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The COVID-19 outbreak greatly limited human activities and reduced primaryemissions particularly from urban on-road vehicles, but coincided with Beijingexperiencing pandemic haze, raising the public concerns of the validity andeffectiveness of the imposed traffic policies to improve the air pollution.Here, we explored the relationship between local vehicle emissions and thewinter haze in Beijing before and during the COVID-19 lockdown period based onan integrated analysis framework, which combines a real-time on-road emissioninventory, in-situ air quality observations and a localized chemical transportmodeling system. We found that traffic emissions decreased substantiallyaffected by the pandemic, with a higher reduction for NOx (75.9%, 125.3 Mg/day)compared to VOCs (53.1%, 52.9 Mg/day). Unexpectedly, our results show that theimbalanced emission abatement of NOx and VOCs from vehicles led to asignificant rise of the atmospheric oxidizing capacity in urban areas, but onlyresulting in modest increases in secondary aerosols due to the inadequateprecursors. However, the enhanced oxidizing capacity in the surrounding regionsgreatly increased the secondary particles with relatively abundant precursors,which is mainly responsible for Beijing haze during the lockdown period. Ourresults indicate that the winter haze in Beijing was insensitive to the localvehicular emissions reduction due to the complicated nonlinear response of thefine particle and air pollutant emissions. We suggest mitigation policiesshould focus on accelerating VOC and NH3 emissions reduction and synchronouslycontrolling regional sources to release the benefits on local traffic emissioncontrol.

Significant reduced traffic in Beijing failed to relieve haze pollution during the COVID-19 lockdown: implications for haze mitigation