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Case study

Ozone pollution: a persistent challenge in large cities like Mexico City, Los Angeles and Beijing

Authors
  • Erik Velasco orcid logo (Molina Center for Energy and the Environment, Boston, MA, USA)
  • Armando Retama orcid logo (Molina Center for Energy and the Environment, Boston, MA, USA)
  • Luisa T. Molina orcid logo (Molina Center for Energy and the Environment, Boston, MA, USA)

This is version 1 of this article, the published version can be found at: https://doi.org/10.14324/111.444/ucloe.3448

Abstract

Over the past decade or so, many large cities around the world have made little to no progress in lowering ground-level ozone concentrations, despite significant reductions in key precursor pollutants directly emitted into the atmosphere. Ozone comes from complex chemical reactions in the air that make it difficult to control. Current control measures implemented in some cities have apparently reached their limit. While stricter emission regulations, improvements in technology and cleaner fuels have prevented a return to previous ozone levels, they have not reduced them further. They have triggered changes in the mixture of precursor species (i.e., nitrogen oxides and volatile organic compounds) due to differences in the extent to which their emissions have been reduced, emerging emission sources and the increasing relevance of emissions previously overlooked, such as those related to cooking and the use of household cleaning and personal care products. Similarly, as the contribution of typical emission sources (e.g., combustion of fossil fuels) has decreased, biogenic contributions have become more important, as well as the influence of regional and transboundary pollution. These changes have also responded to increasing urbanisation in the face of a changing climate that favours ozone production. There is no recipe that all cities can follow to tackle ambient ozone; however, it is necessary to review why ozone concentrations have not decreased in some cities and what they are doing about it in order to use it as a reference to update, improve and develop control measures tailored to local conditions, as actions implemented in one city may be ineffective or impractical in another. In such a context, this article examines the cases of three metropolitan areas: the Mexico City Metropolitan Area, the Los Angeles Basin and the Beijing–Tianjin–Hebei (Jing–Jin–Ji) region. These urban conglomerations, with different geography, meteorology, socioeconomic conditions and governance, have succeeded in reducing concentrations of many regulated pollutants to levels near or below air quality standards set to protect public health, but not for ozone. While these cities have robust and timely air quality management (i.e., air quality monitoring, emission inventories and air quality models), the current ozone challenge requires even greater efforts to understand the physical and chemical processes at the local and regional scales. This will enable informed actions that can adjust to changing environmental, social and economic scenarios, following a science-policy approach with a perspective of human rights and social justice.

Keywords: ground-level ozone, ambient ozone, surface ozone, air quality, air quality management, environmental policy, atmospheric chemistry, urban air pollution

1337 Views

Published on
30 Oct 2025
Peer Reviewed
License
Creative Commons Attribution 4.0

 Open peer review from Dan Osborn

Review
This is a set of descriptive case-studies of three metropolitan areas - Beijing, "Greater" Mexico City and Los Angeles. All are large areas with high populations and are known for the issues all of them face with air pollution and its consequent impacts on health and wellbeing. They all have different relationships with neighbouring areas in terms of interactions with other major settlements in the vicinity and factors linked to their location and its topographic nature. In addition to the case-studies there is a summary of the lessons that cities might learn when tackling low-level ozone pollution (that which affects the air in the city that people breath).

The paper has several strengths and some weaknesses. Overall, if some improvements can be made I feel this submission should be ready for full publication. Technically, I have asked for major revisions but authors should note I feel the general thrust of the paper and the lessons learnt sections are all valuable and worth publishing. So, making the improvements to the necessary standard should not be too difficult.

Strengths include the clear description of the situation of each city and an approach to each example that is reasonably consistent across the three. The paper does not include a great deal of new information on this complex subject but by juxtaposing the three metropolitan centres and drawing out lessons the submission does add to the field that uses science as a basis for policy and public health interventions. There are places where referencing is good and other areas - such as the sections describing policy measures - where more references would certainly help the reader and increase the rigour within the text.

Weaknesses that need some improvement are as follows:

1. Each example could be improved by including (where these are absent) references to reports or policy papers that support statements made about policies and interventions. At the bottom of PDF page 18, for example, there is a paragraph about the important monitoring co-ordination in the Beijing area. It would help to have one or two references to that system if possible. If the system was set out in the ref State Council, 2021 then please repeat that. Similar improvements could be made elsewhere such as: the paragraph at the bottom of page 17; the sections on CARB from p13 to p14 - if these all from the Parrish ref please make that clear to avoid any suggestion of inadvertent plagiarism and if not then please add references where possible.

2. Figures are based on data that have been displayed graphically. It is not clear how the figure was created from the data and a section needs to be inserted to explain that. Please insert a new section that describes how this was done or amend the Figure legends. This is important so that the reader can understand the methodology. Also, where limits or thresholds or standards of various kinds are referred to it would be useful to set out what these in terms of WHO or case specific ones. It is possible that some of these are local and not international and that was not clear to me.

3. The first paragraph of each case-study contains a good deal of general description and some material that is almost a conclusion. The authors should look at each of these paragraphs in order to see if the three can be made a little more consistent and less dependent on presenting any concluding material.

4. In each of the case studies there is discussion of the factors that might be limiting further falls in low-level ozone. The paper would be further strengthened by just a sentence or two on some of the physical processes that might be in train in one or all case studies. For example, all vehicles have an abrasive action involving braking and interaction with road surfaces. This generates particulate matter and, at least theoretically, EVs (especially being heavier than fossil fuel vehicles) might still be contributing to particulates in urban areas. Apologies if I missed this in the text.

5. The link between VOCs and nitrogen oxides is important in all cases of urban air pollution and it would help the reader if a few sentences could be inserted that explain the author's position on what constitutes a situation where one of the oxides becomes "limiting".

6. I note that the name of the Institution and the name of the last author are the same. A statement in an appropriate section (Conflicts of Interest?) should be inserted to explain any connection between the institution name and the last author.

7. I would ask the authors to add some references to the bullet points on future interventions that can be found on pages 21-23 of the PDF text. I think it likely that some of these points have already been suggested by others even if the compilation of the overall list is part of the paper's contribution to understanding the link between science and policy and associated interventions.

8. Some readers might argue that comparing these three urban areas that are so different is too difficult. The authors have shown it is not by drawing out some lessons to be acted on. However, in Table 1 the difference in geographical area is very clear and maybe does not reflect that fact that for MCMA (my "Greater" Mexico City) other urban areas also have a role to play in addition to the MCMA area itself. In contrast, it is clear that the China example includes both Beijing and linked urban areas. It would be good if the text and the Table were consistent in that respect as at present the reader is left with some uncertainty. If the authors want to keep the Table as it is then the text or Table legend needs to offer some clarification.

Note:
This review refers to round 1 of peer review.

 Open peer review from Kai Wang

Review
This manuscript presents the case studies and review on the challenges in controlling ground-level O3 pollution in large cities. Such topic is significant. Three metropolitan areas, namely the Mexico City Metropolitan Area (MCMA), the Los Angeles Basin (LA Basin), and the Beijing-Tianjin-Hebei Region (BTH, also known as Jing-Jin-Ji) are chosen as the case studies to examine their current O3 problems, dicsucss and review the associated impacts, main interventions and future challenges. The whole manuscript is well structured and well written, with very comprehensive review and discussions. I enjoyed reading it.
While I believe the manuscript is already in good form, there is a small issue, with regards to the abstract part, that could be further improved. Current abstract spends most of the words on the background of the research. It would be better to add a bit more on the general key findings, such as the key learnings and recommendations in section 5 and 6.

Note:
This review refers to round 1 of peer review.