Insights for Air Quality Management from Modeling and Record Studies in Cuenca, Ecuador

On-road traffic is the primary source of air pollutants in Cuenca (2500 m. a.s.l.), an Andean city in Ecuador. Most of the buses in the country run on diesel, emitting high amounts of NOx (NO + NO2) and PM2.5, among other air pollutants. Currently, an electric tram system is beginning to operate in this city, accompanied by new routes for urban buses, changing the spatial distribution of the city’s emissions, and alleviating the impact in the historic center. The Ecuadorian energy efficiency law requires that all vehicles incorporated into the public transportation system must be electric by 2025. As an early and preliminary assessment of the impact of this shift, we simulated the air quality during two scenarios: (1) A reference scenario corresponding to buses running on diesel (DB) and (2) the future scenario with electric buses (EB). We used the Eulerian Weather Research and Forecasting with Chemistry (WRF-Chem) model for simulating the air quality during September, based on the last available emission inventory (year 2014). The difference in the results of the two scenarios (DB-EB) showed decreases in the daily maximum hourly NO2 (between 0.8 to 16.4 μg m−3, median 7.1 μg m−3), and in the 24-h mean PM2.5 (0.2 to 1.8 μg m−3, median 0.9 μg m−3) concentrations. However, the daily maximum 8-h mean ozone (O3) increased (1.1 to 8.0 μg m−3, median 3.5 μg m−3). Apart from the primary air quality benefits acquired due to decreases in NO2 and PM2.5 levels, and owing to the volatile organic compounds (VOC)-limited regime for O3 production in this city, modeling suggests that VOC controls should accompany future NOx reduction for avoiding increases in O3. Modeled tendencies of these pollutants when moving from the DB to EB scenario were consistent with the tendencies observed during the COVID-19 lockdown in this city, which is a unique reference for appreciating the potentiality and identifying insights for air quality improvements. This consistency supports the approach and results of this contribution, which provides early insights into the effects on air quality due to the recent operability of the electric tram and the future shift from diesel to electric buses in Cuenca.


Introduction
On-road traffic is one of the most important sources of air pollutants in cities located in Ecuador [1,2]. Emissions from this source are exacerbated for cities located in the Andean region of the country, owing to their altitude, where the content of atmospheric oxygen is lower compared to at sea level. Therefore, combustion processes emit more primary pollutants in these cities [3].
for studying the influence of six planetary boundary layer schemes for modeling the air quality in Cuenca [18]. Modeling is also a powerful approach for foreseeing the effects on air quality, due to changes in the emission inventories [19], which can help define policies, programs, and projects for air quality management.

The Air Quality from Cuenca
The air quality stations are mainly located in the urban area. There is one automatic station located in the historic center (MUN station, Figure 1), which, since 2012, has monitored the short-term air quality (CO, NO 2 , PM 2.5 , and O 3 ) and meteorology [20]. Additionally, there are about 20 passive stations for measuring monthly-mean air quality concentrations (NO 2 and O 3 ). Measurement of the air quality is based on the methods established in the Ecuadorian air quality regulation under the responsibility of the Municipality of Cuenca, which, for this purpose, is the entity accredited by the National Environmental Authority. The air quality network's current equipment is described in the report on the air quality from 2019 [20], which corresponds to the directives established by the USA Environmental Protection Agency and European. As part of its operation, corresponding quality assurance and quality control activities are permanently performed.
From 2012 to 2019, during fourteen days, the PM 2.5 concentrations (24-h mean) were higher than the WHO guideline (25 µg m −3 ). The annual mean PM 2.5 concentrations varied between 6.1 and 10.8 µg m −3 . During four years in this period, this mean was higher than the WHO guideline (10 µg m −3 , [11]). Studying the growing historical dataset or air quality records promotes the understanding of the complex behavior of air pollutants in Cuenca. Based on the records from 2013 to 2015, the weekend effect (WE), which is a phenomenon characterized by increased concentrations of O 3 during weekends, although the emissions of NO x and VOC are typically lower in comparison to weekdays, was identified in the urban area of Cuenca [27], suggesting the presence of a VOC-limited regime for O 3 production. This finding provided the first insights into the influence of decreased on-road traffic emissions during weekends.

