The outbreak of coronavirus named COVID-19, initially identified in Wuhan, China in December 2019, has spread rapidly at the global scale. Most countries have rapidly stopped almost all activities including industry, services and transportation of goods and people, thus decreasing air pollution in an unprecedented way, and providing a unique opportunity to study air pollutants. While satellite data have provided visual evidence for the global reduction in air pollution such as nitrogen dioxide (NO 2) worldwide, precise and quantitative information is missing at the local scale. Here we studied changes in particulate matter (PM 2.5 , PM 10), carbon monoxide (CO), NO 2 , sulfur dioxide (SO 2) and ozone (O 3) at 10 urban sites in Hangzhou, a city of 7.03 million inhabitants, and at 1 rural site, before city lockdown, January 1-23, during city lockdown, January 24-February 15, and during resumption, February 16-28, in 2020. Results show that city lockdown induced a sharp decrease in PM 2.5 , PM 10 , CO, and NO 2 concentrations at both urban and rural sites. The NO 2 decrease is explained by reduction in traffic emissions in the urban areas, and by lower regional transport in rural areas during lockdown, as expected. SO 2 concentrations decreased from 6.3 to 5.3 μg m −3 in the city, but increased surprisingly from 4.7 to 5.8 μg m −3 at the rural site: this increase is attributed both to higher coal consumption for heating and emissions from traditional fireworks of the Spring Eve and Lantern Festivals during lockdown. Unexpectedly, O 3 concentrations increased by 145% from 24.6 to 60.6 μg m −3 in the urban area, and from 42.0 to 62.9 μg m −3 in the rural area during the lockdown. This finding is explained by the weakening of chemical titration of O 3 by NO due to reductions of NO x fresh emissions during the non-photochemical reaction period from 20:00 PM to 9:00 AM (local time). During the lockdown, compared to the same period in 2019, the daily average concentrations in the city decreased by 42.7% for PM 2.5 , 47.9% for PM 10 , 28.6% for SO 2 , 22.3% for CO and 58.4% for NO 2 , which is obviously explained by the absence of city activities. Overall, we observed not only the expected reduction in some atmospheric pollutants (PM, SO 2 , CO, NO 2), but also unexpected increases in SO 2 in the rural areas and of ozone (O 3) in both urban and rural areas, the latter being paradoxically due to the reduction in nitrogen oxide levels. In other words, the city lockdown has improved air quality by reducing PM 2.5 , PM 10 , CO, and NO 2 , but has also decreased air quality by augmenting O 3 and SO 2 .
