Study On Characteristics And Causes Of Carbonaceous Aerosol Concentration Variation During The Heating Season In Baoding City

In the autumn and winter heating season,heavy air pollution events frequently occur in the Beijing-Tianjin-Hebei region of China,and PM_2.5 is the main pollutant.Among the various components of PM_2.5 in the Beijing-Tianjin-Hebei region,the concentration of carbon components is relatively high,especially in Baoding City,where the proportion and concentration of PM_2.5 and carbon components in the autumn and winter seasons are higher than in other cities in the region,such as Beijing,Tianjin,and Langfang.Since 2013,with the implementation of a series of clean air measures,the concentration of PM_2.5 in the Beijing-Tianjin-Hebei region has shown a significant downward trend.During this period,Baoding City has always ranked low in the comprehensive air quality index and suffered from severe air pollution.However,since2015,Baoding City has implemented a series of clean air measures,including the "clean heating" policy,and the air quality has improved significantly.The comprehensive air quality index has risen,and the trend of PM_2.5 concentration in Baoding City is consistent with the timeline of the implementation of the "clean heating" policy.Therefore,in order to understand the characteristics and reasons for the change and reduction of carbon aerosol concentration during the heating season in Baoding City,this paper collected PM_2.5 data from 2013 to 2022,conventional air pollutant data from2017 to 2022,and online data of OC,EC in PM_2.5,and analyzed the PM_2.5 membrane samples of OC,EC,and PAHs in Baoding City in 2014,2019,and 2022.Combined with mathematical and analytical methods,we studied the characteristics,sources,and formation reasons of carbon aerosol concentration in the heating season in Baoding City.The main conclusions of the paper are as follows:From 2013 to 2022,the PM_2.5 in Baoding City decreased by 68.1 μg/m³,and the air quality improved significantly.Analysis of the online concentrations of OC,EC,PM_2.5,PM10,SO₂,NO₂,O₃ and other pollutants from 2017 to 2022 revealed that the concentrations of OC and EC,as well as their proportions in PM_2.5,showed a downward trend during the four heating seasons from 2018 to 2022,while the concentrations of OC and EC during non-heating seasons from 2017 to 2020 did not decrease.Both OC and EC showed significant diurnal variations during the heating season,and the proportion of OC and EC in PM_2.5 under polluted weather conditions during the heating season continued to decrease.The changes in SO₂ and POC were consistent with those in OC and EC,showing a significant downward trend during the four heating seasons.NO₂ showed a similar diurnal variation with OC and EC,while O₃ showed an upward trend.SOC showed a trend of first decreasing and then increasing,indicating that atmospheric oxidation had increased,and that atmospheric pollution had developed into combined pollution.The Mann-Kendall mutation test revealed that the mutation points of OC and EC during the heating season from 2018 to 2020 were all after the end of the March heating season,while the mutation points during the two heating seasons from2020 to 2021 and from 2021 to 2022 were two months earlier than the previous two heating seasons,indicating that the sources of OC and EC during the heating season in Baoding City had changed due to the reduction in coal and biomass burning and the increase in the use of natural gas,and the "clean heating" measures reducing the emissions of carbon-containing aerosols from coal sources into the atmosphere.The concentrations of OC and EC in PM_2.5 were 60.9 μg/m³ and 18.2 μg/m³ in2014,36.6 μg/m³ and 6.1 μg/m³ in 2019,and 10.4 μg/m³ and 4.9 μg/m³ in 2022,respectively.Compared with 2014,the concentrations of OC and EC decreased by 39.9%and 66.6% in 2019,and by 88.9% and 77.8% in 2022,respectively.Air quality assessment by urban box model showed that the meteorological factors in 2019 and2022 were unfavorable for the decrease of pollutant concentrations compared with 2014.The decrease in OC and EC concentrations in Baoding City was mainly due to the adjustment of energy structure and the reduction of anthropogenic emissions.Compared with the Beijing-Tianjin-Hebei region and the "2+26" cities,the decrease in OC and EC concentrations in Baoding City was greater than that in Tianjin,Shijiazhuang,and the "2+26" cities,and the decrease in EC concentration in Baoding City was greater than that in OC.The decrease in concentration after the completion of the "dual substitution" for clean heating in 2022 was greater than that during the implementation process in 2019.Analysis of OC1～OC₄,EC1～EC₃,and SOC showed that the carbon-containing aerosols from coal and biomass burning that were emitted once in recent years have significantly decreased in Baoding City.The analysis of polycyclic aromatic hydrocarbons（PAHs）during the sampling period showed that in the heating season of 2014,the main concentration of PAHs in Baoding City was in 4-5 rings,in the heating season of 2019,it was mainly concentrated in 4 rings,and in the heating season of 2022,it was mainly concentrated in 5-6 rings.Analysis of the concentration of PAHs in urban and Zhuozhou stations with different progress of clean heating showed that the sources of PAHs in city stations during the heating season were mainly from coal sources,while those in Zhuozhou were mainly from biomass burning.The contribution of coal sources to PAHs in the city with low clean heating rate was greater than that in Zhuozhou with high clean heating rate.The progress and location of the implementation of the "clean heating" policy have an impact on the magnitude of the decrease in PAHs.The use of characteristic ratio method revealed that the main sources of PAHs in Baoding City in 2014 and 2019 were coal and biomass burning,while in 2022,the main source of PAHs was from vehicle sources.From the above analysis,it can be concluded that in recent years,the concentrations of PM_2.5,OC,and EC in Baoding City have continued to decrease during the process of clean heating and atmospheric pollution control measures,and the air quality has continued to improve.The significant decrease in carbon-containing aerosols emitted from coal and biomass burning in PM_2.5,consistent trend of PM_2.5 and carbon aerosol reduction with the timeline of the "clean heating" policy implemented in Baoding City,indicate that the clean heating measures taken in Baoding City have been effective in controlling carbon aerosols in PM_2.5.