Characterizing near-ground-layer structure changes during a heavy air pollution episode in a remote urban area of northwest China

Abstract

The composition of air pollutants and features of the near–ground layer were investigated from four 100 m high towers during a heavy pollution event in 2013 in Urumqi, northwest China. The PM_2.5, PM₁₀, SO₂, NO₂, and CO concentrations increased rapidly during the heavy pollution event, with the exact amounts dependent on air pollutant emissions and meteorological conditions. Ozone concentrations increased in northern rural areas, and were affected by the emission and conversion of primary pollutants from urban areas. The maximal mixing layer was slightly lower than 1000 m. A strong temperature inversion intensity of 6.8, 2.5, 4.2, and 4.7 °C/100 m in suburban, urban, northern suburban, and rural sites, respectively, occurred during the heavy pollution event. A significant mountain–valley wind in the urban and northern suburbs was an important factor affecting the heavy pollution. During heavy pollution days, the wind was almost always from the south and east, and the wind speed was mostly over 7 m·s⁻¹ in the southern suburbs. Conversely, it was lower than 2 m s⁻¹ in the urban and northern rural sites, where the wind directions were variable. There was a positive correlation between pollutant concentrations and air temperature, but pollutant concentrations were negatively correlated with relatively humidity. Dynamic and thermal turbulence (friction velocity and vertical kinematic eddy heat flux) favored the diffusion and accumulation of heavy pollution in Urumqi. The friction velocity was strong in the southern suburbs (0.5 m s⁻¹), but was weak at the northern rural (0.11 m s⁻¹) site, which were located on high and low terrain, respectively. The turbulent thermal effect was the main atmospheric condition associated with the formation of heavy pollution events, which had little relationship with the turbulent dynamic effect. A backward trajectory analysis of the atmospheric motion showed that heavy pollution events were more likely to be caused by local pollutant transport.