On The Atmospheric Boundary Layer And PM_2.5 Interaction In China: A Perspective From Radiosonde Observations

The planetary boundary layer?PBL?–aerosol interaction is one of the major causes for the deterioration of air quality.Nevertheless,it remains lacking for the comprehensive observational studies on the relationship between surface pollution–PBL on a national level.Elucidating the relationship between pollution–PBL becomes essential for improving the prediction of air quality.Ground-based air quality and meteorological data,in combination with L-band high resolution?1-sec?radiosonde measurements and satellite observational data from2014 to 2017,were used to study the fine structures of PBL,and the relationships between boundary layer height?BLH?and PM_2.5 from national and regional spatial scales.The results show that both the BLH and PM_2.5 exhibit distinct seasonal and diurnal variations throughout China.The BLH is the highest in summer and in the afternoon,with the lowest in winter and in the morning.PM_2.5 concentrations is the opposite.By and large,the BLHs are found to negatively correlate with PM_2.5 concentrations in China,albeit quite different time scales.During the course of a day,the correlation between BLH and PM_2.5 varies dramatically.The negative correlation in the afternoon in summer becomes the strongest,when the PBL is fully developed at this time.Spatially speaking,the negative correlation between PBL and PM_2.5 concentration uniformly spread across China,albeit with quite different R values.The North China Plain experiences the strongest negative correlation coefficient?-0.5?at noon,followed by some large cities with intensive anthropogenic activities.Contrary to our expectations,the positive correlation has been seen at East China,several sporadic inland cities and coastal sites.Due to the different meteorological conditions over China,there is no uniform mechanism to account for the relationship observed between BLH and PM_2.5.5 from a national-level perspective.This suggestes that the effects of meteorological variables on the correlation between BLH and PM_2.5 are much more complicated than expected.Using the sounding at 1400 BJT in summer,the relationships between BLH under different thermal stability conditions,including convective boundary layer?CBL?,stable boundary layer?SBL?and neutral boundary layer?NBL?,and PM_2.5.5 have been systematically analyzed.The CBL height?1527±627 m?is typically higher than the NBL height?1422±668m?,while the corresponding PM_2.5 concentration for CBL is lower than that for NBL,31.06±24?g m^-3 and 32.73±26.48?g m^-3 respectively.In contrast,the SBL height is the lowest?116±112m?.Compared with NBL,the negative correlation reaching-0.92 between the CBL and PM_2.5is the strongest,which is conducive to the diffusion and dispersion of atmospheric pollutants in North China and East China.Noteworthy is that the positive correlation of SBL is 0.93.High cloud cover tends to stabilize the PBL,which in turn transitions to the SBL regime with the increasing relative humidity.Under the conditions of unstable atmospheric stability,low cloud cover,and strong wind tend to promote the formation of CBL with the reducing relative humidity.The more unstable atmospheric stability,the thicker the CBL,combined with the lower relative humidity,which is conducive to the diffusion of pollutants.There is a significant difference in the PBL–PM_2.5 interaction in difference regions.On the inner-annual timescale,the strongest negative correlation is observed over the North China Plain?NCP?with highly polluted conditions,followed by the Yangtze River Delta?YRD?.Meanwhile,the air quality of Qinghai-Tibet Plateau?TBP?is relatively clean,where it shows the lowest negative correlation coefficient.The correlation coefficient are-0.34,-0.25 and-0.18 for the NCP,the YRD and the TBP.Diurnaly,negative correlation is observed in the NCP,while the positive correlation is over the YRD.Interesintly,without significant correaltion is found over the TBP in the afternoon when the PBL is fully developed.In the heavily polluted region,the pollutant–PBL–meteorology is closely related,indicating that their interaction is most obvious.Under clean conitions,no dinstinct interaction can be found between BLH and PM_2.5.5 in the TBP,whereas the meteorological conditions show a greater impact on the development of PBL.The PBL–pollution interaction depends in the degree of pollution and the background value of BLHs,and the relationship between PM_2.5 and BLHs is nonlinear,which is more obvious with high concentration of pollutants or the low BLHs.Of course,this relationship is also significantly affected by other meteorological varibles.