| Carbonaceous aerosol is one of the important chemical components in atmospheric fine particulate matter(PM2.5),which has adverse effects on air quality,atmospheric visibility,climate change and human health.The Beijing-Tianjin-Hebei(BTH)region and its surrounding areas are one of the three key regions of air pollution control in China,suffered from severe air pollution and extensive anthropogenic emissions.Since 2013,a series of strict air pollution mitigation control measures have been implemented in the BTH region and its surrounding areas.Through satellite data and observation data,it has been confirmed that the air quality of BTH region and its surrounding areas has been improved during 2013-2017 under the implementation of the Air Pollution Prevention and Control Action Plan(APPCAP).However,there is still a lack of assessment of the pollution characteristic and variation of carbonaceous aerosol during the implementation of the Three-year Action Plan to Fight Air Pollution.As an important air and climate pollutant,which has a good homology with carbon dioxide(CO2),how did the composition,sources,and optical properties of carbonaceous aerosol change under the background of "blue sky protection campaign" and reduction of pollution and carbon emissions?The real measurement data during different years will be helpful to evaluate the effectiveness of air quality supervision from the perspective of observation,and has important significance for better air pollution prevention and control and accurate understanding and assessment of the carbonaceous aerosol radiative effect in the future.In this study,PM2.5 samples were collected simultaneously in Tianjin and Handan in three consecutive winters from 2018 to 2020.Organic carbon(OC),elemental carbon(EC)and water-soluble organic parbon(WSOC)mass concentrations in PM2.5 were measured with the same methods and analysis protocols.The light absorption coefficient of EC was analyzed and calculated based on thermo-optical method.The absorption spectrum and fluorescence spectrum of WSOC were analyzed by using the liquid waveguide capillary cell spectrum monitoring system and three-dimensional fluorescence photometer.The concentrations of primary organic carbon(POC)and secondary organic carbon(SOC)were estimated by EC tracer method and Deming regression.Sources and potential source regions of carbonaceous aerosols were analyzed with multiple techniques,such as OC/EC ratio method,source emission inventory combined with EC/CO ratio method,correlation analysis of gaseous pollutants and meterological factors,and potential source contribution function(PSCF).And,the chromophore composition of WSOC was identified based on parallel factor analysis(PARAFAC)combined with three-dimensional fluorescence spectrum.The results of this study showed that(1)In terms of the carbonaceous aerosol compositions:significant reductions in POC and EC concentrations were observed both in Tianjin and Handan,with decreasing rates of 0.65 and 2.95 μg m-3 yr-1 for POC and 0.13 and 0.64 μg m-3 yr-1 for EC,respectively.In contrast,the estimated SOC concentrations decreased first and then increased in both Tianjin and Handan,accounting for more than half of the total OC concentrations in winter of 2020-2021 and with increasing contributions especially in highly polluted days.It indicated thatthe emission of primary pollutants was effectively controlled during the pollution mitigation period,while the contributions by secondary pollutants might increase.(2)In terms of the carbonaceous aerosol sources:EC in the two cities was relatively more related to coal combustion and industrial sources.The reductions of primary carbonaceous components over the study period might be attributed more to the strong air quality regulations targeting coal combustion and industrial sources emissions in BTH area.PSCF analysis results indicated that the major source areas of OC and EC in Tianjin were the southwest regions of the sampling site,while the southeast areas for Handan.(3)In terms of the carbonaceous aerosol optical properties:The measured absorption coefficients of EC(babs,EC)also decreased year by year in two cities,with a decreasing rate of 1.82 and 6.16 Mm-1 yr-1 in Tianjin and Handan,respectively.SOC was recognized as one of key factors influencing EC light absorption through the establishment of multiple regression model analysis.The correlations between the light absorption coefficient(babs,365,wsoc)of WSOC and organic carbonaceous components were ranked from high to low as WSOC>SOC>POC(p<0.01).Humic-like substances(HULIS)was the main chromophobe of WSOC in Tianjin and Handan,among which less-oxygenated HULIS exhibited the highest contribution.The sources of fluorescent components in WSOC in Tianjin were more complex compared to those in Handan,and the overall humification level was higher in Tianjin.The fluorescent components in both sites have strong autogenesis characteristics and were mainly derived from primary emissions and fresh aerosols,such as anthropogenic and biological emissions.Through real measurement data,this study proved that the primary carbonaceous pollutants emission in typical polluted cities in the BTH region has been effectively controlled,and the light absorption of carbonaceous aerosols has also been weakened to a certain extent under the pollution emission mitigation control,and this study highlighted the needs for further control and future in-depth investigation of SOC formation mechanism,and strengthen regional joint prevention and control. |
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