Numerical Simulation Study of Winter Pollutant Transport Characteristics over Lanzhou City, Northwest China

Abstract

Air pollution levels are severe in Lanzhou due to the valley topography and the semi-arid climate. A comprehensive understanding of pollutant transport characteristics, which are affected by atmospheric circulation, can help explain the reason for the air pollution to some extent. Using the Weather Research and Forecast (WRF) model coupled with the FLEXible PARTicle (FLEXPART) dispersion model, the authors of this paper simulated the transport pathways of pollutants discharged from local sources and analyzed the diffusion efficiency over Lanzhou during six winters from 2002 to 2007. Flow field analysis showed that a divergence and convergence region formed in the Lanzhou valley during the day and at night, respectively. The Lanzhou valley was dominated by an easterly wind. Based on transport trajectories from FLEXPART, five main transport pathways, namely, the southwest pathway (SW), west pathway (W), south pathway (S), southeast pathway (SE), and northeast pathway (NE), were identified over Lanzhou. Compared with static weather, it was easier for pollutants to cross the south mountain (i.e., along the southeast pathway) during the strong cold air process. The percentage of particles moving out of the urban valley after 12 h of transport and the ratio of particles moving back into the urban valley showed significant diurnal variability. This indicates that the air pollution over Lanzhou may be reduced to some extent by artificially controlling the emission time of pollutants.

Keywords

• WRF
• FLEXPART
• transport trajectory
• local circulation