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the Creative Commons Attribution 4.0 License.
| 01 Dec 2025
Ozone trends and drivers at a Southern Hemisphere background site in Chile
Abstract. Tropospheric ozone (O3) is a significant anthropogenic climate forcer with uncertain distribution in the Southern Hemisphere due to sparse observations. This study analyzes 28 years of in situ ozone, methane, carbon monoxide, and meteorological data at Tololo (30.17° S, 70.80° W, 2154 m a.s.l.), Chile, integrating reanalysis and atmospheric chemistry modeling. Here we identify a rising ozone trend of 2.1 ± 0.8 ppbv per decade since 2006, primarily driven by increasing background methane. We quantify contributions from biomass burning and stratosphere-to-troposphere transport, each adding approximately 5 ppbv per event during late winter and spring O3 maximum. Stratospheric intrusions are linked to synoptic-scale troughs and cutoff lows, modulated by El Niño Southern Oscillation phases. These findings enhance understanding of ozone variability in the Southern Hemisphere free troposphere and underscore the importance of sustained observations at Tololo to monitor tropospheric ozone dynamics amid climate change.
Competing interests: Maria Kanakidou is an editor of Atmospheric Chemistry and Physics
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.-
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-5643', Anonymous Referee #1, 23 Dec 2025
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AC2: 'Reply on RC1', Laura Gallardo, 07 Apr 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5643/egusphere-2025-5643-AC2-supplement.pdf
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AC2: 'Reply on RC1', Laura Gallardo, 07 Apr 2026
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RC2: 'Comment on egusphere-2025-5643', Anonymous Referee #2, 20 Jan 2026
This manuscript provides a thorough analysis of the long-term surface ozone trend at Tololo, Chile. Tololo is a very important and unique monitoring site, the topic is within the scope of the journal, and the findings are of value to the research community. In general, the findings of the study are supported by the data and the analysis, but there are three issues that need to be addressed before the paper is published, as described below.
Major Comments:
1) The authors conclude that rising methane is the most likely explanation for the increase in ozone, but they don’t evaluate this conclusion in terms of studies that have quantified the impact of rising methane on the long-term tropospheric ozone trend. For example, model assessments suggest that a 100 ppbv enhancement in methane mixing ratios could lead to an increase of 2.57-3.86 Tg/yr in the tropospheric ozone burden (West et al., 2007; Fiore et al., 2008; Zhang et al., 2016). Is your observed increase of ozone from 2006 to 2023 consistent with these model estimates?
Fiore, A. M., et al. (2008), Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality, J. Geophys. Res., 113, D08307, doi:10.1029/2007JD009162.
West, J.J., et al., 2007. Ozone air quality and radiative forcing consequences of changes in ozone precursor emissions. Geophysical Research Letters, 34(6).
Zhang, Y., et al. (2016), Tropospheric ozone change from 1980 to 2010 dominated by equatorward redistribution of emissions, Nature Geoscience, 9(12), p.875, doi: 10.1038/NGEO2827.
2) Figure 3 indicates that specific humidity has decreased since 1996. Given that drier air has greater ozone concentrations than moist air, could the ozone increase simply be explained by a shift in circulation that is bringing drier air to the site? I recommend selecting a specific humidity threshold value to filter out moist air (perhaps throw out the wettest 25% of data). You will have to use the same threshold for each year, which will result in more data being thrown out in the early part of the record, but this is necessary in order to be certain that you are looking at similarly dry air masses in each year. Then calculate the ozone trend for the dry air masses. If the ozone trend diminishes then one can conclude that the increased frequency of dry air masses was contributing to the trend. But if the trend stays the same then you have clear evidence that ozone is increasing in the dry air masses, consistent with increasing methane concentrations.
3) Regarding the identification of stratospheric intrusions, I have no doubt that aged stratospheric air reaches Tololo, however, I am very skeptical that the simple method of flagging events with high ozone and low water vapor can distinguish between a stratospheric intrusion and aged air from the mid- or upper troposphere. None of the ozone values observed at Tololo are very high (do any values exceed 60 ppbv?) so we can conclude that fresh intrusions do not reach the site. Rather, any intrusion that reaches the site would be heavily diluted with tropospheric air. I think that all the authors can say is that they have selected data that are likely to be indicative of the mid- and upper troposphere, and that these air masses contain an indeterminant quantify of stratospheric ozone.
