Lidar Observation of PBL Height under Severe Air Pollution and its Comparison with Radiosonde and Numerical Simulation

The planetary boundary layer (PBL) height determines the environmental capacity for the diffusion of atmospheric pollutant, and has always been a hot issue in the study of air pollution. The comparisons of PBL height using lidar, radiosonde and WRF model during a haze episode in Beijing are presented. The PBL heights estimated from four different variables: potential temperature, wind, turbulence quantity and pollutants are expressed and distinguished by H_θ, Hu, Ht and Hc respectively. H_θ decreased from approximately 1250 m to 450 m and Hc also dropped below 500 m, resulting in deteriorating pollution situation from 26 to 31 December. And on the whole, Hc calculated by the gradient method was in good agreement with the values of H_θ. Ht estimated from turbulence quantities simulated by WRF model mainly approximated Hu, H_θ and Hc in the daytime during haze pollution, approaching 600 m; however, Ht was seriously underestimated for the nocturnal boundary layer height in the winter. Generally, Hc was relatively lower than H_θ and Hu, despite a high bias caused by the existence of the residual layer, the multilayer aerosol, or lower inversion intensity.

The averaged PBL heights according to the pollution level showed that Hc, H_θ, Hu and Ht differed greatly on clean days, and the maximum value of PBL height exceeded 1400 m. The observed inversion intensities were lower on clean days, and moreover the indistinct inversion layer weakened the correlation between Hc determined by pollutant concentration and H_θ determined by temperature. However, Hc and Ht were very close, approximately 1100 m. The decrease of PBL height led to heavy pollution, Hc, H_θ and Ht were almost close to 700 m. Hu was slightly higher, but Hu also decreased by approximately 450 m during heavy pollution. The detailed analyses and comparisons of the PBL height from different variables can help improve the rational application of different methods in the determination of PBL height.