On the Relationship Between Aerosol and Boundary Layer Height in Summer in China Under Different Thermodynamic Conditions

The observed relationships between boundary layer height (BLH) and PM2.5 on a national scale remain unclear due to the dearth of observations. Here we investigated this relationship from a unique perspective of thermodynamic stability in the planetary boundary layer (PBL), using summertime (June-August) soundings from China for the period from 2014 to 2017. For all three times of soundings (0800, 1400, and 2000 Beijing time [BJT]), positive (negative) PM2.5 concentrations anomalies were found to correlate with negative (positive) BLHs anomalies relative to daily means. The negative correlation was strongest at 1400 BJT, followed by 2000 BJT and 0800 BJT. Overall, the PM2.5 was found to nonuniformly anticorrelate with BLH across China at 0800 and 2000 BJT. The strongest anticorrelation occurred in the North China Plain at 1400 BJT, in sharp contrast to the much weaker correlation in other regions characterized by much less polluted regions. The averaged PM2.5 in neutral boundary layers was higher than that in convective boundary layers (CBLs). The CBL, where the anticorrelation was the strongest, was conducive to dissipating more aerosol in the heavily polluted area in China than neutral boundary layer. The higher CBL formed under low cloud cover, low surface humidity, and strong wind speed was favorable for the dispersion of aerosol, in contrast to the stable boundary layers that happen under the highest cloud cover. Also, positive correlation was seen between stable boundary layer and PM2.5. The findings call for attention that the thermodynamical condition of PBL should be considered when examining the aerosol-PBL interactions.