Several of these authors are also authors on a recent ACP paper that investigates hemispheric differences in ozone across the stratosphere–troposphere exchange region (Seguel et al., 2025, https://doi.org/10.5194/acp-25-8553-2025). Seguel et al. show observed ozone vs. H2O in the southern hemisphere UTLS region, and it’s clear that for ozone mixing ratios between 100 and 200 ppbv (indicative of the lowermost stratosphere) there is a very wide range of H2O mixing ratio values (see their Figure 2b). How do your values of ozone vs H2O (within intrusion events) compare to the range of values reported by Seguel et al.? Are your values indicative of the lowermost stratosphere? Or do they simply resemble the typical range of values encountered in the mid- and upper troposphere?
Minor Comments:
The paper states that the site is affected by upslope winds during the day, and downslope winds at night. Previous studies (e.g. Gaudel et al., 2018) have used nighttime data at mountaintop sites in order to focus on the free troposphere and to limit the impact of local air masses. Have you filtered your data so that you only focus on nighttime observations?
The title gives no indication if this paper focuses on the troposphere or stratosphere. I recommend changing the title to “Tropospheric ozone trends and drivers…”
page 2, line 11
Innes et al. 2015 is missing from the list of references
page 2, line 14
Please specify if you are just talking about ozone trends in the free troposphere, because if we look at Figure 2.8 of IPCC AR6 WG1 we see that ozone trends in the lower troposphere have a range of positive and negative values at northern midlatitudes.
page 2, line 17
This statement “particularly variations in stratosphere-troposphere exchange” gives the impression that STE is a major driver of ozone trends, but it’s most likely only a minor component. STE only contributes 10% to the global tropospheric ozone budget, so if STE changed by an unrealistically large amount of 10%, it would only have a 1% impact on the tropospheric ozone budget. The paper by Skerlak et al. only looks at the changes in STE flux from 1979 to 2011. While their calculation shows an increase in net STT for 2001-2011, we have no way of knowing if this is a short-lived anomaly, as their analysis ended 15 years ago. The paper by Li et al. 2024 focuses on wintertime when ozone production is limited, so any ozone trend driven by STE is not representative of the full year.
page 2, line 20
An important paper that explores the impact of climate variability on STE is Neu et al., 2014.
Neu et al., 2014, Tropospheric ozone variations governed by changes in stratospheric circulation, Nature Geoscience, DOI: 10.1038/NGEO2138
Page 2, line 23
When discussing ozone increases since the 20th century the key TOAR paper to cite is Tarasick et al. 2019, with further consolidation of evidence assessed by IPCC AR6, WGI, Chapter 2 (Gulev et al., 2021).
Page 3, line 6
The key study that demonstrated the impact on ozone of emissions shifting from mid-latitudes toward the equator is Zhang et al., 2016.
Zhang, Y., et al. (2016), Tropospheric ozone change from 1980 to 2010 dominated by equatorward redistribution of emissions, Nature Geoscience, 9(12), p.875, doi: 10.1038/NGEO2827.
Page 3, line 21
A key study on ozone production over the South Pacific is Schultz et al., 1999, On the origin of tropospheric ozone and NOx over the tropical South Pacific, JGR, 104(D5)
Figure S3
The figure legend says Tololo data are in orange, but the figure caption says the Tololo data are colored blue. Which is it?
Page 7, line 2
In the statement, “This index is elaborated by NOAA Physical Sciences Laboratory”, the word elaborated is not used correctly, and I’m not sure what you are trying to say. Can you just say “This index is provided by the NOAA Physical Sciences Laboratory”?
page 26, line 13
“growing median trend” implies that the trend is changing over time and becoming stronger, for both the 1996-2006 period and the 2006-2023 period. I don’t think this is what you are trying to say. If the trend value is not changing just say “positive ozone trend”.
Citation: https://doi.org/10.5194/egusphere-2025-5643-RC2 -
AC1: 'Reply on RC2', Laura Gallardo, 07 Apr 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5643/egusphere-2025-5643-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Laura Gallardo, 07 Apr 2026
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-5643', Anonymous Referee #1, 23 Dec 2025
This manuscript investigates the ozone trends and drivers at a Southern Hemisphere background site in Chile. It is well structured and written and illustrates original and interesting results. I suggest accepting the manuscript for publication after taking into consideration the following comments.
Comments
Page 2, lines 18-19: The sentence needs elaboration. The changes of the net stratospheric influx in STE are linked to changes of the stratospheric Brewer–Dobson Circulation and the amount of ozone in the lowermost stratosphere, which are strongly influenced in a changing climate by the emissions of ODSs and GHGs (Butchart 2014, Banerjee et al 2016, Morgenstern et al 2018, Meul et al 2018, Akritidis et al 2019).
