Evolution of open air quality of urbanized territories under Covid-19 pandemic conditions

Current state of world affairs in 2020 during quarantine enforced due to COVID-19 pandemic is characterized, on the one hand, with economic recession, but on the other hand, also with improvement of ecological state of environment. Thus a unique opportunity came up to study processes of open air conditions formation in settlements in circumstances of restricted economic activity and limitations imposed on all kinds of transportation. This article presents results of research of trends of formation of open air quality of urbanized territories (using city of Rivne as an example) during quarantine enforced due to COVID-19 pandemic. It is determined that due to introduction of quarantine measures air quality has improved. So, during the quarantine as well as after relaxation of quarantine measures only concentration of formaldehyde in open air of Rivne city did exceed average daily MAC. Primary source of open air pollution in Rivne city is motor vehicles which comprise 79% of total amount of pollutant emissions. Main pollutants which influence formation of open air quality in the city are: dust, sulfur dioxide, nitrogen dioxide, hydrogen fluoride and formaldehyde. They exceeded average daily MAC by factor of 1.3 to 32. Complex air pollution index (CAPI) was equal to 5.4 during quarantine restrictions but grew to 5.7 when the restrictions were partially loosened. Thus, air pollution level was evaluated as "mild pollution". During the corresponding periods of 2019 this index was changing from 6.9 ("mild air pollution") to 7.7 ("polluted air") respectively. Open air pollution level is determined by a complex of constituents including pollutant emission amounts, their specifics and dependency on meteorological factors. Due to this aspect we have performed statistical examinations to determine dependency of open air pollutant concentrations on meteorological conditions using multiple correlation coefficients. Strong correlation was exhibited for nitrogen oxide, hydrogen chloride and ammonia: multiple correlation coefficients fall within 0.76-0.80 range; moderate correlation was seen for dust, sulfur dioxide, nitrogen dioxide, hydrogen disulphide, phenol and formaldehyde with multiple correlation coefficients varying in 0.51-0.70 range.


Introduction
Quality of atmospheric air is a determinant of formation of favorable and comfortable conditions for human beings. Yet in conditions of ever growing man-induced environmental footprint, increasing motor vehicles emissions state of environment is worsening, and that applies to open air quality as well. Open air pollution is one of the major contemporary ecological challenges that manifests itself most badly within urbanized territories. Therewith, open air pollution level depends not only on volumes of industrial and motor vehicle emissions but also on the character of their vertical and horizontal dispersion which are driven first of all by meteorological conditions (Kiptenko et al., 2013).
Year 2020 has already became history as one of the years of most significant economic crisis, caused by COVID-19 pandemic, that has spread to virtually all countries of the world. Quarantine declared by World Health Organization as one of the means to decrease rate of disease propagation has resulted in decrease or suspension of production at industrial plants as well as restriction or prohibition of population movement using both mass transit and personal vehicles. Thus at this conjuncture, on one hand, economics went into recession, but on the other hand, improvement of environmental situation was observed. This is because anthropogenic load on the natural environment, including state of open air, has substantially decreased during the quarantine. Ministry of Ecology and Natural Resources of Ukraine had reported on its website after only a month of quarantine restrictions about lowered average concentrations of certain pollutants is some cities; these were: nitrogen dioxide, sulfur dioxide, airborne (suspended) dust and formaldehyde in open air. A number of scholarly works is dedicated to research of evolution of open air quality in large cities, including works by following scientists: Y. Novikov (Novikov, 2005), V. Kucheryavy (Kucheryavy, 1999), O. Adamenko (Adamenko et al., 2004), V. Baharev (Baharev et al., 2012, M. Klymenko (Klymenko et al., 2006), A. Pryschepa (Pryschepa et al., 2017), O. Brezhytska (Brezhytska, 2013) and others. As noted by L. Nadtochiy, roles of anthropogenic emission sources and their contribution to general dynamics of evolution of open air pollution vary substantially at different time scales. Such effects are not always univocal, complicating determination of interdependence between meteorological conditions and pollutant concentrations, therefore effect of meteorological characteristics on pollution levels must be studied separately in every city (Nadtochiy, 2020). Many scientists abroad have studied effects of meteorological conditions on athmospheric pollution levels and their temporal dynamics (Jacobson, 2002;Moller, 2010;Akimoto, 2016;Lazaridis, 2011;Liss et al., 2014;Liu et al. 2015).
Scientific works by A. Hvesyk, A. Stepanenko (Hvesyk et al., 2014;, Z. Herasymchuk (Herasymchuk and Oleksyuk, 2007), A. Kachynsky (Kachynsky, 2001), V. Shevchuk (Shevchuk et al. 2004) present developments of theoretical, methodological principles and applied guidances for ensuring sufficient level of ecological safety, including safety in regard to open air, for Ukraine as a whole as well as on regional basis (Boyko, 2016). Situation that has developed in 2020 during the period of quarantine introduced due to COVID-19 pandemic is essentially unique. It provides an opportunity to study and quantify possible improvement of open air state in settlements due to restrictions of economic activities and all kinds of transportation. Therefore research in this direction is quite relevant.

