In the Beijing-Tianjing-Hebei (BTH) region of China, haze has become especially serious in recent years, which has attracted much attention. To protect public health, the red alert has been implemented since 7 a.m. Tuesday (8th Dec. 2015), and the warning is due to be lifted at noon on Thursday (10th Dec. 2015). Since this is the first-ever red alert for smog, PM2.5 monitoring, including both concentration and pollutants’ source characterization, is of great significance for environmental departments and the public to validate the effect of red alert.