Opposite impact of emission reduction during the COVID-19 lockdown period on the surface concentrations of PM 2.5 and O 3 in Wuhan, China.

To prevent the spread of the COVID-19 epidemic, the Chinese megacity Wuhan has taken emergent lockdown measures starting on January 23, 2020. This provided a natural experiment to investigate the response of air quality to such emission reductions. Here, we decoupled the influence of meteorological and non-meteorological factors on main air pollutants using generalized additive models (GAMs), driven by data from the China National Environmental Monitoring Center (CNEMC) network. During the lockdown period (Jan. 23 - Apr. 8, 2020), PM _2.5 , PM ₁₀ , NO ₂ , SO ₂ , and CO concentrations decreased significantly by 45 %, 49 %, 56 %, 39 %, and 18 % compared with the corresponding period in 2015-2019, with contributions by S(meteos) of 15 %, 17 %, 13 %, 10 %, and 6 %. This indicates an emission reduction of NO _x at least 43 %. However, O ₃ increased by 43 % with a contribution by S(meteos) of 6 %. In spite of the reduced volatile organic compound (VOC) emissions by 30 % during the strict lockdown period (Jan. 23 - Feb. 14, 2020), which likely reduced the production of O ₃ , O ₃ concentrations increased due to a weakening of the titration effect of NO. Our results suggest that conventional emission reduction (NO _x reduction only) measures may not be sufficient to reduce (or even lead to an increase of) surface O ₃ concentrations, even if reaching the limit, and VOC-specific measures should also be taken.
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