Optical properties and radiative forcing of the Eyjafjallajökull volcanic ash layer observed over Lille, France, in 2010

Abstract

[1] In this work we characterize optical properties and assess the direct radiative effect of an ash plume observed on April 17, 2010 by AERONET, lidar and broadband solar flux measurements collocated on the roof of the Laboratory of Atmospheric Optics in Lille, northern France. These measurements allowed experimental evaluation of ash radiative impact and validation of simulations. The derived aerosol model of ash is characterized by a bi-modal size distribution dominated by coarse mode centered at a radius of 1.5 mm and by relatively strong absorption at short wavelengths (single scattering albedo of 0.81 AE 0.02 at 440 nm as opposed to 0.92 AE 0.02 at 670, 870 and 1020 nm). Due to relatively low aerosol optical thickness during the ash plume transport ($0.26 at 440 nm), which is unfavorable for AERONET retrievals, the uncertainties in derived ash aerosol model were additionally evaluated. The complex refractive index of ash was derived assuming that effective refractive index retrieved by AERONET for externally mixed bi-component aerosol can be approximated as an average of refractive indices of two components weighted by their volume concentrations. Evaluation of the accuracy of this approximation showed acceptably small errors in simulations of single scattering albedo and aerosol phase function over the range of scattering angles observed by the AERONET almucantar. Daily average radiative forcing efficiency of ash calculated for a land surface reflectance representing Lille was about À93 AE 12 Wm À2 t 550 À1 and À31 AE 2 Wm À2 t 550 À1 at the bottom and top of the atmosphere; the values for an ocean surface reflectance are also provided.

Retrieval of the Eyjafjallajökull volcanic aerosol optical and microphysical properties from POLDER/PARASOL measurements

“…Properties of a downwind volcanic plume observed over Lille on 17 April 2010 were analyzed in detail using groundbased measurements (Derimian et al, 2012). In this study, the AERONET, lidar, and broadband flux observations in conjunction with numerical simulations were used to assess the volcanic aerosol optical properties.…”

“…This value is close to the one (0.004) prescribed by Schumann et al (2011), which was derived from the joint analysis of ground-based lidar and airborne in situ measurements acquired for volcanic plumes observed on 16 and 17 April. For the imaginary refractive index we use an a priori of the maximal possible value of 0.02, which relies on findings reported in Derimian et al (2012). Because the a priori information for Z a varies from one case to another, we mention it in the paper as necessary.…”

“…We assume that the imaginary refractive index is 0.012/0.007 times smaller at 670 nm than at 490 nm. This ratio is based on the results from Derimian et al (2012), which analyzed the properties of a volcanic plume observed on 17 April 2010 using Sun photometer measurements. This study also found a rather small spectral dependence for the absorption between 670 nm and 865 nm.…”

Initial Assessment of the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR)-Based Aerosol Retrieval: Sensitivity Study

“…The largest uncertainties of the AERONET-based aerosol retrievals and, consequently, of downwelling flux estimations, were obtained in regions dominated by mineral dust and mixed aerosols [16]. The AERONET-based microphysical and optical properties of natural and anthropogenic aerosols collected around the world were applied for the DARF assessment [11,17].…”

“…Abundant literature exists on the DARF assessment using aerosol properties obtained from satellite [8,9] and surface [10,11] observations, including aerosol retrievals from the Aerosol Robotic Network (AERONET; [12]; http://aeronet.gsfc.nasa.gov/) and a suite of instruments supported by the Atmospheric Radiation Measurement (ARM) Program ( [13]; http://www.arm.gov/instruments). The AERONET-supported sunphotometer measurements at four specific wavelengths (0.44, 0.674, 0.87 and 1.02 μm) have been used for deriving aerosol optical properties from the retrieved size distributions and complex refractive index [14,15].…”