Seasonal variation of PM₁₀ and the causes of the PM₁₀ long-term trend in Lanzhou valley were studied using the air pollutant monitoring data during 1999-2001 in Lanzhou and the statistical data of dust storm frequency in Hexi Corridor. Results indicated that the dust storm events in Hexi Corridor area had an important influence on PM₁₀ concentration of Lanzhou, and the PM₁₀ concentration showed higher value in Spring and Winter while exhibited lower value in Summer. Sand-dust frequency monitoring data during 1985-2001 demonstrated that the annual average frequentcy of dust events in the sixteen years in middle-east part of the Hexi Corridor was about 37.6 days, and the annual average days of the dust events in the sixteen years in the Lanzhou Valley was about 24.4 days, and there was a significant correlation between them (R=0.67, P=0.003). Statistical analysis of hourly mean PM₁₀ concentration in 2001 in the Lanzhou Valley showed that PM₁₀ peak values more than 1.0 mg·m^-3 were mostly caused by the process of dust storm in Hexi Corridor area, however, PM₁₀ peak values between 0.5-1.0 mg·m^-3  were mainly caused by the emission of industrial pollutant SO₂, NO_x and some other particles. In order to effectively improve the air quality in Lanzhou for ever, it is advisable to not only establish effective control strategies of emission but also improve surrounding ecological environment.

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