Regional ozone (O3) pollution has drawn increasing attention in China over the recent decade,but the contributions from urban pollution and biogenic emissions have not been clearly elucidated.To better understand the formation of the regional O3 problem in the North China Plain (NCP),intensive field measurements of O3 and related parameters were conducted at a rural site downwind ofJi’nan,the capital city of Shandong province,in the summer of 2013.Markedly severe O3 pollution was recorded,with the O3 mixing ratios exceeding the Chinese national ambient air quality standard on 28 days (a frequency of 78％) and with a maximum hourly value of 198 ppbv.Extensive regional transport of well-processed urban plumes to the site was identified.An observation-constrained chemical box model was deployed to evaluate in situ photochemical O3 production on two episodes.The results show that the in situ formation accounted for approximately 46％ of the observed O3 accumulation,while the remainder (-54％) was contributed by regional transport of the O3-laden urban plumes.The in situ ozone production was in a mixed controlled regime that reducing either NOx or VOCs would lead to a reduction of ozone formation.Biogenic VOCs played an importantrole in the local ozone formation.This study demonstrates the significant mixed effects of both anthropogenic pollution from urban zones and biogenic emission in rural areas on the regional O3 pollution in the NCP region,and may have general applicability in facilitating the understanding of the formation of secondary pollution over China.