Early data showed decreases in air pollution as cities went into lockdown during the pandemic. After controlling data for weather variables, researchers say three pollutants are worse than previously believed.
(CN) — At the beginning of the pandemic, dramatic air quality improvements seemed like a silver lining during an otherwise bleak moment. Scientists collected promising data: Air pollution was at its lowest levels in years while offices closed, people remained home, and cars stayed off the road.
Despite the early data, however, air quality has not improved significantly in the long term, according to a study published Wednesday in Science Advances.
Researchers examined changes in the concentrations of nitrogen dioxide, ozone and fine particles in New York, London, Beijing and eight other big cities.
Using machine learning, the team of scientists controlled their data based on weather patterns and analyzed hourly concentrations of the three pollutants between December 2015 and May 2020.
The new data overturns the early claims about what the pandemic did to alter air pollution. For example, by April 2020, nitrogen dioxide had dropped by as much as 50% in parts of Europe, the European Space Agency reported at the time.
Nitrogen dioxide can cause respiratory health problems and contribute to the development of asthma. It largely comes from burning fuel in cars, trucks and power plants — so the idea that levels would drop as cities shut down seems logical.
Once researchers accounted for weather patterns that shape emissions, however, reductions in nitrogen dioxide turned out to be smaller than expected. In some areas, the rate was as low as a 10% drop.
“Weather changes can mask changes in emissions on air quality,” said lead study author Zongbo Shi, professor of atmospheric biogeochemistry at the University of Birmingham, in a statement.
Shi and colleagues studied hourly temperature, humidity, atmospheric pressure, wind speed and wind direction in various cities. They also looked at cloud cover, solar radiation and precipitation.
“Importantly,” Shi said, “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.”
Ozone is also harmful to health and hinders the growth of crops. In the new study, researchers found that ozone levels actually increased in all cities in the study, with most concentrations growing between 2% and 30% greater.
Study co-author William Bloss, professor of atmospheric sciences at Birmingham, said that the ozone finding did not come as a surprise.
“This is what we expect from the air chemistry,” he said, “but this will counteract at least some of the health benefit from NO2 reductions.”
The third air pollutant studied, PM2.5, varied from city to city. But the findings offered some good news: concentrations of the fine particulate matter, which can worsen asthma and heart disease, decreased for the most part.
In London and Paris, however, PM2.5 grew to higher concentrations.
Because of the variation across cities, there should be different approaches to curbing air pollution based on location, Bloss said. That way cities can “maximize the overall benefits of air quality changes to human health.”
Across the planet, air pollution is the single biggest risk to human health posed by the environment. It contributes to 6.7 million deaths annually and costs the global economy $3 trillion.
The study included New York, Los Angeles, Beijing, Wuhan, Milan, Rome, Madrid, London, Paris, Berlin and Delhi.
The instant changes brought on by Covid-19 pandemic lockdowns gave researchers a previously impossible opportunity to study air pollutants, say the authors of the new study.
“Rapid, unprecedented reduction in economic activity provided a unique opportunity to study the impact of interventions on air quality,” Shi said.
“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.”