[Analysis of particulate pollution characteristics and its causes in Lanzhou, Northwest China].

Seasonal characteristics, annual variations and trend of PM10 and the causes of the PM10 long-term trend in Lanzhou were studied for the period of 2001-2007 using daily air pollution index (API) data and detrending technique. Results indicate that air quality in Lanzhou has been improved since 2001. The annual averaged PM10 concentration decreased from 236 microg x m(-3) in 2001 to 127 microg x m(-3) in 2007 and the number of days satisfying the Chinese Grade I and II air quality standard increased 2.3 times as 2001, but there still have 25% days exceeding the national Grade II air quality standard. Monthly mean PM10 concentration undergoes a seasonal change characterized by higher values in November, December and January to March. The maximum monthly average PM10 concentrations appear in December (271 microg x m(-3)) followed by March (245 microg x m(-3)) while it is low during summer months (May to October) with monthly average PM10 concentrations below 150 microg x m(-3). 61.2% of moderate pollution and 50.6% of serious pollution days occurred in winter and 67.4% of Grade I and II days occurred in summer and autumn. Analyses show that annual mean PM10 concentrations decreased at a rate of 12.6% over the past 7 years which correlates well with the reductions in anthropogenic dust emissions. The occurrence of blowing dust and floating dust can modulate the overall trend of PM10, which explains about 21% of the inter-annual variations of PM10 during 2001-2007. The changes of local meteorological conditions have little effect on the overall trend of PM10 concentrations. The improvement of air quality in Lanzhou is mainly due to the reduction of local emissions, with contributions from the variation of natural dust emissions due to annual variations of blowing and floating dust events. To further improvement of the air quality in Lanzhou, it is advisable to not only implement effective emission control strategies but also improve surrounding ecological environment.