Model calculations of non-cloud radiative forcing due to anthropogenic sulphate aerosol

Anthropogenic sulphate aerosol particles scatter incoming solar radiation thereby perturbing the radiative budget, hence climate. We have used a three dimensional radiative transfer model together with the sulphate concentration fields simulated by two independent chemistry-transport models to calculate the annual cycle of the radiative forcing due to anthropogenic sulphate aerosol. The calculated forcing pattern shows large peaks over the eastern United States, southeast Europe and eastern China. The calculated annual global-mean radiative forcing is -0.50 Wm-2 for Langner and Rodhe (1991) data and -0.49 Wm-2 for Penner et el. [1994 (a&b)] data. The forcing was found to vary with season, with a larger forcing during northern hemispheric summer than winter. Sulphate aerosol also appreciably perturbs the lower tropospheric heating rates over northern hemispheric mid-latitudes. The forcing was also found to be sensitive to the global cloud cover and to the optical properties of the aerosol. The possible sources of the differences in magnitude with previous estimates are discussed. Over northern hemispheric mid-latitudes, the negative radiative forcing due to the direct effect of aerosols appreciably offsets the positive forcing due to increase in greenhouse gases. A 26-layer radiative-convective model (RCM) was also used to examine the equilibrium temperature profiles due to sulphate aerosols and increase in greenhouse gases. It was found that the effect of sulphate aerosols is the cooling of surface-troposphere system. Sulphate aerosols reduce the tropospheric warming and enhance the stratospheric cooling caused by increase in greenhouse gases.

Photometric Observations of Aerosol Optical Properties and Emission Flux Rates of Stromboli Volcano Plume during the PEACETIME Campaign

The characterisation of aerosol emissions from volcanoes is a crucial step towards the assessment of their importance for regional air quality and regional-to-global climate. In this paper we present, for the first time, the characterisation of aerosol emissions of the Stromboli volcano, in terms of their optical properties and emission flux rates, carried out during the PEACETIME oceanographic campaign. Using sun-photometric observations realised during a near-ideal full plume crossing, a plume-isolated aerosol optical depth of 0.07–0.08 in the shorter-wavelength visible range, decreasing to about 0.02 in the near infrared range, was found. An Ångström exponent of 1.40 ± 0.40 was also derived. This value may suggest the dominant presence of sulphate aerosols with a minor presence of ash. During the crossing, two separate plume sections were identified, one possibly slightly affected by ash coming from a mild explosion, and the other more likely composed of pure sulphate aerosols. Exploiting the full crossing scan of the plume, an aerosol emission flux rate of 9–13 kg/s was estimated. This value was 50% larger than for typical passively degassing volcanoes, thus pointing to the importance of mild explosions for aerosol emissions in the atmosphere.

Innovative Box-Wing Aircraft: Emissions and Climate Change

The PARSIFAL project (Prandtlplane ARchitecture for the Sustainable Improvement of Future AirpLanes) aims to promote an innovative box-wing aircraft: the PrandtlPlane. Aircraft developed adopting this configuration are expected to achieve a payload capability higher than common single aisle analogues (e.g., Airbus 320 and Boeing 737 families), without any increase in the overall dimensions. We estimated the exhaust emissions from the PrandtlPlane and compared the corresponding impacts to those of a conventional reference aircraft, in terms of Global Warming Potential (GWP) and Global Temperature Potential (GTP), on two time-horizons and accounted for regional sensitivity. We considered carbon dioxide, carbonaceous and sulphate aerosols, nitrogen oxides and related ozone production, methane degradation and nitrate aerosols formation, contrails, and contrail cirrus. Overall, the introduction of the PrandtlPlane is expected to bring a considerable reduction of climate change in all the source regions considered, on both the time-horizons examined. Moreover, fuel consumption is expected to be reduced by 20%, as confirmed through high-fidelity Computational Fluid Dynamics (CFD) simulations. Sensitivity of data, models, and metrics are detailed. Impact reduction and mitigation strategies are discussed, as well as the gaps to be addressed in order to develop a comprehensive Life Cycle Assessment on aircraft emissions.

