atmosphere Article Mosses as Bioindicators of Heavy Metal Air Pollution in the Lockdown Period Adopted to Cope with the COVID-19 Pandemic Nikita Yushin 1, Omari Chaligava 1,2, Inga Zinicovscaia 1,3,* , Konstantin Vergel 1 and Dmitrii Grozdov 1 1 Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Russia; [email protected] (N.Y.); [email protected] (O.C.); [email protected] (K.V.); [email protected] (D.G.) 2 Faculty of Exact and Natural Science, Ivane Javakhishvili Tbilisi State University, Chavchavadze ave. 3, 0179 Tbilisi, Georgia 3 Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, 30 Reactorului, MG-6 Bucharest-Magurele, Romania * Correspondence: [email protected]; Tel.: +7-496-216-5609 Received: 9 October 2020; Accepted: 1 November 2020; Published: 3 November 2020 Abstract: The coronavirus disease, COVID-19, has had a great negative impact on human health and economies all over the world. To prevent the spread of infection in many countries, including the Russian Federation, public life was restricted. To assess the impact of the taken actions on air quality in the Moscow region, in June 2020, mosses Pleurosium shreberi were collected at 19 sites considered as polluted in the territory of the region based on the results of the previous moss surveys. The content of Cd, Cr, Cu, Fe, Ni, and Pb in the moss samples was determined using atomic absorption spectrometry. The obtained values were compared with the data from the moss survey performed in June 2019 at the same sampling sites. Compared to 2019 data, the Cd content in moss samples decreased by 2–46%, while the iron content increased by 3–127%. The content of Cu, Ni, and Pb in mosses decreased at most sampling sites, except for the eastern part of the Moscow region, where a considerable number of engineering and metal processing plants operate. The stay-at-home order issued in the Moscow region resulted in a reduction of vehicle emissions affecting air quality, while the negative impact of the industrial sector remained at the level of 2019 or even increased. Keywords: COVID-19; air pollution; metals; industry; moss survey; biomonitoring 1. Introduction On March 11, 2020, the World Health Organization declared COVID-19 as a global pandemic [1]. On March 12, based on a decree of the Government of the Moscow region, a self-isolation regime was introduced. According to the official decision, only essential services such as healthcare, logistics, food supply, public transport, and industrial enterprises due to technical reasons did not cease to operate. A significant part of the regular national and international flights and train services was cancelled. Since the majority of the population switched to remote working, the number of vehicles has dropped significantly. New rules adopted in many countries to help slow the spread of COVID-19 resulted in a decrease in the negative impact on the environment in some regions of the world [2]. Nadzir et al. [3] measured the concentrations of CO, PM2.5, and PM10 in Malaysia and observed that the concentration of pollutants declined significantly, by 20 to 60%, during the Control Order in Malaysia (MCO) days at most studied ≈ locations. At the same time, in Kota Damansara, the level of pollutants significantly increased due to local anthropogenic activity. COVID-19 resulted in a worldwide decrease in the concentration of CO2 Atmosphere 2020, 11, 1194; doi:10.3390/atmos11111194 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 1194 2 of 10 in March 2020 by 7% in comparison with the monthly concentration in 2019 [4]. In three Chinese cities (Chongqing, Luzhou, and Chengdu) concentrations of PM2.5, PM10, SO2, CO, and NO2 in February 2020 were lower by 17.9%–62.1% than the values determined in February 2017–2019 [5]. In three other cities in China (Wuhan, Jingmen, and Enshi) concentrations of the abovementioned pollutants measured in February 2020 declined by 27.9% 61.4% compared to February 2017–2019. At the same − time, in the period of January–March 2020, an increase of O3 concentration of up to 15% was noticed [6]. In Turkey, the concentrations of PM10 and NO2 in 2016 and 2020 were compared, and no significant difference in PM10 concentrations was observed. In the after-lockdown period, the NO2 concentrations were lowered by 11.8% [7]. A decrease in the PM2.5 concentration in Wuhan, Daegu, and Tokyo by 29.9%, 20.9%, and 3.6%, respectively, took place after one month of COVID-related restrictions. The concentration of NO2 also declined in all three cities, with the most pronounced decrease being by 53.2% in Wuhan [2]. In Brazil, a remarkable decrease in CO and NO2 concentrations was observed during the lockdown period [8]. PM contains various metals, including some elements recognized as human carcinogens. Sources of metals in the atmosphere can be natural: soil dust, volcano eruptions, and forest fires, or they can be anthropogenic, which include road traffic, industry, and thermal power plants [9]. It is often difficult to conduct measurements of concentrations of atmospheric