Beijing city issued a yellow alert for sandstorm as a severe sandstorm smothered the capital and other northern Chinese regions Monday morning. The meteorological agency called it the “the strongest sandstorm in the past decade" which has reached the level of a strong storm. The concentration of PM10 in six downtown districts in Beijing reached over 8,100, while the Air quality index (AQI) hit 500 and above is considered as severe air pollution, reducing visibility to less than 1,000 meters. Data from Flymaster showed that more than 400 flights have been cancelled at the two international airports in Beijing. Photos: Li Hao/GT