Urban Air Pollution in the Global South: A Never-Ending Crisis?

The article is the result of good research work and has the appropriate structure and necessary innovation. It is better to consider the following points: 1- It is better to analyze the geographical distribution of variables if the necessary data is available. 

We added an additional section 3.3 to discuss the geographical distribution of all studied air pollutants, included a Table 1 in the main text, and added 10 new figures in Appendix A (Figures A2-11) that deepen the analysis of spatial and temporal distribution of pollution trends during the study period. This new section specifically addresses the geographical distribution of pollutants across Quito’s eight monitored districts, revealing significant spatial variability driven by local emission sources and environmental conditions. For instance, Table 1 highlights stark differences, such as a 255% increase in SO₂ in Carapungo, an industrial area, compared to more moderate increases in residential valleys like Los Chillos (112.5%), reflecting the influence of urban layout and activity types. The annulus plots (Figures A7-11) further illustrate these spatial patterns, showing how pollutants like NO₂ are concentrated in densely populated areas like Centro and Belisario due to traffic emissions, while O₃ increases are more pronounced in suburban areas like Tumbaco due to photochemical reactions. We believe these additions provide a comprehensive view of the geographical distribution of air pollutants, aligning with the reviewer’s suggestion while maintaining the focus of the study on the broader air quality challenges in Quito. 

2- It is better to discuss the impact of environmental conditions on temporal and spatial changes.  

Apart from the analysis on general impact of meteorological conditions on air pollution in Quito over the study period, which answers the “temporal part of the question”, we now added an additional section (i.e., Subchapter 3.3) to discuss spatial variations in air pollution. We here also discuss the impact of wind direction on all pollutants in all studied districts, using annulus plots (Figures A7-11, Appendix A). For example, these plots reveal how wind patterns contribute to the spatial distribution of pollutants, with areas like Centro and Belisario experiencing higher NO₂ concentrations due to the accumulation of traffic-related emissions under low wind speeds, while suburban areas like Los Chillos show elevated O₃ levels influenced by wind-driven transport of precursors and enhanced photochemical activity. Additionally, throughout the manuscript, we discuss the role of other environmental conditions, such as the El Niño-driven drought in 2023–2024, which exacerbated wildfires and reduced hydropower availability, leading to increased PM₂.₅ and SO₂ levels during the electricity crises (Sections 3.1, 3.2.2, and 3.2.5). We also note the impact of thermal inversions, particularly in densely populated districts, which trap pollutants and amplify temporal peaks during dry seasons (Section 3.1 and Section 3.3). 

We hope it is quite informative and sufficient, as it seems a bit more complex to keep adding more figures for each of the eight locations, for five pollutants, six years, and each meteorological or other environmental variable. Such an extensive analysis would risk diluting the study’s primary focus on illustrating the 'air quality rollercoaster' driven by new socio-political and climate-related challenges, while also exceeding the practical scope of this paper in terms of length and complexity. Nonetheless, we believe the current additions, including the discussion of wind direction and other environmental factors, provide a robust understanding of how environmental conditions shape both temporal and spatial changes in air pollution across Quito. 

3- The impact of climate change on time series is discussed. 

Yes, we do discuss it in the manuscript, as it is quite an important point of this study. While quite lucky to have this air pollution monitoring network, we do not have enough data to directly study the impact of climate change on air quality in this city. However, based on other studies, we know that the Amazonian record drought of 2023/24 is caused by climate change, a phenomenon that has exacerbated local issues in terms of hydropower production and seasonal wildfires. We show that over the study period, the levels of most air pollutants increase, which unfortunately undermines all local sustainability efforts (e.g., improved fuel and technology quality, etc.). Which is the main message of this study – underline the gaps in urban environmental quality resilience while facing new challenges.  

This paper focused on the air pollution spatiotemporal distribution of high-elevation tropical metropolis, Quito. Starting from the complex impact of multiple crises on urban air quality, such as climate change, energy crisis, social and political turbulence, and the COVID-19, the study explored the challenges of developing countries in climate adaptation and energy planning. The structure and logic of the paper are clear, and the research has certain social reference value. I still have some questions to raise here. 

