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Legras, Bernard
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- PublicationRestrictedThe impact of Mount Etna sulfur emissions on the atmospheric composition and aerosol properties in the central Mediterranean: A statistical analysis over the period 2000–2013 based on observations and Lagrangian modelling(2017)
; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ;; ;The emission of gases and aerosols due to volcanic activity may impact significantly atmospheric composition, cloud occurrence and properties, and the regional and global climate. While the effects of strong explosive (stratospheric) eruptions are relatively well known, limited information on the impacts of small to moderate volcanic activities, including passive degassing, is available. In this paper, the downwind impact of Mount Etna's sulfur emissions on the central Mediterranean is investigated on a statistical basis over the period 2000e2013 using: (a) daily sulfur dioxide emission rates measured near crater at Mount Etna with ground-based ultraviolet spectrophotometers, (b) Lagrangian trajectories and simulated plume dispersion obtained with the FLEXPART (FLEXible PARTicle dispersion) model, and (c) long-term observations of column SO2 concentration and aerosol Ångstr€om exponent a at Lampedusa (35.5 N, 12.6 E). This statistical analysis has allowed, for the first time, the characterization of decadal impact of Mount Etna's sulfur emissions on the sulfur dioxide and the aerosol microphysical/optical properties in the central Mediterranean. On average, statistically significant higher SO2 concentrations and smaller aerosol sizes are present when air masses from Mount Etna overpass Lampedusa. Despite being upwind of Lampedusa for only 5% of the time, Mount Etna is potentially responsible for up to 40% and 20% of the SO2 and a extreme values (exceedances of a fixed threshold), respectively, at this location. The most important factor determining this perturbation is the prevailing dynamics, while the magnitude of the SO2 emission rates from Mount Etna appears to be likely important only for relatively strong emissions. The observed perturbations to the aerosol size distribution are expected to produce a direct regional radiative effect in this area.179 6 - PublicationOpen AccessQuantitative Retrieval of Volcanic Sulphate Aerosols from IASI Observations(2021)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ;We developed a new retrieval algorithm based on the Infrared Atmospheric Sounding Interferometer (IASI) observations, called AEROIASI-H2SO4, to measure the extinction and mass concentration of sulphate aerosols (binary solution droplets of sulphuric acid and water), with moderate random uncertainties (typically 35% total uncertainty for column mass concentration estimations). The algorithm is based on a self-adapting Tikhonov–Phillips regularization method. It is here tested over a moderate-intensity eruption of Mount Etna volcano (18 March 2012), Italy, and is used to characterise this event in terms of the spatial distribution of the retrieved plume. Comparisons with simultaneous and independent aerosol optical depth observations from MODIS (Moderate Resolution Imaging Spectroradiometer), SO2 plume observations from IASI and simulations with the CHIMERE chemistry/transport model show that AEROIASI-H2SO4 correctly identifies the volcanic plume horizontal morphology, thus providing crucial new information towards the study of volcanic emissions, volcanic sulphur cycle in the atmosphere, plume evolution processes, and their impacts. Insights are given on the possible spectroscopic evidence of the presence in the plume of larger-sized particles than previously reported for secondary sulphate aerosols from volcanic eruptions.164 9 - PublicationOpen AccessVolcanic Emissions, Plume Dispersion, and Downwind Radiative Impacts Following Mount Etna Series of Eruptions of February 21–26, 2021(2023)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; During the extended activity of Mount Etna volcano in February–April 2021, three distinct paroxysmal events took place from February 21 to 26, which were associated with a very uncommon transport of the injected upper-tropospheric plumes toward the north. Using a synergy of observations and modeling, we characterized the emissions and three-dimensional dispersion for these three plumes, monitored their downwind distribution and optical properties, and estimated their radiative impacts at selected locations. With a satellite-based source inversion, we estimate the emitted sulfur dioxide (SO2) mass at an integrated value of 55 kt and plumes injections at up to 12 km altitudes, which qualifies this series as an extreme event for Mount Etna. Then, we combine Lagrangian dispersion modeling, initialized with measured temporally resolved SO2 emission fluxes and altitudes, with satellite observations to track the dispersion of the three individual plumes. The transport toward the north allowed the height-resolved downwind monitoring of the plumes at selected observatories in France, Italy, and Israel, using LiDARs and photometric aerosol observations. Volcanic-specific aerosol optical depths (AODs) in the visible spectral range ranging from about 0.004 to 0.03 and local daily average shortwave radiative forcing (RF) ranging from about −0.2 to −1.2 W m −2 (at the top of atmosphere) and from about −0.2 to −3.0 W m −2 (at the surface) are found. The composition (possible presence of ash), AOD, and RF of the plume have a large inter-plume and intra-plume variability and thus depend strongly on the position of