Investigation On The Vertical Distribution And Typical Processes For Aerosols In Zibo City

Zibo City is located in the east of the North China Plain,which is a transmission channel city with atmospheric pollutant for the Beijing-Tianjin-Hebei region.In this study,the field campaign was conducted in the site named atmospheric compound pollution comprehensive monitoring station in Zibo City.Ground-based remote sensing instruments（e.g.,polarized micro-pulse lidar and coherent Doppler lidar）and ground observation instruments were utilized to measure aerosols and relevant parameters in2021-2022.In Combination with Fengyun 4 satellite,ERA5 reanalysis data,forward trajectory and backward trajectory and other auxiliary data,the causes of aerosol pollution in Zibo City were comprehensive investigated.The main results are as follows:（1）Based on the ground and lidar observation data of four seasons in Zibo City,the three-dimensional distribution characteristics of particulate matter in each season were obtained and their influencing factors were explored.The season sequence of ground PM_2.5 and PM₁₀ concentrations in Zibo City were in accordance with:winter>spring>summer>autumn.The peaks of vertical extinction coefficient and depolarization ratio of aerosol in four seasons all occurred at about 300 m.The peak of the extinction coefficient profile in winter was the largest(0.85 km^-1),while the peak of the extinction coefficient profile in autumn was the lowest(0.53 km^-1).The diurnal variation of extinction coefficient was obvious,and along with the increase of boundary layer height,the extinction coefficient at low altitude gradually decreased during daytime,while the range of high value for extinction coefficient in the vertical direction gradually expanded.Affected by high-altitude aerosol transportation,the diurnal variation of depolarization ratio in springtime was not obvious,while the depolarization ratio showed obvious diurnal variation in winter.Meanwhile,in the vertical direction,the range of high value for depolarization ratio gradually expanded during daytime,and the value of low-altitude depolarization ratio decreased correspondingly.The four seasons of Zibo City were mianly to varying degrees affected by the potential pollution from Inner Mongolia,Shanxi,Hebei to Shandong in the northwest direction.（2）The vertical distribution characteristics of particulate matter and the causes of pollution were obtained by monitoring the environmental air particles during different pollution processes and major activities.The ground PM_2.5 concentration during the air quality guarantee period of the Beijing Winter Olympics decreased by 29.40%compared with that before the pre-control period.In general,the value sequence of extinction coefficient was followed by:pre-control period>mid-control period>late-control period,while the sequence of high-altitude depolarization ratio,temperature,relative humidity and horizontal wind speed were followed with:late-control period>mid-control period>pre-control period.Also,the result showed that temporary emission control measures and meteorological conditions jointly promoted the reduction of aerosol pollution.On March 4,2022,the dust transmission process was monitored in Zibo City.The high-altitude depolarization ratio was as high as 0.30,and the non-spherical characteristics of particles were obvious.Meanwhile,the lidar could monitor the high-altitude dust transmission process earlier 7 hours than the ground observation.It was found that the dust was mainly affected by the cold front system,which developed from Inner Mongolia and Mongolia,and had an impact on North China,Huanghuai and Jianghuai from northwest to southeast,and the the dust transport process was very fast and strong.In addition,serious fine particulate matter pollution occurred in Zibo City from 01:00 on December 14 to 12:00 on December 16,2021,with an extinction coefficient of more than 2.00 km^-1.During the pollution period,the aerosol depolarization ratio in the boundary layer was relatively low,which mainly was composed of anthropogenic particulate matter.The depolarization ratio of 1000-2000m was higher than that of low altitude,while it did not cause large-scale settlement due to the static and stable state of vertical wind.Additionally,factors（e.g.,nighttime ground inversion and high relative humidity）further aggravated the fine particle pollution in this process.（3）Based on the aerosol lidar and coherent Doppler lidar,the horizontal and vertical transmission intensities were calculated,and the transmission intensities and routes of aerosol in clean and polluted weather were obtained.During the observation period,the distribution of average transmission intensity in different vertical heights and horizontal directions was followed by:clean days(1.297 ks^-1)>polluted days(0.765 ks^-1),while the net average transmission intensity in the vertical direction was was followed by clean days(0.006 ks^-1,downward direction)>polluted days(0.005 ks^-1,upward direction).In the horizontal direction:aerosols were mainly affected by a single direction on a clean day,while the discrepancy in transmission effects in all directions on a polluted day was not obvious,leading to that aerosols were more likely to stay and accumulate in the local area,and aggravating atmospheric particulate matter pollution over local scale.In the vertical direction:the high value of updraft transmission intensity was more likely to occur in the morning and noon,while the high value of downdraft transmission intensity was more likely to occur at night and afternoon.