# Changes of Air Pollution between Countries Because of Lockdowns to Face COVID-19 Pandemic
## 3 Results and Discussion
### General Observations  
Considering the restriction policies imposed in manifold countries under study, when the pollutant concentration is compared with the pre-COVID-19 period, the maximum CO reduction was in France with a rate of 4668% in January 2020, 2021, and 2022, followed by Canada with 1010% in January 2019, 2021 and 2022. Comparative analysis also showed that Australia and China have a maximum and equal decrease of 222% in January 2022 [95]. The highest declining rate of SO\({}_{2}\) (generated by both natural sources, such as volcanoes, and anthropogenic sources, such as coal-burning power plants, smelters, and oil refineries) is for Canada (413%) in February 2021, Colombia (224%) in March 2021, and Germany (191%) in January 2022 [96]. The study here also shows that the country with the maximum decrease (\(-343\%\)) in ozone concentration at ground level (comparing values during the COVID-19 pandemic with lockdown and the data of the 2015-2018 period) is Colombia, which also showed a maximum decrease in NO\({}_{2}\) (\(-21\%\)) and CO (\(-151\%\)) concentrations. The countries with maximum declines in PM emissions are Canada, with a decrease of 112% for PM\({}_{2.5}\) over March 2022, and Turkey, with a decrease of 159% for PM\({}_{10}\) over March 2021. Table 2 summarizes the main effects of the lockdown policy to face COVID-19 on air pollution between different countries. The maximum reduction in CO emissions was recorded in India, Israel, Canada, France, Germany, and Spain; the minimum was in China, Poland, and Australia. Moreover, the maximum reduction in NO\({}_{2}\) emissions was observed only in China and Australia, whereas the minimum decrease was observed in Chile. Maximum reductions for SO\({}_{2}\) emissions were recorded in Japan, South Korea, America, Brazil,  
Figure 5: Average concentrations of air pollutant in a country of the Oceania geoeconomic region. Croatia, Netherlands, Poland, and the United Kingdom, while minimum reductions were recorded in Croatia, Spain, and Turkey. In addition, no country recorded a maximum reduction in O\({}_{3}\) and PM\({}_{2.5}\) concentrations. The minimum reduction in O\({}_{3}\) concentrations was observed in Japan and Croatia, while Israel, South Korea, the Netherlands, and England were the countries where the minimum reduction was observed in PM\({}_{2.5}\) among air pollutants. As for PM\({}_{10}\), Colombia and Turkey were the countries representing the maximum decrease in its concentrations; by contrast, Canada, Brazil, America, France, and Germany are the countries showing the highest decrease in PM\({}_{10}\) concentrations. Overall, when the maximum reduction in pollutant concentrations is evaluated in terms of amount, then CO, with a rate of 655.5%, is superior to others, whereas, if the sum of the countries is evaluated, SO\({}_{2}\) shows a decrease in nine different countries. [PERSON] et al. [97] investigated the impact of quarantine measures on air pollution levels, and they found that NO\({}_{2}\) concentrations displayed a reduction on a global scale, while PM\({}_{2.5}\) and O\({}_{3}\) did not show any reduction in their concentrations based on the restrictions. [PERSON] and [PERSON] [98] investigated air quality levels globally, and they indicated that lockdown had a positive impact on air quality by decreasing NO\({}_{2}\) and PM\({}_{2.5}\) concentrations. [PERSON] et al. [99] compared global PM\({}_{2.5}\) concentrations in 2020 based on lockdown measures of pandemic control with the concentrations in 2018 and 2019 for different geoeconomic regions: in general, PM\({}_{2.5}\) concentrations declined in the examined countries. [PERSON] et al. [79] investigated variations in the concentration of air pollutants globally, comparing data from the lockdown period in 2020 and baseline period (2015-2019). They found that concentrations of PM\({}_{2.5}\) and NO\({}_{2}\) decreased, whereas O\({}_{3}\) concentration increased.