the sampled section of the plumes.128 99 - PublicationOpen AccessDeep convective influence on the UTLS composition in the Asian Monsoon Anticyclone region: 2017 StratoClim campaign results(2020)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; The StratoClim stratospheric aircraft campaign took place in summer 2017 in Nepal (the 27th of July - 10th of August) and provided a wide dataset of observations of air composition inside the Asian Monsoon Anticyclone. In the framework of this project, with the purpose of modelling the injection of pollutants and natural compounds into the stratosphere, we performed a series of diffusive back-trajectories runs along the flights’ tracks. The availability of in-situ measurements of trace gases has been exploited 5 to evaluate the capability of the trajectory system to reproduce the transport in the Upper Troposphere - Lower Stratosphere (UTLS) region. The diagnostics of the convective sources and mixing in the air parcel samples have been derived by integrating the trajectories output with high-resolution observations of cloud tops from the Meteosat Second Generation (MSG1) and Himawari geostationary satellites. Back-trajectories have been calculated using meteorological fields from European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-Interim and ERA-5) at 3h and 1h resolution, using both kinematic and diabatic vertical motion. The comparison among the different trajectory runs shows, in general, a higher consistency with observed data, as well as a better agreement between the diabatic and kinematic version, when using ERA-5 based runs with respect to ERA-Interim. Overall, a better capacity in reproducing the pollution features is finally found in the diabatic version of the ERA-5 runs. Adopting this setting for the analysis, a large variety of transport conditions have been individuated during the 8 flights of the campaign. The larger influence by convective injections is found from the continental sources of China and India. Only a small contribution appears to be originated from maritime regions, in particular the South Pacific and the Bay of Bengal that, unexpectedly, was not particularly active during the period of the campaign. Thin filamentary structures of polluted air, characterized by peaks in CO, are observed, mostly associated with young convective air (age less than a few days) and a predominant South-China origin. Observed air from continental India, on the contrary, is often linked to a lower concentration of the trace gas and to air masses that recirculated within the anticyclone for 10 to 20 days. Vertical stratification in the age of air is observed: up to 15 km, the age is less than 3 days and these fresh air masses constitute almost the totality of the air composition. A transition layer is then individuated between 15 km and 17 km, where the convective contribution is still dominant and the ages vary between one and two weeks. Above this level, the mean age of the air sampled by the aircraft is estimated to be 20 days. There, the convective contribution rapidly decreases with height, and finally became negligible around 20 km.100 32 - PublicationOpen AccessSynergistic use of Lagrangian dispersion and radiative transfer modelling with satellite and surface remote sensing measurements for the investigation of volcanic plumes: the Mount Etna eruption of 25–27 October 2013(2016)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ; ; ;; ;; ;In this paper we combine SO2 and ash plume dispersion modelling with satellite and surface remote sensing observations to study the regional influence of a relatively weak volcanic eruption from Mount Etna on the optical and micro-physical properties of Mediterranean aerosols. We analyse the Mount Etna eruption episode of 25–27 October 2013. The evolution of the plume along the trajectory is investigated by means of the FLEXible PARTicle Lagrangian dispersion (FLEXPART) model. The satellite data set includes true colour images, retrieved values of volcanic SO2 and ash, estimates of SO2 and ash emission rates derived from MODIS (MODerate resolution Imaging Spectroradiometer) observations and estimates of cloud top pressure from SEVIRI (Spinning Enhanced Visible and InfraRed Imager). Surface remote sensing measurements of aerosol and SO2 made at the ENEA Station for Climate Observations (35.52 N, 12.63 E; 50ma.s.l.) on the island of Lampedusa are used in the analysis. The combination of these different data sets suggests that SO2 and ash, despite the initial injection at about 7.0 km altitude, reached altitudes around 10–12 km and influenced the column average aerosol particle size distribution at a distance of more than 350 km downwind. This study indicates that even a relatively weak volcanic eruption may produce an observable effect on the aerosol properties at the regional scale. The impact of secondary sulfate particles on the aerosol size distribution at Lampedusa is discussed and estimates of the clear-sky direct aerosol radiative forcing are derived. Daily shortwave radiative forcing efficiencies, i.e. radiative forcing per unit AOD (aerosol optical depth), are calculated with the LibRadtran model. They are estimated between 39 and 48Wm^-2 AOD^ -1 at the top of the atmosphere and between 66 and 49Wm^-2 AOD^-1 at the surface, with the variability in the estimates mainly depending on the aerosol single scattering albedo. These results suggest that sulfate particles played a large role in the transported plume composition and radiative forcing, while the contribution by ash particles was small in the volcanic plume arriving at Lampedusa during this event.301 100