Articles | Volume 17, issue 13
Research article
14 Jul 2017
Research article |  | 14 Jul 2017

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

Andrew T. Prata, Stuart A. Young, Steven T. Siems, and Michael J. Manton

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 (Sp) at 532 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 characterization of the optical and geometric properties of the volcanic aerosol layers over a time period of 1–2 weeks. For the volcanic ash-rich layers produced by the Puyehue-Cordón Caulle eruption (June 2011), we obtain mean and median particulate lidar ratios of 69 ± 13 sr and 67 sr, respectively. For the sulfate-rich aerosol layers produced by Kasatochi (August 2008) and Sarychev Peak (June 2009), the means of the retrieved lidar ratios were 66 ± 19 sr (median 60 sr) and 63 ± 14 sr (median 59 sr), respectively. The 532 nm layer-integrated particulate depolarization ratios (δp) observed for the Puyehue layers (δp = 0.33 ± 0.03) were much larger than those found for the volcanic aerosol layers produced by the Kasatochi (δp = 0.09 ± 0.03) and Sarychev (δp = 0.05 ± 0.04) eruptions. However, for the Sarychev layers we observe an exponential decay (e-folding time of 3.6 days) in δp with time from 0.27 to 0.03. Similar decreases in the layer-integrated attenuated colour ratios with time were observed for the Sarychev case. In general, the Puyehue layers exhibited larger colour ratios (χ′ = 0.53 ± 0.07) than what was observed for the Kasatochi (χ′ = 0.35 ± 0.07) and Sarychev (χ′ = 0.32 ± 0.07) layers, indicating that the Puyehue 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 (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) data products.

Short summary
We have studied the optical properties of ash-rich and sulfate-rich volcanic aerosols by analysing satellite observations of three different volcanic eruptions. Our results indicate that ash particles have distinctive optical properties when compared to sulfates. We expect our results will improve space-borne lidar detection of volcanic aerosols and provide new insight into their interaction with the atmosphere and solar radiation.
Final-revised paper