Evolution of atmospheric boundary layer structure and its relationship with air quality in Wuhan, China

Air pollution data, air quality index (AQI) data and L-band sounding data of Wuhan City from January 1 to February 28, 2015, were used in this study. Since air quality is mainly determined by the condition of the atmospheric boundary layer structure (ABLS), a detailed analysis was carried out in order to determine the evolution of this layer and its relationship with air quality. During the investigation period, the highest value of AQI was 307 on January 26 and the lowest was 33 on February 28 indicating 'severe pollution' and 'excellent' air quality, respectively. The concentrations of PM2.5 during the days with the highest and lowest AQI were 142.61 and 9.78 mu g/m(3), respectively. The percentage of days in which the ratio of PM2.5 in PM10 was more than half was 83.05% which means that the greater portion of pollutants were composed of smaller particles. Moreover, four PM2.5 episodes (three or more consecutive days of PM2.5 >= 75 mu g/m(3)) were identified and the average percentage of elementary carbon (EC) in PM2.5 during episode 1 (prior to the episode) was 6.274% (6.276%), episode 2 was 5.634% (7.174%), and episode 4 was 4.067% (7.785%). Higher concentrations of EC prior to episodes suggest biomass burning to be one of the reasons for episodes occurrence. Analysis of the ABLS during polluted days show that the boundary layer was dry and warm and had weak low-level wind and dominance of northerly winds. A different scenario is seen on clean days as the boundary layer is observed to be wet and cool, and there is dominance of strong winds. Back trajectory analysis results show that polluted days were dominated by air mass from north China while on clean days, the dominant air masses were from East China Sea, Mongolia, and west China.