# Changes of Air Pollution between Countries Because of Lockdowns to Face COVID-19 Pandemic
###### Abstract
Regionally, most monitoring sites in Europe, the USA, China, and India displayed reductions in the concentration of air pollutants containing CO, NO\({}_{2}\), SO\({}_{2}\), PM\({}_{2.5}\), and PM\({}_{10}\) during the quarantine period, whereas only O\({}_{3}\) concentrations increased during the same period. In Valencia, Spain, [PERSON] et al. [34] assessed the effects of restrictions to cope with COVID-19 on air quality and pollutant emissions, including nitrogen oxides (NO\({}_{\rm x}\)), nitric oxide (NO), NO\({}_{2}\), PM\({}_{10}\), PM\({}_{2.5}\), and O\({}_{3}\), comparing the same periods in 2020 and 2019. The highest reduction of the PM\({}_{10}\) and PM\({}_{2.5}\) levels for the Valencia Centre, Valencia Avd Francia, and Valencia Pista de Silla was given at 58-42%, 56-53%, and 60-41%, respectively. Similarly, a remarkable decline in NO levels was also recorded in all air monitoring stations. In particular, NO\({}_{\rm x}\), NO\({}_{2}\), and NO concentrations decreased in the range of 37.4-65.5%, 35.7-67.7%, and 35.3-63.5%, respectively, in 2020. Finally, O\({}_{3}\) levels were reduced during the lockdown period. [PERSON] et al. [35] analyzed the variations of air quality parameters (PM\({}_{2.5}\), PM\({}_{10}\), NO\({}_{2}\), and SO\({}_{2}\)) in five large cities of Poland, comparing data of the lockdown period in 2020 to data from 2018 and 2019. They showed decreasing levels in aerosol concentrations in April and May 2020 of approximately \(-23\)% and \(-18\)% compared to the 2018-2019 period. For PM\({}_{2.5}\) and PM\({}_{10}\), the reductions were from \(-11.1\)% to \(-26.4\)% and \(-8.6\)% to \(-33.9\)% in April 2020 and from \(-8.7\) to \(-21.1\)% and \(-8.5\)% to \(-31.5\)% in May 2020, as compared to the same months in 2019. [PERSON] and [PERSON] [36] examined the variations in the concentration of gaseous air pollutants containing NO\({}_{2}\), benzene, and ammonia-based during lockdown restrictions applied in the Po Valley of Northern Italy, comparing data of the COVID-19 pandemic crisis to the previous 6 years, on monthly, daily and hourly bases. Results showed improvements in air quality during the spring of 2020 because of the reductions in nitrogen oxides and benzene emissions, associated with \(-50\)% of road traffic in urban areas. [PERSON] et al. [37] analyzed the effects of COVID-19 measures of restriction on concentrations of PM\({}_{1}\), PM\({}_{10}\) and their chemical components in some urban regions of the Western Mediterranean, by comparing data during the lockdown of 2020 with the previous five years. Results revealed that the average reduction in NO\({}_{\rm x}\) and volatile organic compounds was higher than 50%, while ground-level ozone concentrations did not display considerable differences during the study period. Moreover, a 35% drop in PM\({}_{1}\) and PM\({}_{10}\) levels was recorded when Saharan dust events were excepted from the period under study. Hence, traffic limitations associated with lockdown measures contributed to significant declines in elemental carbon and heavy metal concentrations resulting from road dust. In addition, nitrate showed the largest reductions for the decline in regional emissions of NO\({}_{x}\) regarding secondary inorganic aerosols. [PERSON] et al. [38] investigated the impact of lockdown measures in Avellino, Italy on air pollution by analyzing the concentrations of CO, O\({}_{3}\), PM\({}_{2.5}\), PM\({}_{10}\), C\({}_{6}\)H\({}_{6}\), and NO\({}_{2}\) during the period January-December 2020. They showed significant reductions in CO, C\({}_{6}\)H\({}_{6}\), and NO\({}_{2}\) pollutants during the examined period, in particular March 2020. In a study implemented in Kabul, Afghanistan, [PERSON] [39] analyzed the city's air quality by examining air quality parameters containing PM\({}_{10}\), PM\({}_{2.5}\), CO, SO\({}_{2}\) NO\({}_{2}\), and O\({}_{3}\) for the pre-and post-COVID-19 period.