Abstract. Severe ozone (O3) pollution episodes plague a few regions in eastern
China at certain times of the year, e.g., the Yangtze River Delta (YRD). However, the formation
mechanisms, including meteorological factors, contributing to these severe pollution events
remain elusive. A severe summer smog stretched over the YRD region from
22 to 25 August 2016. This event displayed hourly surface O3 concentrations that
exceeded 300 µg m−3 on 25 August in Nanjing, an urban area in the western
YRD. The weather pattern during this period was characterized by near-surface
prevailing easterly winds and continuous high air temperatures. The formation
mechanism responsible for this O3 pollution episode over the YRD region, particularly
the extreme values over the western YRD, was investigated using
observation data and by running simulations with the Weather Research and Forecasting
model with Chemistry (WRF-Chem). The results showed that the extremely high
surface O3 concentration in the western YRD area on 25 August was
largely due to regional O3 transport in the nocturnal residual
layer (RL) and the diurnal change in the atmospheric boundary layer.
On 24 August, high O3 levels, with peak values of
220 µg m−3, occurred in the daytime mixing layer over the eastern YRD region. During
nighttime from 24 to 25 August, a shallow stable boundary layer formed near
the surface which decoupled the RL above it from the surface. Ozone in the
decoupled RL remained quite constant, which resulted in an O3-rich
“reservoir” forming in this layer. This reservoir persisted due to the absence of O3 consumption from nitrogen
oxide (NO) titration or dry deposition during nighttime. The prevailing
easterly winds in the lower troposphere governed the regional transport of
this O3-rich air mass in the nocturnal RL from the eastern to the western YRD. As
the regional O3 transport reached the RL over the western YRD, O3
concentrations in the RL accumulated and rose to 200 µg m−3 over the
western Nanjing site during the sunrise hours on 25 August. The development of
the daytime convective boundary layer after sunrise resulted in the
disappearance of the RL, as the vertical mixing in the convective boundary layer
uniformly redistributed O3 from the upper levels via the
entrainment of O3-rich RL air down to the O3-poor air at the ground.
This net downward transport flux reached up to 35 µg m−3 h−1,
and contributed a considerable surface O3 accumulation, resulting in severe daytime
O3 pollution during the summer smog event on 25 August in the western YRD region.
The mechanism of regional O3 transport through the nocturnal RL revealed
in this study has great implications regarding understanding O3 pollution
and air quality change.
Ozone and related gaseous pollutants in the boundary layer of eastern China: Overview of the recent measurements at a rural site
