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 The eruption of the Eyjafjallajökull Icelandic volcano during the period 14 April to 21 May 2010 caused unprecedented disruption to European airspace. This was due to the advection of volcanic ash particles over much of Europe, which are known to damage jet engines if encountered at sufficient concentrations [Guffanti et al., 2010; Witham et al., 2007]. During this episode the monitoring of ash spatial distributions from satellite radiance measurements played an important role in the tracking of aerosol plumes and in the validation of ash dispersion forecasts. As well as highlighting the importance of satellite data in ash detection the incident motivated further research into the quantitative retrieval of ash concentrations from space.
 Operational retrievals of aerosol optical depths and column burdens have been extensively developed using multiple wavelengths in the visible region of the spectrum on polar orbiting satellite platforms (e.g., the Advanced Very High Resolution Radiometer (AVHRR) [Mishchenko et al., 1999]; Clouds and the Earth's Radiant Energy System (CERES) [Loeb and Kato, 2002]; the Moderate Resolution Imaging Spectrometer (MODIS) [Remer and Kaufman, 2006]; the Multiangle Imaging Spectro-Radiometer (MISR) [Kahn et al., 2001]). Geostationary satellite retrieval algorithms have also been developed at solar wavelengths [e.g., Brindley and Ignatov, 2006] and offer far superior temporal sampling. While the algorithms differ in detail, these retrievals rely on the wavelength-dependent reflection by aerosol of incident sunlight back to space and are only possible during the hours of sunlight in the absence of cloud. Additionally, many of the visible retrieval algorithms are only performed over well-characterized dark surfaces such as oceans although the use of multiple views [Kahn et al., 2001] or UV wavelengths [Hsu et al., 2006] can eliminate this problem.
 If aerosols are of large enough sizes, in addition to their impact on solar wavelengths of electromagnetic radiation they may significantly perturb the terrestrial radiation budget by absorbing and emitting terrestrial radiation. This has led to the development of aerosol retrievals for mineral dust and volcanic ash that use thermal infrared channels on polar orbiting [e.g., Prata and Grant, 2001; Watkin, 2003] and geostationary platforms [e.g., Prata and Kerkmann, 2007]. The detection algorithms for multichannel imagers exploit brightness temperature differences between channels at 8–10 μm (sensitive to ash and dust) and 11–12 μm (sensitive to water and ice clouds) and typically use data from two or three wavelengths. When mounted on geostationary platforms, infrared sensors are capable of providing high temporal (e.g., every 15 min for the EUMETSAT Spinning Enhanced Visible and Infrared Imager (SEVIRI) RGB product) monitoring of volcanic ash plumes [see Francis et al., 2012].
 A new generation of spaceborne hyperspectral sounders, such as the Atmospheric Infrared Sounder (AIRS) on the Aqua platform and the Infrared Atmospheric Sounding Interferometer (IASI) on MetOp, offer greater information content through much higher spectral resolution. The very high spectral resolution from hyperspectral measurements has already proven extremely useful in monitoring and tracking the evolution of SO2 from large volcanic eruptions which can be used to validate numerical model simulations [e.g., Haywood et al., 2010]. The unique signature of volcanic ash in hyperspectral data allows parameters such as aerosol effective radii, concentrations and mass to be remotely sensed with greater confidence than if only a few wavelengths are utilized [Clarisse et al., 2010a; Prata et al., 2010].
 The initial explosive eruption of Eyjafjallajökull commenced on 14 April 2010 closing significant amounts of UK and European airspace [Dacre et al., 2011; Schumann et al., 2011; Ansmann et al., 2010]. Unfortunately, the FAAM BAe-146 was out of service when the eruption occurred, but was made operational as soon as possible and commenced active flying on 20 April, making a total of twelve flights dedicated to remote sensing and in situ measurements of volcanic ash (B. T. Johnson et al., In situ observations of volcanic ash clouds from the FAAM aircraft during the eruption of Eyjafjallajökull in 2010, submitted toJournal of Geophysical Research, 2011).
