The planetary boundary layer (PBL) height mainly determines the environmental capacity for the diffusion of atmospheric pollutants, and has always been a hot issue in the study of air pollution. However, there still remains great uncertainty, partly because different PBL heights definitions and the PBL heights are obtained by various measurement instruments. Pollutants are the substances emitted, different from the atmospheric background physical properties such as wind, temperature and turbulence flux that always exist even without pollution. It is very important to distinguish PBL heights obtained from wind, temperature, turbulence quantities and the concentration of pollutants. In this paper, we express the PBL heights determined on the above four parameters as Hu, H_θ, Ht and Hc respectively, and compare them during a heave haze pollution process in Beijing using observation data and simulation results. The comparison results show that: (1) H_θ, namely the inversion layer height, decreased from approximately 1250 m to 450 m from 26 to 30 December, resulting in deteriorating pollution situation. Hc, calculated by lidar and characterizes the maximum depth of vertical diffusion of particulates, also dropped below 500 m, and on the whole, the values of Hc estimated by gradient method and H_θ were in good agreement; (2) Generally, Hc was relatively lower than H_θ and Hu, despite a high bias caused by the existence of the residual layer, multilayer aerosol structure, or lower inversion; (3) Ht estimated from turbulence quantities simulated by WRF model mainly approximated Hu, H_θ and Hc in the daytime during haze pollution, however for the nocturnal boundary layer height in the winter, Ht was seriously underestimated. 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 PBL height Hu exceeded 1400 m. On clean days, the inversion intensities observed were lower, so the blocking effect of the inversion layer to pollutant diffusion was not strong enough, H_θ (886 m) deviated from Hc (1111 m)_. 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 700 m. Hu was slightly higher and reduced by about 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.

The study and control of air pollution involves measuring the structure of the atmospheric boundary layer (ABL) to understand the mechanisms of the interactions occurring between the atmospheric boundary layer and air pollution. Beijing, the capital of China, experienced heavy haze pollution in December 2016, and the city issued its first red-alert air pollution warning of the year (the highest PM_2.5 concentrations were later found to exceed 450 µg m⁻³). In this paper, the vertical profiles of wind, temperature, humidity and the extinction coefficient (reflecting aerosol concentrations), as well as ABL heights and turbulence quantities under heavy haze pollution conditions, are analyzed, with data collected from lidar, wind profile radar (WPR), radiosondes, a 325 m meteorological tower (equipped with a 7-layer ultrasonic anemometer and 15-layer low-frequency wind, temperature, and humidity sensors) and ground observations. The ABL heights obtained by three different methods based on lidar extinction coefficient data (H_c) are compared with the heights calculated from radiosonde temperature data (H_θ), and their correlation coefficient can reach 72 %. Our results show that H_θ measured on heavy haze pollution days was generally lower than that measured on clean days without pollution, but H_c increased from clean to heavy pollution days. The time changes in friction velocity (u_*) and turbulent kinetic energy (TKE) were clearly inversely correlated with PM_2.5 concentration. Momentum and heat fluxes varied very little with altitude. The nocturnal sensible heat fluxes close to the Earth surface always stay positive. In the daytime of the haze pollution period, sensible heat fluxes were greatly reduced within 300 m of the ground. These findings will deepen our understanding of the boundary layer structure under heavy pollution conditions and improve the boundary layer parameterization in numerical models.

To investigate the evolution of the nocturnal boundary layer (NBL) and its impacts on the vertical distributions of pollutant particulates, a combination of in situ observations from a large tethered balloon, remote sensing instruments (aerosol lidar and Doppler wind lidar) and an atmospheric environment-monitoring vehicle were utilized. The observation site was approximately 100 km southwest of Beijing, the capital of China. Results show that a considerable proportion of pollutant particulates were still suspended in the residual layer (RL) (e.g., the nitrate concentration reached 30 μg m⁻³) after sunset. The NBL height calculated by the aerosol lidar was closer to the top of the RL before midnight because of the pollutants stored aloft in the RL and the shallow surface inversion layer; after midnight, the NBL height was more consistent with the top of the surface inversion layer. As the convective mixing layer gradually became established after sunrise the following day, the pollutants stored in the nocturnal RL of the preceding night were entrained downward into the mixing layer. The early morning PM_2.5 concentration near 700 m in the RL on 20 December decreased by 83% compared with the concentration at 13:34 on 20 December at the same height. The nitrate concentration also decreased significantly in the RL, and the mixing down of nitrate from the RL could contribute about 37% to the nitrate in the mixing layer. Turbulence activities still existed in the RL with the bulk Richardson number (Rb) below the threshold value. The corresponding increase in PM_2.5 was likely to be correlated with the weak turbulence in the RL in the early morning.

The removal process of urban air pollution is very rapid even under the strong obstruction of various buildings in the city. In this study, based on high-accuracy ultrasonic anemometer observations at seven different altitudes on a 325 m tower, we proposed a physical mechanism in which turbulent wind gust and its coherent structure may play important roles in this phenomenon. Pollutants suspended within the urban canopy layer UCL cannot be quickly removed by means of basic flow but diffuse and crosse the UCL by means of the coherent structure of gust wind and then propagate further under strong basic flows above the UCL. With a continuous increase in wind speed in the whole layer, combined with gust-wind and turbulence mixing, the rapid transformation from heavily polluted days to clean days in urban areas can take place in just a few hours. Turbulent wind gust and its coherent structure were extracted by third-order Daubechies wavelet transformation. The downwind wind was decomposed into three separate components, basic flow u ¯ ( t ) , gust-wind u ′ g ( t ) , and turbulence u ′ t ( t ) . The results show that in the early rapid drop of urban surface pollutant concentration, horizontal wind speed at 8 m was still low, and sinking movements dominated in the lower layer. Friction velocity calculated from gust-wind u * g significantly affected the surface PM_2.5 concentration, and was the most obvious factor with an inverse correlation coefficient −0.56. The correlation coefficient of coherent structure of gustiness Ru _g w _g at 8 m basically changed to positive values before the rapid decline of the surface PM_2.5 concentration. The concentration of PM_2.5 decreased significantly when Ru _g w _g > 0, and exhibited an increasing trend when Ru _g w _g < 0. The close relationship between the concentration of PM_2.5 and Ru _g w _g indicated the important role of the coherent structure of gustiness in the rapid removal process of urban pollutants. The research deepens the vertical exchange process of matter and energy in the UCL, and provides a scientific reference for the rapid dissipation mechanism of urban air pollution.