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Ambient air quality monitoring data and radar tracking sonde data were used to study the atmospheric boundary layer structure (ABLS) and its changing characteristics over Wuhan. The boundary layer structure index (BLSI), which can effectively describe the ABLS, was accordingly developed and its ability to describe the near-surface air quality was analyzed. The results can be summarized as follows. (1) An analysis of the ABLS during seriously polluted cases revealed that the ABLS was usually dry and warm with a small ventilation index (VI); meanwhile, the ABLS during clean cases was usually wet and cold with a large VI. (2) The correlation between the air quality and BLSI at 100~300 m was good and passed the confidence level limit at 99%. Moreover, the correlation coefficient increased with the altitude at 10~250 m and showed a downward trend at 250~500 m. The correlation between the BLSI at 250 m and the ground air quality was the most significant (r = 0.312), indicating that the layer ranging from 0 to 250 m is essential for determining the ground air quality. (3) The BLSI considers both the vertical diffusion capability and horizontal removal capability of the atmosphere. Therefore, it is highly capable of describing the ABLS and the ground air quality.

In this study, we investigated six air pollutants from 21 monitoring stations scattered throughout Wuhan city by analyzing meteorological variables in the atmospheric boundary layer (ABL) and air mass backward trajectories from HYSPLIT during the pollution events. Together with this, ground meteorological variables were also used throughout the investigation period: 1 December 2015 to 30 November 2016. Analysis results during this period show that the city was polluted in winter by PM2.5 (particulate matter with aerodynamics of less than 2.5 microns) and in summer by ozone (O3). The most polluted day during the investigation period was 25 December 2015 with an air quality index (AQI) of 330 which indicates ‘severe pollution’, while the cleanest day was 26 August 2016 with an AQI of 27 indicating ‘excellent’ air quality. The average concentration of PM2.5 (O3) on the most polluted day was 265.04 (135.82) µg/m3 and 9.10 (86.40) µg/m3 on the cleanest day. Moreover, the percentage of days which exceeded the daily average limit of NO2, PM10, PM2.5, and O3 for the whole year was 2.46%, 14.48%, 23.50%, and 39.07%, respectively, while SO2 and CO were found to be below the set daily limit. The analysis of ABL during PM2.5 pollution events showed the existence of a strong inversion layer, low relative humidity, and calm wind. These observed conditions are not favorable for horizontal and vertical dispersion of air pollutants and therefore result in pollutant accumulation. Likewise, ozone pollution events were accompanied by extended sunshine hours, high temperature, a calm wind, a strongly suspended inversion layer, and zero recorded rainfall. These general characteristics are favorable for photochemical production of ozone and accumulation of pollutants. Apart from the conditions of ABL, the results from backward trajectories suggest trans-boundary movement of air masses to be one of the important factors which determines the air quality of Wuhan.