Why is the weather in Chengdu always strange?

Chengdu is located in the plain, and Sichuan is a basin topography. Chengdu is surrounded by mountains. The airflow from high places can't invade Chengdu, but it can't take away the excess carbon dioxide from Chengdu, and Chengdu is developing into an industrialized city, which has caused serious pollution in Chengdu in recent ten years and the climate is becoming more and more unpredictable.