（1）I don’t quite agree with your view point about ‘there is a notable scarcity of longitudinal research in developing nations’. Please clearly identifies gaps in longitudinal air quality studies in developing nations, particularly in unlisted cities like Quito.  

We agree, we should have been a bit more specific and sustain it by references. While some studies from the developing countries exist, most are focused on large cities and megacities (Quito population is 2.7 million). We added some extra information and more references, as follows: “Though numerous studies have documented air pollution trends in developed and developing countries [28], [29], [33], [34], there is a notable scarcity of longitudinal research in small and mid-size cities of the Global South [35]. Gaps in air quality data hamper a comprehensive understanding of air quality dynamics in these regions [36], [37]. Inspired by the "Air Quality Stripes" project [38], this study aims to provide a perspective on air pollution variability in Quito, Ecuador, an unlisted city in global air quality assessments.” y focusing on Quito, our study addresses this research gap, offering a longitudinal analysis of air quality trends from 2019 to 2024 and highlighting the compounded impacts of climate change, energy crises, and socio-political instability on a mid-size city in the Global South, thereby contributing to a more balanced understanding of urban air pollution challenges across diverse urban contexts. 

(2) I think if you can link climate-driven energy crises to pollution spikes, that would emphasize systemic vulnerabilities.  

You are completely right. We did that in Figure 2 by shading the electrical crises periods in red, which helps visualize the impact of crises on air quality. This is exactly the main idea of the study, systemic vulnerability to new challenges that undermine any previous nationwide efforts to clear the air and again exposes the population to bad air quality. Then even further, every next figure for each of the criteria pollutant diurnal trends, in almost every case, shows an increase during the months with electricity crisis. Additionally, the new Figures A2-11 in Appendix A (discussion in the Section 3.3) illustrate how these spikes vary spatially, with industrial areas like Carapungo experiencing a 255% SO₂ increase, underscoring the systemic vulnerability of Quito’s energy infrastructure to climate-driven disruptions and the urgent need for resilient energy planning to mitigate such pollution surges. 

(3) The causal mechanisms linking crises to pollution trends require deeper analysis. While temporal correlations may suggest the occurrence of state changes, robust policy recommendations require a deeper analytical foundation to establish causality and contextualize driving mechanisms. 

Thank you for the valuable suggestion. This study focuses on describing how air quality changed between 2019 and 2024 in possible relation to multiple crises we lived through, rather than establishing direct causal links. We acknowledge that a deeper causal analysis would strengthen the foundation for policy recommendations, as the reviewer suggests. However, our descriptive approach still provides critical insights into the 'air quality rollercoaster' experienced in Quito, identifying key temporal correlations that highlight systemic vulnerabilities. For example, Figure 2 shows SO₂ peaking at 100 µg/m³ during the 2023–2024 electricity crises, linked to increased thermoelectric power use, which informs the urgent need for resilient energy planning. While we have explored causality methodology, based on Machine Learning, in previous work (e.g., air pollution and COVID-19 mortality), that lies beyond the scope of this paper. Nonetheless, we agree that deeper causal analysis is important and will consider it in future research. 

（4）Air pollution impacts from crisis events exhibit spatial disparities across regions within a nation. By relying on aggregated data from eight monitoring sites, the analysis captures overall trends but risks masking localized variations. To address this, we recommend prioritizing spatial heterogeneity in the analysis to better inform region-specific mitigation strategies. 

We recognize that aggregated data may mask spatial disparities, and we fully agree with the reviewer’s concern. To address this, we maintained the city-wide analysis using averaged data to capture overall trends, while incorporating a robust new Subchapter 3.3, Table 1, and Figures A2-11 in Appendix A, which prioritize spatial heterogeneity. As described in Section 2.3, these analyses use annulus plots to assess spatial variations across all eight monitoring sites, incorporating wind direction to reveal the extreme variability in Quito’s air quality, and box-plots to examine spatio-temporal trends, including annual medians and maxima (Section 3.3). 