Actions for Controlling Air Pollutant Emissions
One of the most relevant controls of air pollutant emissions in Cuenca is the technical vehicular revision (RTV, due to its acronym in Spanish). According to the RTV regulation, it is mandatory that Atmosphere 2020, 11, 998 6 of 21 vehicles running in Cuenca must demonstrate each year that their exhaust emissions are lower than the levels established in the national regulation as a requirement to allow their use. Currently, through the RTV, CO and HC emissions from gasoline cars and the opacity of diesel vehicle emissions are measured. Today, NO x emissions from on-road traffic are not controlled.
Another component currently affecting air pollutant emissions is the operation of an electric tram, conceived as the new core of the public transportation system of Cuenca that aims to solve the problems associated with on-road traffic. The building of this facility began at the end of 2013. However, different problems and conflicts have appeared related to restricted mobility and effects on local commercial activities [28]. After several years of delay, the electric tram began to work during the time of writing this manuscript. The electric tram alleviates the air pollutant emissions along its route, which includes the historic center. However, the displaced buses will move their emissions to new routes.
In the future, under the framework of the Ecuadorian energy efficiency law [29], all vehicles incorporated into the public transportation system must be electric by 2025. The shift from diesel to electric buses will eliminate or reduce the exhaust emissions from diesel buses, decreasing, as a consequence, the NO x and PM 2.5 emissions.

The Forced Lockdown Owing to COVID-19
To reduce the spread of COVID-19, several measures, such as lockdowns, quarantine, stay at home, and transportation restrictions were applied world-wide. The corresponding effects on air quality have begun to be reported from different regions. Nakada and Urban (2020) [30] [33] reported reductions in PM 10 , SO 2 , and NO 2 in Salé (Morocco).
In Ecuador, based on the infected people and the pandemic declaration by WHO, the government declared (decree 1017) the exception status on 16 March 2020 [34]. One of the measures of this status was a restriction on mobility. During the following days, on-road traffic and other activities notably reduced, therefore decreasing the emission of air pollutants.
Although the exception status was officially maintained in Cuenca until 24 May 2020, some activities restarted on 17 May 2020 [35]. On 25 May 2020, the status was relaxed, alleviating the restriction of on-road traffic and other activities, and allowed buses employed for regional transportation to reactivate their service. Since 01 June 2020, urban buses in Cuenca have returned to service.
The air quality records from the exception status provide a unique opportunity to learn and appreciate the potentiality for air quality improvement as a consequence of the decrease in activities, such as on-road traffic.
This contribution explores the following issues of the air quality in Cuenca: • Verification of the presence of the WE in Cuenca after 2015; • The effects on air quality due to the future shift from diesel to electric buses; • The air quality during the COVID-19 lockdown, and its comparison to previous weeks and years; • A holistic analysis of these interrelated components to identify insights for air quality management.

WE in Cuenca
To complete the analysis of 2013 to 2015, and based on the same approach presented in Parra (2017) [27], we obtained mean-daily profiles for CO, NO 2 , PM 2.5 , and O 3 for each year of the period 2016 to 2019, and considering weekdays, Saturdays, and Sundays. We obtained the maximum 8-h mean O 3 concentrations to quantify the variation (percentage) of this pollutant from Saturdays and Sundays, compared to weekdays. These O 3 concentrations were obtained from the hourly values obtained between 9:00 and 16:00, considering that, during this period, typically, the O 3 concentrations are higher. This approach was used in a study of the WE in Santiago (Chile) [36] and Quito (Ecuador) [37].

