Meteorological Causes Of Compound Pollution And Characteristics Of Vertical PM_2.5-O₃ In Guangzhou

In recent years,the concentration of PM_2.5 and the number of days with heavy pollution had been greatly reduced.However,the O₃ pollution problem in the Pearl River Delta is becoming more and more serious.PM_2.5 and O₃ had become the main factors affecting the air quality of cities and regions in China.Focusing on improving air quality,studying high PM_2.5 and high O₃ pollution in the boundary layer is of great reference significance for local formulation of pollution prevention and control plans.In this study,Guangzhou winter（December 6,2017-January 3,2018）particulate matter lidar,as well as（January 1,2020-December 31,2020）differential absorption ozone lidar remote sensing observation data,combined with surface observations meteorological elements and conventional air quality element data were used to quantitative analysis the meteorological conditions of compound pollution,and to conduct in-depth research on the vertical variation characteristics of compound pollution.The mainly research results are as follows:（1）Meteorological conditions of compound pollution:high temperature,low humidity,high actinic radiation,and static and stable weather conditions were favorable for high PM_2.5 and high O₃.There was a strong negative correlation between PM_2.5 and wind speed,and the relationship with temperature and humidity was not significant.Daytime O₃ was positively correlated with T,actinic radiation,and negatively correlated with RH.With the increasing of T,actinic radiation and RH,O₃ concentration increased and decreased in a stepwise manner,respectively.T,actinic radiation and RH were the key meteorological factors affecting O₃.Statistics showed that the temperature of 28～37℃and the humidity of 18～51%were the most favorable for ozone generation.The correlation between O₃ and NO₂photolysis rate JNO₂ with a lag of 2 hours was 0.67.When JNO₂ was the largest,O₃ did not reach the peak synchronously,indicating that the impact of physical processes on ground O₃cannot be ignored in addition to photochemical effects.（2）Vertical distribution characteristics of extinction coefficients in the boundary layer of the compound pollution day:In the boundary layer,the particle lidar observation results showed that the aerosols continue to mix upwards after sunrise on the compound pollution day,and an obvious mean area appeared,about 330-800 m.In the afternoon,with the weakening of the radiation intensity,the development of the mean area was restricted,and the extinction coefficient above the mean area increased with the height.During the day,the extinction coefficient concentration in the vertical direction first increased and then decreased with height.Especially on days with severe pollution,a more obvious peak appeared near the top of the boundary at night,and a temperature inversion phenomenon was observed at the top of the layer.（3）The vertical distribution characteristics of O₃ in the boundary layer of the compound pollution day:From the time scale,the O₃ concentration on the compound pollution day within 3 km had obvious diurnal variation characteristics.Spatially,the distribution of O₃ in the vertical direction first increased and then decreased with height.There was an obvious high-concentration air mass of O₃ at 255-1000m on the compound pollution day.（4）The influence of airflow transport on the concentration of PM_2.5-O₃:The compound pollution days were mostly static and stable weather conditions.Combined with the 72-hour backward trajectory cluster analysis of the three height,it was found that the low-level trajectories had shorter air masses and moved slow.medium and high air mass transmission speed was fast and relatively clean,air mass at a height of10m in Guangzhou had experienced a certain degree of subsidence or moved close to the ground,and the slow transmission speed of near-surface air mass basically belonged to local transportation,indicating that there was less external transmission,which was unfavorable.The local diffusion conditions were poor under the background of the meteorological field,and the pollutants were more likely to accumulate near the ground.（5）Interaction between aerosols and O₃ on compound pollution days:after excluding the large attenuation of actinic radiation by clouds,compared the SSA of clean days,compound pollution days,and hazy days,typical haze days,and compound pollution solarized radiation fluxes,NO₂ photolysis rate diurnal variation.During the study period,the attenuation ratios of typical haze days and compound pollution solarized radiation peaks were 34.31%and 31.91%,respectively,and the compound pollution solarized radiation attenuation ratio was smaller than that of haze days,and the photolysis rate had the same law.In the three cases,the SSA is the largest on the clean day,the SSA on the compound pollution day increased rapidly,and the SSA on the compound pollution day was greater than that on the haze day,which indicated that the aerosol had an attenuation effect on the radiation and the scattering components in the aerosol had an effect on the actinic radiation.It had a certain promoting effect,which was another manifestation of the mutual promotion of PM_2.5 and O₃ on compound pollution days.