Spatial Regionalization on Surface Ozone in the Yangtze River Delta of China

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. Eight spatial regions with distinct patterns of O 3 variations were identified. 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. 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 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 regional O 3 pollution mainly occurred in spring, with higher peaks of MDA8 O 3 being observed during April ~ May for most subregions. 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 ). During the whole study period, the spatial differences of weekly/monthly amplitudes of MDA8 O 3 were similar to those of NO 2 among the eight subregions, indicating the significant influence of NO 2 on O 3 over YRD on the yearly timescale. This influence was especially evident in warm seasons for the coastal subregions, where O 3 production was controlled by NO x . While for the west inland subregion, O 3 was less affected by NO 2 variation, indicating a NO x -saturated characteristic of O 3 formation.