Review for "Extended Aerosol Optical Depth (AOD) time series analysis in an Alpine Valley: A Comparative Study from 2007 to 2023" by Wagner et al. (discussion 30 Jan 2024)

Valuable results in a well written paper. Very few mountain sites with such a long records of AOD. Methodology, clear and simple. Data at both sites (Innsbruck and Davos) have sufficient statistics and can be comparable. Already for this comparative aspect and the trends obtained for the AOD variability - it is worth publishing.

However, I'd like authors to rethink following:

Trends as both sites are provide as linear fit, maybe non-linear would be better? Why should one expect the linearly declining trend for the entre period? From the point of view of the low-elevation sites (AOD dominated by boundary layer aerosols) one maybe could expect that with the dimming (improving air quality in last decades) we get less AOD.  For the mountain site the AOD is related rather to the long-term aerosol transport, which at both sites can be expected significant, if not completely dominating. I feel this could be more properly addressed. I would less focus of the "remarkably similar trends" but try to explain better the differences and similarities  in Fig.8 in a way to explain why it is so.

Limitations of the study in terms of not being ably to estimate of which % of AOD load is form boundary layer aerosols and which form free troposphere is not discussed. Taking into account that the Davos rural site measures only in free-troposphere (can this be assumed?), the Innsbruck urban site has a strong contribution form boundary layer aerosol. So are they comparable and to what extend. Are the similarities at both sites due only to high-tropospheric aerosol? Taking into account the latter site being one of the ACTRIS sites, it would be good to mention that continuous lidar observations of aerosol extinction profiles could help in such distinction.

minor/technical comments:

Please check reference Tiw 2023, line 13

Fig.1 and Fig.2  Caption - pls check denoted colors are not in the figure

Longer data gaps occur at both sites de to device failures or calibration - can you quantify (e.g. 24 days due to X and 45 days due to y) to betetr assess on the instrument reliability?

Fig.5 the black dots are no visible

line 64 and 76 - same info, pls avoid repetitions (please check also elsewhere)

It is a comparative study, so it would be better to plot Figs.1 and 2 as one figure with two panels one above other. Similar  for e.g. Figs. 5 and 6, I would plot one next to other. This way you will reduce the length of paper and also ease reader life. Also, the most important result is the trend in Fig.8 but you have too many figures and this message gets lost.

In this study, the authors compared the aerosol optical depth (AOD) time series between two alpine valleys from 2007 to 2023. The paper is well written. The methods used and the results obtained are reasonably well documented. The long data series is a treasure in this field of aerosol research. I therefore recommend the manuscript for publication in the journal Aerosol Research.

The authors, however, should consider the following questions and recommendations when preparing the final draft. (In specific cases I refer to the page and line numbers of the draft I received.)

A more precise description of the two sites would be important because many readers do not know anything about the two sites. Characterize the site, such as type of town, e.g. industrial area, cultural center, population, etc. Furthermore, please provide detailed information on the meteorological conditions.

Page 2 Figure 1 and page 3 Figure 2: Both sites are missing data around 2013. Please explain.

The markers in Figure 1 and 2 are difficult to understand. Please correct.

Page 3, Line 47: The monthly data is 73.2 % in the text while 83.2% in Table 1. Please correct it.

In Figure 8 the markers cannot be followed. Which is the black dot and which is the blue one? Please clear them.

The authors use the term correlation several times in the text. In these cases I would expect correlation values with significance levels. Please correct them.

Review for "Extended Aerosol Optical Depth (AOD) time series analysis in an Alpine Valley: A Comparative Study from 2007 to 2023" submitted to Aerosol Research by Wagner et al.

Synopsis:

This manuscript which analyses two long-term datasets from Aerosol Optical Depth observations in the Alps is a valuable contribution to the assessment of aerosol trends in mountainous areas.

General comments:

• The manuscripts lacks a description of the dataset concerning measurement uncertainties, especially concerning long-term stability of the observations and possible biases.
  
  I suppose that the AERONET sun photometer in Davos has been regularly calibrated according to AERONET standards. There is no information about the instrument in Innsbruck, concerning instrument type and associated uncertainties. Please provide information about the uncertainties of both instruments and a summary of the calibrations performed during the 17-year period.
• Please provide a (very brief) introduction into how AOD is defined and about the general measurement principle of sun photometers, as there might be readers who are not so familiar with these topics.
• As you have data only from about 80% of the months: Are these missing months equally distributed over the year? And what is the main reason for missing months? Are the gaps caused by instrument failures or calibration periods? Because I could imagine that you get nearly every month 5 cloud-free days where you can derive AOD. Please comment on that!
• I do not see any negative AOD trend after 2014 anymore (Fig. 8). Do you have an explanation for the decline until 2013, but no more change afterwards? Are the trends the same if you separate into winter and summer seasons?
• What is the statistical significance of the trends? Please give some information on that!

Minor comments:

• Line 47 and Table 1: In the table you write for valid months: 83.2% and 79.4%, whereas in the text you give 73.3%/79.4%. Please check these values!
• Line 69: Do you think that there are more convective clouds “during the melting period”? I would rather say “after the melting period”, as heating of the ground is inhibited as long as there is snow cover - at least in the higher parts of the Alps.

Technical comments:

• Line 8: typo in “latitude”
• Figures 1,2: Please check the colors – I don’t see the blue color of the individual measurements, for me the triangles for the monthly means look blue
• Figure 2 has another size than fig. 1: Please provide a larger version of fig. 2