A haze frequently hangs over China, particularly over the highly populated North China Plain in the autumn and winter months. The concentration of haze pollution particulates (PM2.5) in this region has reached over twice the national standard for safe air quality. Chinese scientists probed deeper into the haze pollution patterns and observed a short-term "seesaw" phenomenon in autumn and early winter, with high pollution levels in September and October coupled with low haze levels in November and December—or vice versa.