Lanzhou is the capital and largest city of Gansu Province in Northwest China. The prefecture-level city, located on the banks of the Yellow River, is a key regional transportation hub, connecting areas further west by rail to the eastern half of the country. Historically, it has been a major link on the Northern Silk Road. The city is also a center for heavy industry and petrochemical industry. Lanzhou was previously ranked as one of the cities with the worst air quality in the world, due to industrial pollution and its situation in a narrow river valley. Government measures to reduce the pollution have had their effect, and in 2015 the city was awarded China's climate progress title.