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China has been experiencing severe ozone pollution problems in recent years. While a number of studies have focused on the ozone-pollution-prone regions such as the North China Plain, Yangtze River Delta, and Pearl River Delta regions, few studies have investigated the mechanisms modulating the interannual variability of ozone concentrations in Shandong Province, where a large population is located and is often subject to ozone pollution. By utilizing both the reanalysis dataset and regional numerical model (WRF-CMAQ), we delve into the potential governing mechanisms of ozone pollution in Shandong Province—especially over the major port city of Qingdao—during summer 2014–2019. During this period, ozone pollution in Qingdao exceeded the tier II standard of the Chinese National Ambient Air Quality (GB 3095-2012) for 75 days. From the perspective of meteorology, the high-pressure ridge over Baikal Lake and to its northeast, which leads to a relatively low humidity and sufficient sunlight, is the most critical weather system inducing high-ozone events in Qingdao. In terms of emissions, biogenic emissions contribute to ozone enhancement close to 10 ppb in the west and north of Shandong Province. Numerical experiments show that the local impact of biogenic emissions on ozone production in Shandong Province is relatively small, whereas biogenic emissions on the southern flank of Shandong Province enhance ozone production and further transport northeastward, resulting in an increase in ozone concentrations over Shandong Province. For the port city of Qingdao, ship emissions increase ozone concentrations when sea breezes (easterlies) prevail over Qingdao, with the 95th percentile reaching 8.7 ppb. The findings in this study have important implications for future ozone pollution in Shandong Province, as well as the northern and coastal areas in China. 

Abstract. In the summer of 2017, heavy ozone pollution swamped most of the North ChinaPlain (NCP), with the maximum regional average of daily maximum 8 h ozoneconcentration (MDA8) reaching almost 120 ppbv. In light of the continuingreduction of anthropogenic emissions in China, the underlying mechanisms forthe occurrences of these regional extreme ozone episodes are elucidated fromtwo perspectives: meteorology and biogenic emissions. The significantpositive correlation between MDA8 ozone and temperature, which is amplifiedduring heat waves concomitant with stagnant air and no precipitation,supports the crucial role of meteorology in driving high ozoneconcentrations. We also find that biogenic emissions are enhanced due tofactors previously not considered. During the heavy ozone pollution episodesin June 2017, biogenic emissions driven by high vapor pressure deficit(VPD), land cover change and urban landscape yield an extra mean MDA8 ozoneof 3.08, 2.79 and 4.74 ppbv, respectively, over the NCP, which togethercontribute as much to MDA8 ozone as biogenic emissions simulated using theland cover of 2003 and ignoring VPD and urban landscape. In Beijing, thebiogenic emission increase due to urban landscape has a comparable effect onMDA8 ozone to the combined effect of high VPD and land cover change between2003 and 2016. In light of the large effect of urban landscape on biogenicemission and the subsequent ozone formation, the types of trees may becautiously selected to take into account of the biogenic volatile organic compound (BVOC) emission during the afforestation of cities. This study highlights the vital contributions ofheat waves, land cover change and urbanization to the occurrence of extremeozone episodes, with significant implications for ozone pollution control ina future when heat wave frequency and intensity are projected to increaseunder global warming.