The Influence of Shallow Foehn on Atmospheric Diffusion Conditions and Air Quality over Urumqi in Winter

<p>Using hourly air pollutants concentration from six environmental monitor stations, meteorological data and wind profile radar data in winter during 2013-2015, the influences of shallow foehn on diffusion conditions and air pollution concentration over Urumqi were analyzed. The results showed the occurrence frequency of shallow foehn was 57.3% in Urumqi in winter. The flow depth, base height and top height of shallow foehn were about 1500 m, 600 m and 2100 m, respectively. The maximum mixing layer depth, the inversion depth, the temperature difference between the top and bottom of inversion layer on foehn days were 200 m lower, 344m thicker and 4.4℃ higher than the corresponding values on non-foehn days, respectively. However, the differences of wind speed and inversion intensity between on foehn days and on non-foehn days were slight. Also, the frequency of each pollution level on foehn days was higher than on non-foehn days with extra frequency of 18% from level Ⅲ to level Ⅵ. Moreover, there was foehn existence on days with air pollution level Ⅵ. Except for O<sub>3</sub>, the other five air pollutant concentrations at each environmental station on foen days were all higher than on non-foehn days but with similar diurnal variation. The spatial distributions of six air pollutants on foehn days and non-foehn days were almost same. Overall, the air quality at south urban area was relative excellent than other areas.</p>