The topic of the pandemic has recently been practically removed from the agenda. However, lockdown introduced in most major cities of the world provided a unique opportunity to assess the consequences of reducing anthropogenic emissions affecting the environment. The scientists at Moscow State University together with foreign colleagues for the first time studied changes in the physicochemical properties, ionic composition, and concentrations of potentially toxic elements in Moscow's atmospheric precipitation in the spring and summer of 2020 – during the period of restrictive measures to combat the spread of COVID-19.
Already published scientific papers have confirmed a decrease in the level of dustiness of the air and its gas pollution, more frequent meetings with songbirds, a reduction in noise and light pollution (especially in city centers), as well as other positive results of lockdowns. On the other hand, many studies have shown an increased environmental impact due to more intensive incineration of household waste in the private sector, accelerated ozone formation, rapid growth in the mass of medical waste and used personal hygiene products, etc. However, it was still unclear whether the chemical composition of precipitation in Russian cities changed during the pandemic and how big these changes were. The answers to these questions are important for understanding the contribution of precipitation to the purification of the atmosphere from gaseous pollutants and solid particles while reducing anthropogenic impact.
Москва-Сити. Фото: Яна Буракова, участница конкурса РГО "Самая красивая страна"
The paper, published in the journal Urban Climate, for the first time examines the content of major ions and potentially toxic elements (PTEs) in the precipitation of the world's highest-latitude metropolis – Moscow – before, during, and after the lockdown in the winter and summer of 2020. Two phases of precipitation were studied: undissolved (suspended), represented by solid particles (with a diameter larger than 0.45 microns) that remain on the filter when filtering a sample, as well as dissolved (liquid) passing through the filter.
The undissolved phase enters precipitation mainly when atmospheric particles suspended in the cloud layer are scavenged. The chemical composition of the liquid phase is a good indicator of contamination of precipitation during its formation inside clouds (the so-called in-cloud scavenging).
“Our study showed the ambiguity of the lockdown effect on the chemical composition of precipitation in Moscow. On the one hand, the level of precipitation pollution has decreased as a result of a decrease in anthropogenic activity, primarily due to a sharp drop in traffic intensity on highways. On the other hand, in some periods there was an increase in pollution of precipitation with certain toxic elements," said Dmitrii Vlasov, one of the authors of the study, senior researcher at the Department of Landscape Geochemistry and Soil Geography of the Faculty of Geography, Moscow State University.
The decrease in precipitation pollution with potentially toxic elements (by 10-99% for the dissolved ones and by 9-61% for the undissolved PTEs) is due not only to a decrease in anthropogenic emissions, but also to the purification of the atmosphere from aerosols during their prolonged scavenging by precipitation, since the end of May and June 2020 were among the rainiest in 200 years of observations in Moscow. At the same time, the movement of air into the capital from the suburbs, where wood, coal, household and agricultural waste were burned during the lockdown in April, contributed to an increase in precipitation pollution with insoluble P, Pb, Cd, soluble P, Ag, Pb, Sb, As, Cd, as well as chlorine and potassium ions.
For the first time in Moscow, the contribution of various sources to the chemical composition of atmospheric precipitation has also been determined. The lifting of restrictive measures reduced the contribution of industrial sources to the content of soluble potentially hazardous elements from 38-66% to 6%. This was due to an increase in the contribution of road dust and non-exhaust emissions of transport during the abrasive wear of brakes and tires, as well as soil particles resuspension, the burning of waste and fuel, vehicle emissions.
Главное здание МГУ в Москве. Фото: Павел Водопьянов, участник конкурса РГО "Самая красивая страна"
“The results revealed the need for further studies of the chemical composition and properties of precipitation in the city in similar periods for the correct separation of the influence of social, economic, emission, meteorological, and physico-chemical factors on the content and ratio of forms of PTEs in the rains,” said Dmitrii Vlasov.
One of the authors of the paper was First Vice-President of the Russian Geographical Society, Chairman of the Scientific and Expert Councils of the Russian Geographical Society, Academician of the Russian Academy of Sciences Nikolay Kasimov. The research was supported by a grant from the Ministry of Education and Science within the framework of the national project "Science and Universities".