Rodrigo Seguel a, b, Charlie Opazo a, b y Lucas Castillo a, b
a Center for Climate Science and Resilience Research
b Department of Geophysics, Faculty of Physical and Mathematical Sciences, University of Chile
Edited by: José Barraza, CR2 Science Communicator
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Trends in Surface Ozone in Major South American Cities
Over the past decade, surface ozone levels in major cities of South America have increased or remained stable. In the Metropolitan Region, surface-level ozone has shown an upward trend since 2017.
Context
The Tropospheric Ozone Assessment Report (TOAR), developed by the International Global Atmospheric Chemistry (IGAC),project, provides an up-to-date scientific assessment of the global distribution and trends in tropospheric ozone, which spans from the Earth’s surface to approximately 10 to 15 kilometers in altitude.
The first phase of TOAR (2014-2019) produced an open-access database with easily accessible web services for evaluating ozone metrics at all available monitoring sites worldwide. This database offers the scientific community a global view of surface ozone based on observational data.
TOAR is currently in its second phase (TOAR-II, 2020-2025). involving more than 150 researchers from 31 countries, organized into 16 working groups [1]. This phase aims to update the global distribution and trends in tropospheric ozone, including its precursors (gases that, through chemical reactions, produce ozone). Like the first phase, TOAR-II aims to quantify the impacts of tropospheric ozone on climate, human health, and vegetation.
State of Knowledge
Recent research shows that global tropospheric ozone levels have increased by approximately 45% since 1850 due to emissions of anthropogenic precursors (Szopa et al., 2021). Additionally, surface ozone has increased by 32% to 71% (with significant uncertainty) in the atmosphere over rural areas in the Northern Hemisphere compared to historical observations (1896-1975) (Tarasick et al., 2019). Since the mid-1990s, free tropospheric ozone (at altitudes of approximately 3 to 12 kilometers) [2] has increased by 1 to 4 nmol mol-1 per decade across most mid-latitude regions in the Northern Hemisphere and by 1 to 5 nmol mol-1 per decade in tropical areas (high-confidence data) (Gulev et al., 2021).
In the Southern Hemisphere, limited monitoring station coverage has hindered estimating ozone trends. However, tropospheric ozone column observations since the mid-1990s suggest an increase of less than 1 nmol mol-1 per decade in mid-latitudes, with medium confidence (Cooper et al., 2020; Gulev et al., 2021).
South America
From a global perspective, South America is considered by the scientific community as an under-studied region where ozone trend estimates have rarely been addressed comprehensively. For this reason, the Tropospheric Ozone Precursors Working Group has focused part of its efforts on estimating trends in surface ozone and its precursors in South America since the early 21st century. Results published in the 2024 special issue of Copernicus yield the following conclusions:
- Trends in surface ozone in monitored major South American cities have either increased or remained stable, with no evidence of reduction in the past decade.
- Rising trends can be attributed to photochemical regimes that efficiently transform anthropogenic precursors into chemical products favoring ozone accumulation.
- These results suggest a phenomenon termed “climate penalty” for ozone, whereby extreme events tend to increase ozone levels, worsening air quality. In Chile, meteorological conditions favoring forest fires led to the emission of ozone precursors. In southern Brazil, this penalty is associated with extended heatwaves capable of increasing tropospheric ozone formation.
Relevant Findings for Chile
- In the Metropolitan Region, surface ozone levels decreased by 2 nmol mol-1 per decade from 1997 to 2017 (with very high confidence). However, from 2017 onwards, ozone trends increased by 0.6 nmol mol-1 per year (with high confidence), representing a cumulative increase of 3 nmol mol-1 over five years. Thus, the past five years have seen a reversal equivalent to 20 years of progress in ozone reduction (Figure 1). This increase over five years is partly explained by warmer summers, ozone precursors emitted in forest fires, the pandemic’s effects on anthropogenic emissions, and inconsistent control of nitrogen oxides and volatile organic compounds (represented by carbon monoxide in Figure 1), among other variables.
- In Los Andes, the highest short- and long-term ozone exposure risk levels were recorded at 88 and 58 nmol mol-1 , respectively. These values far exceed the World Health Organization’s recommended short- and long-term metrics of 51 and 31 nmol mol-1, respectively (WHO, 2021).
- The Tololo station, located in the Coquimbo region at an altitude of 2.2 km, is one of the few South American stations with a long enough time series to evaluate changes in the baseline ozone level. Between 2006 and 2014, a cumulative increase of 2 nmol mol-1 was observed, signaling regional and hemispheric changes in the baseline ozone level in the Southern Hemisphere’s troposphere.
