Environment: Variation in ground-level nitrogen dioxide pollution during COVID-19 lockdowns

Reductions in ground-level nitrogen dioxide (NO₂) concentrations during COVID-19 lockdowns varied considerably by region and emission sector with decreases in mean country-level population weighted NO₂ levels being about a third larger in countries with stricter COVID-19 lockdown measures, compared to those without restrictions. The findings, published in Nature, improve our knowledge of NO₂ exposure estimates and provide an opportunity for advances in air quality health assessment.

NO₂ is an important contributor to air pollution and human exposure is associated with adverse health outcomes, including respiratory infections, asthma and lung cancer. Decreases in atmospheric and ground-level NO₂ have previously been reported as a result of lockdown measures to reduce the spread of COVID-19. However, questions remain about the relationship between atmospheric NO2 levels, measured using satellite data, and human health-relevant ground-level NO₂ concentrations that are often only measured in higher income countries.

Matthew Cooper and colleagues used high-resolution satellite data to derive global ground-level NO₂ concentrations and assess individual cities during COVID-19 lockdowns in 2020 compared to 2019. They then used the satellite measurements to quantify the NO₂ level changes in over 200 cities, including 65 cities without available ground level monitoring, largely in lower income regions. The authors found that decreases in mean country-level population-weighted NO₂ concentrations were about 29% larger in countries with strict lockdown conditions, such as North America and China, than in those without. NO₂ decreases during COVID-19 lockdowns were also found to exceed recent annual mean year-to-year decreases from emission controls, comparable to about 15 years of reductions globally.

The results also suggest that the sensitivity of NO₂ to lockdowns varies by country and emission sector (power plants or transportation, for example), demonstrating the need for high-resolution satellite-derived information of ground level concentration estimates.