Zugspitze ozone 1978–2020: The role of stratosphere-troposphere transport
Abstract. The pronounced increase of ozone observed at the Alpine station Zugspitze (2962 m a.s.l.) since the 1970s has been ascribed to an increase of stratospheric air descending to the Alps. In this paper, we present a re-analysis of the data from 1978 to 2011 for both ozone and carbon monoxide, extended until 2020 by the data from the Global Atmosphere Watch site Schneefernerhaus (UFS, 2671 m a.s.l.) just below the Zugspitze summit. The analysis is based on data filtering utilizing the isotope 7Be (measured between 1970 and 2006) and relative humidity (1970 to 2011, UFS: 2002 to 2020). We estimate both the influence of stratospheric intrusions directly descending to the northern rim of the Alps from the full data filtering and the aged (“indirect”) intrusions from partial filtering with the 7Be data. The evaluated total stratospheric contribution to the annual-average ozone rises roughly from 12 ppb in 1970 to 24 ppb in 2003. It turns out that the increase of stratospheric influence is particularly strong in winter. A lowering in positive trend is seen afterwards, almost parallel to the beginning decrease of solar irradiation. The air masses hitting the Zugspitze summit have become drier in the percentile range up to at least 25 % until 2003, and we see the growing stratospheric contribution as an important factor to this drying. Both an increase of lower-stratospheric ozone and a growing width of the intrusion layer departing downward from just above the tropopause must be taken into consideration. Carbon monoxide in intrusions did not change much during the full measurement period 1990 to 2020, with perhaps a slight increase until 2005 and an almost constant behaviour afterwards. This is remarkable since outside intrusions a decrease by approximately 44 % was found, indicating a substantial improvement of the tropospheric air quality.
Thomas Trickl et al.
Status: open (until 25 Mar 2023)
- RC1: 'Comment on acp-2022-783', Anonymous Referee #1, 14 Mar 2023 reply
Thomas Trickl et al.
Thomas Trickl et al.
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review of manuscript acp-2022-783
Title: Zugspitze ozone 1978–2020: The role of stratosphere-troposphere transport
Authors: Thomas Trickl et al.
The manuscript presents an interesting and extensive analysis of more than 40 years of ozone data measured at the Zugspitze summit and the nearby Schneefernerhaus research station in the German Alps. The present manuscript builds on a series of publications by some of the authors. The manuscript reveals some interesting phenomena like that increasing stratospheric contribution to the O3 levels at Zugspitze over time. However, the paper is hard to follow as it is rather wordy, uses some vague terminology, mixes previously published results, methods and new findings, and includes an excessive number of figures. See some examples in the specific comments below.
To my mind, the paper is within the scope of ‘Atmospheric Chemistry and Physics’ but can profit from considerable shortening and removal or movement of some of the figures to the supplementary material and, thus, requires major revisions prior to its acceptance for ACP.
Line 19: it is confusing that changes between 1970 and 2003 are reported while the title of manuscript refers to the 1978 to 2020 period.
Lie 21ff: "The air masses hitting the Zugspitze summit have become drier in the percentile range up to at least 25 % until 2003 …" difficult/impossible to understand. Please rephrase.
Lines 23-24: "… a growing width of the intrusion layer departing downward from just above the tropopause must be taken into consideration …" difficult to understand here (and on page 20)
Line 25: "… perhaps a slight increase …". Very vague wording. Is it statistically significant?
Lines 46 ff and Fig. 1, lower panel: It is uncommon to refer to figures already in the introduction. I do not see the added value of showing the Wank data here. It does not really contribute to the discussions and conclusions drawn here.
Lines 89-90: add reference to the "far below 1%"-statement.
Lines 121 ff.: Awkward sentence (in grammatical terms). Please rephrase.
Lines 179-179: add references or urls, according to the WCC webpage (at Empa), there was another audit in 2020.
Line 182: add url of the data centre.
Line 199: add location or url for Deuste-Steininger.
Line 201: add year 2020? See above.
Lines 217-2018: "The periods are time shifted to 7:30 to 19:00 CET and 19:30 to 7:00. This does not matter since the data filtering is carried out on a half-hour basis." Difficult to understand since the filtering is only explained below.
Line 220: "… both averages determined on a half-hour grid …" This is confusing. What does it mean? 7Be were measured as 12h or 24h bulk samples.
Lines 220-221: "This means that the 12-h DWD data were calibrated to match the 24-h IFU data. It could not be clarified why this is the case." What does that mean? The DWD were scaled to match the IFU data? You cannot call this a calibration. Your last sentence is weird … and I do not understand to what it refers. To the application of a scaling factor? If so, don't you think that it was done to homogenize the two records?
Line 228-229: Something is linguistically wrong in this sentence.
Lines 231-232: "… which turned out to be highly necessary near the temporal boundary (end) of our analysis." Weird wording. How about "… which were useful to apply to the filters (see below) to the whole dataset."
Chapter 3 is can be considerably shortened as it largely repeats previous findings. However, it also mixes methods, previous findings and new results/interpretation, which make it really hard to follow.
Line 284: "The specific activity per intrusion has changed over the years …" why?
Line 347: what is a short gap?
Lines 361-363: Move this statement somewhere upfront.
Lines 387-402: could be summarized by one or two sentences.
Lines 407-411: isn't it trivial when 7Be is steadily increasing as pointed out in line 378? Remove Fig. 4?
Line 435: " The monthly minimum RH …" This is the lowest half-hourly RH reading for each month, right? Is this a useful number? I don't think so. You even state that it might be subject to different sensors. Remove from Fig. 6.
Line 485: did Yuan et al. measure in a tunnel? Does that make sense?`
Line 718: Stohl, 2001: reference does not exist in the bibliography.
Lines 718-720: did you compare with CO data from IAGOS?
Line 750: Claude, 2003: reference does not exist in the bibliography.
Lines 756 ff: "… increasingly wider layers have separated from the range just above the tropopause …" Where is this conclusion coming from? Add reference.
Table 1: What does year "1970.50" etc. mean? Simply write "1970" …?
Table 1 and Figs. 14 and 15: "annual ozone averages in intrusions …". Maybe I missed it in the manuscript but these numbers have to be understood as "annual mean contributions to the ozone levels", correct?
Figure 12, caption: criterion needs to read "RH-criterion", not "7Be-RH-criterion", right?
Fig. 14: this figure (along with the figure caption) is very confusing when first looking at the figure without reading the whole manuscript as it is makes the reader think that O3 is depleted during stratospheric intrusions (since numbers here are lower than in e.g. in Fig. 1 top panel).
Figs. 16, 17, 18 and 20: reword captions to make it clear what is shown (O3 mole fractions at Zugspitze and their individual contributions).
Fig. 2: it is textbook knowledge that O3 and 7Be rise while rh drops during stratospheric intrusions. Does it really need a case study figure here?
Figs. 6 and 7: delete one of them or move at least to the sup mat.
Figs. 8 and 9: delete one of them or move at least to the sup mat.
Figs. 16 and 17 are pretty much redundant.