Comment on cp-2021-78 Anonymous Referee # 1 Referee comment on " The blue suns of 1831 : was the eruption of Ferdinandea , near Sicily , one of the largest volcanic climate forcing events of the nineteenth century ?

This is a well written and interesting study on the origin of numerous observations of blue/green suns in 1831. There will certainly always be some uncertainty, but in my opinion this is a convincing study and according to the evidence presented in the manuscript it seems indeed highly plausible that the 1831 eruption was the one of Ferdinandea. The paper is well suited for Climate of the Past and I recommend publishing the article after considering the following (mainly minor) comments.

This is a well written and interesting study on the origin of numerous observations of blue/green suns in 1831. There will certainly always be some uncertainty, but in my opinion this is a convincing study and according to the evidence presented in the manuscript it seems indeed highly plausible that the 1831 eruption was the one of Ferdinandea. The paper is well suited for Climate of the Past and I recommend publishing the article after considering the following (mainly minor) comments.
Apart from the specific comments below it would be good if the paper would also briefly address the following points: Were there reports of wildfires in Europe in August 1931? How does the time lag between the eruption and the occurrence of colored suns differ between Ferdinandea and other eruptions, e.g. Krakatoa?
Specific comments: Lines 77 -80: Perhaps you can here already refer to the appendices. When reading these sentences I was asking myself: How many newspapers and journals were searched and which ones? This information is provided later, but it would also fit here.
Line 122: "These observations have accordingly not been included in the present analysis." Probably because Fig. 1 is right below this sentence I was asking myself, whether the point in China refers to the observations of this Mandarin compendium (which is not the case, as I learned later). Figure 5: what is the meaning of the color of the circles. This is not explained as far as I can tell. In particular: What is the black circle in the "no blue sun" area? This refers to longitude, right? I suggest mentioning this explicitly to avoid confusion. Section 3.5: I would add that zonal winds in the stratosphere at mid-latitudes depend on the seasons and are easterly (westward) in summer and westerly (eastward) in winter. In August the winds would be easterlies, i.e. westward winds, which supports your hypothesis. The zonal wind reversal in the middle atmosphere typically occurs in September. This information can be found in a standard text book on atmospheric dynamics.
Equation (1): what are the limitations of this equation? What assumptions is it based on? There must be limits to the parameter ranges in which the equation is applicable, e.g. if Delta M ist very small, tau may be negative, which does not make sense.
Line 242: "These three observational phases" It's not entirely clear which three phases you mean here.
Line 258: "and using the corresponding magnitude range (either 8 < \Delta M < 12 or 12.5 < \Delta M < 16.5)" Which range is used in which case? Figure 8 and related explanations in the text: I don't fully understand, how the optical depths and their error bars are estimated. You use equation 1 and obtain information on the solar zenith (or elevation angles), but how is \Delta M determined for each case? This seems highly arbitrary and should introduce significant uncertainties.
Please also explain, how the error bars in Fig. 8 were determined.
Line 289: "extinction co-efficient Q" Q needs to be dimension-less for equation (2) to yield the correct units. This already implies that Q is the "extinction efficiency", not an "extinction co-efficient". Coefficients typically have units of 1/length, X-sections of length^2 and the Mie efficiencies (scattering or extinction) are dimension-less.
Line 340: "The eruption has been assigned a VEI of 3, which is associated with a total volume of erupted tephra of the order of 0.1 km^3." This amount of erupted tephra is certainly probably associated with large uncertainties, right? I suggest mentioning this.
Line 364: "This hypothesis" Which hypothesis do you mean here? That Ferdinandea was the source of the aerosol leading to the blue sun observations? Or your hypothesis on the additional release of sulfur?