 In accompanying papers in this special section [Turnbull et al., 2012; Marenco et al., 2011] we describe the in situ and downward-looking lidar data gathered during the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 flight on 17 May 2010, and the corresponding data analysis required to obtain quantitative estimates of ash size distribution, optical extinction and mass loading. Part 1 of this case study [Turnbull et al., 2012] describes the in situ airborne observations on 17 May 2010 obtained from the FAAM BAe-146 and Deutsches Zentrum für Luft- und Raumfahrt (DLR) Falcon aircraft.Turnbull et al. [2012, Figure 3] illustrate how geostationary SEVIRI imagery was able to track the volcanic ash plume, with light north to northwesterly winds carrying the ash over the North Sea in otherwise clear sky conditions. In this (Part 2) paper we concentrate on radiation measurements obtained on 17 May 2010 comprising aircraft and satellite radiances and irradiances in the presence of ash, spanning the infrared to visible spectral range. We seek to demonstrate radiative closure between (1) the radiation observations; (2) collocated profiles of aerosol extinction derived from lidar backscatter measurements; (3) aerosol optical properties based on a representative particle size distribution and choice of ash complex refractive index; and (4) radiative transfer simulations. Further, we verify the performance of hyperspectral retrievals of ash mass loadings using IASI observations, providing independent verification of such methods for the first time.
 In section 2 of this paper we summarize the aircraft observations relevant to this work, including in situ ash properties and concentrations derived from aircraft probes and lidar backscatter measurements. We describe in section 3 observations from the Met Office airborne radiometers operating at solar and terrestrial wavelengths and show results from radiative transfer modeling. Observations from the IASI infrared sounder on the MetOp satellite and the results of ash mass loading retrievals are detailed in section 4, and we conclude in section 5.
2. Measurements and Modeling of Ash Properties
2.1. In Situ Ash Measurements
 Observations of airborne ash from the eruption of Eyjafjallajökull were made from the UK's FAAM aircraft. A comprehensive overview of the in situ measurements during the flights in April–May 2010 is made by Johnson et al. (submitted manuscript, 2011) while Turnbull et al.  present analysis of aircraft data from this case study on 17 May 2011. We restrict ourselves here to a brief summary of the aircraft data and derived ash properties.
 Aerosol concentrations were measured by wing-mounted optical particle counters: the Particle Measuring System (PMS) Passive Cavity Aerosol Spectrometer Probe 100X (PCASP), with size bins covering the range 0.1–0.6μm nominal diameter i.e., fine mode aerosol, and the University of Manchester Cloud and Aerosol Spectrometer (CAS) probe covering the size range 0.6–35 μm, i.e., coarse mode. CAS is one of the instruments that comprises the Cloud Aerosol and Precipitation Spectrometer (CAPS). The Small Ice Detector (SID-2H) was also fitted and used to determine the asphericity of particles with diameters greater than 2μm. The coarse mode aerosols are assumed to be ash, with ash mass concentration derived from integrating over the CAS size distribution; this requires the solid density of ash to be known, in this case an assumed value of 2300 kg/m3, and is also sensitive to assumptions regarding particle shape and complex refractive index.
 The refractive index used in the present CAS data analysis is based on the mineral dust data set of Balkanski et al. with a 1.5% level of hematite. The use of these high silicate refractive indices is justified by examining the in-flight statistics of airborne volcanic ash collected by filters on the German Falcon aircraft [Schumann et al., 2011] which was flying in close proximity to the BAe 146 aircraft [Turnbull et al., 2012] and sampled the same volcanic ash plume. Schumann et al. report post-flight analysis of 489 individual particles using a scanning electron microscope with an attached energy dispersive X-ray (EDX) detector. Their analysis reveals that, perhaps rather surprisingly, silicates compose more than 83% of particles between 0.5 and 1.0μm size, 92% of particles between 1.0 and 2.0 μm and 100% of particles larger than 2.0 μm (only 7 particles sampled for this largest size bin). Note that while Schumann et al.  report that the particles are predominantly silicates on 17 May 2010, analysis of the volcanic plume on 2 May 2010 suggest significantly less silicate material present. While our results use the refractive indices from Balkanski et al. , we also present a sensitivity analysis of the impacts on retrievals of volcanic ash using other assumed refractive indices and show that our choice gives optimal results across the terrestrial spectrum.