Shift from Diesel to Electric Buses
As an early and preliminary assessment of the impact owing to the shift from diesel to electric buses, established in the Ecuadorian efficiency law, we simulated the air quality in Cuenca under two scenarios: (1) A reference scenario corresponding to buses running on diesel (DB) and (2) the future scenario with electric buses (EB). We used the Eulerian Weather Research and Forecasting with Chemistry (WRF-Chem V3.2) model [38] for simulating the air quality during September of 2014, based on the most recent emission inventory of Cuenca [4]. WRF-Chem is a state-of-the-art chemical transport model that requires, as the input, hourly maps of speciated emissions. Hourly maps were built using factors for activity data, considering the differences between weekdays and weekends and the influence of meteorology on vegetation emissions. September was selected because its on-road traffic and other activity levels are representative for the other months. Additionally, during specific days of September 2015 (2) and September 2017 (3), O 3 concentrations were higher than the WHO guideline (100 µg m −3 , maximum 8-h mean), partly due to the high levels of near zenith solar radiation reaching the region of Ecuador during this month.
For the DB scenario, all of the emissions sources of Table 1 were included when building the hourly emissions maps. For the EB scenario, as a first assumption, all of the combustion emissions (NO x , CO, NMVOC, SO 2 , PM 10 exhaust, and PM 2.5 exhaust) from buses (Table 2) were eliminated.
Initial and boundary conditions were generated using the final National Centers for Environmental Prediction (NCEP FNL) Operational Global Analysis data [39]. Meteorological simulations were carried out using a master domain of 70 × 70 cells (27 × 27 km each) and three nested sub-domains. The third sub-domain (100 × 82 cells of 1 km each, and 35 vertical levels) covers the region of the Cantón Cuenca ( Figure 1). For the third sub-domain, the option of WRF-Chem for the chemical transportation of pollutants was activated, selecting the carbon bond mechanism-Z (CBMZ) [40] for gaseous pollutants and the model for simulating aerosols interactions and chemistry (MOSAIC) for aerosols [41]. One crucial feature of WRF-Chem is the possibility to apply online approach modeling, allowing simultaneous treatment with feedback between meteorological and air quality variables. For this study, the option for direct effects between aerosols and meteorology was activated, using four aerosol bins. Working with direct effects improved the performance when modeling the air quality in Cuenca, compared to modeling without feedback [18]. Table 3 indicates the physics options used for the simulation. We selected the Yonsei University (YSU) for the planetary boundary layer scheme-one of the options of WRF-Chem-which provided the best modeling performance for modeling the air quality in Cuenca [18].

Air Quality during the COVID-19 Lockdown
We compared the short-term air quality (maximum 8-h mean CO, maximum 1-h mean NO 2 , 24-h mean PM 2.5 , and maximum 8-h mean O 3 ) records obtained from the MUN station during 17 March 2020 to 16 May 2020 (61 days), with records from the following periods: • Weeks before the exception status (01 January 2020 to 16 March 2020), and; • From 17 March to 16 May, of previous years, from 2015 to 2019. Although there is information available from 2012, we selected 2015 onwards, because records after this year covered at least 70% of days. We selected this percentage to assure the representativeness of records.
The short-term air quality concentrations used for comparison are congruent with the WHO guidelines [9,11] and the Ecuadorian air quality regulation. The maximum 1-h mean corresponds to the maximum hourly mean concentration per day. The maximum 8-h mean corresponds to the maximum mean concentration for eight consecutive hours per day. We conducted Wilcoxon tests to establish if the distributions were statistically equal or different.

WE in Cuenca
In agreement with the results from 2013 to 2015 [27], from 2016 to 2019, the mean-daily profiles of CO, NO 2 , and PM 2.5 showed lower concentrations on Saturdays and Sundays, compared to weekdays. However, O 3 profiles were higher. Figure

