Redha Belhadji’s research while affiliated with Université Paris Cité and other places

The intense wildfires in Australia, during the 2019–2020 fire season, generated massive Pyro-cumulonimbus (pyro-Cb) clouds, and injected an unprecedented amount of smoke aerosols into the upper troposphere–lower stratosphere (UTLS). The smoke aerosols produced a self-sustaining confined anticyclonic vortex, that ascended up to 35 km altitude by March 2020 by diabatic heating of radiation absorbing aerosols. This vortex transported ozone-poor tropospheric air into the ozone-rich stratosphere, thus forming a transient ozone mini-hole. This study investigates the early evolution of the dynamically-generated ozone mini-hole, using satellite and ground-based observations, supported by modelling information. Ozone anomalies within the vortex are tracked and quantified by satellite observation. In particular, ad-hoc in-vortex observations are derived by coupling the IASI (Infrared Atmospheric Sounding Interferometer) satellite observations and meteorological reanalysis information of the vortex. With these observations, a 30–40 % ozone depletion is observed in a 6-km partial stratospheric column, which exponentially decreased to ~7 % by the end of January with an e-folding time of about one week, as the vortex ascended in the stratosphere. A total ozone column depletion of ~7 %, immediately after the pyro-Cb injection, was observed with IASI and the TROPOMI (TROPOspheric Monitoring Instrument) satellite instrument. Consistently, ground-based measurement at Lauder, New Zealand showed a localised ozone depletion reaching ~10 % (total column) and ~20 % (in-vortex stratospheric partial column) associated with two vortex overpasses. These results provide insights into the impacts of extreme wildfires and pyro-Cbs on the dynamics and composition of the stratosphere.

Plain Language Summary The eruption of the submarine Hunga volcano in January 2022 polluted the global stratosphere with a large amount of water vapor and significantly perturbed the stratospheric aerosol layer. In this paper, we present a 1‐year long aftermath study of the stratospheric sulfur pollution from this volcanic eruption using observations from the Infrared Atmospheric Sounding Interferometer (IASI) satellite‐borne instrument. Gaseous and aerosol sulfur emissions are observed simultaneously using the specific potential of this sensor. These observations provide unique capabilities to characterize the aerosol type in the Hunga plume and the sulfur cycle associated with the volcanic emissions. An extremely rapid conversion of gaseous sulfur emissions to aerosols is observed, leading to larger than expected and persistent anomalies of the stratospheric aerosol layer (compared with a consistent long‐term climatology), still noticeable in the Southern Hemisphere after 1 year. The total mass of the emitted sulfur in gas and aerosol state is also simultaneously estimated, for the first time.

The Hunga volcano violently erupted on January 15th, 2022, and produced the largest stratospheric aerosol layer perturbation of the last 30 years. One notable effect of the Hunga eruption was the significant modification of the size distribution (SD) of the stratospheric aerosol layer with respect to background conditions and other recent moderate stratospheric eruptions, with larger mean particles size and smaller SD spread for Hunga. Starting from satellite-based SD retrievals, and the assumption of pure sulphate aerosol layers, in this work we calculate the optical properties of both background and Hunga-perturbed stratospheric aerosol scenarios using a Mie code. We found that the intensive optical properties of the stratospheric aerosol layer (i.e., single scattering albedo, asymmetry parameter, aerosol extinction per unit mass and the broad-band average Ångström exponent) were not significantly perturbed by the Hunga eruption, with respect to background conditions. The calculated Ångström exponent was found consistent with multi-instrument satellite observations of the same parameter. Thus, the basic impact of the Hunga eruption on the optical properties of the stratospheric aerosol layer was an increase of the stratospheric aerosol extinction (or optical depth), without any modification of the shortwave and longwave relative absorption, angular scattering and broad-band spectral trend of the extinction, with respect to background. This highlights a marked difference of the Hunga perturbation of the stratospheric aerosol layer and those from other larger stratospheric eruptions, like Pinatubo 1991 and El Chichon 1982. With simplified radiative forcing estimations, we show that the Hunga eruption produced an aerosol layer likely 3–10 times more effective in producing a net cooling of the climate system with respect to Pinatubo and El Chichon eruptions, due to more effective shortwave scattering. As intensive optical properties are seldom directly measured, e.g. from satellite, our calculations can support the estimation of radiative effects for the Hunga eruption with climate or offline radiative models.

