Characteristics of Winter Heavy Pollution Episodes and Meteorological Causes and Structures of Boundary Layer in Tianjin

The Beijing-Tianjin-Hebei area, including Tianjin, incurs severe haze pollution. In this study, the mass concentrations of PM2.5 and gaseous pollutants for seven sites in Tianjin, together with pollutants in Beijing, ground meteorological factors, visibility, temperature, and humidity profiles, wind profiles, and backward trajectories during 20-28 February 2013, are used to analyze the pollution characteristics, meteorological causes, and boundary layer characteristics of heavy haze episodes. The average concentrations of PM2.5, SO₂, NO₂, CO, and O₃ were 150, 87, 56, 2.4, and 22 μg m^-3, respectively. Although the variation of gaseous pollutants in different sites was large and significant, only SO₂ concentrations were in non-attainment, according to air quality grade limit of 50 μg m^-3. The PM2.5 variation over various sites in Tianjin showed regional and synchronous characteristics. PM2.5 concentrations during the research period far exceeded the grade limit value of 35 μg m^-3 and were 2-8 times higher, with a maximal hourly mean of 364 μg m^-3. High PM2.5 concentration is a key factor contributing to low visibility; the threshold PM2.5 mass concentration, corresponding to visibility < 10 km, is 50 μg m^-3. PM2.5 concentration began to increase because of the local emission accumulating under weak winds and high humidity and rose steadily under sustained south winds; in additon, a fluctuating increase was found under weak north and east winds. The south and north winds then sheared quickly, and the peak value of PM2.5 reached the maximum because of the regional transport and repeated circulation and accumulation processes during the heavy pollution period. High PM2.5 concentration is associated with advection inversion, except when the advection inversion is caused by a prevailing strong east wind. During haze pollution episodes, high humidity occurred in the low and moderate layers of less than 500 m, and decreased quickly with height. The low-level jet between 200 m and 600 m curtailed the growth in PM2.5 concentration to some degree; in particular, the persistent, strong east wind contributed to a steady drop in PM2.5 to Grade II level. Pollution was removed effectively and quickly only when the entire layer was controlled by the strong northwest wind. Pollutants transported from Beijing and central and southwest area of Hebei province surrounding Tianjin made remarkable contributions to heavy pollution in Tianjin.