## 4 Discussion
As well, it was observed over the major cities of India, such as Delhi, Mumbai, Hyderabad, Kolkata and Chennai, that a decline in PM\({}_{2.5}\) during the lockdown period registered a significant improvement in air quality, which provides important information to the cities' administration about the implementation of regulations [14]. Other studies conducted during the lockdown suggested the main sources of atmospheric particulate matter PM\({}_{10}\) and PM\({}_{2.5}\) (include fossil fuel combustion, motor vehicle exhaust emissions, industrial production, secondary particulate matter generation, among others) experienced a significant reduction up to \(-\)48.9% in three of China's provinces [66]. The decline in PM\({}_{2.5}\) emissions due to the lockdown to control the spread of SARS-CoV-2 in New York, Los Angeles, Zaragoza, Rome, Dubai, Delhi, Mumbai, Beijing and Shanghai reflected the positive changes that contributed to improve air quality [67]. BC emissions reduction can be attributed to on-road diesel sources [68], so the mitigation of transportation-related BC emissions decreased the global emissions significantly [69]. In this study, acidifying gases (NH\({}_{3}\), SO\({}_{2}\)) also showed a significant emissions reduction, up to \(-\)23% for NH\({}_{3}\). Other studies found that decreasing emissions were identified in Kannur district, India (\(-\)16%), due to a complete shutdown of traffic and industrial activities [70], as NH\({}_{3}\) emissions come mainly from heavy-duty diesel vehicles [65]. In addition, SO\({}_{2}\) emissions registered a decrease (\(-\)19%), which was identified in China as a decrease attributed to lower emissions from traffic and coal combustion [62]. Kannur, India, reported decreased emissions (\(-\)62%), and a diurnal variation most pronounced during peak traffic hours was absent during the lockdown owing to the roads being deserted [70]. Figure 4 shows NO\({}_{2}\) emissions increased in the northern Colombian region due to events of long-range pollution transport, like regional biomass burning beginning at the end of March, during the lockdown, according to recent studies [33; 34]; the air quality improvement shown in this period was partially annulled by the impact of these events. Despite an emissions reduction in the four pollutant groups selected, an increase of ozone concentration was observed in urban areas of Bogota. This result was consistent with recent studies, in which Sao Paulo (Brazil) urban areas, highly influenced by road transportation, had an increase of approximately 30% in ozone emissions [60]. The increase of ozone concentration is related to nitrogen monoxide decreases, which may cause a reduction in ozone consumption during the photochemical reactions [61; 71]. Moreover, VOCs are often the limiting precursors for O\({}_{3}\) production in urban areas [23; 31]. O\({}_{3}\) levels increased up to 57%, probably due to lower titration of O\({}_{3}\) by NO (titration, NO + O\({}_{3}\) = NO\({}_{2}\) + O\({}_{2}\)), and the decrease of NOx added to the increase of solar irradiation and temperatures in this period of the year [61]; ozone levels are a major concern in tropical cities, where the temperature and insolation favor the atmospheric processes leading to O\({}_{3}\) formation [31]. In this sense, recent studies also showed that reductions in PM\({}_{2.5}\) during the COVID-19 pandemic favored the formation of O\({}_{3}\) due to a reduction in NOx levels due to reduced transport and an increase in solar radiation [31; 72]. On the other hand, the increase in ozone seems to be associated with the decrease in PM\({}_{2.5}\), because the sinking of hydroperoxyl radicals is slowed down, and therefore, ozone production accelerates [73].