metals over large territories. Passive moss biomonitoring proved to be a suitable technique to study the spatial distribution of metals over the large territories [10]. This technique is well recognized and widely applied in many European countries [11]. Although data obtained using moss biomonitors do not correspond to the direct quantitative measurement of metal deposition [12], this information is very useful for regions where it is difficult to obtain official data related to air pollution. In the Moscow region, moss survey studies were performed in 2004 [13], 2014 [14], and 2019 (data not yet published). The aim of the present study was to find out whether moss can be used as a tool to assess the impact of the restrictions due to the lockdown imposed to face the pandemic emergency on air quality in a relatively short time (2.5 mounts). For this purpose, the data obtained for moss samples collected in the Moscow region in June 2020 were compared with the data obtained in 2019 for the same collection sites. 2. Experiments 2.1. Study Area and Sampling The Moscow region, which is a subject of the Russian Federation, is one of the most densely populated and industrially developed regions of the country [14]. The Moscow region can be considered the economic core of the country, with about 70% of Russia’s financial wealth concentrated here. It is located in the center of railway and air networks, reaching Transcaucasia, Central Asia, and the Pacific, and it is connected by rivers and canals to the Baltic, White, Azov, Black, and Caspian Seas [15]. In the center of the region is Moscow, the capital of the Russian Federation, and the most important industrial city, transportation hub, educational and cultural center of the region [13,15]. The industrial sector of the Moscow region includes metallurgical, oil refining, mechanical engineering, food, energy, and chemical enterprises [14]. The main industrial centers in the Moscow region are: Krasnogorsk, Lyubertsy, Mytishchi, Klin, Noginsk, Pavlovsky Posad, Voskresensk, Kolomna, Dmitrov, Klin, Elektrostal, Balashikha, and Sergiyev Posad. In 2019 and 2020, moss Pleurosium shreberi was collected on the sheared tree bark. Moss sampling was performed in June 2019 and June 2020 at 19 sampling sites (Figure1) in accordance with [ 16]. The selection of the sampling sites in 2020 was based on the moss survey data obtained in 2019 (analyzed but not yet published). Moss samples were collected at sites with the highest metal concentrations in 2019. After collection, samples were cleaned of soil particles and other contaminants. The upper 3–4 cm of green and green-brown shoots from the top of the moss were separated and dried at 105 ◦C to constant weight. For analysis, approximately 0.2 g of moss was placed in a Teflon vessel and treated Atmosphere 2020, 11, 1194 3 of 10 Atmosphere 2020, 11, x FOR PEER REVIEW 3 of 10 withperoxide. 2 mL The of concentrated Teflon vessels nitric were acid put and into 1 mL a ofmicrowave hydrogen peroxide.digestion Thesystem Teflon (Mars; vessels CEM, were USA) put intofor acomplete microwave digestion. digestion Digestion system (Mars;was perf CEM,оrmed USA) in fortwo complete steps: (1) digestion. ramp: temperature Digestion was 180 performed °C, time 15 in twomin, steps:power (1) 400 ramp: W, and temperature pressure 20 180 bar;◦ C,(2) timehоld: 15 temperature min, power 160 400 °C, W, time and 10 pressure min, power 20 bar; 400 (2) W, hold: and temperaturepressure 20 bar. 160 Digests◦C, time were 10 min,quantitatively power 400 transferred W, and pressure to 100-mL 20 bar.calibrated Digests flasks were and quantitatively made up to transferredthe volume towith 100-mL bidistilled calibrated water. flasks and made up to the volume with bidistilled water. Figure 1. Sampling map (in 2019 and 2020 samples were collected in the same sampling sites).sites). 2.2. Chemical Analysis The content ofof Cd,Cd, Cu,Cu, Pb,Pb, Cr,Cr, Ni, Ni, Fe, Fe, V, V, and and Sb Sb in in the the moss moss samples samples was was determined determined by by means means of atomicof atomic absorption absorption spectrometry spectrometry using using a a Thermo Thermo Scientific Scientific™™ iCE iCE™™ 3400 3400 AAAA spectrometerspectrometer (Thermo Scientific,Scientific , Waltham,Waltham, MA,Massachusetts, USA) with USA) electrothermal with electrothermal (graphite furnace) (graphite atomization. furnace) atomization. Stock solutions Stock (AASsolutions standard (AAS solution;standard Merck,solution; Germany) Merck, Germany) with metal with concentrations metal concentrations of 1 g/L wereof 1 g/L used were to prepareused to calibrationprepare calibration solutions. solutions. The National The National Institute Instit of Standardsute of Standards and Technology and Technology (NIST) reference (NIST) materialsreference 1570amaterials (Trace 1570a Elements (Trace inElements Spinach in Leaves) Spinach and Leaves 1575a) (Pineand 1575a Needles) (Pine were Needles) used towere ensure used the to qualityensure controlthe quality of measurements.