Spatio-temporal Characteristics And Influencing Factors Of Black Carbon In The Atmosphere Of Chongming Island,shanghai Based On Ground Observation And Reanalysis Data

Black Carbon（BC）is an important component of Carbon aerosol with absorption properties,which can change the radiative forcing and influence the climate change.Due to the sub-micron level of BC particles,it is easy to cause human health problems through respiration.As a short-lived climate pollutant,reducing BC emissions is an effective means to mitigate climate change and improve air quality in the short term.Taking Chongming Island,a world-class ecological island planned by the State Council,,as an example,based on ground-based monitoring and reanalysis data,the temporal and spatial distribution characteristics of black carbon in the atmosphere of Chongming Island and its influencing factors were investigated.On the one hand,by breaking through the traditional fixed-sample-point monitoring method,the mobile monitoring method was used to monitor the black carbon concentration in the near-ground atmosphere of Chongming Island,the influence of different population characteristic statistics and background concentration correction on the number of moving samples and the uncertainty it may bring are discussed,the spatial variability of Chongming Island black carbon concentration and its response to underlying surface types are analyzed based on the moving observation data.On the other hand,the modern-era retrospective analysis for research and applications（version 2,Merra-2）was used to analyze the BC data from 1980 to 2019 in Chongming Island,Shanghai,combining the potential source contribution function（PSCF）with the backward trajectory model,the atmospheric trajectory and the potential source of the Chongming BC are determined.The main findings of this study are as follows:（1）The representative analysis method of moving sample band monitoring can effectively reduce the number of sampling times and characterize the BC concentration in the region with limited data sets.The uncertainty of sampling times for mobile monitoring is mainly determined by the demand for the accuracy of the results,that is,the confidence interval.When representative data are required to be within the 50%confidence interval of the total data,the number of times a minimum of 7 sampling times can characterize the BC of the Chongming Island atmosphere,increasing to 75%,the number of times a sample has to be increased to 14-18.With the increase of confidence interval,the difference of sampling times such as arithmetic mean,median and mean of cutting tail decreases.The time variation and spatial variation of BC concentration in Chongming Island can be shown by the statistics of different population characteristics,in which the median concentration is lower than the actual concentration,the trimmed mean can better reduce the adverse effect of random events on the mean and preserve the typical peak values in some regions.As the atmosphere in Chongming Island is relatively clean,the concentration of BC is generally low,the contribution of local emissions to the concentration of BC is not obvious,and the effect of background concentration correction on the uncertainty of sampling times is relatively small,when the confidence interval is large,the distribution of sampling times with different spatial resolution fluctuates little（21-26 times）.（2）Based on a total of 27 mobile monitoring samples taken in 2019,the annual mean concentration of BC in the atmosphere of Chongming Island was 2.8±1.6μg·m^-3,compared with other urban areas such as Shanghai(10.77±3.5μg·m^-3),Sao Paulo(8.5±1.6μg·m^-3),London(11.7μg·m^-3)and Bogotá(10-38μg·m^-3),it was slightly higher than that of the suburbs of Berkeley(1.8μg·m^-3).The concentration of BC in Chongming Island showed the characteristics of winter(2.95±1.5μg·m^-3)>spring(2.74±1.33μg·m^-3)>summer(2.30±1.07μg·m^-3).The spatial distribution of BC concentration in different seasons was generally consistent,which was affected by the spatial heterogeneity of land use types on the underlying surface,the concentrations of BC in the Chongming Island atmosphere are lower in the northwest and southeast and higher in the middle.The correlation between BC concentration and land use types in the buffer zones of Chongming Island was significant in summer（p<0.01）,but weak in spring and winter.The correlation coefficient between land use type and air BC concentration shows a decreasing trend in the buffer zone of 100 m-2000 m,which indicates that the effect of land use type on near-surface air concentration has a distance decreasing effect.The spatial distribution of BC and CO₂ was significantly correlated in summer and winter（p<0.01）.（3）The Merra-2 reanalysis data showed that the annual average concentration of BC in Chongming Island increased from1.50μg·m^-3 to 2.59μg·m^-3 in the past 40 years from 1980 to 2019,and showed an overall upward trend.The results of Mann-kendall trend analysis show that the seasonal variation in 1988-2019 and 1980-2019 shows a significant right-leaning“U”pattern,while the monthly concentration curve of BC tends to be flat in 1980-1987.The seasonal variation was the same as that in 2019 based on moving monitoring,which is winter(3.18±1.22μg·m^-3)>spring(2.37±0.64μg·m^-3)>autumn(2.16±0.90μg·m^-3)>summer(1.99±0.55μg·m^-3).Based on the backward trajectory model and Potential Source Contribution Function（PSCF）analysis,the BC concentration in Chongming Island was mainly influenced by the pollution air mass from the northwest and the clean sea air mass from the east,in summer,53.85%of the air mass is clean air mass transported from the sea.In autumn,it is mainly influenced by air flow in the northeast（15.16%）,northwest（27.38%）and Shandong（37.09%）.In winter,it is influenced by 87.37%of the polluted air mass transported in the north.During the period of low BC pollution concentration,BC mainly came from local local emissions（such as summer）,while Jiangxi,Hunan,Hubei and Anhui still contributed less,the Yangtze River Delta,Hunan,Hubei and North China regions became the major contributors to Chongming Island in Shanghai.The correlation of Chongming BC data based on the mobile monitoring and Merra-2 reanalysis data was 0.80,0.72,0.86 and0.87 in spring,summer and winter,respectively.The daily average concentration of mobile monitoring was lower than that of Merra-2 by 32.9%.