WE in Cuenca
In agreement with the results from 2013 to 2015 [27], from 2016 to 2019, the mean-daily profiles of CO, NO2, and PM2.5 showed lower concentrations on Saturdays and Sundays, compared to weekdays. However, O3 profiles were higher. Figure 2 depicts the profiles from 2018. The profiles of other years presented similar configurations.
From 2013 to 2019, the increase in the maximum 8-h mean O3 concentrations of Saturdays and Sundays compared to weekdays varied between 2.6% and 11.8% and 5.6% and 15.8%, respectively ( Figure 3). Although we limited our analysis to the yearly period, our results confirm the presence of the WE in the urban area of Cuenca, where on-road traffic is the most relevant air pollutant source. More insights can be drawn in the future, through an analysis of the historical records per season or month.   From 2013 to 2019, the increase in the maximum 8-h mean O 3 concentrations of Saturdays and Sundays compared to weekdays varied between 2.6% and 11.8% and 5.6% and 15.8%, respectively ( Figure 3). Although we limited our analysis to the yearly period, our results confirm the presence of the WE in the urban area of Cuenca, where on-road traffic is the most relevant air pollutant source. More insights can be drawn in the future, through an analysis of the historical records per season or month.
Diesel vehicles, representing 10.8% of the total fleet from Cuenca, reduce their activity during weekends. Therefore, significant reductions in NO x and PM 2.5 emissions take place on weekends compared to weekdays. The lower activity of gasoline vehicles, which cover 89.2% of the fleet, during weekends, mainly decreases the emissions of CO and NMVOC. Therefore, the decrease of CO, NO x , and PM 2.5 emissions during weekends produced, on average, lower concentrations of these pollutants (Figure 2). Other sources, such as small industries, also reduce their emissions during weekends, but the decrease in on-road traffic is more significant.
Atmosphere 2020, 10, x FOR PEER REVIEW 9 of 22 Diesel vehicles, representing 10.8% of the total fleet from Cuenca, reduce their activity during weekends. Therefore, significant reductions in NOx and PM2.5 emissions take place on weekends compared to weekdays. The lower activity of gasoline vehicles, which cover 89.2% of the fleet, during weekends, mainly decreases the emissions of CO and NMVOC. Therefore, the decrease of CO, NOx, and PM2.5 emissions during weekends produced, on average, lower concentrations of these pollutants (Figure 2). Other sources, such as small industries, also reduce their emissions during weekends, but the decrease in on-road traffic is more significant. .

Shift from Diesel to Electric Buses
At the location of the MUN station, differences between the simulated scenarios (DB -EB) showed decreases in CO (between 0.00 and 0.14 mg m

Shift from Diesel to Electric Buses
At the location of the MUN station, differences between the simulated scenarios (DB -EB) showed decreases in CO (between 0.00 and 0.14 mg m −3 , median 0.02 mg m −3 ), NO 2 (0.  (Figure 4).
At the passive stations, the results of EB compared to the DB scenario ( Figure 5) showed decreases in mean-monthly NO 2 (0.2 to 5.6 µg m −3 , median 3.8 µg m −3 ), although increases in mean-monthly O 3 (0.0 to 5.7 µg m −3 , median 3.9 µg m −3 ) ( Figure 5). The smallest differences, for both NO 2 and O 3 , were computed at Ictocruz (ICT) and Escuela Héctor Sempértegui (EHS), which are passive stations located in the south and north, respectively, in terms of the consolidated urban area of Cuenca (Figure 1), and, therefore, are only influenced by on-road traffic emissions to a small degree. Figure 6 shows the modeled maps of NO 2 (1-h mean at 7:00 local time (LT)) and O 3 (maximum 8-h mean) concentrations of the DB and EB scenarios, from 12 September 2014. The Supplementary Materials section shows movies of the hourly modeled concentrations of NO 2 and O 3 from 12 September 2014, for both the DB and EB scenarios.

