![]() However, during the COVID-19 lockdown, the air quality index (AQI) frequently hit extremely unhealthy levels in Beijing, including January 25–29 and February 9–13, with a peak daily PM 2.5 concentration reaching 218.3 μg/m 3 on February 12, which is more than eight times the World Health Organization’s recommended level of 25 μg/m 3 for 24 h average concentrations. (7) As a result, the PM 2.5 annual concentration in Beijing decreased from 89.5 μg/m 3 in 2013 to 42.0 μg/m 3 in 2019, and heavy pollution days were also reduced from 58 days in 2013 to 4 days in 2019, (8) providing confidence in source–receptor mechanisms supporting pollution control strategies. Following scientific instructions, control measurements were undertaken over the past 5 years to reduce the sources of aerosol pollution. The unexpected heavy pollution cast doubt on the current understanding of the source–receptor relationship in Beijing. (5,6) However, severe air pollution episodes occurred in Beijing during “the most silent spring,” leading to query the response of air pollution to anthropogenic activities. (3,4) Generally, pandemic lockdowns led to clearer skies in China and other places in the world. Because the lockdown policy significantly affected various human activities, its impacts on the anthropogenic emissions and air quality probably exceeded the effects of the well-known “activity-limited” events in the past, such as the US copper smelter strike or the Olympic Games in Beijing. The Spring Festival holiday and the Coronavirus restrictions led to widespread shutdowns and a near-halt in normal life and economic activities in Beijing and its surrounding cities. (2) In addition, a two-week compulsory quarantine was implemented for travelers to Beijing. Soon afterward, the lockdown was applied to the whole country. The stay-at-home orders were initially imposed in Wuhan on January 23, 2020, one day before the eve of the Spring Festival. The Spring Festival migration reduced significantly the population in Beijing, with an estimated 39% decrease from the 22 million residents in normal times, (1) while the Coronavirus pandemic lockdown further reduced human activities. The unexpected COVID-19 pandemic in 2020, which coincided with the Spring Festival, the most important holiday in China, put the Chinese economy into a rapid stall. ![]() ![]() ![]() We recommend that mitigation policies should focus on accelerating VOC emission reduction and synchronously controlling regional sources to release the benefits of local traffic emission control. Moreover, the enhanced oxidizing capacity in the surrounding regions greatly increased the secondary particles with relatively abundant precursors, which was transported into Beijing and mainly responsible for the aggravated haze pollution. We found that traffic emissions decreased substantially during the COVID-19 pandemic, but its imbalanced emission abatement of NO x (76%, 125.3 Mg/day) and volatile organic compounds (VOCs, 53%, 52.9 Mg/day) led to a significant rise of atmospheric oxidants in urban areas, resulting in a modest increase in secondary aerosols due to inadequate precursors, which still offset reduced primary emissions. This paper explores the relationship between local vehicle emissions and the winter haze in Beijing before and during the COVID-19 lockdown based on an integrated analysis framework, which combines a real-time on-road emission inventory, in situ air quality observations, and a localized numerical modeling system. The COVID-19 outbreak greatly limited human activities and reduced primary emissions particularly from urban on-road vehicles but coincided with Beijing experiencing “pandemic haze,” raising the public concerns about the effectiveness of imposed traffic policies to improve the air quality. ![]()
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