Physical and optical properties of 2010 Eyjafjallajökull volcanic eruption aerosol: ground-based, Lidar and airborne measurements in France

Abstract

Abstract. During the Eyjafjallajökull eruption (14 April to 24 May 2010), the volcanic aerosol cloud was observed across Europe by several airborne in situ and ground-based remote-sensing instruments. On 18 and 19 May, layers of depolarizing particles (i.e. non-spherical particles) were detected in the free troposphere above the Puy de Dôme station, (PdD, France) with a Rayleigh-Mie LIDAR emitting at a wavelength of 355 nm, with parallel and crossed polarization channels. These layers in the free troposphere (FT) were also well captured by simulations with the Lagrangian particle dispersion model FLEXPART, which furthermore showed that the ash was eventually entrained into the planetary boundary layer (PBL). Indeed, the ash cloud was then detected and characterized with a comprehensive set of in situ instruments at the Puy de Dôme station (PdD). In agreement with the FLEXPART simulation, up to 65 µg m −3 of particle mass and 2.2 ppb of SO 2 were measured at PdD, corresponding to concentrations higher than the 95 percentile of 2 yr of measurements at PdD. Moreover, the number concentration of particles increased to 24 000 cm −3 , mainly in the submicronic mode, but a supermicronic mode was also detected with a modal diameter of 2 µm. The resulting optical properties of the ash aerosol were characterized by a low scatteringÅngström exponent (0.98), showing the presence of supermicronic particles. For the first time to our knowledge, the combination of in situ optical and physical characterization of the volcanic ash allowed the calculation of the mass-to-extinction ratio (η) with no assumptions on the aerosol density. The mass-to-extinction ratio was found to be significantly different from the background boundary layer aerosol (max: 1.57 g m −2 as opposed to 0.33 ± 0.03 g m −2 ). Using this ratio, ash mass concentration in the volcanic plume derived from LIDAR measurements was found to be 655 ± 23 µg m −3 when the plume was located in the FT (3000 m above the sea level -a.s.l.).This ratio could also be used to retrieve an aerosol mass concentration of 579 ± 60 µg m −3 on 19 April, when LIDAR observations detected the ash cloud at 3000 m a.s.l. in correspondence with model simulations (FLEXPART). On 22 April, another ash plume entered the BL, and although it was more diluted than during the May episode, the French research aircraft ATR42 that passed over Clermont-Ferrand in the PBL confirmed the presence of particles with a supermicronic mode, again with a modal diameter at 2 µm.This data set combining airborne, ground-based and remote sensing observations with dispersion model simulations shows an overall very good coherence during the volcanic eruption period, which allows a good confidence in the characteristics of the ash particles that can be derived from this unique data set.Published by Copernicus Publications on behalf of the European Geosciences Union. M. Hervo et al.: Ground-based, Lidar and airborne measurements of volcanic ashes in France

Experimental Evidence of the Feeding of the Free Troposphere with Aerosol Particles from the Mixing Layer

“…These errors are estimated from calibrations, truncation errors for the scattering coefficient, and from noise. More details of these calculations can be found in Hervo et al (2012). The value of ω0 did not vary considerably over the measurement period in the FT, with mean values of 0.88 ± 0.02.…”

“…The LIDAR instrument is a Raymetrics Rayleigh Mie Raman, emitting at 355 nm, with a spatial resolution of 7.5 m. More detailed descriptions of this instrument are available in Hervo et al (2012) and Freville et al (2015). The LIDAR instrument was used to determine the height of the ML using the wavelength covariance technique (Brooks, 2003).…”

In situ physical and chemical characterisation of the Eyjafjallajökull aerosol plume in the free troposphere over Italy

“…New significant eruptions occurred on 4-9 May and 14-19 May. The first of these events mainly influenced western Europe, from Great Britain to the Iberian Peninsula (Pappalardo et al, 2013), while the second travelled over Ireland and Great Britain and reached central Europe (Bukowiecki et al, 2011;Hervo et al, 2012). Volcanic layers were observed in the central and eastern Mediterranean area, including Greece and Turkey, in the days from 18 to 22 May Papayannis et al, 2012).…”

“…A 60 % volcanic contribution is reported at a high altitude station in France (Puy de Dome, 1465 m a.s.l.). According to the authors (Hervo et al, 2012), the 33 % Flexpart estimate at Puy de Dome, reported in Table 2 for the same site, would represent an underestimate, partly explained by the fact that the model does not simulate the formation of secondary aerosols. By contrast, the April episode denotes a higher degree of variability in the volcanic contributions calculated at different sites.…”

“…For the typical ash collected during the explosive phase of the eruption near the source, diameter peaked at approximately 500 µm, with at least 20 % of mass < 10 µm (Gislason et al, 2011), therefore able to stay airborne while being transported over long distance. Far from the source, aircraft measurements over France between 19 and 22 April showed increased particle concentrations both in the submicronic (∼ 100 nm) and in supermicronic (∼ 2 µm) modes in the PBL (Hervo et al, 2012).…”