Study On The Evolution Of Physical And Chemical Properties Of Atmospheric Fine Particles In The Northern Suburbs Of Nanjing During Winter From 2017 To 202

The atmospheric fine particulate matter(PM_2.5)is one of the most important factors causing air pollution in China.Its main chemical components include water-soluble ions and carbonaceous components（including brown carbon,i.e.,Br C）,which not only affect atmospheric visibility and global climate,but also pose a certain threat to human health.In recent years,with the implementation of a series of air pollution control action plans and policies in China,PM_2.5has undergone significant changes in concentration levels,chemical composition,optical properties and sources.Winter is a period of severe haze pollution,so this study specifically investigated the PM_2.5 samples in the northern suburbs of Nanjing during the winter seasons from 2017 to 2021,and a total of 255 valid samples were collected.Comprehensive offline analysis was conducted based on multiple instruments,including a total organic carbon/total nitrogen analyzer,an organic carbon/elemental carbon analyzer,an ion chromatograph,a UV-visible spectrophotometer,a fluorescence spectrometer,and a soot particle aerosol mass spectrometer.The main changes in the chemical composition,optical properties and sources of PM_2.5 during the five-year period were studied,and the main conclusions are as follows:（1）Regarding the evolution of the overall composition of PM_2.5,the average temperature in the northern suburbs of Nanjing during the entire sampling period showed an increasing trend,the average wind speed decreased slightly but remained at a low level,and the relative humidity fluctuated within the range of 33%to 85%without significant changes.PM_2.5 concentration showed a significant decrease trend,from 97.68μg m^-3 in 2017 to 73.14μg m^-3 in 2021,a decrease of 25.12%.All measured components accounted for 75.69%to 92.96%of PM_2.5,of which water-soluble inorganic ions contributed the most,reaching 68.17%in the winter of 2021,followed by organic matter（20.38%to 25.17%）and elemental carbon（1.94%to 2.57%）.（2）Regarding the evolution of different chemical components,the average concentration of water-soluble ions showed a decreasing trend overall,from 54.02μg m^-3 in 2017 to 49.86μg m^-3 in 2021.NO₃^-、SO₄^2-and NH₄⁺had the highest proportion among the total water-soluble ions,and showed an overall increasing trend,indicating that secondary components were becoming more important in the total water-soluble ions.The ratio of NO₃^-/SO₄^2-in 2021was 2.45 times that in 2017,indicating that the contribution of mobile sources to particulate matter has increased significantly compared with fixed sources.In terms of mass concentration,organic carbon（OC）showed the highest concentration in 2018 and the lowest concentration in2021.The SOC/OC ratio increased year by year,reaching the highest value（60%）in the winter of 2020,indicating that secondary organic carbon gradually became the dominant component in the organic carbon.（3）Regarding the evolution of Br C optical properties,the light absorption of water-soluble organic carbon（WSOC）and methanol-soluble organic carbon（MSOC）both decreased with the increase of wavelength.The average mass absorption efficiency of Br C showed the minimum value in the winter of 2021,indicating that the light absorption efficiency of Br C has decreased in recent years.Three humic-like substances（C1,C2,C3）and one protein-like substance（C4）were identified in both WSOC and MSOC.C4 contributed more to the overall color groups of OC in MSOC,which was about twice or more than that in WSOC,indicating that C4 in MSOC made a larger contribution to the color groups of OC,which may mean that protein-like substances are more soluble in methanol solvent than in water.（4）Regarding the evolution of PM_2.5 sources,the positive definite matrix factor analysis was used to classify the sources of water-soluble organic aerosol（WSOM）into four categories,namely,coal combustion sources（CCOA）,traffic sources（HOA）,relatively low oxidation OA（OOA1）and relatively high oxidation OA（OOA2）in winter 2017,and fossil fuel combustion sources（FFOA）,cooking sources（COA）,OOA1,and OOA2 in winter 2018-2021.During the study period,the mass contribution of FFOA showed an increasing trend,with the highest contribution in 2021（32.41%）,while the mass concentration of COA was lower than FFOA.The total contribution of OOA1 and OOA2 decreased from 74.15%to 51.36%in 2021,indicating a decrease in the proportion of winter secondary organic aerosols over these 5 years.In addition,in the winter of 2017,the sum of CCOA and HOA contributions to Br C absorption was 34.50%;from 2018 to 2021,the average absorption contribution of FFOA was highest in2020（26.20%）,and the absorption contribution of COA was between 7.18%and 13.40%,indicating that primary emissions have an important contribution to Br C absorption.Although OOA2 has a similar mass contribution to WSOM as OOA1,the Br C absorption contribution of OOA2 is much higher than that of OOA1.Furthermore,it was found that although the mass contribution of secondary organic aerosols decreased during the study period,their contribution to the Br C absorption was still important,with more than 60%of the WSOM absorption being attributed to secondary organic aerosols.