Two air pollution events in the coastal city of Tianjin, north China plain

Air pollution has become a serious problem driven by the mesoscale-circulation system in the Beijing-Tianjin-Hebei area over the North China Plain. In this study, the particulate matter (PM) levels and gaseous pollutant concentrations during 2018 were examined in Tianjin. The vertical structure of meteorological data at the 250-m tower and the characteristics of turbulence evolution were used to analyse the pollutant occurrence and development during the severe pollutant cases. The maximum PM2.5 values observed in March (similar to 7.1 mu g m(-3)) and November (82.4 mu g m(-3)) as well as the higher PM10 concentration in March-April (similar to 100.0 mu g m(-3)) highlight the contribution of both fine mode and coarse mode particles over Tianjin. The prevailing southerly wind in April was mainly sea winds with larger wind speed at surface and high-altitudes (1.35 m s(-1) to 7.98 m s(-1)), and changed to land breeze in September south-westerly with lower relative humidity (37.6%-46.5%) at 250 m. The photochemical reaction of O-3 was affected by the moderate-to-high air temperature, and the distribution of NO2 and SO2 was attributable to the emission and gas-particle transformation under different humidity conditions. The hourly PM2.5 concentration sharply increased to 2.6-5.8 times and lasted for 28-38 h (exceeded 200 mu g m(-3)) during the two pollution events. The variation of turbulent characteristics at different heights could be depressed due to the development of heavy pollution and otherwise could be enhanced by solar radiation. These findings could facilitate understanding the fine structure of temperature-humidity systems and the turbulence characteristics in the boundary layers during typical pollutant events, and their regional climate effects over coastal northern plain cities.