| As one of the main components of atmospheric aerosols,carbonaceous aerosols have an important impact on regional and global climate,environment,and human health.With the enactment of policies such as the“Action Plan for Air Pollution Prevention and Control”and the“Three-Year Action Plan to Win the Blue Sky Defense War”,and the implementation of policies such as reduction of carbon dioxide emissions China,regional air quality has been effectively improved.Therefore,it is important to understand what factors cause the improvement of regional air quality?Have the characteristics and sources of carbonaceous aerosols changed after the policies were implemented?It will be beneficial for us to understand the regional pollution characteristics more clearly and provide a reference for air quality management.To this end,this study takes Beijing,Xi’an and Linfen,the representative cities in northern China,as examples,analyzes the trends of urban air quality and major air pollutants from 2014 to 2020,as well as the factors of air quality improvement;then analyzes the elemental carbon（EC）,organic carbon（OC）,saccharides compounds,stable carbon isotopes(13C)and radiocarbon isotopes(14C)in PM2.5 from January 2018 to April 2019,and combined with Latin hypercube sampling technique（LHS）and positive moment factor decomposition（PMF）to resolve the sources of carbonaceous aerosols and saccharides compounds under different seasons and pollution conditions,and whether their sources have changed in recent years.The specific results are as follows:（1）The average AQI in Beijing,Xi’an,and Linfen during 2020 decreased by46.2%,40.9%,and 6.6%,respectively,compared to 2014;the concentrations of most air pollutants in Beijing and Xi’an decreased by 29.6%to 82.9%compared to 2014;the average O3 concentrations in Xi’an and Linfen increased by 29.2%and 69.1%,respectively,compared to 2014.Principal component analysis showed that the yearly reduction in AQI in cities in Guanzhong was attributed mainly to reductions in industrial emissions,followed by reductions in emissions from homes and motor vehicle exhausts.During the COVID-19 pandemic of February to May 2020,lockdown measures in cities in Guanzhong resulted in reductions of approximately18.4%,24.2%,and 17.9%in the AQI,PM2.5,and PM10 concentrations compared to the same period in 2019.（2）The average PM2.5 mass concentrations during the sampling period were 72.1±44.9μg/m3,98.6±64.5μg/m3 and 148.1±118.8μg/m3 in Beijing,Xi’an and Linfen cities,respectively,and the total carbon（TC）concentrations in PM2.5 were 12.5±11.8μg C/m3,14.6±7.5μg C/m3 and 35.7±36.5μg C/m3,all of which are characterized by high winter and low summer.The mean OC/EC values of 4.0±1.4,9.0±6.1,and 6.6±2.0 for the three sites,indicate that carbonaceous aerosols contain a significant contribution from SOC or biomass combustion.（3）Among the saccharides compounds,dehydrated saccharides compounds（levoglucosan,mannitol,etc.）had higher proportions（>56%）in the Beijing,Xi’an,and Linfen city samples and were elevated by 17.4～35.0%in autumn and winter,indicating an increased contribution of biomass combustion in autumn and winter.The concentrations（21.6～46.9μg/m3）and proportions（15.7～42.8%）of saccharides alcohols（mannitol,arabinitol,etc.）were higher in spring and summer.The higher concentrations of sugar alcohols in the Beijing and Linfen samples compared to the Xi’an samples indicate that the contribution of fungal aerosols may be higher at both sites;the concentrations（13.9～31.1μg/m3）and proportions（10.6～26.4%）of monosaccharides（glucose,fructose,etc.）indicate that farming and biological activities are more intense in spring and summer around Beijing and Xi’an,especially in Beijing,where the spring The high proportion of simple saccharides（35.2%）may be related to the early spring cultivation in the nearby countryside.the PMF source resolution results indicate that biomass burning is the main source of saccharides compounds with a contribution of 49.7～58.2%.The analysis of biomass fuel structure using the ratio of different dehydrated sugar compounds showed that biomass fuel sources were more biased towards hardwoods and crops in Beijing,more towards softwoods and crops in Xi’an,and between the three types in Linfen.（4）The best estimate of source apportionment showed that non-fossil sources have become an important component of carbonaceous aerosols in the samples from Beijing,Xi’an and Linfen cities,accounting for 54.9±10.6%,54.4±10.3%and 37.3±14.1%of the TC,respectively.In Xi’an,44.8±26.8%of the non-fossil sources were attributed to biomass combustion.Among the fossil sources,motor vehicle emissions accounted for 29.3±12.7%,24.9±18.0%and 20.9±12.3%of the TC in the Beijing,Xi’an and Linfen samples,respectively,while coal combustion contributed 15.5±8.8%,20.9±18.0%and 42.9±19.4%,respectively.Comparison with data from past research revealed that the contribution of fossil sources of TC in Beijing aerosols decreased by 13.1%and the contribution of motor vehicle emissions（to fossil sources）decreased by 23.0%since the implementation of the Air Pollution Control Action Plan,indicating the effectiveness of relevant air quality management.The highest contributors to OC in Linfen and Xi’an were fossil sources（74.2±9.6%and 43.2±10.8%,respectively）,whereas that in Beijing was non-fossil sources（66.8±13.9%）.The main contributors to EC were fossil sources,accounting for 91.4±7.5%,66.8±23.8%,and 88.4±10.8%in Beijing,Xi’an,and Linfen,respectively.（5）The best estimate of source apportionment during the haze events and nonhaze periods in January 2019 showed that the contribution of vehicle emissions（49.5±4.8%）to TC in Beijing during the haze event was higher than that of coal combustion（7.3±4.2%）,and it increased significantly（29.8%）compared to the nonhaze period.The contribution of vehicle emissions（19.2±9.2%）was close to that of coal combustion（18.1±9.8%）during the haze event in Xi’an,and the coal combustion contribution increased by 8.2%compared to the nonhaze period.Biomass burning contributed 21.1±10.5%during the haze event in Beijing and 40.9±6.6%in Xi’an（with an increase of 3.3%）.The contribution of fossil fuel sources to secondary organic carbon increased by 17.8%during the haze event in Beijing and by 19.1%in Xi’an compared to the nonhaze period.Therefore,it is recommended to strengthen the specific control measures targeted to coal combustion and vehicle emissions in different cities,strengthen the regulation of the use of biomass fuels,and optimize the structure and production methods of biomass fuels,with a view to seeing further improvements in air quality in northern China in the future. |
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