Using eddy-covariance data from the Beijing 325-m meteorological tower(39°58′N,116°22′E),we compare urban boundary-layer turbulence characteristics between cleanand hazy episodes in Beijing(1–31 January,2013).Differences are demonstrated in theatmospheric stability,heat/moisture fluxes,and turbulent mixing intensity.From an atmosphericphysics perspective,wintertime haze pollution owes directly to weakened mechanical removal ofpollutants in calm weather.Thermally unstable(daytime)and stable(nighttime)stratifications inhazy episodes likely yield,respectively,an increased reaction rate of precursor chemicals and areduced capacity of turbulent dispersal.The findings presented are instrumental in promotingmechanistic understanding of the occurrence of urban haze pollution.