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https://doi.org/10.5194/egusphere-2025-5643
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2025-5643
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
01 Dec 2025
| 01 Dec 2025
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
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.
How to cite. Gallardo, L., Opazo, C., Menares, C., Basoa, K., Daskalakis, N., Kanakidou, M., Vega, C., Huneeus, N., Rondanelli, R., and Seguel, R.: Ozone trends and drivers at a Southern Hemisphere background site in Chile, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-5643, 2025.
Competing interests: Maria Kanakidou is an editor of Atmospheric Chemistry and Physics
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Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
Charlie Opazo
Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
Camilo Menares
Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
Kevin Basoa
Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Ministry of the Environment, Chile
Nikos Daskalakis
Laboratory for Modeling and Observation of the Earth System (LAMOS), Institute of Environmental Physics (IUP), University of Bremen, Bremen, Germany
Maria Kanakidou
Laboratory for Modeling and Observation of the Earth System (LAMOS), Institute of Environmental Physics (IUP), University of Bremen, Bremen, Germany
Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, Heraklion, Greece
Center for the Study of Air Quality and Climate Change, Foundation for Research and Technology – Hellas, Patras, Greece
Carmen Vega
Dirección Meteorológica de Chile
Nicolás Huneeus
Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
Roberto Rondanelli
Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
Rodrigo Seguel
Center for Climate and Resilience Research (CR2, FONDAP 15110009), Santiago, Chile
Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile
Short summary
We assert the role of methane and other drivers of change in explaining the growing tropospheric ozone (O3) trend at Tololo (30.17° S, 70.80° W, 2154 m a.s.l.), and we quantify the contributions of biomass burning and stratosphere-to-troposphere transport on O3, particularly during the late winter and spring. These findings enhance understanding of O3 variability in the Southern Hemisphere free troposphere and underscore the importance of sustained observations at Tololo amid climate change.
We assert the role of methane and other drivers of change in explaining the growing tropospheric...
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.