 Different assumptions of particle shape, namely spheres and an irregularly shaped model comprising a mixture of hexagonal prisms and polyhedral particles, hereinafter referred to as the “irregular model” (for details of the irregular model, see Osborne et al. ), have been tested separately. Ash mass concentrations of up to approximately 500 μg/m3 (irregular model; 700 μg/m3 when spheres are assumed with the same effective diameter and extinction but increased number) were encountered during a series of profiles around 54°N, 0–2°E on 17 May 2010. The mass concentrations have an estimated uncertainty of a factor of 2 (Johnson et al., submitted manuscript, 2011) due to uncertainties in particle properties and instrument sizing accuracy.
 Additionally, aerosol scattering coefficients were determined at three wavelengths (450 nm, 550 nm, 700 nm) using a TSI 3563 nephelometer on the FAAM BAe-146 via an inlet sampling the external air. Using optical properties calculated from the CAS/PCASP size distributions the aerosol mass was derived from the nephelometer measurements. Combining several intercepts of the ash layer the nephelometer-derived column ash loading was in the range 0.29–0.72 g/m2, compared with 0.22–0.71 g/m2 for the CAS data [Turnbull et al., 2012] when obtained with the default irregular model.
2.2. Lidar Observations
 The Leosphere ALS450 elastic backscatter lidar was deployed on the FAAM BAe 146–301 research aircraft during the Eyjafjallajökull eruption, in a nadir-viewing geometry.Marenco et al.  describe the methodology for converting lidar beam returns at 355 nm wavelength into profiles of aerosol extinction, which excludes molecular Rayleigh scattering and cloud returns. It is estimated that the uncertainty in the derived extinction is ±30%.
 Although it is not directly measured by the lidar, it is possible to infer the ash mass concentration by combining lidar extinction with in situ measurements. Key to this derivation are the specific extinction coefficient, kext (units m2/g) at the lidar wavelength, and the fraction of aerosol extinction associated with the coarse mode (ash), fc, which were determined from PCASP and CAS measurements for each FAAM flight (Johnson et al., submitted manuscript, 2011). Values of kext and fc are dependent on assumptions of particle shape, complex refractive index and density. Marenco et al. [2011, Table 2] show how these parameters vary from flight to flight, with Mie-Lorenz calculations (spheres) and with the more sophisticated irregular particle model. The assumption of spheres leads to slightly higher lidar-derived mass estimates than with the irregular model (by approximately 30%). An overall uncertainty of a factor of two is estimated for the lidar mass concentrations derived in this manner.
 On 17 May 2010, a peak ash concentration of 800 μg/m3 (assuming the irregular model), 1100 μg/m3 (spherical model), was determined from lidar returns, i.e., locally higher than CAS peak values where the FAAM BAe 146 intercepted the ash. (We note that this discrepancy can be explained by the highly spatially variable concentrations of aerosol observed.) The ash plume was observed [Marenco et al., 2011] mainly between 3.5 and 5.6 km altitude and had a typical depth of 1.3 km (up to 2.0 km at times). Its measured east-west horizontal cross section was found to be over 500 km, with a large concentration feature (>500μg/m3) only 85 km long. The lidar-derived ash column loading was typically 0.3 g/m2, peaking at 0.75 g/m2, assuming irregular particles (0.4 g/m2 peaking at 1.0 g/m2 assuming spheres) compared with CAS values of up to 0.7 g/m2 (irregulars), 1.0 g/m2 (spheres). A map of observed ash column loads derived from the lidar for 17 May is displayed in Figure 4e of Marenco et al. .
