| Black carbon（BC）is one of the components of atmospheric fine particulate matter（PM）,which has significant impacts on global climate change,atmospheric visibility,air quality and human health.In this work,continuous observations of BC concentrations in Nanjing from January2019 to December 2021 were conducted using an Aethalometer（Magee Scientific,Model AE33）.And the characteristic,interannual,seasonal,diurnal variation of BC was discussed.Also,the correlations between BC and meteorological factors were investigated.Subsequently,the Aethalometer model was then utilized to quantify the contribution of fossil fuel combustion and biomass（coal）combustion to concentration of BC,and the drivers of polluted weather in Nanjing were explored by combining the observed data with the FLEXPART（FLEXible PARTicle）model.Finally,to explore the long-term BC trends,the random forest（RF）model was used to reproduce the BC concentrations since 2014,and the effects of meteorological factors and emissions on the long-term BC trends were quantified by combining KZ（Kolmogorov-Zurbenko）filtering and multiple linear regression.The results showed that:（1）The average concentration of BC in Nanjing was 2.5±1.6μg m-3 during the study period,which was higher than that of the southern cities and lower than that of the northern cities,and its concentration frequency shifted toward to lower values during the study period.The interannual concentration of BC decreased gradually,and the three-year average concentrations were 3.2±2.0,2.3±1.4,2.0±1.1μg m-3,respectively.The seasonal variation showed a higher value in winter（2.6±1.5μg m-3）,and lower levels were found in spring(2.4±1.5μg m-3)summer（2.3±1.3μg m-3）and autumn（2.2±1.5μg m-3）.The diurnal variation of BC shows higher levels at night and lower values during the day,which is mainly affected by the morning and evening peaks of traffic and the height of the boundary layer.The correlation analysis showed that the meteorological factors influenced the BC concentration from strong to weak as follows:boundary layer height（BLH）>wind speed（WS）>relative humidity（RH）>temperature（T）.（2）The average value of the absorption（?）ngstr（?）m exponent（AAE）of BC is 1.25±0.14.Because of the seasonal variation of emission source,with AAE being the lowest in summer（1.13±0.14）and higher in the rest of the seasons.The Aethalometer model results show that the contribution of fossil fuel combustion to BC is as high as 80%,with the highest contribution in summer（85%）and the lowest in winter（72%）.Both BC/PM2.5 and BC/CO results proved that the contribution of fossil fuel combustion to BC was higher.The concentrations of BC,BCff,and BCbb on polluted days were 189%,182%,and 230%of the clean period,respectively,and the changes of BC/PM2.5 and BC/CO on clean and polluted days indicated that the polluted days in Nanjing were caused by the increase of biomass and coal combustion.The FLEXPART model results indicated that the polluted days in Nanjing were influenced by both local emissions and long-range transport.（3）The R2 between the predicted concentrations of the RF model and the observed concentrations of BC,BCff,and BCbb ranged from 0.84 to 0.86,and the MSE ranged from 0.03to 0.30,indicating that the RF model predictions were reliable.The results of the Mannkendall trend test showed a significant（P<0.001）decreasing trend for the concentrations of BC,BCff,and BCbb.The concentrations of BC,BCff,and BCbb decreased from 2014 to 2021 by 50.4%,54.0%,and 46.4%,with slopes of-4.56%yr-1(-0.16μg m-3yr-1),-4.57%yr-1(-0.13μg m-3yr-1),and-4.55%yr-1(-0.03μg m-3yr-1),respectively.The contribution of BCffto the decrease of overall BC concentration was as high as 81.3%,indicating that the reduction of traffic emissions was the main reason for the decrease of BC concentration in Nanjing.k Z filter results showed that the emission-related long-term component contributed 88%to the reduction of BC concentration while the meteorological-related long-term component contributed 12%to the decrease of BC. |
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