Comments to Carbonate content and stable isotopic composition of aerosol carbon in the Canadian high arctic

I am a scientist who specialize more in the area of radiocarbon instrumentation and the analytical method.

Introduction

The claim of line 62-63 is too strong or even false about the influence of carbonates on the atmosphere in the Arctic of not been studied yet. Just go to Google scholar and type arctic carbonate aerosol and you will get several publications.

The claim in line 73-74 is misleading. There are many publications that attribute the higher delta13C values to carbonates in the Artic.

There is a conceptual point that the authors do not deal with which is important because this work is about carbonates. If EC is the same as BC but carbonate are colorless thus carbonate is not part of BC then in which group, do we set the carbonates? If the answer is that carbonates are part of EC then this proves that EC is not the same as BC, right? Carbonates belong to what fraction of the carbonaceous matter?

Results

Weak demonstration of quantitative analysis of carbonate with Sunset.

I have problems believing that carbonates can be quantitatively analyzed by the proposed Sunset method due to the Sunset lower temperature comparing with elementar analyzer (1000 deg). I do believe that certain amount of carbonate evolves qualitatively during Sunset OC and EC fractions but it is hard to believe that 100% of carbonates becomes CO2 at EC2 temperature.

After seeing Chow et al 2007 and Hu et al 2023, I can not find a method developed and rigorously proved about the quantitatively analysis of carbonate with the Sunset temperature in any of the provided references. Of course, the claim is correct in line 141 “These are temperature steps during which we should expect the release of carbonate carbon”. But that does not prove that it is release quantitatively to accurately measure CC as TC-TC_HCl

Jankowski et al. shows qualitatively that CC evolves at 600-650 deg but the quantitatively part was done measuring TC that includes CC at 1000 deg.

The result in figure S1 (left) comparing TC Sunset before any HCl treatment look to me in average lower than the TC from EA-IRMS. The slope (1.014) looks high due to the outlier at approximately 1.1 ug/m3. By removing this outlier with the strongest magnitude, you will get a lower slope and the method from this work (Sunset carbonate) is underestimating, in average. The same applies to figure S4 where the outlier, strongest in magnitude, is lifting up the slope to 0.95.

If you check the publication from Baudin et al. 2023 (doi 10.2516/stet/2023038), you can see that the temperature depends on the type of carbonate (Fig. 5, Fig. 6) where they increased all the way until 1200 degrees. Yes, it is a fact that the carbonate evolve temperature decreases with the amount (Fig. 1). Because your manuscript is dealing with very small amounts then you oxidize most of it but conceptually and empirically (Fig. S1 of the manuscript), I believe that your method is losing a small part and it is not quantitative. If true, you should mention this.

Source apportionment

It is hard to believe that the origin of CC is mainly from North America as claimed in the abstract and conclusions. In Fig. 7, it seems that the stronger changes of d13C are due to North America but with episodes coming from Greenland too. For example, in Fig. 7, at 01/03/2018 there is a very strong episode (peak of d13C before HCl) colored in green with yellow (Greenland, Europe). Or a mixture of both, for example at 01/09/2016 the peak of d13C before HCl is a mix of green and pink (Greenland, North America). Later in the text, the authors did explain that the influence is also coming from Greenland, Europe but why the conclusion of North America as main source in the abstract and conclusion?

The manuscript “Carbonate content and stable isotopic composition of aerosol carbon in the Canadian High Arctic” by Vodička et al., utilizes measurements from the arctic to quantify the contribution of carbonate to total suspended particle mass.  The manuscript is clearly written and of interest to the community.  My comments are included below

- The authors mention some of the interferences for carbonate quantification, such as oxalates.  Is it possible to provide a bound on the uncertainty arising from this.  For example, in the text the authors state that oxalates peaks likely appear in the OC4, while CaCO₃ appears in EC2.  The mass lost in the EC2 fraction may provide a lower bound for the carbonate fraction.   

- Line 140: The authors state that for some samples the largest material loss in EC2, while for others it is in EC1 or OC4.  Is the presence of the CaCO₃ peak in EC2 repeatable, or is there a situation where it is shifted to lower temperatures, like EC1.  Asked another way, is the loss of material at these lower temperatures suggestive that compounds other than CaCO₃ dominates the material lost during HCl fumigation.   

- Were any tests of HCl fumigation performed on the carbonate standards or are there references for this procedure to ensure there is quantitative removal of carbonate. 

- One of the important concluding points in section 4 is that the mis-identification of CC as either EC or OC will result in incorrect estimates of aerosol effects on radiative forcing.  Are there any previous reports on the optical properties of carbonate aerosol to help expand on this point?

- Figure 3:  I find this figure hard to interpret, and I recommend simplifying it, or splitting into different panels.  There are a number of items on it that are not discussed.  For example, the laser intensity.  Also, the textboxes “PC”,  “HCl” and “EC” and the short dashed lines around 600 seconds are not defined.  There are also a few periods (OC3 and OC4) where the HCl treated is higher than the untreated.  Is there an explanation for this?

Figure 7: Are the outliers for the calculated δ¹³C_CC that are >100 ‰ simply due to the low abundance of carbonates during those periods? If so, it may be worthwhile to filter out those points as they are misleading. 

Typographical:

Line 57: This is not a complete sentence

Line 280: should this be particles?