Air pollution is a big problem in China, with smog from factories and cars routinely blanketing entire cities.
On Tuesday, while countries were meeting to discuss climate change in Paris, Beijing and other northern cities recorded some of the worst smog of the year.
In September, China announced a version of the cap-and-trade system as a way to reduce greenhouse gas emissions.
This month's Paris summit is intended to bring countries together in a global deal to curb emissions and limit temperature rises as a result of climate change.