While for east YRD, O 3 pollution was most serious in summer, when its monthly MDA8 O 3 reached the highest value (141.0 µg/m 3). The regional O 3 pollution mainly occurred in spring, with higher peaks of MDA8 O 3 being observed during April ~ May for most subregions. The spatial disparity of O 3 over YRD was strongly affected by local emissions, with larger precursor emissions contributing to higher frequencies of O 3 pollution and larger amplitudes of temporal variations. The central and east YRD were the most polluted subregions with higher incidence of O 3 episodes, while the southwest and southeast YRD were relatively clean. The exceedances over the Grade I (100 µg/m 3) and Grade II (160 µg/m 3) National Ambient Air Quality Standard of maximum daily average 8 h O 3 (MDA8 O 3) varied within 99 ~ 192 and 1 ~ 41 days, respectively, for different subregions. Eight spatial regions with distinct patterns of O 3 variations were identified. In order to better understand the tempo-spatial characteristics of surface ozone (O 3) on the regional scale of Yangtze River Delta (YRD), China, cluster analysis was applied to surface O 3 monitored from 2016 to 2017 at 110 state-controlled stations.
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