Air pollution in cities fell less than expected during the corona-related restrictions in spring 2020. According to a new analysis, the weather and the usual changes in the transition from winter to spring had a marked effect on the levels of nitrogen dioxide (NO 2 ), ozone (O 3 ) and particulate matter (PM 2.5 ). Researchers working with Zongbo Shi from the University of Birmingham (United Kingdom) published their results in the journal Science Advances .

“Weather changes can overlay changes in emissions related to air quality,” Shi said. That is why his team used machine learning to calculate the weather impact from the air quality measurement data. The computer model learned from measurement data and associated weather data from the years 2016 to 2019 to recognize the effects caused by the weather and to adjust the results accordingly, as can be seen from the research results.

The scientists applied this to the series of air measurements before and after the start of a lockdown in eleven cities. You therefore did not compare the data of 2020 with the previous year’s periods, but the values ​​before the lockdown with those during it. The cities were in Asia (Beijing, Wuhan, Delhi), Europe (London, Berlin, Milan, Rome, Paris, Madrid) and North America (New York, Los Angeles).

NO 2 – decline less high Furthermore, the researchers calculated the Influence of seasonal changes. This means that in spring there are fewer and fewer pollutants and dirt particles from heating, but more pollen, spores and other organic substances are on the move. After taking these influencing factors into account, the decrease in NO 2 during the restrictions was lower than in previous studies.

This was reported that the NO 2 value in Wuhan (China) during the lockdown at 93 Percent has decreased. Taking into account the weather, however, the decrease was only 33, 9 percent and taking into account the seasonal changes only 33, 9 percent. In Berlin there was a decrease in the NO 2 value by 28 .1 percent has been measured. Excluding the weather influence, however, the value only fell by 25, 4 percent, after taking seasonal changes into account 11, 3 percent.

Weather influence on ozone values ​​ On the other hand, according to the new calculations, the increase in ozone values ​​due to the lockdown is no longer as high as previously indicated . For example, an increase in ozone levels of 43, 8 percent was measured for Berlin during the lockdown. If you factor out the weather influence, this increase is only 28, 9 percent, if you factor out the seasonal influence only 2, 6 percent. The higher ozone levels in Berlin during the lockdown in spring 2019 were mainly due to the weather and the change of seasons.

Because reactions with NO 2 cause a breakdown of ozone, lower NO 2 values ​​in the course of the lockdown also lead to higher ones Ozone levels. Solar radiation causes the formation of ozone, which is why the ozone values ​​increase as the days get longer in spring anyway.

Low impact on air quality In the data on particulate matter (PM 2.5 ), the researchers found no clear trend, but rather clear differences from city to city. In London and Paris, particulate matter even increased after the start of the lockdown. Future mitigation measures required a systematic approach to air pollution control in terms of NO 2 , O 3 and particulate matter PM 2, 5 , which is tailored to individual cities, emphasize the researchers. “Changes in emissions related to the limitations of the early lockdowns led to abrupt changes in air pollution levels, but their impact on air quality was more complex than expected and less than expected,” Shi summarizes the study.

(olb)