Review of the manuscript "Opinion: The strength of long-term comprehensive observations to meet multiple grand challenges at different environments and in the atmosphere" from Markku Kulmala et al.

Indeed, the manuscript deals with a relevant issue, and the author's team is excellent. It deals with the need for a comprehensive global network of observations for the Grand Challenges. The manuscript describes the concept of SMEAR (Station for Measuring Earth surface – Atmosphere Relations).

The SMEAR concept is not new, and several papers from the same group have already discussed the same idea. Section 2 describes the concept and refers to the paper from Hari and Kulmala of 2005, which had already presented a very similar concept. The title of that paper is Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II). So, what new ideas arise after 18 years from the same concept in this new manuscript? Other than the example of COVID and progress on NPF, the concept is basically the same.

The manuscript frequently discusses the "open DATA" issue when the discussion should be "Open SCIENCE." This indicates a quite narrow view since only having an open data policy does not promote open science.

Additionally, the manuscript proposes a global network of aerosol and other environmental observations. The GLOBAL is essential to the proposition. But 95% of the paper deals only with the approach adopted in one single monitoring station (Hyytiälä) in Finland. It almost exclusively discusses a single, very specific monitoring station. Not even other Scandinavian stations are mentioned. There is no discussion of global networks of stations such as GAW, NOAA, and others. Several examples globally are pretty similar to the SMEAR II station approach in Finland. I think this is an important limitation in the manuscript.

Later, the manuscript describes the COBACC feedback loop, which is discussed only in terms of boreal forests and only with data from Finland. The manuscript would be much richer and more useful if the discussion on COBACC is done more broadly.

Then on page 9, the manuscript discusses the COVID restrictions and the strength of the SMEAR concept. Again, it takes the example of the AHL/BUCT laboratory, where the first author also works. Furthermore, the authors choose a limited view of an important issue instead of a broader, European, or Global approach. This discussion would be much richer if done with a more comprehensive view.

The discussion that NPF is a very important issue in a proposed global monitoring network? What about aerosol radiative forcing changes? What about the aerosol optical properties linked to global heating/cooling? Certainly, many other aspects than NPF could have been discussed in the SMEAR and COBACC concepts.

Section 5 on "future perspectives and possibilities" also fell short of a global view. It would be important to analyze the GAW network, the European initiatives such as ACTRIS, ICOS, and the overall COPERNICUS. They are good examples, of course, and the manuscript could be more useful for readers if a critical analysis with recommendations for these initiatives could be included in Section 5. The need to have global networks for climate change monitoring as well as detailed atmospheric monitoring, including air pollutants, could be really good for the manuscript.

Also important in the manuscript is the list of references. Of the 120 references, nearly 95% are from the same author, Kulmala. The manuscript could again profit from a broader list of European and American authors that have extensively worked on the paper subject. The proposal to have a global atmospheric observation network should not be done with a Finnish approach but certainly with a European or global perspective. The manuscript would profit significantly if thought much broader. The Global Challenges can only be fulfilled with a Global view.

This paper provides a discussion of the role of long-term environmental monitoring in understanding global environmental challenges. The value of long-term measurements in understanding environmental change still needs to be better recognised and this paper helps support this important argument.

The paper is well written and discusses a number of important topics. The focus is on one specific approach to long-term monitoring, namely the SMEAR (Station for Measuring Earth surface – Atmosphere Relations) concept. This focus results in the paper being heavily weighted to reviewing papers written by the lead author and co-authors, who lead the SMEAR approach.

The section on the impacts of COVID lockdowns on atmospheric composition is very interesting and clearly demonstrates the value of long-term measurements in understanding complex and unexpected environmental change. In particular, Figure 3 very clearly shows the value and need of long-term measurements across a very wide range of atmospheric components to help interpret and understand complex atmospheric and environmental interactions.

I have a few comments that I would ask the authors to consider.

Major comments

The lead author and co-authors are productive researchers who have written a number of important review and opinion articles that cover similar ground to this paper. It would be helpful if the authors clearly identified how this article builds upon and is different to previous articles. In particular, it would be very helpful if the authors added a clear statement of the novel findings / focus of this study compared to previous papers.

The paper focuses heavily on research conducted by the lead author and co-authors. This is understandable given the focus of the paper on the SMEAR concept established and run by the authors. Despite this, where relevant a wider acknowledgement of the work done by other groups, particularly other long term atmospheric monitoring, would strengthen the paper.

Minor comments

Page 7, Section 3.2. Figure 3 is a powerful way of illustrating the power of multiple long term observations in understanding a complex environmental issue. I missed an equivalent figure for the role of long-term measurements in helping to understand the COBACC and other environmental feedback loops. Is there an equivalent figure that demonstrates how measurements from SMEAR can be combined to help understandd a component of COBACC? If so, I think this would make a valuable addition to the paper. 

Page 13, Fig 4. It is interesting that there is no observed long-term trend in monoterpene concentrations during 2010-2022. Whilst this is a short period and the data is discontinuous I wonder if this can help say something about the sensitivity of monoterpene emissions to environmental drivers. Would theory/models expect an increasing trend over this short period? A short discussion on this might be useful.