Page 6, line 16: Please clarify within the text the meteorological reasons for this decision.
Page 7, line 29: Please describe at first place what were the meteorological fields to drive the simulations of TM4-ECPL.
Page 8, lines 16-18: The criterion for the identification of SI is arbitrary and I suggest to comment the limitations. It could be possible that subsidence/transport of upper tropospheric air could possible have a similar result and can be accounted as SI. Another important issue for the identification is the lifetime of the filamentary structures of stratospheric origin in the troposphere which can persist for several days in the troposphere before cascading down to smaller scales and getting mixed with the surrounding air. Due to this dissipation process of stretched filamentary structures and the irreversible mixing associated with deep stratospheric intrusions, the characteristic signs of stratospheric air with high O3 content, high values of potential vorticity (PV) and low humidity dilute and disappear over the period of a few days. This complicates the observation of stratospheric intrusions in the lower troposphere and especially within the atmospheric boundary layer and near surface unless in cases of direct and intense deep intrusion events that can result to distinct spikes in measured stratospheric tracer concentrations (e.g. see Stohl et al., 2003).
Page 9, lines 24-25: Maybe you also mention here that the explanatory variables used are indicated in Table 2. This will help the reader while reading this paragraph.
Page 14, lines 20-23: Since TM4-ECPL has a dedicated stratospheric ozone tracer, you may look also O3s at Tololo for El Nino and La Nina years.
Page 15, lines 10-11: I am rather confused with the plots of anomalies (Figures 6 and 7) with respect to the 12-day period mean. I think that if you want to clearly illustrate more thoroughly the passage of the trough, I would rather suggest plotting the actual fields of geopotential height and vertical velocity at 500 hPa.
Page 18, lines 1-4: There is a misunderstanding here. Stratospheric air penetrating into the troposphere is characterized by high PV- values (not negative as discussed here). I would rather suggest plotting the actual PV values rather than the anomalies to see the evolution of the filament. Potential vorticity generally provides a good indication of air of recent stratospheric origin. Threshold values for dynamical tropopause reported in the literature range from 1.0 or 1.6 pvu (Stohl et al. 2000) to 3.5 pvu (Hoerling et al. 1991) with a value of 2 pvu used most often (Hoskins and Berrisford 1988; Stohl et al. 2003; Akritidis et al. 2019). Partly, this value depends on the vertical resolution of the meteorological data, and partly it depends on the synoptic situation and the geographical location (Hoinka 1997).
Page 22, line 25: The way is written here sounds more like a chemical effect (e.g. ozone chemical loss because of more water vapour in unpolluted environment). But later on you attribute it to dynamical effect from more efficient mixing upwards of boundary layer air rich in water vapour and low in ozone. Please clarify.
Citation: https://doi.org/10.5194/egusphere-2025-5643-RC1 -
AC2: 'Reply on RC1', Laura Gallardo, 07 Apr 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5643/egusphere-2025-5643-AC2-supplement.pdf
-
AC2: 'Reply on RC1', Laura Gallardo, 07 Apr 2026
-
RC2: 'Comment on egusphere-2025-5643', Anonymous Referee #2, 20 Jan 2026
This manuscript provides a thorough analysis of the long-term surface ozone trend at Tololo, Chile. Tololo is a very important and unique monitoring site, the topic is within the scope of the journal, and the findings are of value to the research community. In general, the findings of the study are supported by the data and the analysis, but there are three issues that need to be addressed before the paper is published, as described below.
Major Comments:
1) The authors conclude that rising methane is the most likely explanation for the increase in ozone, but they don’t evaluate this conclusion in terms of studies that have quantified the impact of rising methane on the long-term tropospheric ozone trend. For example, model assessments suggest that a 100 ppbv enhancement in methane mixing ratios could lead to an increase of 2.57-3.86 Tg/yr in the tropospheric ozone burden (West et al., 2007; Fiore et al., 2008; Zhang et al., 2016). Is your observed increase of ozone from 2006 to 2023 consistent with these model estimates?
Fiore, A. M., et al. (2008), Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality, J. Geophys. Res., 113, D08307, doi:10.1029/2007JD009162.
West, J.J., et al., 2007. Ozone air quality and radiative forcing consequences of changes in ozone precursor emissions. Geophysical Research Letters, 34(6).
Zhang, Y., et al. (2016), Tropospheric ozone change from 1980 to 2010 dominated by equatorward redistribution of emissions, Nature Geoscience, 9(12), p.875, doi: 10.1038/NGEO2827.