Shift from Diesel to Electric Buses
At the location of the MUN station, differences between the simulated scenarios (DB -EB) showed decreases in CO (between 0.00 and 0.14 mg m −3 , median 0.02 mg m −3 ), NO2 (0.8 to 16.4 µg m −3 , median 7.1 µg m −3 ), and 24-h mean PM2.5 (0.2 to 1.8 µg m −3 , median 0.9 µg m −3 ) concentrations. However, the maximum 8-h mean O3 increased (1.1 to 8.0 µg m −3 , median 3.5 µg m −3 ) (Figure 4).  At the passive stations, the results of EB compared to the DB scenario ( Figure 5) showed decreases in mean-monthly NO2 (0.2 to 5.6 µg m −3 , median 3.8 µg m −3 ), although increases in meanmonthly O3 (0.0 to 5.7 µg m −3 , median 3.9 µg m −3 ) ( Figure 5). The smallest differences, for both NO2 and O3, were computed at Ictocruz (ICT) and Escuela Héctor Sempértegui (EHS), which are passive stations located in the south and north, respectively, in terms of the consolidated urban area of Cuenca (Figure 1), and, therefore, are only influenced by on-road traffic emissions to a small degree.   At the passive stations, the results of EB compared to the DB scenario ( Figure 5) showed decreases in mean-monthly NO2 (0.2 to 5.6 µg m −3 , median 3.8 µg m −3 ), although increases in meanmonthly O3 (0.0 to 5.7 µg m −3 , median 3.9 µg m −3 ) ( Figure 5). The smallest differences, for both NO2 and O3, were computed at Ictocruz (ICT) and Escuela Héctor Sempértegui (EHS), which are passive stations located in the south and north, respectively, in terms of the consolidated urban area of Cuenca (Figure 1), and, therefore, are only influenced by on-road traffic emissions to a small degree.   advisable to model other months or even the entire yearly period.
Another scenario deserving exploration is the control of emissions from heavy diesel vehicles, which contributed the largest percentages of on-road emissions of NOx (36.9%) and PM2.5 (63.4%) in 2014. The effects of NMVOC controls should be explored for gasoline cars, especially for older vehicles. Due to their emissions, other sources deserving dedicated assessments are industrial activities, the power facility, and the handcrafted production of bricks.  The operation of the tram project will produce changes in the public transportation system of Cuenca. The routes of buses need to be appropriately redesigned to define the best way to incorporate them. Emissions from buses will be redistributed, alleviating their magnitude on the historic center, although moving emissions to areas under the influence of new routes. Table 4 presents a comparison with other assessments on the influence of moving to electric vehicles. Minet [46] applied an approach based on changes in emissions when assessing the effects in São Paulo (Brazil) and Cluj-Napoca (Romania), respectively. These assessments, summarized in Table 4, reported decreases in NOx emissions. Apart from decreasing their short-term concentrations, lowering the NO 2 and PM 2.5 will also reduce their annual mean levels, promoting the attainment of the WHO guidelines and the air quality regulation. We highlight the benefits of reducing air pollution and particulate matter, owing to their carcinogenicity to humans [12,13], and the effects of particulate matter on the brain, which, according to recent literature, is the component of air pollution that appears to be the most concerning [17]. The modeled results and VOC-limited regime for photochemical O 3 production suggest that VOC controls should accompany future NO x reduction to avoid an increase in O 3 levels in the urban area of Cuenca.
In the future, the RTV should incorporate both NO x and VOC emission controls to verify the proper condition of exhaust catalysts for gasoline cars. Additionally, the RTV should incorporate NO x and PM 2.5 controls for diesel vehicles.
The direction of change in pollutants between the DB and EB scenarios was consistent with that observed during the COVID-19 lockdown. Although other sources reduced their activities, the absence of buses allowed, to a high degree, a reduction in NO x and PM 2.5 in the urban area of Cuenca. This consistency supports the validity of the approach used in this contribution to assess the effects of the future shift from diesel to electric buses in Cuenca.
The modeled results provide a preliminary estimation of air quality benefits based on the assumption that all diesel buses belonging to public transportation in the future will be replaced by electric buses. An updated emission inventory and the proposal of an appropriate future electric bus fleet (electric and hybrid) configuration will refine these results. Another limitation of our study is the period employed for modeling. Although September was considered representative, it is advisable to model other months or even the entire yearly period.
Another scenario deserving exploration is the control of emissions from heavy diesel vehicles, which contributed the largest percentages of on-road emissions of NO x (36.9%) and PM 2.5 (63.4%) in 2014. The effects of NMVOC controls should be explored for gasoline cars, especially for older vehicles. Due to their emissions, other sources deserving dedicated assessments are industrial activities, the power facility, and the handcrafted production of bricks.
The operation of the tram project will produce changes in the public transportation system of Cuenca. The routes of buses need to be appropriately redesigned to define the best way to incorporate them. Emissions from buses will be redistributed, alleviating their magnitude on the historic center, although moving emissions to areas under the influence of new routes. Table 4 presents a comparison with other assessments on the influence of moving to electric vehicles.  [46] applied an approach based on changes in emissions when assessing the effects in São Paulo (Brazil) and Cluj-Napoca (Romania), respectively. These assessments, summarized in Table 4, reported decreases in NO x emissions.
Although the replacement of diesel buses by electric buses will reduce the emissions along the routes used by these vehicles, the generation of electricity will produce air pollution in the areas of influence of the fossil fuel power facilities belonging to the Ecuadorian mix. From 2001 to 2018, electricity came from renewable sources (43.5% to 73.6%), fossil fuels (26.2% to 52.2%), and importations (0.1% to 11.5%) [47]. Non-renewable sources include the combustion of fuel oil, diesel, naphtha, natural gas, bunker, oil, and liquid petroleum gas. The impact on air quality due to the electricity produced in Ecuador is a topic deserving of further research.