Materials and Methods
Object of this work is to determine trends of evolution of open air quality in urbanized territories (using the example of Rivne city) in conditions of quarantine imposed due to COVID-19 pandemic as well as to determine role of meteorological constituents in processes that determine quality of open air. City of Rivne is a district (oblast) center of Rivne oblast, located in northwestern part of Ukraine. City population is 246.5 thousand (as of 2019), its area is 63 sq. km. While Rivne oblast has remarkably agrarian specialization, it has quite developed industrial production. Dominant industries in region's economy are: power industry, chemical industry, consumer goods (light) industry, timber industry, food manufacturing, construction materials manufacturing, metal industry and machine-building industry. A lot of manufacturing plants are located on Rivne city territory or in its surroundings. Enterprises of Rivne city have different structure of output. The region is intersected by international and national highways "Kyiv -Warsaw", "Kyiv -Brest", "Kyiv -Lviv", "Lviv -Zhytomyr", "Kyiv -Chernivtsi". There are 588 km of maintained railway track in the region as well as 7535 km of motor roads. Thus mobile sources of emissions make substantial contribution to pollution of open air in Rivne oblast (about 79% of total emissions in the region), and their amounts are growing each year due to increased numbers of motor vehicles and growing traffic loads. According to data of regional Central statistics authority, total amount of pollutant emissions into open air in the region in year 2019 from stationary sources was 9.9 thousand tons, or 0.8 thousand tons greater than in 2018. Dynamics of pollutant emissions by stationary and mobile pollution sources on the territory of Rivne city during years 2000 -2019 is presented on Figure 1.

Results and discussion
To determine level of pollution of open air in Rivne city with top-priority pollutants, we used complex air pollution index (CAPI). Numeric index value lets one evaluate pollution level from "clean air" to "extremely polluted". Results of the study are presented on Figure 4.  By comparing these data with data for corresponding periods of 2019 we have determined that MACa.d. was exceeded for nitrogen dioxide (by factor of 1.09 during February-April and by factor of 1.02 during May-July); for hydrogen fluoride -by factor of 1.7 and by factor of 1.4 respectively; for formaldehyde -by factor of 2.2 and by factor of 2.6 respectively. We have performed CAPI calculation for two studied periods (Fig. 6). Complex air pollution index (CAPI) was equal to 5.4 during quarantine restrictions but grew to 5.7 when the restrictions were partially loosened. Air pollution level was evaluated as "mild pollution". During the corresponding periods of 2019 this index was changing from 6.9 ("mild air pollution") to 7.7 ("polluted air") respectively. We have determined that higher level of open air pollution in May-July period compared to respective February-April period was observed both in 2019, when there were no quarantine restrictions implemented, as well as in 2020 during the quarantine. Research was performed using archive materials of Rivne Oblast Hydrometeorology Centre for the 2000-2020 period. Following meteorological parameters were taken into account: air temperature, total atmospheric precipitation, air humidity and wind velocity.
List of pollutants and research findings are presented in Table 1.  Nature of open air pollution is determined by specifics of each individual pollutant, by its potential for effect summation and by peculiarities of its dispersion depending on meteorological conditions. Dependency of concentration of pollutants emitted into open air on meteorological factors was established. Strong correlation was exhibited for nitrogen oxide, hydrogen chloride and ammonia: their multiple correlation coefficients fall within 0.76-0.80 range; moderate correlation was seen for dust, sulfur dioxide, nitrogen dioxide, hydrogen disulphide, phenol and formaldehyde with multiple correlation coefficients varying in 0.51-0.70 range.