Record-breaking wildfires ravaged south-eastern Australia during the fire season 2019–2020. The intensity of the fires reached its paroxysmal phase at the turn of the year 2019–2020, when large pyro-cumulonimbi developed. Pyro-convective activity injected biomass burning aerosols and gases in the upper-troposphere–lower-stratosphere (UTLS), producing a long-lasting perturbation to the atmospheric composition and the stratospheric aerosol layer. The large absorptivity of the biomass burning plume produced self-lofting of the plume and thus modified its vertical dynamics and horizontal dispersion. Another effect of the in-plume absorption was the generation of compact smoke-charged anticyclonic vortices which ascended up to 35 km altitude due to diabatic heating. We use observational and modelling description of this event to isolate the main vortex from the dominant Southern Hemispheric biomass burning aerosol plume. Entering this information into an offline radiative transfer model, and with hypotheses on the absorptivity and the angular scattering properties of the aerosol layer, we estimate the radiative heating rates (HRs) in the plume and the vortex. We found that the hemispheric-scale plume produced a HR of 0.08±0.05 K d-1 (from 0.01 to 0.15 K d-1, depending on the assumption on the aerosol optical properties), as a monthly average value for February 2020, which is strongly dependent on the assumptions on the aerosol optical properties and therefore on the plume ageing. We also found in-vortex HRs as large as 15–20 K d-1 in the denser sections of the main vortex (8.4±6.1 K d-1 on average in the vortex). Our results suggest that radiatively heated ascending isolated vortices are likely dominated by small-sized strongly absorbing black carbon particles. The hemispheric-scale and in-vortex HR estimates are consistent with the observed ensemble self-lofting (a few kilometres in 4 months) and the main isolated vortex rise (∼ 20 km in 2 months). Our results also show evidence of the importance of longwave emission in the net HR of biomass burning plumes.

Plain Language Summary We study the stratospheric aerosol plume produced by the Hunga Tonga–Hunga Ha'apai eruption on 15 January 2022 based on the high quality solar occultation measurements of the instrument SAGE III (Stratospheric Aerosol and Gas Experiment) onboard the International Space Station. These data reveal that the aerosol sizes are about twice as large as after other documented volcanic eruptions and that the total mass of H2SO4 in the liquid droplets of sulfate in the stratosphere has been very stable from March 2022, when it started to be well homogeneized in longitude, to November 2022, when it started to decay. The total mass of 0.66 Tg of H2SO4 is in good agreement with the early estimates of a stratospheric emission of 0.4–0.5 Tg of SO2. The implication is that the aerosol radiative impact will not mask the persisting warming effect of the water vapor injected in the stratosphere by the eruption.

The Hunga Tonga-Hunga Ha’apai volcano violently erupted on 15 January 2022, producing the largest perturbation of the stratospheric aerosol layer since Pinatubo 1991, despite the estimated modest injection of SO2. Here we present novel SO2 and sulphate aerosol (SA) co-retrievals from the Infrared Atmospheric Sounding Instrument, and use them to study the dispersion of the Hunga Tonga plume over the entire year 2022. We observe rapid conversion of SO2 (e-folding time: 17.1±0.6 days) to sulphate aerosols (SA), with an initial injected burden of >1.0 Tg. This points at larger SO2 injections than previously thought. A long-lasting SA plume was observed, with a meridional dispersion of marked anomalies from the tropics to the higher southern hemispheric latitudes. A very small SA removal is observed after 1-year dispersion. The total SA mass burden was estimated at 1.6 ± 0.1 Tg in total column, with a build-up e-folding time of about 2 months.

The Tonga eruption of 15 January 2022 has released a long-lived stratospheric plume of sulfate aerosols. More than 15 months after, we focus on the high quality data series of SAGE III/ISS to determine the mean radius and size distribution of the aerosols and their total mass. We show that the persisting aerosols – with a mode width of 1.25 and an effective radius of 0.4 µm – differ from the significantly smaller background aerosols and from those measured during other recent stratospheric eruptions. The sulfuric acid mass between 50°S and 30°N is estimated to be very stable in spite of considerable redistribution in latitude at a value of 0.66 Tg ± 0.1 Tg, corresponding to an initial sulfur dioxide emission of 0.44 Tg. Such properties are expected to induce a small negative aerosol radiative forcing, thus facilitating the persistence of a climate warming due to the volcanic water vapour.

