Greenpeace East Asia says average concentrations of air particulates in 189 Chinese cities fell by 10 percent in 2015, a sign of decreasing pollution overall even as catastrophic levels of smog this winter in northern China effectively shut down schools and roads.

Overall pollution levels in Beijing were down for the second consecutive year but skyrocketed in December, when a thick gray soup enveloping the capital prompted the government to issue a first-ever "red alert" warning, limiting automobile use and closing schools.

The Greenpeace study released Wednesday found the northern Chinese region surrounding Beijing has seen concentrations of microscopic PM2.5 particles drop by a quarter since 2013.

Still, 80 percent of Chinese cities did not meet national air quality standards, the group found.