The pattern and mechanism of an unhealthy air pollution event in Lanzhou, China

To study the patterns and mechanisms of unhealthy air pollution events in Lanzhou, China, meteorological factors, synoptic conditions and regional transportation circumstances were analysed. The results showed that the average daily mass concentrations of PM10 and PM2.5 were 3646.0 μg/m3 and 668.0 μg/m3, respectively; the average value of PM2.5/PM10 was less than 30%, indicating that this event was caused by PM10. WPSCF and WCWT analyses showed that the contribution sources of PM2.5 and PM10 were diverse. A large-scale high-pressure ridge lasted for a long period and was prone to long-term calm winds that reduced the transportation, dilution and diffusion of pollutants. The formation of this unhealthy air pollution event was closely related to the large-scale high-pressure ridge and the trough-shaped topography of the valley.