Air Quality during the COVID-19 Lockdown
The concentrations of CO and NO 2 were lower during the COVID-19 lockdown compared to previous records from 2020 ( Figure 7). The maximum 8-h mean CO mean decreased from 0.74 (median) to 0.60 mg m −3 . The maximum 1-h mean NO 2 decreased from 36.8 (median) to 16.3 µg m −3 . The distributions of CO and NO 2 during the lockdown period were statistically different, with lower levels compared to distributions from 01 January 2020 to 16 March 2020 (Table 5).
Additionally, the concentrations of PM 2.5 were lower during the restriction (Figure 7). The 24-h mean PM 2.5 decreased from 9.6 (median) to 5.7 µg m −3 . The peak after 17 March 2020 can be associated with the arrival of volcanic ash from the Cayambe-one of the currently active volcanoes in Ecuador [48]-which produced light ash fallout in Cuenca on 24 March 2020 [49]. This peak influenced the PM 2.5 records from 17 March to 16 April 2020, which showed a distribution statistically equal to records from 01 January to 16 (Table 4). However, the distribution of O 3 from 17 March to 16 May 2020 was statistically equal, showing similar levels to the distribution from 01 January to 16 March 2020.
The seasonal behavior of the maximum 8-h mean O 3 in Cuenca shows a decrease during April and May, with the lowest concentrations during June and July (Figure 8). After this, O 3 increases, typically reaching the highest values during September. Therefore, the O 3 decrease during the second month of the lockdown relates to its seasonal variation. Figure 8 shows that the O 3 levels during the COVID-19 lockdown were higher than the concentrations from previous years (2015 to 2019). Figure 8 shows the mean profile (maximum 8-h mean) of O 3 concentrations deduced from the records of the period 2015 to 2019 and the concentrations from 2020. The profile from 2020 shows, in general, higher concentrations compared to the mean of the previous years. From 01 January to 16 March, the O 3 concentrations from 2020 were 10.0 µg m −3 (median) higher than the mean profile from previous years. During the lockdown (17 March to 16 May 2020), this difference increased to 13.9 µg m −3 (median), indicating a net increase of 3.9 µg m −3 , which is consistent with the increase (3.5 µg m −3 , median) obtained by modeling when assessing the air quality effects of moving from DB to EB.
During the first days of the lockdown, the O3 concentrations increased. The maximum 8-h mean O3 rose from 52.2 to 55.7 µg m −3 , with the last value being the median from the first month after 17 March 2020. The median from 17 March 2020 to 16 May 2020 was 47.1 µg m −3 . The distribution of O3 from 17 March to 16 April 2020 was statistically different, showing higher values compared to the distribution from 01 January to 16 March 2020 (Table 4). However, the distribution of O3 from 17 March to 16 May 2020 was statistically equal, showing similar levels to the distribution from 01 January to 16 March 2020.
The seasonal behavior of the maximum 8-h mean O3 in Cuenca shows a decrease during April and May, with the lowest concentrations during June and July (Figure 8). After this, O3 increases, typically reaching the highest values during September. Therefore, the O3 decrease during the second month of the lockdown relates to its seasonal variation. Figure 8 shows that the O3 levels during the COVID-19 lockdown were higher than the concentrations from previous years (2015 to 2019). Figure 8 shows the mean profile (maximum 8-h mean) of O3 concentrations deduced from the records of the period 2015 to 2019 and the concentrations from 2020. The profile from 2020 shows, in general, higher concentrations compared to the mean of the previous years. From 01 January to 16 March, the O3 concentrations from 2020 were 10.0 µg m −3 (median) higher than the mean profile from previous years. During the lockdown (17 March to 16 May 2020), this difference increased to 13.9 µg m −3 (median), indicating a net increase of 3.9 µg m −3 , which is consistent with the increase (3.5 µg m −3 , median) obtained by modeling when assessing the air quality effects of moving from DB to EB.        The distributions of CO and NO 2 during the lockdown period from 2020 were statistically different, showing lower concentrations compared to the distributions of the same period from 2015 to 2019 (Table 6). Similarly, the distribution of O 3 was statistically different, showing higher levels compared to the previous years. The distribution of PM 2.5 during the lockdown period from 2020 was statistically equal, only showing similar concentrations to 2016.
The low PM 2.5 median from 2015 (4.2 µg m −3 ) can be associated with the reduction in traffic-especially buses-in the historic center, due to activities of the construction of the electric tram. This project's construction activities caused the closing of streets and changes in the routes of buses and limited the use of particular vehicles [28]. The operation of this project will produce changes in the public transportation system of Cuenca. At the time of writing this manuscript, the tram was being tested, and it will officially start working during the upcoming weeks.
The decrease in CO, NO 2 , and PM 2.5 from 17 March to 16 May 2020, compared to previous weeks (01 January to 16 March 2020), was consistent with the decrease of these pollutants compared to previous years (2015 to 2019). Although other activities, such as some industries, probably reduced their activities, these changes can be associated, to a high degree, with reductions in on-road traffic. During the restriction, all types of vehicles reduced their activity. Buses did not work, and, therefore, there were essential reductions in NO 2 and PM 2.5 .
On the other hand, the increase in O 3 concentrations is consistent with the hypotheses behind the WE [50]. Among them, the results suggested that the following could have a leading role:

