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Photochemical aging of volatile organic compounds(VOCs)in the atmosphere is an impor-tant source of secondary organic aerosol(SOA).To evaluate the formation potential of SOA at an urban site in Lyon(France),an outdoor experiment using a Potential Aerosol Mass(PAM)oxidation flow reactor(OFR)was conducted throughout entire days during January-February 2017.Diurnal variation of SOA formations and their correlation with OH radical exposure(OHexp),ambient pollutants(VOCs and particulate matters,PM),Relative Humid-ity(RH),and temperature were explored in this study.Ambient urban air was exposed to high concentration of OH radicals with OHexp in range of(0.2-1.2)x 1012 molecule/(cm3·sec),corresponding to several days to weeks of equivalent atmospheric photochemical aging.The results informed that urban air at Lyon has high potency to contribute to SOA,and these SOA productions were favored from OH radical photochemical oxidation rather than via ozonolysis.Maximum SOA formation(36 μg/m3)was obtained at OHexp of about 7.4 x 1011molecule/(cm3·sec),equivalent to approximately 5 days of atmospheric oxidation.The correlation between SOA formation and ambient environment conditions(RH&temper-ature,VOCs and PM)was observed.It was the first time to estimate SOA formation potential from ambient air over a long period in urban environment of Lyon.