the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jan 2025
Satellite-based evidence of dust emission over Northern Canada
Abstract. High latitude dust (HLD) is receiving growing research interest as its relative impact in the Arctic has been elucidated. Precise knowledge of HLD emission locations is limited in both field studies and satellite observations, leading to a general lack of representation in global models. Using the Frequency of Occurrence (FoO) of above-average Dust Optical Depth (DOD > 0.5) from twenty years (2002–2022) of high-resolution MODIS observations derived for this study (0.1° x 0.1°), we present quantitative evidence that dust sources are widespread across the Canadian Arctic. Additionally, we present qualitative supporting evidence from aerosol type ‘dust’ classifications in VIIRS and CALIPSO satellite data products, as well as some challenges of comparing MODIS AOD to two co-located AERONET sites.
The HLD hotspots identified in the “Canadian Arctic Dust Belt” correspond to surfaces with high potential for dust emission in the G-SDS-SBM dataset. There are more areas where hotspots are observed but emission potential is low than the opposite case; additionally, two considerable areas of dust emission are identified at lower latitudes in mainland Canada. When spatially averaged across the broad dust producing region (65° N – 85° N, 125° W – 70° W), annual mean time series of FoO of MODIS DOD > 0.5 suggest an increase in the frequency of dustiness in the latter decade, consistent with our understanding that HLD emissions are increasing in a warming climate. These results further motivate model development to include HLD sources and provide an observational basis for evaluating them.
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Status: final response (author comments only)
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CC1: 'Comment on egusphere-2024-3828', Seyed Ali Sayedain, 04 Feb 2025
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AC1: 'Reply on CC1', Aldona Wiacek, 12 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3828/egusphere-2024-3828-AC1-supplement.pdf
- CC2: 'Reply on AC1', Seyed Ali Sayedain, 16 Feb 2025
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AC1: 'Reply on CC1', Aldona Wiacek, 12 Feb 2025
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RC1: 'Comment on egusphere-2024-3828', Anonymous Referee #1, 14 Apr 2025
In this study, the authors attempted to identify HLD sources across the Canadian Arctic Archipelago and adjacent areas by using FoO of DOD from 20-yr high-resolution MODIS retrievals as well as VIIRS aerosol type product, CALIOP products, and observations from two AERONET sites. The authors also compared the spatial distribution of FoO with G-SDS-SBM dataset and showed an overall agreement. There are also significant areas of disagreement between FoO from MODIS and VIIRS and between satellite retrievals and G-SDS-SBM dataset. Overall, I think this study provides insights for identifying HLD sources and can benefit future modeling and observational studies. I have a few comments for the authors to consider.
General Comments:
- I wonder if MODIS products using the Dark Target algorithm (DT alone or combined DT/DB) can change the FoO of DOD and contribute to the uncertainties in identifying potential HLD sources, especially for coastal regions having both land and ocean.
- I would suggest providing more details or high-level formulas to better explain how SI from G-SDS-SBM is calculated which may be helpful to explain the differences between the datasets and satellite retrievals. It seems to be mostly dependent on soil properties. It is not clear to me if SI depends on surface wind. SI being 1 indicates maximum capacity in cases of strong surface winds. However, the authors state that the derivation of SI does not need surface wind. What about the surface wind/friction velocity in the large green domain in Figure 6c compared to that in the red domain in Figure 6b and 6c. I also wonder how native (~0.008°) SI data were regridded to 0.1°.
Specific comments:
Line 9, please check if multiplication sign is used instead of letter x. Please check other places as well.
Lines 32-33, there are some more recent studies addressing this issue. I would suggest adding them. For example, Shi et al. (2022). Relative importance of high-latitude local and long-range-transported dust for Arctic ice-nucleating particles and impacts on Arctic mixed-phase clouds. Atmospheric Chemistry and Physics, 22, 2909-2935.
Line 235, why FoO DODB16 > 0.5 is much lower than FoO DODPG16 > 0.5 in the east of 95°W high-latitudes?
Line 238, for DODPG16, is it calculated when w < 1? Does that mean it is filtered as well?
Figure 3, I would suggest changing the color bar for the three bottom figures. It is just showing black over most places and gives less info.
Figure 5. I feel it gives limited information compared to lat-lon plot. How about showing similar lat-lon spatial distribution of FoO of CALIOP DOD using the algorithms from literature (e.g., Yu et al., 2015)?
Lines 362-363, this sentence is not clear to me. Could you explain a bit more and rephrase it?
