Abstract. In the summer of 2017, heavy ozone pollution swamped most of the North China
Plain (NCP), with the maximum regional average of daily maximum 8 h ozone
concentration (MDA8) reaching almost 120 ppbv. In light of the continuing
reduction of anthropogenic emissions in China, the underlying mechanisms for
the occurrences of these regional extreme ozone episodes are elucidated from
two perspectives: meteorology and biogenic emissions. The significant
positive correlation between MDA8 ozone and temperature, which is amplified
during heat waves concomitant with stagnant air and no precipitation,
supports the crucial role of meteorology in driving high ozone
concentrations. We also find that biogenic emissions are enhanced due to
factors previously not considered. During the heavy ozone pollution episodes
in June 2017, biogenic emissions driven by high vapor pressure deficit
(VPD), land cover change and urban landscape yield an extra mean MDA8 ozone
of 3.08, 2.79 and 4.74 ppbv, respectively, over the NCP, which together
contribute as much to MDA8 ozone as biogenic emissions simulated using the
land cover of 2003 and ignoring VPD and urban landscape. In Beijing, the
biogenic emission increase due to urban landscape has a comparable effect on
MDA8 ozone to the combined effect of high VPD and land cover change between
2003 and 2016. In light of the large effect of urban landscape on biogenic
emission and the subsequent ozone formation, the types of trees may be
cautiously selected to take into account of the biogenic volatile organic compound (BVOC) emission during the afforestation of cities. This study highlights the vital contributions of
heat waves, land cover change and urbanization to the occurrence of extreme
ozone episodes, with significant implications for ozone pollution control in
a future when heat wave frequency and intensity are projected to increase
under global warming.
Typhoon-boosted biogenic emission aggravates cross-regional ozone pollution in China
