Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
Submicron volcanic ash detected in sulfate aerosol mixtures observed by the Atmospheric Chemistry Experiment satellite
Michael Lecours,Chris Boone,and Peter Bernath
Abstract. Mixtures of volcanic ash and stratospheric sulfate aerosol (SSA) have been observed by the Atmospheric Chemistry Experiment (ACE), a remote sensing satellite mission in operation since 2004. The migration of the volcanic plumes from the Puyehue-Cordón Caulle eruption in June of 2011 and the Calbuco eruption of April 2015 aligned well with ACE’s sampling pattern, providing extended sampling of volcanic aerosol from these two eruptions. Weak volcanic ash spectral signatures are observed in the presence of SSA in the Southern Hemisphere up to 165 days following the Calbuco eruption and 125 days following the Puyehue-Cordón Caulle eruption. ACE residual spectra of ash-sulfate mixtures were simultaneously modelled to retrieve the line-of-sight column densities and the median particle radii for both ash and SSA. Pure volcanic ash from the Puyehue-Cordón Caulle eruption was modelled with a bimodal lognormal distribution.
Received: 22 Jul 2026 – Discussion started: 14 Aug 2026
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Comments on “Submicron volcanic ash detected in sulphate aerosol mixtures observed
by the Atmospheric Chemistry Experiment satellite”
by Michael Lecours, Chris D. Boone, and Peter F. Bernath
This paper presents some analyses from the ACE instrument of particles (ash and sulfates) in dispersing volcanic plumes originating from two volcanoes in Chile. The eruptions of Calbuco and Puyehue Cordon-Caulle (PCC) occurred in 2015 and 2011, respectively, both more than 10 years ago.
This made me wonder why the analyses are being presented now–was there some new retrieval technique allowing fresh insights into the fate of the particles, or are the analyses correcting some previous analyses? Neither of these are apparent in the paper. The best justification for the analyses seems to be that they do not invalidate earlier published results concerning sulfates. That is, the identification of an ash signature in the ACE spectra have minimal impact on earlier sulfate retrievals. I’m not convinced that there is enough new science in this paper that make it worthy of publication as a scientific contribution. It may be a valuable technical contribution, but the methods and results are not widely applicable to other instruments. The retrieval of a sub-micron ash signature is, as the authors state (without reference) fairly common, so this aspect is not new. There is little in the way of treatment of errors–they are mentioned but not quantitative assessment is provided.
General comments.
There appear to be several assumptions in the retrieval scheme that make me believe the retrieval is severely under-constrained and that there may be a number of solutions that satisfy the data to within experimental error. In particular the assumption that the particles are non-scattering is at odds with previous work on ash and sulfate extinction in the IR. The radiative transfer for 10 µm wavelengths and particles with r~1 µm is within the Mie regime. The authors vary parameters in a bimodal log-normal distribution (assumed) and choose the lowest chi squared solution. But they do not show whether this is a unique minimum or how well defined the minimum is. Could there be other distributions or alternate optical values that yield similar results?
No other corroborating data is presented. The Calbuco and PCC eruptions were very well observed by other satellite instruments (e.g. AIRS and MODIS) and the locations of ash plumes were observed by these instruments for many days.
I certainly believe that there are submicron ash particles in dispersing plumes but there are also much larger particles (>1 µm) observed several days after the initial eruption. The authors cite one paper (Bignami et al) and suggest that the results of observations of submicron ash particles are in disagreement. I don’t think that comment is justified as they are not comparing simultaneous observations and from what I could tell the ACE observations are discussing older particles (but I have to say it was not easy to find the dates and locations of the ACE observations).
There appears to be an assumption of the habit of the distributions of the two types of particle identified. In some places the assumption seems to be that they are co-habiting (“mixtures”) while in other parts of the m/s the particles are treated as separate. It is not clear, nor is it demonstrated, that the transmittance can be determined by simply adding the optical depth contributions (multiplying the transmittances) based on each particles optical parameters. It could also be that the sulfate is coating a submicron ash particle and that the infrared extinction is entirely due to the optical properties of sulfates (with modified optical parameters and size distribution).
Figure S1 seems to be a crucial figure that shows the strong dependence of spectral transmittance on particle size. This figure should be in the main paper and not in a Supplementary.
Mixtures of volcanic ash and sulfate aerosol have been observed by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), a remote sensing satellite mission. Ash-sulfate mixtures are observed in the Southern Hemisphere up to 165 days following the Calbuco eruption of 2015 and 125 days following the Puyehue-Cordón Caulle eruption of 2011. The line-of-sight column densities and median particle radii for both ash and sulfate were retrieved from ACE measurements.
Mixtures of volcanic ash and sulfate aerosol have been observed by the Atmospheric Chemistry...
Comments on “Submicron volcanic ash detected in sulphate aerosol mixtures observed
by the Atmospheric Chemistry Experiment satellite”
by Michael Lecours, Chris D. Boone, and Peter F. Bernath
This paper presents some analyses from the ACE instrument of particles (ash and sulfates) in dispersing volcanic plumes originating from two volcanoes in Chile. The eruptions of Calbuco and Puyehue Cordon-Caulle (PCC) occurred in 2015 and 2011, respectively, both more than 10 years ago.
This made me wonder why the analyses are being presented now–was there some new retrieval technique allowing fresh insights into the fate of the particles, or are the analyses correcting some previous analyses? Neither of these are apparent in the paper. The best justification for the analyses seems to be that they do not invalidate earlier published results concerning sulfates. That is, the identification of an ash signature in the ACE spectra have minimal impact on earlier sulfate retrievals. I’m not convinced that there is enough new science in this paper that make it worthy of publication as a scientific contribution. It may be a valuable technical contribution, but the methods and results are not widely applicable to other instruments. The retrieval of a sub-micron ash signature is, as the authors state (without reference) fairly common, so this aspect is not new. There is little in the way of treatment of errors–they are mentioned but not quantitative assessment is provided.
General comments.
Specific comments.
Please see annotated m/s.