Abstract: It is well known that Urumqi is one of the cities with the severest air pollution in China and even over the world. A large numbers of researches on calculation methods of maximum mixing depth (MMD) and relations between MMD and air pollutant concentration have been performed in the world in recent 50 years. However, there have been only a few studies focusing on the characteristics of MMD over Urumqi and the impacts of MMD on local air quality so far. According to the fundamentals of estimating MMD by the assumption of a dry adiabatic lapse rate and method of stepwise approach, in this paper the MMD values in Urumqi were calculated and their variable characteristics and relations with air pollution were analyzed based on the daily surface and sounding meteorological data observed during the period from January 2003 to April 2009. The result reveals that the annual change of monthly mean MMD was singly periodic. That is, the minimum value (389 m) of monthly mean MMD occurred in January but the maximum value (2 071 m) occurred in June. The occurrence frequency of MMD over 2 000 m during the period from April to September was higher than 50%. Moreover, the occurrence frequency of MMD below 500 m during the period from November to next February was relatively high. There was a negative correlation between air pollution index and MMD. As the air pollution level was increased, the MMD values with peak occurrence frequency was correspondingly decreased, that is, the MMD is one of the important factors affecting local air pollution, and there is an exponential relation between the daily mean PM₁₀ concentration and the MMD.

Key words: mixing depth, monthly mean, occurrence frequency, air pollution, correlation analysis, Urumqi