A combination photo shows the weather conditions respectively on Feb 23, 2017, and May 4, 2017 in Beijing. Beijing issued a blue alert for sandstorm on Thursday. Beijing has a four-tier color alert system for pollution, with red being the highest, followed by orange, yellow and blue. The blue alert means the air quality index is forecast to reach between 200 and 300 PM2.5 for one day. (Photo/Ecns.cn)