•
There is a VOC-limited regime, with a VOC/NO x ratio lower than 8. Under this regime, VOC limits O 3 production, and NO x reduction promotes O 3 production, and; • Less O 3 is titrated because NO x emissions are lower compared to weekdays.
Other mechanisms, such as the reduction in soot, can contribute to higher O 3 concentrations. More studies are required to define the participation of these and other hypotheses behind the WE in Cuenca. Figure 10 shows the mean profiles of global solar radiation from 2017 to 2020 (MUN station), corresponding to the period 17 March to 16 May. These profiles indicate the mean levels of solar radiation from 9:00 to 16:00, representing the hours when O 3 concentrations are typically higher. The profile of 2020 did not show higher values compared to previous years. The corresponding Wilcoxon tests indicated that the distributions of global radiation records from 2020 were statistically equal compared to the previous three years. These results indicate that the increase in O 3 concentrations during 2020 is not related to higher solar radiation levels. Apart from changes in the emissions of precursors, another factor potentially involved is the long-range transport of O 3 . From 1 January to 16 May of 2020, Terra and Aqua satellites [51] identified forest fires, mainly in Colombia and Venezuela, toward the northeast of Ecuador. In addition, forest fires were mostly identified in the north of Peru and the center of Brazil. At the latitude of Cuenca, forest fires were less abundant and mainly at the center and west of South America. Although the influence of forest fires is outside the scope of this study, their occurrence from 1 January to 16 May suggests their emissions were not the leading cause of O 3 increases during the COVID-19 lockdown.
The effects of the COVID-19 lockdown and modeled results presented in this contribution provide an early reference for the potential changes in the air quality of Cuenca during the next few years.
Although we focused our analyses on Cuenca, our results can act as a preliminary reference for other medium-large Ecuadorian cities, which share similar features with regards to their vehicular fleets and emission contributions [1,2].

