Abstract. Landfall typhoons can significantly affect O3 in the Yangtze
River Delta (YRD) region. In this study, we investigate a unique case
characterized by two multiday regional O3 pollution episodes related to
four successive landfall typhoons in the summer of 2018 in the YRD. The
results show that O3 pollution episodes mainly occurred during the
period from the end of a typhoon to the arrival of the next typhoon. The
time when a typhoon reached the 24 h warning line and the time when the
typhoon dies away in mainland China can be roughly regarded as time
nodes. Meanwhile, the variations of O3 were related to the track,
duration and landing intensity of the typhoons. The impact of typhoons on
O3 was like a wave superimposed on the background of high O3
concentration in the YRD in summer. When a typhoon was near the 24 h warning
line before it landed on the coastline of the YRD, the prevailing wind
originally from the ocean changed to be from inland, and it transported lots
of precursors from the polluted areas to the YRD. Under influences of the
typhoon, the low temperature, strong upward airflows, more precipitation and
wild wind hindered occurrences of high O3 episodes. After the passing
of the typhoon, the air below the 700 hPa atmospheric layer was warm and
dry, and the downward airflows resumed. The low troposphere was filed with
high concentration of O3 due to O3-rich air transported from the
low stratosphere and strong photochemical reactions. It is noteworthy that
O3 was mainly generated in the middle of the boundary layer
(∼ 1000 m) instead of at the surface. High O3 levels remained in
the residual layer at night, and would be transported to the surface by
downward airflows or turbulence by the second day. Moreover, O3 can be
accumulated and trapped on the ground due to the poor diffusion conditions
because the vertical diffusion and horizontal diffusion were suppressed by
downward airflows and light wind, respectively. The premature deaths
attributed to O3 exposure in the YRD during the study period were 194.0,
more than the casualties caused directly by the typhoons. This work has
enhanced our understanding of how landfall typhoons affect O3 in the
YRD and thus can be useful in forecasting O3 pollution in regions
strongly influenced by typhoon activities.
Remote impacts from the tropical Indian Ocean on haze pollution in January over the Yangtze River Delta
