Afforestation for Valley Urban Air Quality Improvement

Lanzhou is one of the major cities in northwest China and the capital of Gansu Province and located at a narrow (2-8 km width), long (40-km), NW-SE oriented valley basin (elevation: 1,500- 1,600-m) with the Tibetan plateau in the west, Baita mountain (above 1,700-m elevation) in the north, and the Gaolan mountain in the south. Due to topographic and meteorological characteristics, Lanzhou is one of the most polluted cities in China. Meteorological conditions (low winds, stable stratification especially inversion), pollutant sources and sinks affect the air quality. Afforestation changes the mountain-valley local circulation system, destabilizes the atmosphere, and weakens the inversion. Besides, it may absorb some pollutants (sink). Lanzhou local government carried out afforestation and pollutant-source reduction (closing several heavy industrial factories) to improve the air-quality for the past two decades. Numerical model (MM5-HYPACT) simulates the effect of afforestation on the air pollution (TSP, SO2, NOx …) control.