The first Covid-19 lockdowns led to significant changes in urban air pollution levels in major cities around the world but the changes were smaller than expected, according to a new study led by researchers at Birmingham University in the UK.
After developing new corrections for the impact of weather and seasonal trends, such as reduced NO2 emissions from winter to summer, the researchers evaluated changes in ambient NO2 , O3 and fine particle (PM2.5) concentrations arising from lockdown emission changes in 11 global cities: Beijing, Wuhan, Milan, Rome, Madrid, London, Paris, Berlin, New York, Los Angeles and Delhi.
The researchers used machine learning to strip out weather impacts and seasonal trends before analyzing the data, which consisted of site-specific hourly concentrations of key pollutants from December 2015 to May 2020.
The team of scientists discovered that the beneficial reductions in NO2 due to the lockdowns were smaller than expected, after removing the effects of weather. In parallel, the lockdowns caused (weather-corrected) concentrations of ozone in cities to increase.
Publishing their findings in Science Advances, the research team also found that concentrations of PM2.5, which can worsen medical conditions such as asthma and heart disease, decreased in all cities studied except London and Paris.
Lead-author Zongbo Shi, Professor of Atmospheric Biogeochemistry at the University of Birmingham, commented, “Rapid, unprecedented reduction in economic activity provided a unique opportunity to study the impact of interventions on air quality. Emission changes associated with the early lockdown restrictions led to abrupt changes in air pollutant levels but their impacts on air quality were more complex than we thought, and smaller than we expected.
“Weather changes can mask changes in emissions on air quality. Importantly, our study has provided a new framework for assessing air pollution interventions, by separating the effects of weather and season from the effects of emission changes.”
William Bloss, Professor of Atmospheric Sciences and a co-author on the paper, added, “We found increases in ozone levels due to lockdown in all the cities studied. This is what we expect from the air chemistry, but this will counteract at least some of the health benefit from NO2 reductions. The changes in PM2.5 differ from city to city. Future mitigation measures require a systematic air pollution control approach towards NO2 , O3 and PM2.5 which is tailored for specific cities, to maximize the overall benefits of air quality changes to human health.”