There are many grammar errors in the paper that need to be addressed. 

We have done a grammar check, hopefully it has fixed all the errors. 

This study analyzes five criteria pollutants—carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), and particulate matter ≤ 2.5 µm (PM2.5)—during 2019-2024 in Quito, Ecuador, a high-elevation tropical metropolis. Despite long-term efforts to regulate emissions, air pollution levels continue to rise, driven by overlapping crises, including energy shortages, political unrest, and extreme weather events. The persistent failure to improve air quality underscores the vulnerability of developing nations to climate change-induced energy instability and the urgent need for adaptive, diversified, and resilient future energy planning. Without immediate shifts in climate adaptation policies, cities like Quito will continue to experience worsening air quality, with severe implications for public health and environmental sustainability.It is interesting and it is relevant and within the scope of the journal.The paper has certain novelty and advantages for this field research work, and has value for publishing in the Journal. Therefore, I suggest that this paper be accepted with further modification. 

I have some suggestions as follows: 

1.What is the average annual decrease and proportion of air pollution from 2019 to 2024? 

We added Table 1 (Subsection 3.3) to show pollution changes over the study period for each district of Quito, this table provides the proportion of change for each pollutant from 2019 to 2024 across all districts. We also prepared five new figures that show average concentration and maximum level evolution to deepen the analysis for temporal distribution of pollution trends during the study period (See Figures A2-6, Appendix A). 

2 . What is the relationship between this sentence in the abstract and the title? Can be included in the introduction? 

Among the current challenges that human population needs to address are threats of global pandemics, increasing socioeconomic inequality, especially in developing countries, and the anthropogenic climate change. The effect of the latter has been amplified with the arrival of 2023/24 El Niño, causing an exceptional drought in the whole Amazon basin significantly affecting fire conditions and hydroelectric power production in several South American countries, including Ecuador. 

Thank you for your insightful comment.  The sentences in the abstract about global challenges, including pandemics, socioeconomic inequality, and climate change amplified by the 2023/24 El Niño, directly relate to the title 'Urban Air Pollution in the Global South: A Never-Ending Crisis?', by framing the systemic pressures driving the air quality crisis in Quito. They highlight how climate-driven events, like the climate change and El Niño-induced drought, exacerbate pollution through reduced hydroelectric power and increased wildfire risks, aligning with the 'never-ending crisis' theme of the study (Sections 3.1, 3.2). These sentences are already included in the second paragraph of the Introduction, where we discuss global challenges affecting air quality in the Global South, such as the climate crisis and its impact on hydroelectric production in Ecuador. We retained them in the abstract to provide critical context for the study’s focus on overlapping crises, ensuring readers immediately grasp the broader systemic drivers behind Quito’s air quality challenges. 

3.There should be some new references. 

 [1]The Characteristics of Air Quality Changes in Hohhot City in China and their Relationship with Meteorological and Socio-economic Factors. Aerosol and Air Quality Research. 2024.https://doi.org/10.4209/aaqr.230274 

 [2]Comparative Analysis of Multiple Deep Learning Models for Forecasting Monthly Ambient PM2.5 Concentrations: A Case Study in Dezhou City, China. Atmosphere 2024, 15(12), 1432. https://doi.org/10.3390/atmos15121432 

We agree, these studies are a good reflection of increasing research not only in developed but also rapidly developing countries like China, for instance. We added the references and corrected the sentences of introduction, as follows:  

“Though numerous studies have documented air pollution trends in developed and developing countries [28], [29], [33], [34], there is a notable scarcity of longitudinal research in small and mid-size cities of the Global South [35]. Gaps in air quality data hamper a comprehensive understanding of air quality dynamics in these regions [36], [37]. Inspired by the "Air Quality Stripes" project [38], this study aims to provide a perspective on air pollution variability in Quito, Ecuador, an unlisted city in global air quality assessments.” 

1. Through further proofreading, the quality of this article should be greatly improved. 

We did the proofreading, fixed several grammatical and style errors. Thank you.