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. We conducted the first real-time continuous verticalmeasurements of particle extinction ( bext) , gaseous NO 2 , and blackcarbon (BC) from ground level to 260 m during two severe winter hazeepisodes at an urban site in Beijing, China. Our results illustrated fourdistinct types of vertical profiles: (1) uniform vertical distributions(37 % of the time) with vertical differences less than 5 %, (2) highervalues at lower altitudes (29 %), (3) higher values at higher altitudes(16 %), and (4) significant decreases at the heights of ∼  100–150 m (14 %). Further analysis demonstrated that vertical convection asindicated by mixing layer height, temperature inversion, and local emissionsare three major factors affecting the changes in vertical profiles.Particularly, the formation of type 4 was strongly associated with thestratified layer that was formed due to the interactions of different airmasses and temperature inversions. Aerosol composition was substantiallydifferent below and above the transition heights with ∼  20–30 % higher contributions of local sources (e.g., biomass burning andcooking) at lower altitudes. A more detailed evolution of vertical profilesand their relationship with the changes in source emissions, mixing layerheight, and aerosol chemistry was illustrated by a case study. BC showedoverall similar vertical profiles as those of bext ( R2=0 .92 and0.69 in November and January, respectively). While NO 2 was correlatedwith bext for most of the time, the vertical profiles of bext ∕  NO 2 varied differently for different profiles, indicating theimpact of chemical transformation on vertical profiles. Our results alsoshowed that more comprehensive vertical measurements (e.g., more aerosol andgaseous species) at higher altitudes in the megacities are needed for abetter understanding of the formation mechanisms and evolution of severehaze episodes in China.

Vertically resolved characteristics of air pollution during two severe winter haze episodes in urban Beijing, China