2) Figure 3 indicates that specific humidity has decreased since 1996. Given that drier air has greater ozone concentrations than moist air, could the ozone increase simply be explained by a shift in circulation that is bringing drier air to the site? I recommend selecting a specific humidity threshold value to filter out moist air (perhaps throw out the wettest 25% of data). You will have to use the same threshold for each year, which will result in more data being thrown out in the early part of the record, but this is necessary in order to be certain that you are looking at similarly dry air masses in each year. Then calculate the ozone trend for the dry air masses. If the ozone trend diminishes then one can conclude that the increased frequency of dry air masses was contributing to the trend. But if the trend stays the same then you have clear evidence that ozone is increasing in the dry air masses, consistent with increasing methane concentrations.
3) Regarding the identification of stratospheric intrusions, I have no doubt that aged stratospheric air reaches Tololo, however, I am very skeptical that the simple method of flagging events with high ozone and low water vapor can distinguish between a stratospheric intrusion and aged air from the mid- or upper troposphere. None of the ozone values observed at Tololo are very high (do any values exceed 60 ppbv?) so we can conclude that fresh intrusions do not reach the site. Rather, any intrusion that reaches the site would be heavily diluted with tropospheric air. I think that all the authors can say is that they have selected data that are likely to be indicative of the mid- and upper troposphere, and that these air masses contain an indeterminant quantify of stratospheric ozone.
Several of these authors are also authors on a recent ACP paper that investigates hemispheric differences in ozone across the stratosphere–troposphere exchange region (Seguel et al., 2025, https://doi.org/10.5194/acp-25-8553-2025). Seguel et al. show observed ozone vs. H2O in the southern hemisphere UTLS region, and it’s clear that for ozone mixing ratios between 100 and 200 ppbv (indicative of the lowermost stratosphere) there is a very wide range of H2O mixing ratio values (see their Figure 2b). How do your values of ozone vs H2O (within intrusion events) compare to the range of values reported by Seguel et al.? Are your values indicative of the lowermost stratosphere? Or do they simply resemble the typical range of values encountered in the mid- and upper troposphere?
Minor Comments:
The paper states that the site is affected by upslope winds during the day, and downslope winds at night. Previous studies (e.g. Gaudel et al., 2018) have used nighttime data at mountaintop sites in order to focus on the free troposphere and to limit the impact of local air masses. Have you filtered your data so that you only focus on nighttime observations?
The title gives no indication if this paper focuses on the troposphere or stratosphere. I recommend changing the title to “Tropospheric ozone trends and drivers…”
page 2, line 11
Innes et al. 2015 is missing from the list of references
page 2, line 14
Please specify if you are just talking about ozone trends in the free troposphere, because if we look at Figure 2.8 of IPCC AR6 WG1 we see that ozone trends in the lower troposphere have a range of positive and negative values at northern midlatitudes.
page 2, line 17
This statement “particularly variations in stratosphere-troposphere exchange” gives the impression that STE is a major driver of ozone trends, but it’s most likely only a minor component. STE only contributes 10% to the global tropospheric ozone budget, so if STE changed by an unrealistically large amount of 10%, it would only have a 1% impact on the tropospheric ozone budget. The paper by Skerlak et al. only looks at the changes in STE flux from 1979 to 2011. While their calculation shows an increase in net STT for 2001-2011, we have no way of knowing if this is a short-lived anomaly, as their analysis ended 15 years ago. The paper by Li et al. 2024 focuses on wintertime when ozone production is limited, so any ozone trend driven by STE is not representative of the full year.
page 2, line 20
An important paper that explores the impact of climate variability on STE is Neu et al., 2014.
Neu et al., 2014, Tropospheric ozone variations governed by changes in stratospheric circulation, Nature Geoscience, DOI: 10.1038/NGEO2138
Page 2, line 23
When discussing ozone increases since the 20th century the key TOAR paper to cite is Tarasick et al. 2019, with further consolidation of evidence assessed by IPCC AR6, WGI, Chapter 2 (Gulev et al., 2021).
Page 3, line 6
The key study that demonstrated the impact on ozone of emissions shifting from mid-latitudes toward the equator is Zhang et al., 2016.
Zhang, Y., et al. (2016), Tropospheric ozone change from 1980 to 2010 dominated by equatorward redistribution of emissions, Nature Geoscience, 9(12), p.875, doi: 10.1038/NGEO2827.
