## 3 Results and Discussion  
### Inter-Period Observational Approach  
Ground-based observations are reported from seven well-distributed monitoring sites around the city. To better visualize the evolution of spatial observational data, 12-day average data inverse distance weighting images for pre-strike (20 September 2019-1 October 2019), during the strike (2 October 2019-13 October 2019) and post-strike (14 October 2019-25 October 2019) are presented in Figure 2. Numerical analysis of percentage changes is also presented in Table 1. It can be seen that CO (\(-\)19.85 \(\pm\) 6.67%) and NO2 (\(-\)32.36 \(\pm\) 15.38%) concentrations showed a much higher reduction during the strike when compared to the 12-day average before the strike (see Table 1 and Figure 2a,b,e,f). Meanwhile, O3 and SO2 showed much smaller decreases, \(-\)5.18 \(\pm\) 7.32% and \(-\)7.20 \(\pm\) 7.48%, respectively (see Table 1 and Figure 2c,d,g,h). For concentrations of SO2, there is a visible reduction in most of the sites (\(-\)7.2 \(\pm\) 7.48%), except for the two sites showing an increase in SO2, one of them near the thermoelectric power plant in S7-Chillos (Figure 2h) showing no changes in electricity production. We note that the latter site reaches the highest SO2 concentrations in the city, which is due to crude oil and poor-quality diesel fuel burned at the plant. Figure 2: Spatial variation of trace gas (NO\({}_{2}\), CO, O\({}_{3}\) and SO\({}_{2}\)) concentrations in Quito, Ecuador. The maps represent data averaged over 12-day periods of pre-strike (20 September 2019–1 October 2019), during the strike (2 October 2019–13 October 2019) and post-strike (14 October 2019–25 October 2019). Similar results were observed in other studies carried out during strike events involving a suspension of motorized fleet circulation. A significant reduction in CO concentrations was observed in the city of Hyderabad in India. Explicitly, a CO level reduction of \(-40\%\) was registered in 2009 [34] and \(20\%\) in 2010 [21]. In a study in Sao Paulo, Brazil, an even higher reduction, \(-67\%\), of CO levels was reported [18]. A reduction of NO\({}_{2}\) concentrations was also observed in Sao Paulo ranging from \(-7\%\)[18] to \(-55.5\%\)[35]. As the findings of our study, low reductions in SO\({}_{2}\) (\(-11\%\)) were reported in another study in Sao Paulo [36]. On the other hand, during the truck strike event in 2009 in Hyderabad [34] a significant reduction in O\({}_{3}\) (\(-50\%\)) was observed, while in Sao Paulo, the O\({}_{3}\) concentration reduction was lower (\(-16\%\)) although still superior to our findings [18]. As expected, after the strike ended, most of the gaseous criteria pollutants returned to the pre-strike levels (Figure 2i-l and Table 1). When NO\({}_{2}\) concentrations still showed a slight reduction (\(-2.79\)\(\pm\) 5.82%), CO and SO\({}_{2}\) increased a little by \(+1.62\)\(\pm\) 5.83% and \(+3.29\)\(\pm\) 13.56%, respectively (Table 1). These overall fluctuations, however, are quite negligible from the pre-strike scenario, thus suggesting a return of human activities to normal levels and a return of anthropogenic emissions to the urban air shed. Some hot spots in the city can be pinpointed. For example, for NO\({}_{2}\) concentrations, the central part of the city flairs back up post-strike (see Figure 2i), almost indistinguishably as before the strike (Figure 2a). This indicates a return of the traffic to the city streets and roads [37; 38]. The northwestern part of the city stays less polluted, where most of the urban sprawl to residential, less dense areas is found. A similar effect can be observed for the combustion-related CO concentrations that returned to the industrial part of the city after the strike (Figure 2j). Compared to the pre-strike levels (Figure 2c), the concentrations of O\({}_{3}\) decreased during (\(-5.18\)\(\pm\) 7.32%) and after the strike (\(-25.77\)\(\pm\) 12.01%) (Figure 2g,k).