Abstract: Effect of meteorological conditions of atmospheric boundary layer on air pollutant dispersion is significant, but there have been few studies on atmospheric boundary layer characteristics over Urumqi yet. During December 11-20, 2005, the sounding experiment in urban atmospheric boundary layer over Urumqi City was performed six times a day. Atmospheric boundary layer conditions, such as wind, temperature, mixing layer depth and temperature inversion, were analyzed. The results show that wind speed near ground surface was often lower than 3 m/s, and it was increased gradually with the increase of height below 600 m. The temperature inversion of ground-based and low level-elevated was higher than 1 ℃/100 m, and the daily mean mixing layer depth was 274 m. The meteorological conditions in two durations of air pollution at severe and light levels were compared. Wind speed was lower and temperature was higher during severe air pollution than that during light air pollution. In addition, the temperature inversion intensity during light air pollution was very high (4.5 ℃/100 m), but its inversion depth was low (166 m). The results reveal that the severe air pollution in Urumqi was mainly caused by the simultaneous decrease of wind speed in atmospheric boundary layer, labilization of the prevailing wind direction near ground surface, continuous surface temperature increase, and formation of unstable stratification below exhaustsmoke height of chimneys. The formation and stable continuance of the thick temperature inversion resulted in a long duration of severe air pollution.

Key words: winter, atmospheric boundary layer, air pollution, meteorological conditions, Urumqi