News Release 

Enhanced secondary haze by emission reduction during COVID-19 lockdown in China

Science China Press


IMAGE: Conceptual model showing how the secondary formation offsets the reduction of primary pollutions in China with different processes during the day and night. The upper and lower panels, indicated by... view more 

Credit: ©Science China Press

As one of the first epicenters of this disease, China has announced the most efficient prevention measures to slow down the spread of the COVID-19 lockdown by restricting the movement of populations nationwide after the Chinese New Year 2020, which caused tremendous economic losses and also a substantial reduction in emission of air pollutant from vehicles and factories. Surprisingly, despite such large decreases in primary pollution, air quality in China was not as clean as expected. Heavy multi-day haze pollution still covered the northern and eastern China for several times, raising questions about the well-established relationship between human activities and air quality and even China's efforts on air pollution control that have been extensively implemented in the past few years.

Focusing on this air pollution issue concerned by the public and policymakers, the study, published in the journal National Science Review (18th June), reveals that a sharp emission reduction in transportation caused more secondary formation of particle by enhanced atmospheric oxidizing capacity in the eastern China, which offset the effort of primary emission reduction.

This study integrated multiple observational datasets, including ambient air quality monitoring network, chemical composition measurements, and satellite retrievals, together with up-to-date emission estimation and dozens of numerical simulation experiments, to comprehensively analyze the non-linear response of air pollution to emission reduction during the COVID-19 lockdown.

Dynamic economic and industrial statistics in the first two months of 2020 were collected to estimate emission reduction due to COVID-19 lockdown. The thermal power generation in January to February was 8.9% lower than that in 2019, while cement production in the first two month was 29.5% lower than that in 2019. Comparatively, on-road transportation has fallen off a cliff during the lockdown, with a decline in national traffic volume of 70%.

Such a large drop of anthropogenic emission led to substantially decreased primary pollution in the atmosphere, particularly nitric oxide (NOx) emitted from vehicles. The nonlinear response of ozone, one important atmospheric oxidant, to NOx increased the atmospheric oxidizing capacity and subsequently accelerated the chemical production of secondary particle. Under unfavorable meteorological conditions, faster oxidation offset the emission reduction and thus gave rise to still-severe haze pollution in the eastern China. Findings also reveal that a synchronous control of volatile organic compounds (VOCs) can serve as an effective way to overcome deteriorating haze pollution with NOx reduction.

The study shows direct and consistent "observational" and modeling evidences on the non-linear relationship of emission reduction and secondary haze pollution in the real world. The unique natural experiment of dramatic emission reduction during China's COVID-19 lockdown indicated that the arbitrary reductions in NOx and other air pollutant emissions led to substantial increases in ozone, which in turn increased atmospheric oxidizing capacity and enhanced formation of secondary particles. In China, winter haze and summer ozone pollution are the two major air quality challenges, but with different and separated control policies. Emissions regulation imposed since 2013 have successfully reduced haze pollution in eastern China, while the study suggests that the benefit of proposed further reductions in primary emissions might be offset by enhanced secondary formation of particle. A non-linear tipping-point of NOx chemistry may bring about more challenge to China's haze mitigation than anticipated. Only with coordinated and collaborative control on various primary emissions can China achieve long-term haze mitigation.


This work was funded by the National Natural Science Foundation of China (41725020, 41922038, 41625020, 91744311, and 41921005), the Ministry of Science and Technology of the People's Republic of China (2016YFC0200500) and Jiangsu Provincial Fund on air pollution mitigation.

See the article: Huang X, Ding A and Gao J et al., 2020. Enhanced secondary pollution offset reduction of primary emissions during COVID-19 lockdown in China. National Science Review,

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