Despite the large reduction in anthropogenic activities due to the outbreak of
COVID-19, air quality in China has witnessed little improvement and featured great
regional disparities. Here, by combining observational data and simulations, this work
aims to understand the diverse air quality response in two city clusters, Yangtze River
Delta region (YRD) and Pearl River Delta region (PRD), China. Though there was a
noticeable drop in primary pollutants in both the regions, differently, the maximum
daily 8 h average ozone (O
3
) soared by 20.6–76.8% in YRD but decreased
by 15.5–28.1% in PRD. In YRD, nitrogen oxide (NOx) reductions enhanced
O
3
accumulation and hence increased secondary aerosol formation. Such an
increment in secondary organic and inorganic aerosols under stationary weather reached
up to 36.4 and 10.2%, respectively, which was further intensified by regional transport.
PRD was quite the opposite. The emission reductions benefited PRD air quality, while
regional transport corresponded to an increase of 17.3 and 9.3% in secondary organic and
inorganic aerosols, respectively. Apart from meteorology, the discrepancy in
O
3
–VOCs–NOx relationships determined the different
O
3
responses, indicating that future emission control shall be regionally
specific, instead of one-size-fits-all cut. Overall, the importance of regionally
coordinated and balanced control strategy for multiple pollutants is highly
emphasized.
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