When the Chinese government imposed a lockdown in Hubei province to combat the spread of COVID-19, scientists saw something unique amid the beginnings of a global pandemic: They saw an opportunity.

The lockdown brought people indoors, suspended or otherwise limited transportation, and curbed overall economic activity in ways never seen before. In other words, as people stayed home and traveled less, the lockdown created a novel situation for studying changes in pollutant emissions and, in turn, the potential short-term impacts on air quality and human health.

To study these changes, scientists turned to satellite measurements collected by various NASA instruments, including the Ozone Monitoring Instrument (OMI) and the Microwave Limb Sounder (MLS). They looked at measurements of ozone, carbon monoxide (CO), nitrogen dioxide (NO2), nitric acid (HNO3), and sulfur dioxide (SO2) collected across all Chinese provinces over a 16-year period leading up to 2020, as well as surface observations during the lockdowns, and they assimilated the data into a chemical transport model.

The results were dramatic: Nitrogen oxide (NOX) emissions dropped by 36% between early January and mid-February in 2020, with more than 80% of reductions occurring after the respective lockdown in most provinces. Emissions typically drop during this timeframe, an annual occurrence due to the Chinese New Year, but the emission decline in 2020 was steep—about two times larger than the average from the same timeframe in 2005–2019. Timing matters: Decreases in emissions generally lead to decreases in ozone, except during winter. The emission declines in early 2020 actually spurred increases in ozone, by up to 16 ppb over northern China. In addition, particulate matter at 2.5 micron (PM2.5) was reduced by up to 23 µgm-3 nationwide. (Ozone and PM2.5 are the primary culprits when it comes to air pollution.) Scientists found that significant short-term impacts to human health came with these swings: Changes in human exposure were associated with about 2,100 more ozone-related and at least 60,000 fewer PM2.5-related morbidity incidences, primarily from asthma cases. These changes augmented efforts to reduce hospital admissions and alleviate negative impacts from potentially delayed treatments.

These findings illustrate unprecedented changes in pollutant emissions, and they show that reducing emissions can benefit human health—all critical information for air quality planning and policy-making.

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Citation: Miyazaki, K., Bowman, K., Sekiya, T., Jiang, Z., Chen, X., Eskes, H., et al. (2020). Air quality response in China linked to the 2019 novel coronavirus (COVID‐19) lockdown. Geophysical Research Letters, 47, e2020GL089252. https://doi.org/10.1029/2020GL089252