| 05 Dec 2025
Multi-decadal ozone air quality and the role of temperature in Switzerland during summertime
Abstract. Tropospheric ozone (O3) is a greenhouse gas and air pollutant. Despite efforts to control O3 precursor emissions, O3 levels frequently exceed the Swiss air quality standards. We present multi-decadal summertime measurements of O3 and its precursors across Switzerland from 12 NABEL (Nationales Beobachtungsnetz für Luftfremdstoffe) stations, which are representative of traffic, (sub)urban, rural and background conditions. Average O3 levels have decreased at rural and background sites, remained constant at (sub)urban sites and increased under traffic conditions over the past two decades. Traffic, (sub)urban and rural sites exhibited a pronounced weekend effect at the beginning of the century, which has weakened over time and only persists under traffic conditions today, suggesting that O3 formation is becoming more NOx-sensitive. O3 exhibits a strong dependence on temperature (dO3/dT), which has weakened uniformly at all site types over time. At polluted sites, this effect could be associated with the decreasing influence of titration. While reductions of precursor levels have shifted the probability of O3 exceedances to higher temperatures, O3 is still frequently exceeded on hot summer days and the number of days exceeding 30 °C has tripled since 2000. Ozone formation has been suppressed due to the titration by NO in many locations in the past but is dominated by NOx-sensitive O3 chemistry in background, rural, and (sub)urban environments today. Ozone titration remains dominant under traffic conditions, where O3 levels are currently increasing with NOx and will likely increase for several years before emissions reductions will become effective.
Comments to Authors:
In their manuscript “Multi-decadal ozone air quality and the role of temperature in Switzerland during summertime”, authors investigated changes in summertime ozone and its relationship with temperature from 12 national stations in Switzerland over the past two decades. Decreases in precursor levels have positively affected ozone in remote locations, while ozone is increasing close to busy roads. The ozone formation regime is becoming more NOx-sensitive, and high ozone is associated with hot days. The study is on a topic of relevance and general interest to the readers of ACP. Yet from an ozone chemistry perspective, the findings are not new. Methodologically, the exclusion of any VOC measurement, the omission of ozone production efficiency (OPE) discussion, the lack of quantitative analysis for NOx-temperature dependence discussion and titration discussion, plus correlating trends in ozone with only one key parameter (temperature) is insufficient to reveal photochemical mechanisms that control ozone levels (Line 100). Therefore, I recommend a major revision and am open to review the manuscript again if needed.
Specific comments:
Technical corrections:
- Peak PO3 in VOC-sensitive regime: Los Angeles, CA, U.S.: Stockwell et al., 2025 (https://doi.org/10.5194/acp-25-1121-2025).
- Peak PO3 in NOx-sensitive regime: San Antonio, TX, U.S.: Guo et al., 2020 (https://doi.org/10.1016/j.atmosenv.2021.118624).
- Peak PO3 in VOC- and NOx-sensitive regime in the morning and afternoon respectively, but not the transitional regime in between: Houston, TX, U.S.: Mazzuca et al., 2016 (https://doi.org/10.5194/acp-16-14463-2016).
- Peak PO3 in NOx-sensitive regime: North China Plain, China: Tan et al., 2024 (https://doi.org/10.1016/j.scib.2018.07.001).
I recommend incorporating these studies in the introduction and revise the corresponding lines.
2. Section 3.3.2, Line 265, do you mean “data points above 10 and below 35C” instead?
3. I see no where that ozone production efficiency (OPE) was mentioned, which is modulated by both precursor levels and meteorological conditions (jNO2, temperature, cloud coverage, etc.). With NOx and O3 measurement but without NOy, it won’t be easy to use the regression method to calculate OPE. But it should be considered in the discussion, because when the NOx went down and T went up, the OPE could increase and lead to more rapid ozone production (more propagation cycles) as shown in:
- Kleinman et al., 2002 (https://doi.org/10.1029/2002JD002529)
- Chace et al., 2025 (https://pubs.acs.org/doi/10.1021/acs.est.5c02073)
And also in those referred literature above. It won’t change the main conclusion of your study so I listed it in technical corrections. But it is important to be incorporated and might facilitate some of your discussions (e.g. around Line 365, Line 380, Line 410, and else where).