Surface ozone is a severe air pollution problem in the North China Plain, which is home to 300 million people. Ozone concentrations are highest in summer, driven by fast photochemical production of hydrogen oxide radicals (HO_x) that can overcome the radical titration caused by high emissions of nitrogen oxides (NO_x) from fuel combustion. Ozone has been very low during winter haze (particulate) pollution episodes. However, the abrupt decrease of NO_x emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with maximum daily 8-h average (MDA8) ozone concentrations of 60 to 70 parts per billion. We reproduce this switch with the GEOS-Chem model, where the fast production of ozone is driven by HO_x radicals from photolysis of formaldehyde, overcoming radical titration from the decreased NO_x emissions. Formaldehyde is produced by oxidation of reactive volatile organic compounds (VOCs), which have very high emissions in the North China Plain. This remarkable switch to an ozone-producing regime in January–February following the lockdown illustrates a more general tendency from 2013 to 2019 of increasing winter–spring ozone in the North China Plain and increasing association of high ozone with winter haze events, as pollution control efforts have targeted NO_x emissions (30% decrease) while VOC emissions have remained constant. Decreasing VOC emissions would avoid further spreading of severe ozone pollution events into the winter–spring season.

在拥有3亿人口的华北平原，地表臭氧是一个严重的空气污染问题。夏季，臭氧浓度最高，这是由于快速光化学产生的氢氧自由基（HO _x）可以克服因燃料燃烧产生的大量氮氧化物（NO _x）排放而引起的自由基滴定。在冬季雾霾（颗粒）污染事件中，臭氧含量非常低。但是，NO _x突然减少2020年1月COVID-19锁定后的排放表明，在冬季霾天气期间，臭氧生产已迅速转换，每日平均8小时平均（MDA8）臭氧浓度最高为十亿分之60至70。我们使用GEOS-Chem模型重现了此开关，其中臭氧的快速产生是由甲醛的光解产生的HO _x自由基驱动的，克服了NO _x降低引起的自由基滴定的作用排放。甲醛是通过氧化反应性挥发性有机化合物（VOC）产生的，而挥发性有机化合物在华北平原的排放量很高。在封锁之后，在1月至2月这种向臭氧产生机制的显着转变表明，从2013年到2019年，华北平原冬春季的臭氧含量增加，而高臭氧含量与冬季霾事件之间的联系越来越密切，这是一种更普遍的趋势，这是控制污染的一种趋势。努力有针对性NO _X排放物（30％降低），而VOC排放量保持不变。减少VOC排放将避免严重的臭氧污染事件进一步传播到冬季到春季。