emissions decreased by 44%–58%and 58%–71%in urban areas.The contribution of secondary emissions in non-urban areas decreased of 45%–78%.The main contributing sources of PM_2.5 extinction were secondary emissions and coal combustion before the establishment of the coal banning zone in 2016,and the contribution of both decreased significantly after the establishment of the coal banning zone in 2017.The study found that the further improvement of atmospheric visibility in Beijing–Tianjin–Hebei required the reduction of secondary emissions,biomass burning and dust,especially nitrate emissions from secondary emissions,indicating the future direction of pollution source control in Beijing–Tianjin–Hebei region.It provided a scientific basis for the formulation and implementation of regional air pollution reduction and visibility improvement measures,and provided a scientific reference for the air pollution control in other developing regions and cities.