Lidar ratios of stratospheric volcanic ash and sulfate aerosols retrieved from CALIOP measurements

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Abstract

We apply a two-way transmittance constraint to nighttime CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) observations of volcanic aerosol layers to retrieve estimates of the particulate lidar ratio (<i>S</i><sub><i>p</i></sub>) at 532&amp;thinsp;nm. This technique is applied to three volcanic eruption case studies that were found to have injected aerosols directly into the stratosphere. Numerous lidar observations permitted characterisation of the optical and geometric properties of the volcanic aerosol layers over a time period of 1&amp;ndash;2 weeks. For the volcanic ash layers produced by the Puyehue-Cordón Caulle eruption (June 2011) we obtain mean and median particulate lidar ratios of 72&amp;thinsp;±&amp;thinsp;14&amp;thinsp;sr and 70&amp;thinsp;sr, respectively. For the sulfates produced by Kasatochi (August 2008) and Sarychev Peak (June 2009), the mean of the retrieved lidar ratios were 68&amp;thinsp;±&amp;thinsp;21&amp;thinsp;sr (median 62&amp;thinsp;sr) and 66&amp;thinsp;±&amp;thinsp;15&amp;thinsp;sr (median 61&amp;thinsp;sr), respectively.</br></br> The layer-integrated volume depolarisation ratios (∂<sub><i>v</i></sub>) observed for the Puyehue ash layers (∂<sub><i>v</i></sub>&amp;thinsp;=&amp;thinsp;0.28&amp;thinsp;±&amp;thinsp;0.03) were much larger than those found for the sulfate layers produced by the Kasatochi (∂<sub><i>v</i></sub>&amp;thinsp;=&amp;thinsp;0.08&amp;thinsp;±&amp;thinsp;0.03) and Sarychev (∂<sub><i>v</i></sub>&amp;thinsp;=&amp;thinsp;0.05&amp;thinsp;±&amp;thinsp;0.04) eruptions. However, for the Sarychev layers we observe an exponential decay (<i>e</i>-folding time of 1 week) in ∂<sub><i>v</i></sub> with time from 0.25 to 0.05. The layer-integrated attenuated colour ratios for the Puyehue ash layers (<i>&amp;chi;</i>'&amp;thinsp;=&amp;thinsp;0.54&amp;thinsp;±&amp;thinsp;0.07) were also larger than what was observed for the Kasatochi (<i>&amp;chi;</i>'&amp;thinsp;=&amp;thinsp;0.35&amp;thinsp;±&amp;thinsp;0.07) and Sarychev (<i>&amp;chi;</i>'&amp;thinsp;=&amp;thinsp;0.32&amp;thinsp;±&amp;thinsp;0.07) sulfate layers, indicating that the Puyehue ash layers were generally composed of larger particles. These observations are particularly relevant to the new stratospheric aerosol subtyping classification scheme, which has been incorporated into version 4 of the level 2 CALIPSO data products.

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Prata, A. T., Young, S. A., Siems, S. T., & Manton, M. J. (2017). Lidar ratios of stratospheric volcanic ash and sulfate aerosols retrieved from CALIOP measurements. Atmospheric Chemistry and Physics, 17(13), 8599–8618. https://doi.org/10.5194/acp-17-8599-2017

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