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Volatile organic compounds (VOCs) as precursors of ozone and secondary organic aerosols can cause adverse effects on the environment and human health.However,knowledge of the VOC vertical profile in the lower troposphere of major Chinese cities is poorly understood.In this study,tethered balloon flights were conducted over the juncture of Beijing-Tianjin-Hebei in China during the winter of 2016.Thirty-six vertical air samples were collected on selected heavy and light pollution days at altitudes of 50-1000 meters above ground level.On average,the concentration of total VOCs (TVOCs) at 50-100 m was 4.9 times higher than at 900-1000 m (46.9 ppbV vs.8.0 ppbV).TVOC concentrations changed rapidly from altitudes of 50-100 to 401-500 m,with an average decrease of 72％.With further altitude increase,the TVOC concentration gradually decreased.The xylene/benzene ratios of 34/36 air samples were lower than 1.1,and the benzene/toluene ratios of 34/36 samples were higher than 0.4,indicating the occurrence of aged air mass during the sampling period.Alkenes contributed most in terms of both OH loss rate (39％-71％) and ozone formation potential (40％-72％),followed by aromatics (6％-38％).Finally,the main factors affecting the vertical distributions of VOCs were local source emission and negative dispersion conditions on polluted days.These data could advance our scientific understanding of VOC vertical distribution.