Beijing

Last week I went to Beijing for some work stuff, and had the chance to stay at the new yet-to-open Conrad Beijing. Beijing were suffering from incredibly terrible pollution. Tree days before I arrived, the pm2.5 reading from the US Consulate was in the 700s. But The maximum reading was set only to 500. When we got off the plane, the smog was thick and it smelled like burning trash.
However, the day after we arrived, the sky cleared up. The reading went down to health level.
I wonder what was the magic?