Air pollution is the greatest single environmental health risk worldwide. According to the World Health Organization, ambient air pollution prematurely kills 7 million people per year , with an estimated economic cost of ~ $3 trillion USD, or 3.3% of global GDP . Among air pollutants, fine particulate matter (PM2.5) has the largest health impact in monetized terms: more than half of the global population is exposed to annual-average ambient concentrations exceeding the first interim target from the World Health Organization, 35 μg/m 3 . Efforts to reduce PM2.5 concentrations have not been uniformly successful .