Figure 6, I agree with the key point the authors tried to make. However, I would suggest at least trying to use different thresholds for FoO MODIS DOD and FoO VIIRS ATDUST. There are significant differences between FoO MODIS DOD and FoO VIIRS.
Figure 7, I’m a bit confused about this figure. What’s the unit for FoO shown here. Is it the number of occurrence or the percentage as to total observations? It seems that blue and green lines correspond to the left y-axis while the brown line corresponds to the right y-axis. For right y-axis, is it for number of occurrence or the percentage?
Line 448, I would suggest showing the locations, as well as other selections of region such as the region in Figure 7, in Figure 8 or in the section 2.
For the conclusion part, I would suggest the authors give a high-level summary of some potential HLD sources in the order of confidence (i.e., agreements between MODIS, VIIRS, and G-SDS-SBM).
References
Yu, H., Chin, M., Bian, H., Yuan, T., Prospero, J. M., Omar, A. H., Remer, L. A., Winker, D. M., Yang, Y., Zhang, Y., and Zhang, Z.: Quantification of trans-Atlantic dust transport from seven-year (2007–2013) record of CALIPSO lidar measurements, Remote Sens. Environ., 159, 232–249, https://doi.org/10.1016/j.rse.2014.12.010, 2015.
Citation: https://doi.org/10.5194/egusphere-2024-3828-RC1 -
CC3: 'Comment on egusphere-2024-3828', Lauren Zamora, 02 May 2025
Hello,
Just a quick comment. The authors might want to be aware that the CALIPSO algorithm can sometimes misclassify the diamond dust (small ice crystals in clear sky conditions) as mineral dust. This leads to an overestimation of CALIPSO "dust" in the Arctic when RHi > 100% (e.g., see https://acp.copernicus.org/articles/22/12269/2022/ and https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017JD027530).
Best regards,
-Lauren Zamora
Citation: https://doi.org/10.5194/egusphere-2024-3828-CC3 -
RC2: 'Comment on egusphere-2024-3828', Anonymous Referee #2, 02 Jun 2025
The manuscript titled “Satellite-based evidence of dust emission over Northern Canada” by Ashpole and Wiacek attempted to use multiple satellite products including MODIS, VIIRS, and CALIOP to detect the frequency of occurrence of dust events for mapping possible dust sources. I think the methods to detect dust sources are generally adequate and useful for identifying dust sources. Different from another reviewer, I personally think using the Deep Blue over the land make sense since the authors are looking for dust sources from the land and not from the water bodies. I think this paper is helpful for constraining Canadian dust sources using a suite of satellite retrievals, and the study is helpful for the dust community. I have some general comments below, and I would suggest accepting this manuscript after a major revision.
Major comments:
Line 224: I would imagine that using 0.2 or an even smaller DOD threshold for FoO is needed for HLD sources like Canada. Using 0.5 as a threshold may miss out a lot of small local dust sources.
The difference between DODPG16 and DODB16 are not clear to the reader. The authors need to give more details on the differences between them and why the occurrence of DODB16> 0.5 are generally lower than that of DODPG16>0.5.
Line 317: I am not convinced that VIIRS mixed aerosols are mostly dust in Canada. They mentioned Kok (2011) but Kok showed that most emitted dust are coarse dust, not fine dust. They should stick with VIIRS dust (instead of VIIRS mixed aerosols) when interpreting possible dust sources.
Line 335: The authors used aggregated CALIOP aerosol vertical profiles with a coarse resolution of 5°x2°. A finer resolution might be needed for detecting smaller and local dust events.
Line 336: How did the authors exclude the diamond dust (ice crystal that are mistaken as dust) mistaken by CALIOP? There could be quite some precipitation in the summertime Arctic.
Figure 5: It looks like to me that CALIOP dust aloft be transported from elsewhere (e.g., the lower latitudes). How do you separate local sources from regional transport?
Line 351: The authors used two satellite instruments (MODIS and VIIRS) to generate two FoO maps. It is not clear to the reader which one is better for use. The authors should explain more on the pros and cons of each of them and how people can combine them to better constrain the Canadian dust sources.
Line 433: I would say that the climate change impacts on FoO is not clear from the time series in Fig. 7. I wonder what drives the interannual variability. Are there correlations between the wind / temperature / snow cover time series and the FoO time series?
Citation: https://doi.org/10.5194/egusphere-2024-3828-RC2
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All our comments appear in the attached pdf copy of the Ashpole & Wiacek’s manuscript (key overview comments in red, detailed comments in yellow)