Le Bras et al report results from 30 years of measurements of BTEX in the area of Zurich, Switzerland. They find overall decreasing trends and relate this trend to reduced traffic emissions. While the ozone formation potential by BTEX has gone down significantly from 25% to 8% its contribution remained high at about 80% when compared to overall VOCs.

Obtaining longterm continuous measurement of hydrocarbons is a challenging task to accomplish, and only few groups such as the Reimann group are able to do this with persistent accuracy. The manuscript deserves publication. However, some data interpretations needs to be presented in a more convincing way.

Major remarks

• Based on the title of the paper I had the impression that the paper would be about BTEX only, but major parts of the paper deal with the relative contribution of BTEX with regard to the overall burden of VOCs in ambient air, for instance contribution to OFP and SOA formation (chapter 3), while marginal comments are made on the NMHC measurements beyond BTEX. Neither their measurement details nor their speciation is mentioned. This is a major drawback of this paper, as it seems that OVOCs might have replaced BTEX, at least with regard to OFP. Also, as the authors used full-fledged VOC data, at least for the years 2005, 2015, 2023, and 2024, they could perform source apportionment analysis and yield information about the change of potential emission sources for toluene.
• In L71 the authors mention measurements of total NMHC concentrations, NO_x and CO. Why do the authors not incorporate that wealth of data to support their findings? It would be interesting and easy to do to use these data sets to analyze for temporal trends during the same time period of the BTEX measurements. It would allow to determine how the fraction of BTEX vs NMHC changed from year to year, and whether trends of NO_x and CO showed the same degree of reduction as the BTEX and whether they support the authors’ statement why the toluene/benzene ratio changed over time. Potential reductions in combustion related emission processes should reflect in the NO_x and CO data. As CO is unrelated to solvent emissions, CO could be a better parameter than benzene or at least an additional approach to determine the supposed increasing contributions from solvents for toluene.

Minor remarks:

• L20-25: Some updated thorough NMHC source apportionment analysis have been done in complex urban areas using online NMHC measurement techniques and should be incorporated. You may want to consider some of these:
  • Leuchner and Rappenglueck (2010): VOC Source-Receptor Relationships in Houston during TexAQS-II, Atmos. Environ., 44, 4056-4067, doi:10.1016/j.atmosenv.2009.02.029
  • Ahmed et al. (2021): Source Apportionment of Volatile Organic Compounds, CO, SO₂ and Trace Metals in a Complex Urban Atmosphere, Environ. Adv., 6, DOI: 10.1016/j.envadv.2021.100127
  • Sadeghi et al. (2022): Influence of seasonal variability on source characteristics of VOCs at Houston industrial area, Atmos. Env., 277, DOI: 10.1016/j.atmosenv.2022.119077
• L29: Ozone events are not necessarily confined to heatwaves. Wintertime ozone events have also been observed. Ozone events are primarily driven by the amount and kind of ozone precursors and the availability of solar radiation in combination with stagnant weather conditions.
• L59: What is really meant by “non-continuous sampling” here? Is it a different measurement technique and/or different temporal resolution, for instance once every day or once every week?
• L71: I did not see NO_x, CO or O₃ data incorporated in the discussions of the paper. So there is no real reason to mention them, unless they would be used in the data interpretation, which would strengthen the statements made in the paper. I assume that total NMHC data was used (e.g in L81). If this is true, the specifications of the total NMHC measurements should be described.
• L81: It would be informative to see the longterm trend of NMHCs in a similar way the authors present the BTEX data in Fig 1.
• L85-86: What do the authors exactly mean by “strongest decrease”: (1) absolute decrease, (2) relative decrease, (3) annual decrease, or (4) the point when the annual decrease of BTX tends to become smaller? Depending on the definition, different years could be defined. From what I see is that the annual decrease of BTEX is about the similar magnitude until 2004, afterwards there is less annual decrease, for instance.
• L88-89: I suggest to remove “calculated according to the territorial principle”, as the information is thoroughly described in the subsequent part of this sentence.
• 2 shows overall stronger seasonal variation for all of the BTEX compounds in the time frame 1994-2000 compared with the subsequent time periods. Why would this be the case? Apart from this I am not sure about the value of showing all the individual annual variations in these plots. It only makes the plots hard to decode.
• L90-92: How do the authors know the specific source contributions for benzene? In case it is from the Swiss inventory, then please insert a corresponding reference. Otherwise, please justify these assumptions. Also, the statement made in this sentence should be accompanied by a statement about the overall trend in NMHC emissions. Otherwise, the reader would be misguided that emissions from wood combustion increased in absolute terms, unless this is what has happened.
• Fig 3: I guess the unit should be [t/year] for the x-axis in the left plot. I suggest to remove “calculated according to the territorial principle” in the figure caption.
• L93: It is either “concentrations” or “emissions”, but not “concentrations emitted”.
• L95-96: I suggest the authors do some seasonal analysis similar to the plots shown in Fig. 2 to explore whether there might have been any significant seasonal deviations between these two sites, which eventually could help to explain the deviations.
• L105 and following lines: There are some BTEX datasets from a similar central European city taken in years 1993-1997, including diurnal variations of BTEX ratios. Also references to other cities are made in this paper. It could be worthwile to compare the datasets and to see how both datasets agree for the early period of the Zurich time series.
  • Rappenglueck and Fabian (1999): Non Methane Hydrocarbons (NMH,C) in the Greater Munich Area/Germany, Atmos. Environ., 33, 3843-3857
• L112: Why would biomass burning be negligible during winter between 8 and 9 am? It sounds like during other daytime hours in winter it would be non-negligible.
• Equation 1: during what time of the day is the benzene data taken in the summer? Is it also between 8 and 9 am?
• L121-122: I think this could be further analyzed, when CO data is being considered in addition. I assume that traffic regulations would have an impact on both, CO and toluene, while solvent toluene emissions would be unrelated to CO.
• L131-137: I was confused to see all of a sudden the availability of continous measurements of speciated VOCs, including OVOCs. This dataset was not introduced at any point in the “Material and methods” sections. It appears that there have been four specific years for these measurements (2005, 2015, 2023, 2024). As those are continuous measurements, why just these four years? Are these complete years? It is a critical dataset and should be introduced accordingly. Also, in case you have full-fledged VOC data, you could do VOC emission source apportionment and elucidate the potential change in toluene emissions.
• L134-135: Why did the yearly NMHC average ambient air concentration increase from 2005 to 2024? At this point there was a lot of discussions about efficient BTEX reductions. Why would have OVOC concentrations increased drastically? What are those specific OVOCs?
• L149: I think, it is either “low volatile” or “condensable”, but not “low-condensable”.
• L160: The supposed “significant increase in the contribution of of solvent to the toluene concentrations” requires some more analysis.
• L162-164: Given the significant decrease in OFP by BTEX, which seems to be even less than for alkanes in 2024, it seems that BTEX are not a critical quantity for ozone formation any longer, whereas OVOCs are. And this looks like an important finding.

The measurements report submitted by Le Bras et al evaluates results from 30 years of measurements of BTEX in the suburban area of Zurich in Switzerland. Their long term observations show an overall decreasing trend for all the compounds related to the reduced emissions from automotive exhausts in response to Swiss abatement policies implemented at national level over the years. Authors also evaluated the effect on the total ozone formation potential and on the secondary organic aerosol formation by BTEX, that actually reduced significantly over the years with potential improvements for the air quality and public health at urban scale.