Figure 1 shows surface ozone trends (Panel A), carbon monoxide (Panel D), and nitrogen oxides (Panel E) based on monthly anomalies in Santiago. In these panels, orange points indicate the first three months of each year, the red line corresponds to the 50th percentile (or median), and the blue lines represent the remaining percentiles. A shaded vertical red line represents the trend change point (November 2017 for ozone) and its 95% confidence interval. Notably, Panel A shows that nearly all ozone reductions achieved over the past 20 years were reversed from 2017 onwards. Panels b and c show the trend of each percentile (in intervals of 5) before and after 2017. Up until 2017, the highest percentiles (above 80) showed the most significant decreasing trends (panel b). In contrast, following the change in the 2017 trend, these percentiles demonstrated significant increasing trends (panel c).
Adapted from Seguel et al. (2024): https://doi.org/10.5194/egusphere-2024-328
Ongoing Activities: Regional Tropospheric Ozone Assessment in South America
To continue improving knowledge and confidence in information for South America, the TOAR-II Steering Committee approved a specific assessment for this region [3], this assessment aims to bridge information gaps caused by the limited surface monitoring coverage by using satellite observations and regional models and reporting key risks and findings with corresponding uncertainty estimates.
References
Cooper, O. R., Schultz, M. G., Schröder, S., Chang, K. L., Gaudel, A., Benítez, G. C., Cuevas, E., Fröhlich, M., Galbally, I. E., Molloy, S., Kubistin, D., Lu, X., McClure-Begley, A., Nédélec, P., O’Brien, J., Oltmans, S. J., Petropavlovskikh, I., Ries, L., Senik, I., Sjöberg, K., Solberg, S., Spain, G. T., Spangl, W., Steinbacher, M., Tarasick, D., Thouret, V., & Xu, X. (2020). Multi-decadal surface ozone trends at globally distributed remote locations. Elementa, 8, 23. https://doi.org/10.1525/elementa.420.
Gulev, S.K., P.W. Thorne, J. Ahn, F.J. Dentener, C.M. Domingues, S. Gerland, D. Gong, D.S. Kaufman, H.C. Nnamchi, J. Quaas, J.A. Rivera, S. Sathyendranath, S.L. Smith, B. Trewin, K. von Schuckmann, & R.S. Vose. (2021). Changing State of the Climate System. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 287–422, doi: 10.1017/9781009157896.004.
Organización Mundial de la Salud (OMS). 2021. Global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. World Health Organization, Geneva, ISBN 978-92-4- 003422-8, ISBN 978-92-4-003421-1.
Seguel, R.J., Castillo, L., Opazo, C., Rojas, N., Nogueira, T., Cazorla, M., Gavidia-Calderón, M., Gallardo, L., Garreaud, R., Carrasco-Escaff, T., & Elshorbany, Y. (2024). Changes in South American Surface Ozone Trends: Exploring the Influences of Precursors and Extreme Events. Aceptado en: Atmospheric Chemistry and Physics. DOI: https://doi.org/10.5194/egusphere-2024-328.
Szopa, S., Naik, V., Adhikary, B., Artaxo, P., Berntsen, T., Collins, W.D., Fuzzi, S., Gallardo, L., Kiendler-Scharr, A., Klimont, Z., Liao, H., Unger, N., & Zanis, P. (2021). Short-Lived Climate Forcers. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 817–922, doi:10.1017/9781009157896.008.
Tarasick, D., Galbally, I. E., Cooper, O. R., Schultz, M. G., Ancellet, G., Leblanc, T., Wallington, T. J., Ziemke, J., Liu, X., Steinbacher, M., Staehelin, J., Vigouroux, C., Hannigan, J. W., García, O., Foret, G., Zanis, P., Weatherhead, E., Petropavlovskikh, I., Worden, H., Osman, M., Liu, J., Chang, K. L., Gaudel, A., Lin, M., Granados-Muñoz, M., Thompson, A. M., Oltmans, S. J., Cuesta, J., Dufour, G., Thouret, V., Hassler, B., Trickl, T., & Neu, J. L. (2019). Tropospheric ozone assessment report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties. Elem Sci Anth, 7, 39. https://doi.org/10.1525/elementa.376.
Notes
[1] Current Working Groups: Chemical Reanalysis Focus Working Group, East Asia Focus Working Group, Global and Regional Models Focus Working Group, HEGIFTOM Focus Working Group, Human Health Impacts of Ozone Focus Working Group, Machine Learning for Tropospheric Ozone Focus Working Group, Ozone Deposition Focus Working Group, Ozone over the Oceans Focus Working Group, Ozone and Precursors in the Tropics (OPT) Focus Working Group, Radiative Forcing Focus Working Group, ROSTEES Focus Working Group, Satellite Ozone Focus Working Group, South Asia Focus Working Group, Statistics Focus Working Group, Tropospheric Ozone Precursors (TOP) Focus Working Group, Urban Ozone Focus Working Group.
[2] Expressed in a molar fraction of ozone in the air.
[3] Assessments focused on Health, vegetation, climate, South America, Africa, stratosphere-troposphere exchange, and satellite observations.