2.3. Ash Optical Properties
 A key aim of this paper is to test whether it is possible to obtain radiative closure between: radiation measurements across the visible to infared spectrum; observations of ash profile concentrations; and simulated radiances and irradiances based on derived optical properties. Here we adopt the spherical assumption for particle shape for calculating infrared optical properties. The aerosol optical properties were calculated via Mie-Lorenz theory using the mineral dust refractive index ofBalkanski et al.  (Figure 1). The particle size distribution (PSD) was represented by a lognormal distribution as described by Turnbull et al. . The extinction coefficient, single scattering albedo and asymmetry parameter decrease between solar (<3 μm) and thermal IR wavelengths (>6 μm) and then vary within the IR region as a strong function of refractive index. Note, the infrared wave number region of interest in this study (the atmospheric window situated approximately between 800–1250 cm−1) corresponds to wavelengths of 12.5–8.0 μm. The mineral dust hematite concentration has a significant effect on the single scattering albedo in the shortwave spectrum but has insignificant consequence on the longwave properties or the ratio between shortwave and longwave aerosol extinction. Therefore, for consistency with Johnson et al. (submitted manuscript, 2011) and Turnbull et al.  we adopt the medium (1.5%) hematite content, as used to calculate the optical properties displayed in Figure 1, for all atmospheric radiative transfer modeling in this study associated with the interpretation of broadband and spectrally resolved radiation measurements. Figure 1demonstrates how the Mie-Lorenz calculations relate PCASP/CAS measurements of the PSD and lidar-derived vertical extinction profiles at visible and UV wavelengths with measurements in the mid-infrared spectral region.
 We aim to test the sensitivity of radiative transfer simulations to assumptions of particle shape and PSD. Applying the spherical and irregular models to the interpretation of CAS data results in different parameterized forms of the PSD. Additionally, we make use of independent optical particle counter measurements from the DLR Falcon aircraft on 17 May 2010 [Schumann et al., 2011] for which the coarse mode size distribution is broader than the FAAM data and peaks at larger sizes. Turnbull et al.  present fitted PSDs for these different choices; in the present work we consider three cases:
 FAAM (A) The default assumption, using a PSD derived from CAS data assuming irregular (polyhedral) particles for the coarse mode and applying Mie-Lorenz theory to derive infrared optical properties replacing polyhedrals with equivalent volume spheres.
 FAAM (B) As for FAAM (A) except that spheres are assumed throughout including in the CAS data analysis. This results in an increased geometric mean diameter of 4.0 μm, compared to 3.6 μm for FAAM (A), and increased standard deviation of the lognormal distribution (1.85 c.f. 1.8). Overall this leads to a strengthening of infrared extinction relative to shortwave (0.55 μm) extinction by approximately 10% (at 10 μm) to 19% (at 12.5 μm) compared with the irregular model.
 DLR This case assumes spheres but replaces the CAS-derived PSD with lognormal fits based on DLR airborne data. The coarse mode has a geometric mean diameter of 9.6μm and standard deviation of 2.5. This strengthens the infrared to shortwave extinction ratio by approximately 35% (10 μm) to 134% (12.5 μm) compared with FAAM (A).
 FAAM cases (A) and (B) incorporate the refractive index of Balkanski et al. , with a 1.5% hematite content, for the coarse mode and a refractive index appropriate for sulphuric acid for the fine mode: these are required for the interpretation of CAS (680 nm), PCASP (630 nm) and lidar (355 nm) data. This implies a refractive index of 1.52 + 0.0015i (coarse mode) and 1.43 + 0.00i (fine mode) which is specified across all UV-visible wavelengths.
 For the DLR case we follow Schumann et al.  who used a refractive index of 1.59 + 0.004i (at 632 nm) as a best estimate for deriving PSDs from their optical particle counter data in a uniform analysis of 33 different plume penetrations. They found high variability and size dependence of the refractive index. Larger particles were less absorbing than smaller ones and the refractive index varied during different plume encounters. An imaginary refractive index of 0.008 implied particle sizes larger than observed on particle impactors and larger than expected due to particle sedimentation. Zero absorption could be another reasonable assumption in deriving particle sizes. It is important to recognize that we still use the Balkanski et al.  refractive index for the generation of optical properties at all other wavelengthsusing Mie-Lorenz theory. The parameters used for the three cases are summarized inTable 1.
 Note that even with our use of an irregular model to derive a representative PSD from CAS data for the FAAM (A) case we still assume spheres when applying Mie-Lorenz theory to calculate optical properties over an extended spectral range. In order to justify the use of Mie-Lorenz calculations in this work the scalar optical properties (i.e., extinction cross section,ω0 and g) calculated assuming equal volume spheres have been compared against exact T-matrix [Havemann and Baran, 2001