Evolution of Meteorological Conditions during a Heavy Air Pollution Event under the Influence of Shallow Foehn in Urumqi, China

The air pollution in Urumqi which is located on the northern slope of the Tianshan Mountains in northwestern China, is very serious in winter. Of particular importance is the influence of terrain-induced shallow foehn, known locally as elevated southeasterly gale (ESEG). It usually modulates atmospheric boundary layer structure and wind field patterns and produces favorable meteorological conditions conducive to hazardous air pollution. During 2013–17, Urumqi had an average of 50 d yr^–1 of heavy pollution (daily average PM_2.5 concentration >150 μg m^–3), of which 41 days were in winter. The majority (71.4%) of heavy pollution processes were associated with the shallow foehn. Based on microwave radiometer, wind profiler, and surface observations, the surface meteorological fields and boundary layer evolution during the worst pollution episode in Urumqi during 16–23 February 2013 are investigated. The results illustrate the significant role of shallow foehn in the building, strengthening, and collapsing of temperature inversions. There were four wind field patterns corresponding to four different phases during the whole pollution event. The most serious pollution phase featured shallow foehn activity in the south of Urumqi city and the appearance of an intense inversion layer below 600 m. Intense convergence caused by foehn and mountain–valley winds was sustained during most of the phase, resulting in pollutants sinking downward to the lower boundary layer and accumulating around urban area. The key indicators of such events identified in this study are highly correlated to particulate matter concentrations and could be used to predict heavy pollution episodes in the feature.