## 2 Materials and Methods  
### Study Sites, Data Analysis and Visualization  
Geographically divided by the Andes mountains and the equator, Ecuador (17.3 million inhabitants) is the most densely populated South American country (71 people km\({}^{-2}\)) [26; 27]. Due to a wide range of elevations influencing ecosystems, this region is known for its extremely high biodiversity. The Ecuadorian capital of Quito is positioned at an average elevation of 2850 m above sea level (m.a.s.l.) on the western branch of the Andes cordillera (Figure 1). It spreads over an assortment of heights, varying from the slopes of an active Pichincha volcano (4800 m.a.s.l.) towards the inter-Andean valleys (2300 m.a.s.l.). The change in elevation in the capital city is responsible for an array of precipitation-dependent microclimates [28]. Close to 3,000,000 people inhabit the metropolitan district of Quito (DMQ) [29]. This urban center is characterized by a rapidly growing motorized fleet of vehicles powered by poor-quality fossil fuels affecting air quality [30; 31]. In this study, air quality changes due to the political protests in Quito were measured at seven sites of the environmental monitoring network of the Secretariat of the Environment of DMQ. The study sites are well distributed across the city (north to south): S1-Carapungo (elev. 2660 m.a.s.l, coord. 78\({}^{\circ}\)26\({}^{\prime}\)50\({}^{\prime\prime}\) W, 0\({}^{\circ}\)5\({}^{\prime}\)54\({}^{\prime\prime}\) S), S2-Cotocollao  
Figure 1: Positioning of the study area in: (**a**) the South American continent; (**b**) Ecuador; and (**c**) study sites (black/white markers) in the urban area of Quito, Ecuador. Roadblocks during the national strike are indicated by red cross markers. (elev. 2739 m.a.s.l, coord. 78\({}^{\circ}\)29\({}^{\prime}\)50\({}^{\prime\prime}\) W, 0\({}^{\circ}\)6\({}^{\prime}\)28\({}^{\prime\prime}\) S), S3-Belisario (elev. 2835 m.a.s.l, coord. 78\({}^{\circ}\)29\({}^{\prime}\)48\({}^{\prime\prime}\) W, 0\({}^{\circ}\)11\({}^{\prime}\)4.57\({}^{\prime\prime}\) S), S4-Centro (elev. 2820 m.a.s.l, coord. 78\({}^{\circ}\)30\({}^{\prime}\)36\({}^{\prime\prime}\) W, 0\({}^{\circ}\)13\({}^{\prime}\)12\({}^{\prime\prime}\) S), S5-Camal (elev. 2840 m.a.s.l, coord. 78\({}^{\circ}\)30\({}^{\prime}\)36\({}^{\prime\prime}\) W, 0\({}^{\circ}\)15\({}^{\prime}\)00\({}^{\prime\prime}\) S), S6-Guamani (elev. 3066 m.a.s.l, coord. 78\({}^{\circ}\)33\({}^{\prime}\)5\({}^{\prime\prime}\) W, 0\({}^{\circ}\)19\({}^{\prime}\)51\({}^{\prime\prime}\) S) and S7-Chillos (elev. 2453 m.a.s.l, coord. 78\({}^{\circ}\)27\({}^{\prime}\)36\({}^{\prime\prime}\) W, 0\({}^{\circ}\)18\({}^{\prime}\)00\({}^{\prime\prime}\) S) (Figure 1). Air pollution and meteorological variables (Table A1, Appendix A) were sampled using the United States Environmental Protection Agency approved methods [32], described in detail in past publications [30]. Data from the ground-based sites were gathered from 20 September to 25 October 2019. This includes three 12-day periods: before (20 September 2019-1 October 2019), during (2 October 2019-13 October 2019) and after (14 October 2019-25 October 2019) the national strike of Ecuador. We believe that the S2-Cotocollao site had technical issues in measuring the NO\({}_{2}\) concentrations during the study period, thus it was excluded from the NO\({}_{2}\) analysis. The data for atmospheric variables were processed as 24-h averages and accumulation (rain only) for all seven study sites. In addition, 12-day statistics were processed for four main atmospheric gaseous pollutants: NO\({}_{2}\), CO, O\({}_{3}\) and SO\({}_{2}\). For each pollutant, the concentration change in each site was computed as the mean percent difference between the 12-days before the strike and values measured during (and after) the strike (e.g., % change = (pre-strike average - strike average)/(pre-strike average) \(\times\) 100%). The pre-strike period might be presumed as business-as-usual conditions, due to the normal anthropogenic activity.