Extreme haze episodes shrouded Beijing during the winter of 2013, causing major environmental and health problems. We show that the severe winter haze was driven by stable synoptic meteorological conditions rather than by an abrupt change of emissions; the fast build-up of PM2.5
in Beijing was mainly controlled by the atmospheric transport; and the production of secondary aerosols is enhanced during the haze periods. This enhancement cannot be explained by the weakened photochemistry suggesting a missing source of PM2.5
, which is likely the heterogeneous reaction.