Recent intensification of winter haze in China linked to foreign emissions and meteorology.

Wintertime aerosol pollution in the North China Plain has increased over the past several decades as anthropogenic emissions in China have increased, and has dramatically escalated since the beginning of the 21st century, but the causes and their quantitative attributions remain unclear. Here we use an aerosol source tagging capability implemented in a global aerosol-climate model to assess long-term trends of PM2.5 (particulate matter less than 2.5 µm in diameter) in the North China Plain. Our analysis suggests that the impact of China's increasing domestic emissions on PM2.5 concentrations over the last two decades of 20th century was partially offset (13%) by decreasing foreign emission over this period. As foreign emissions stabilized after 2000, their counteracting effect almost disappeared, uncovering the impact of China's increasing domestic emissions that had been partially offset in previous years by reductions in foreign emissions. A slowdown in the impact from foreign emission reductions together with weakening winds explain 25% of the increased PM2.5 trend over 2000-2014 as compared to 1980-2000. Further reductions in foreign emissions are not expected to relieve China's pollution in the future. Reducing local emissions is the most certain way to improve future air quality in the North China Plain.