Air Quality in the Harbin-Changchun Metropolitan Area in Northeast China: Unique Episodes and New Trends.

Because of the unique geographical, climate, and anthropogenic emission characteristics, it is meaningful to explore the air pollution in the Harbin-Changchun (HC) metropolitan area. In this study, the Air Quality Index (AQI) and the corresponding major pollutant were investigated for the HC cities, based on the air quality data derived from the China National Environmental Monitoring Center. The number of days with the air quality level of "good" gradually increased during recent years, pointing to an improvement of the air quality in HC.