|Response response for Reviewer #1 of the first iteration of “Contribution of dust and elemental carbon to the reduction of snow albedo in the Indian Himalaya and the Finnish Arctic.” by Svensson et al., for ACPD.|
The authors have addressed several of my main comments: they have added a relevant estimate of filter under catch, and satisfactorily addressed most minor comments.
Main comments “leftover”:
1) Carbonates. The authors have added clarification about the decomposition protocol they use, indicating that inspection of thermographs is sufficient to constrain artifacts of carbonates showing up as EC. However, I still recommend that assessments of uncertainty be improved/included.
a) Indeed, I had not understood the specific benefits of the EUSAAR_2 protocol previously; it certainly offers improved rejection of carbonates compared to IMPROVE. However, as pointed out before, Cavelli et al. 2010 deals with atmospheric aerosols that likely have much lower mineral dust loads than the samples here. Examination of their Figure 3 indicates a small response within the higher temperature times of their He/O2 devolution from natural calcite. For mineral dust/EC ratios as in the snow, the question of potentially significant artifacts even with the EUSAAR_2 protocol still requre assessment. Further the authors themselves suggest that ambiguous separation of EC/OC with their technique introduces scatter in their results; to quote from their response:
“The statement that the “good agreement” in fig.5 between the two optical measurements indicate that most of the scatter between TOM and PSAP is related to where the OCEC instrument determines to place the split point between OC and EC, still holds “
I do not understand how this could be reconciled with insensitivity to carbonates.
b) Zhang et al. also provide extra information about this issue. On the one hand, since they use IMPROVE_A, their estimates likely significantly overestimate the artifacts using EUSAAR_2. However, citing their 20% estimate is likely not appropriate beyond their sample ensemble: their snow was significantly higher in EC content than that here (on average ~1500 ng/g, their Figure 4), and it is not known how the mineral content of their samples relates, in amount and composition to those here. I expect that carbonate artifacts scale only with carbonate concentration, not relative to EC load. Another way to present their results could be via examination of their figure 4, where 5 samples were acidified to test carbonate contamination, and on average, shifted downwards in EC content by approximately 200ng/g each.
2) On the strength/relevance of constraints on ambient observations of the laboratory test. I am concerned that the authors did not understand some portions my original major comment 2. To paraphrase: measurements of eBC (i.e. interpretation of light transmission through a complex and uncharacterized filter) with different instruments (PSAP and OC/EC instrument) were carried out. Comparison of these two determinations does not give information about the actual BC or eBC on the filter, but only on the precision and relationship of the light transmission measurement. This is why it is not a clear constraint on the evolved EC measurement, and provides no information about the validity of the separate thermally evolved EC measurement or the relationship to actual eBC. This is why my suggestion of analyzing the results in the context of relative loadings (known from known dilutions) might help: the scaling of results with particulate load for the two different techniques (light attenuation or thermal evolution) could reveal influences of minerals on the BC load determination.
3) On the discussion of the trend of ratio with depth (Figure 13). I don’t think adding the sentence “This observation is rather hypothetical, and it needs to be further explored.” to this section sufficiently addresses my comment that there is no statistically relevant evidence for a trend.
4) It is very unlikely that the R^2 value given for 5 is correct
A separate question I am curious about is the potential impact of carbonate thermal decomposition in the OC stage of the EUSAAR_2 on the optical OC charring constraints; has this been considered?