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O3 and its precursors(NOx,CO,SO2)were measured at six heights of a 40 mtower at Taehwa Research Forest near Seoul Metropolitan Areas during 2011~2014.Volatileorganic compounds(VOCs)were measured using PTR–QMS(proton transferreaction–quadrupole mass spectrometry).The diurnal variations of O3 showed a peak at 3 PM.NOx was increased at 8:00 and 20:00,which was transported from nearby highways.Concentrations of O3 and other pollutants werehigher over the canopy(> 20 m)than below the canopy(< 20 m).The monthly variation of O3showed the maximum concentration in June along with the highest concentrations of isoprene andmonoterpenes below the canopy.These measurements for the four years highlight that landecosystem played role not only in O3 formation but also O3 loss through reactions of biogenicVOCs(BVOCs)such as isoprene and monoterpenes emitted from trees being mixed with NOxemitted from nearby urban areas.