Page 3, line 21
A key study on ozone production over the South Pacific is Schultz et al., 1999, On the origin of tropospheric ozone and NOx over the tropical South Pacific, JGR, 104(D5)
Figure S3
The figure legend says Tololo data are in orange, but the figure caption says the Tololo data are colored blue. Which is it?
Page 7, line 2
In the statement, “This index is elaborated by NOAA Physical Sciences Laboratory”, the word elaborated is not used correctly, and I’m not sure what you are trying to say. Can you just say “This index is provided by the NOAA Physical Sciences Laboratory”?
page 26, line 13
“growing median trend” implies that the trend is changing over time and becoming stronger, for both the 1996-2006 period and the 2006-2023 period. I don’t think this is what you are trying to say. If the trend value is not changing just say “positive ozone trend”.
Citation: https://doi.org/10.5194/egusphere-2025-5643-RC2 -
AC1: 'Reply on RC2', Laura Gallardo, 07 Apr 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5643/egusphere-2025-5643-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Laura Gallardo, 07 Apr 2026
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Charlie Opazo
Camilo Menares
Kevin Basoa
Nikos Daskalakis
Maria Kanakidou
Carmen Vega
Nicolás Huneeus
Roberto Rondanelli
Rodrigo Seguel
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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This manuscript investigates the ozone trends and drivers at a Southern Hemisphere background site in Chile. It is well structured and written and illustrates original and interesting results. I suggest accepting the manuscript for publication after taking into consideration the following comments.
Comments
Page 2, lines 18-19: The sentence needs elaboration. The changes of the net stratospheric influx in STE are linked to changes of the stratospheric Brewer–Dobson Circulation and the amount of ozone in the lowermost stratosphere, which are strongly influenced in a changing climate by the emissions of ODSs and GHGs (Butchart 2014, Banerjee et al 2016, Morgenstern et al 2018, Meul et al 2018, Akritidis et al 2019).
Page 6, line 16: Please clarify within the text the meteorological reasons for this decision.
Page 7, line 29: Please describe at first place what were the meteorological fields to drive the simulations of TM4-ECPL.
Page 8, lines 16-18: The criterion for the identification of SI is arbitrary and I suggest to comment the limitations. It could be possible that subsidence/transport of upper tropospheric air could possible have a similar result and can be accounted as SI. Another important issue for the identification is the lifetime of the filamentary structures of stratospheric origin in the troposphere which can persist for several days in the troposphere before cascading down to smaller scales and getting mixed with the surrounding air. Due to this dissipation process of stretched filamentary structures and the irreversible mixing associated with deep stratospheric intrusions, the characteristic signs of stratospheric air with high O3 content, high values of potential vorticity (PV) and low humidity dilute and disappear over the period of a few days. This complicates the observation of stratospheric intrusions in the lower troposphere and especially within the atmospheric boundary layer and near surface unless in cases of direct and intense deep intrusion events that can result to distinct spikes in measured stratospheric tracer concentrations (e.g. see Stohl et al., 2003).
Page 9, lines 24-25: Maybe you also mention here that the explanatory variables used are indicated in Table 2. This will help the reader while reading this paragraph.
Page 14, lines 20-23: Since TM4-ECPL has a dedicated stratospheric ozone tracer, you may look also O3s at Tololo for El Nino and La Nina years.
Page 15, lines 10-11: I am rather confused with the plots of anomalies (Figures 6 and 7) with respect to the 12-day period mean. I think that if you want to clearly illustrate more thoroughly the passage of the trough, I would rather suggest plotting the actual fields of geopotential height and vertical velocity at 500 hPa.
Page 18, lines 1-4: There is a misunderstanding here. Stratospheric air penetrating into the troposphere is characterized by high PV- values (not negative as discussed here). I would rather suggest plotting the actual PV values rather than the anomalies to see the evolution of the filament. Potential vorticity generally provides a good indication of air of recent stratospheric origin. Threshold values for dynamical tropopause reported in the literature range from 1.0 or 1.6 pvu (Stohl et al. 2000) to 3.5 pvu (Hoerling et al. 1991) with a value of 2 pvu used most often (Hoskins and Berrisford 1988; Stohl et al. 2003; Akritidis et al. 2019). Partly, this value depends on the vertical resolution of the meteorological data, and partly it depends on the synoptic situation and the geographical location (Hoinka 1997).
Page 22, line 25: The way is written here sounds more like a chemical effect (e.g. ozone chemical loss because of more water vapour in unpolluted environment). But later on you attribute it to dynamical effect from more efficient mixing upwards of boundary layer air rich in water vapour and low in ozone. Please clarify.