Conclusions and Summary
Based on records from seven years (2013 to 2019), we confirmed the presence of the WE in the urban area of Cuenca, where on-road traffic is the most relevant air pollutant source. The VOClimited regime for O3 production explains, at least in part, the mean increase in O3 concentrations during weekends, despite the decreased emissions of NOx and VOC, in comparison to weekdays. This regime is behind a counterintuitive variation of O3 owing to the variation of NOx emissions: The increase in NOx emissions decreases O3 concentrations, and the decrease in NOx emissions increases the O3 levels.
Our preliminary assessment, based on the assumption that all diesel buses will be replaced by electric buses, implies the elimination of 1861.2 t y −1 of NOx and 81.7 t y −1 of PM2.5 exhaust emissions ( Table 2). The modeled results indicated decreases in NO2 and PM2.5 but increases in O3 concentrations. The direction of these changes was consistent with the VOC-limited regime presented in Cuenca.
The effects of the limitation of activities during the COVID-19 lockdown also showed consistent variations (NO2 decrease and O3 increase) in comparison to the VOC-limited regime for O3 production.
There was consistency between the WE, the effects on air quality during the COVID-19 lockdown, and the modeled results owing to the future shift in the public transportation system of Cuenca. This consistency supports the modeling approach used in this contribution for assessing

Conclusions and Summary
Based on records from seven years (2013 to 2019), we confirmed the presence of the WE in the urban area of Cuenca, where on-road traffic is the most relevant air pollutant source. The VOC-limited regime for O 3 production explains, at least in part, the mean increase in O 3 concentrations during weekends, despite the decreased emissions of NO x and VOC, in comparison to weekdays. This regime is behind a counterintuitive variation of O 3 owing to the variation of NO x emissions: The increase in NO x emissions decreases O 3 concentrations, and the decrease in NO x emissions increases the O 3 levels.
Our preliminary assessment, based on the assumption that all diesel buses will be replaced by electric buses, implies the elimination of 1861.2 t y −1 of NO x and 81.7 t y −1 of PM 2.5 exhaust emissions ( Table 2). The modeled results indicated decreases in NO 2 and PM 2.5 but increases in O 3 concentrations. The direction of these changes was consistent with the VOC-limited regime presented in Cuenca.
The effects of the limitation of activities during the COVID-19 lockdown also showed consistent variations (NO 2 decrease and O 3 increase) in comparison to the VOC-limited regime for O 3 production.
There was consistency between the WE, the effects on air quality during the COVID-19 lockdown, and the modeled results owing to the future shift in the public transportation system of Cuenca. This consistency supports the modeling approach used in this contribution for assessing future air quality scenarios, owing to changes in the emission inventories. The same approach can be used to assess the effects of reductions in other emission sources, such as old gasoline cars (high NMVOC emissions). Moreover, the modeled results and their consistency with the WE and the effects of the COVID-19 lockdown support the validity of the emission inventory from 2014, which, although not a recent one, is a useful component for modeling purposes. Future emissions inventories should follow the same approach used when building the emission inventory from 2014.
Our findings suggest that VOC emission controls should accompany a future reduction in NO x emissions to avoid an increase in O 3 levels in the urban area of Cuenca. In the future, the RTV should incorporate both NO x and VOC emission controls to verify the proper condition of exhaust catalysts for gasoline cars. Furthermore, the RTV should incorporate NO x and PM 2.5 controls for diesel vehicles.
The operation of the electric tram system will produce changes in the transportation system of Cuenca. Emissions from buses will be redistributed, alleviating their magnitude in the historic center. The effects of the COVID-19 lockdown and modeled results presented in this contribution provide an early reference for the potential changes in the air quality of Cuenca during the upcoming years, due to the recent operability of the electric tram and the future shift from diesel to electric buses in Cuenca.