Characteristics And Influence Factors Of Black Carbon Particles In Suburban Nanjing From 2017 To 2019

As one important species of ambient fine particles,black carbon（BC）has significant adverse effects on air quality,human health and climate.This work reports two-year measurement of BC conducted in northern suburb of Nanjing,China from December 2017 to November 2019.Then the relationship between BC concentration and meteorological factors,local emission sources and surrounding potential sources are discussed.Two-year mean BC concentration was determined to be 2.67±1.35μg/m3.BC concentration presented a“U”shape during the period of observation,lowest in summer and highest in winter.The background value of BC concentration was 1.61μg/m3,which was a significant decrease from previous studies.Averaged BC diurnal patterns was presented the“high peak”and“low valley”.Diurnal pattern revealed that besides a clear emission-induced morning peak,meteorology played a more important role in governing BC changes in the rest of the day.Nighttime BC concentration on average was higher than it in daytime,meanwhile the difference of BC concentration in workday and weekend was similar indicating heavier influence of meteorology than emissions.Meteorological parameters can influenced BC concentrations significantly,high BC concentrations resided mostly in the regime with wind speed<1.5 m/s,temperature<20℃,boundary layer height<400 m but likely at any relative humidity.Influences of meteorological factors on BC were in an order of boundary layer height,wind speed,temperature and relative humidity,and further Granger causality analysis verified that the first three factors were indeed Granger causes of BC,while relative humidity was not.Wind direction studies indicated that industrial parks and expressways in the northeast and east of the observation site may contribute more sources of emissions in spring.The ratio of BC to PM_2.5 was the lowest in heavy pollution,meanwhile the ratio of PM_2.5concentration in heavy haze days to PM_2.5concentration in non-haze days was much higher than that of BC under two conditions which indicated that secondary pollutants has more contribution to the pollution.BC can contribute to the atmospheric visibility degradation and has a greater impact on visibility than wind speed,temperature,and boundary layer height.Based on the diagnostic mass ratios of BC to CO,NO₂ and SO₂,as well as their correlation coefficients,we inferred that diesel vehicles rather than gasoline vehicles were the major BC emission source in suburban Nanjing;contributions from industry coal combustion were relatively less important.Back trajectory and potential source contribution analyses showed that regional rather than long-range transported air parcels led to the BC pollution in this site.Our findings provide useful insights into the reductions of BC and air pollution in this and similar atmospheric environments.It also provides one corresponding scientific basis for the formulation of the original regional BC emissions list.