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BERLIN: Scientists have found that the levels of two major air pollutants drastically reduced globally since lockdowns began in response to the Covid-19 pandemic, but a secondary pollutant -- ground-level ozone -- has increased in China. Two new studies, published in the journal Geophysical Research Letters, found that nitrogen dioxide pollution over northern China, Western Europe and the US decreased by as much as 60 per cent in early 2020 as compared to the same time last year. One of the studies also found that particulate matter pollution -- particles smaller than 2.5 microns -- decreased by 35 per cent in northern China. Such a significant drop in emissions is unprecedented since air quality monitoring from satellites began in the 1990s, said Jenny Stavrakou, an atmospheric scientist at the Royal Belgian Institute for Space Aeronomy in Brussels, and co-author of one of the research papers. The researchers noted that improvements in air quality will likely be temporary, but the findings provide a glimpse into what air quality could be like in the future as emissions regulations become more stringent. "Maybe this unintended experiment could be used to understand better the emission regulations. It is some positive news among a very tragic situation," Stavrakou said. One of the studies, however, found that the drop in nitrogen dioxide pollution has caused an increase in surface ozone levels in China. Although air quality has largely improved in many regions, surface ozone can still be a problem, according to Guy Brasseur, an atmospheric scientist at the Max Planck Institute for Meteorology in Germany, and lead author of one of the new studies. "It means that by just reducing the (nitrogen dioxide) and the particles, you won't solve the ozone problem," Brasseur said. Stavrakou and her colleagues used satellite measurements of air quality to estimate the changes in nitrogen dioxide pollution over the major epicentres of the outbreak: China, South Korea, Italy, Spain, France, Germany, Iran and the US. They found that nitrogen dioxide pollution decreased by an average of 40 per cent over Chinese cities and by 20 to 38 per cent over Western Europe and the US during the 2020 lockdown, as compared to the same time in 2019. However, the study found nitrogen dioxide pollution did not decrease over Iran, one of the earliest and hardest-hit countries. The researchers suspect this is because complete lockdowns weren't in place until late March and before that, stay-at-home orders were largely ignored. The second study looked at air quality changes in northern China where the virus was first reported and where lockdowns have been most strict. Brasseur analysed levels of nitrogen dioxide and several other types of air pollution measured by 800 ground-level air quality monitoring stations in northern China. The researchers found particulate matter pollution decreased by an average of 35 per cent and nitrogen dioxide decreased by an average of 60 per cent after the lockdowns began on January 23. However, they found the average surface ozone concentration increased by a factor of 1.5-2 over the same time period.