Despite extensive efforts into the characterization of air pollution during the lastdecade,real-time characterization of aerosol particle composition above the urban canopy inmegacity Beijing has never been performed to date.Here we conducted the first simultaneousreal-time measurements of aerosol composition at two different heights at the same location inurban Beijing from 19 December 2013 to 2 January 2014.The non-refractory submicron aerosol(NR-PM1)species were measured in-situ by a High-Resolution Aerosol Mass Spectrometer atnear-ground level and an Aerosol Chemical Speciation Monitor at 260 m on the Beijing 325 mmeteorological tower.Secondary aerosol showed similar temporal variations between ground leveland 260 m,whereas much weaker correlations were found for primary aerosol.The diurnalevolution of the ratios and correlations of aerosol species between 260 m and the ground levelfurther illustrated a complex interaction between vertical mixing processes and local sourceemissions on aerosol chemistry in the atmospheric boundary layer.As a result,the aerosolcomposition at the two heights was substantially different.Organic aerosol(OA),mainlycomposed of primary OA(62%),at the ground level showed a higher contribution to NR-PM1(65%)than at 260 m(54%),whereas a higher concentration and contribution(15%)of nitrate wasobserved at 260 m,probably due to the favorable gas–particle partitioning under lowertemperature conditions.In addition,two different boundary layer structures were observed,eachinteracting differently with the evolution processes of aerosol chemistry.