Influence of coastal planetary boundary layer on PM2.5 with unmanned aerial vehicle observation

Field measurement with an Unmanned Aerial Vehicle (UAV) platform was conducted at coastal area of Bohai in winter and summer. 135 flights were obtained and characteristics of boundary layer structure and its influence on PM2.5 were studied. The averaged profiles showed that meteorological parameters and PM2.5 changed significantly below 300 m. Weak suspension inversion existed between 30–300 m in the morning, ground inversion existed below 70 m at night in winter, temperature inversion was only observed below 200 m at night in summer. PM2.5 peaked at 300 m, 100 m and 200 m during morning, daytime and night period, respectively. During the middle and later stages of winter pollution episode, southwest and southeast wind, northwest and northeast wind formed obvious convergence at coastal area in turn, resulted in maximum PM2.5 concentration of 160 μg m−3 and 218 μg m−3 at night and morning, respectively. Isothermal layer was observed below 200 m, the maximum wind speed and high PM2.5 concentration all appeared at 200 m at night. Special stratification between 50–100 m was observed at 11:00, where wind speed decreased and high PM2.5 concentration maintained. During the summer pollution episode, the sea breeze formed at 13:00 and ended at 22:00, sea breeze extend westward 50 km to the central urban area at 16:00. Thermal internal boundary layer formed below 100 m at 15:00 and extended upward to 150 m at 17:00, weak temperature inversion layer was observed between 50 m ∼ 100 m, the maximum wind speed was 8 m s−1, the wind direction changed from southeast to south, and maximum PM2.5 concentrations of 160 μg m−3 were observed at 100 m.