Record-breaking wildfires ravaged south-eastern Australia during the fire season 2019–2020. The intensity of the fires reached its paroxysmal phase at the turn of the year 2019–2020, when large pyro-cumulonimbi developed. Pyro-convective activity injected biomass burning aerosols and gases in the upper-troposphere—lower-stratosphere (UTLS), producing a long-lasting perturbation to the atmospheric composition and the stratospheric aerosol layer. The large absorptivity of the biomass burning plume produced self-lofting of the plume and thus modified its vertical dynamics and horizontal dispersion. Another effect of the in-plume absorption was the generation of compact smoke-charged anticyclonic vortices which ascended up to 35 km altitude due to diabatic heating. We use observational and modelling description of this event to isolate the dominant Southern-Hemispheric biomass burning aerosol plume from the main isolated vortex. Entering this information into an offline radiative transfer model, and with hypotheses on the absorptivity and the angular scattering properties of the aerosol layer, we estimate the radiative heating rates (HR) in the plume and the vortex. We found that the hemispheric-scale plume produced a HR of 0.08±0.05 K/d, which is strongly dependent on the assumptions on the aerosol optical properties and then on the plume ageing, in our simulations. We also found in-vortex HR as large as 15–20 K/d in the denser sections of the main vortex (8.4±6.1 K/d on average in the vortex). Our results suggest that radiatively-heated ascending isolated vortices are likely dominated by small-sized strongly absorbing black carbon particles. The hemispheric-scale and in-vortex HR estimates are consistent with the observed ensemble self-lofting (a few km in 4 months) and the main isolated vortex rise (~20 km in 2 months). Our results also put in evidence the importance of longwave emission in the net HR of biomass burning plumes.

The underwater Hunga Tonga-Hunga Ha-apai volcano erupted in the early hours of 15th January 2022, and injected volcanic gases and aerosols to over 50 km altitude. Here we synthesise satellite, ground-based, in situ and radiosonde observations of the eruption to investigate the strength of the stratospheric aerosol and water vapour perturbations in the initial weeks after the eruption and we quantify the net radiative impact across the two species using offline radiative transfer modelling. We find that the Hunga Tonga-Hunga Ha-apai eruption produced the largest global perturbation of stratospheric aerosols since the Pinatubo eruption in 1991 and the largest perturbation of stratospheric water vapour observed in the satellite era. Immediately after the eruption, water vapour radiative cooling dominated the local stratospheric heating/cooling rates, while at the top-of-the-atmosphere and surface, volcanic aerosol cooling dominated the radiative forcing. However, after two weeks, due to dispersion/dilution, water vapour heating started to dominate the top-of-the-atmosphere radiative forcing, leading to a net warming of the climate system.

As a consequence of extreme heat and drought, record-breaking wildfires developed and ravaged south-eastern Australia during the fire season 2019–2020. The fire strength reached its paroxysmal phase at the turn of the year 2019–2020. During this phase, pyrocumulonimbus clouds (pyroCb) developed and injected biomass burning aerosols and gases into the upper troposphere and lower stratosphere (UTLS). The UTLS aerosol layer was massively perturbed by these fires, with aerosol extinction increased by a factor of 3 in the visible spectral range in the Southern Hemisphere, with respect to a background atmosphere, and stratospheric aerosol optical depth reaching values as large as 0.015 in February 2020. Using the best available description of this event by observations, we estimate the radiative forcing (RF) of such perturbations of the Southern Hemispheric aerosol layer. We use offline radiative transfer modelling driven by observed information of the aerosol extinction perturbation and its spectral variability obtained from limb satellite measurements. Based on hypotheses on the absorptivity and the angular scattering properties of the aerosol layer, the regional (at three latitude bands in the Southern Hemisphere) clear-sky TOA (top-of-atmosphere) RF is found varying from small positive values to relatively large negative values (up to -2.0 W m-2), and the regional clear-sky surface RF is found to be consistently negative and reaching large values (up to -4.5 W m-2). We argue that clear-sky positive values are unlikely for this event, if the ageing/mixing of the biomass burning plume is mirrored by the evolution of its optical properties. Our best estimate for the area-weighted global-equivalent clear-sky RF is -0.35±0.21 (TOA RF) and -0.94±0.26 W m-2 (surface RF), thus the strongest documented for a fire event and of comparable magnitude with the strongest volcanic eruptions of the post-Pinatubo era. The surplus of RF at the surface, with respect to TOA, is due to absorption within the plume that has contributed to the generation of ascending smoke vortices in the stratosphere. Highly reflective underlying surfaces, like clouds, can nevertheless swap negative to positive TOA RF, with global average RF as high as +1.0 W m-2 assuming highly absorbing particles.