An assessment of the likelihood of contrail formation

<p>Air Transport has for a long time been linked to environmental issues like pollution, noise and climate change. Aviation emissions, such as carbon dioxide (CO2), water vapour (H2O), nitrogen oxides (NOx), soot and sulphate aerosols, alter the concentration of atmospheric Greenhouse gases and trigger the formation of contrails and cirrus clouds. The ClimOP collaboration, an Horizon 2020 project, aims to identify, evaluate and support the implementation of mitigation strategies to initiate and foster operational improvements which reduce the climate impact of the aviation sector. To this end, we present a study that assesses the likelihood of contrail formation as a function of key atmospheric variables, at different altitudes.</p>

Return to different climate states by reducing sulphate aerosols under future CO2 concentrations

It is generally believed that anthropogenic aerosols cool the atmosphere; therefore, they offset the global warming resulting from greenhouse gases to some extent. Reduction in sulphate, a primary anthropogenic aerosol, is necessary for mitigating air pollution, which causes atmospheric warming. Here, the changes in the surface air temperature under various anthropogenic emission amounts of sulphur dioxide (SO2), which is a precursor of sulphate aerosol, are simulated under both present and doubled carbon dioxide (CO2) concentrations with a climate model. No previous studies have conducted explicit experiments to estimate the temperature changes due to individual short-lived climate forcers (SLCFs) in different climate states with atmosphere–ocean coupled models. The simulation results clearly show that reducing SO2 emissions at high CO2 concentrations will significantly enhance atmospheric warming in comparison with that under the present CO2 concentration. In the high latitudes of the Northern Hemisphere, the temperature change that will occur when fuel SO2 emissions reach zero under a doubled CO2 concentration will be approximately 1.0 °C, while this value will be approximately 0.5 °C under the present state. This considerable difference can affect the discussion of the 1.5 °C/2 °C target in the Paris Agreement.

The effect of anthropogenic aerosols on the Aleutian Low

Past studies have suggested that regional trends in anthropogenic aerosols can influence the Pacific Decadal Oscillation (PDO) through modulation of the Aleutian Low. However, the robustness of this connection is debated. This study analyses changes to the Aleutian Low in an ensemble of climate models forced with large, idealised global and regional black carbon (BC) and sulphate aerosol perturbations. To isolate the role of ocean feedbacks, the experiments are performed with an interactive ocean and with prescribed sea surface temperatures. The results show a robust weakening of the Aleutian Low forced by a global 10-fold increase in BC in both experiment configurations. A linearised steady-state primitive equation model is forced with diabatic heating anomalies to investigate the mechanisms through which heating from BC emissions influences the Aleutian Low. The heating from BC absorption over India and east Asia generates Rossby wave trains that propagate into the North Pacific sector, forming an upper tropospheric ridge. Sources of BC outside of east Asia enhance the weakening of the Aleutian Low. The responses to a global 5-fold and regional 10-fold increase in sulphate aerosols over Asia show poor consistency across climate models, with a multi-model mean response that does not project strongly onto the Aleutian Low. These findings for a large, idealised step increase in regional sulphate aerosol differ from previous studies that suggest the transient increase in sulphate aerosols over Asia during the early 21st century weakened the Aleutian Low and induced a transition to a negative PDO phase.

A specialised delivery system for stratospheric sulphate aerosols: design and operation

Temporary stratospheric aerosol injection (SAI) using sulphate compounds could help to mitigate some of the adverse and irreversible impacts of global warming. Among the risks and uncertainties of SAI, the development of a delivery system presents an appreciable technical challenge. Early studies indicate that specialised aircraft appear the most feasible (McClelan et al., Aurora Flight Sciences, 2010; Smith and Wagner, Environ Res Lett 13(12), 2018). Yet, their technical design characteristics, financial cost of deployment, and emissions have yet to be studied in detail. Therefore, these topics are examined in this two- part study. This first part outlines a set of injection scenarios and proposes a detailed, feasible aircraft design. Part 2 considers the resulting financial cost and equivalent CO2 emissions spanned by the scenarios and aircraft. Our injection scenarios comprise the direct injection of H2SO4 vapour over a range of possible dispersion rates and an SO2 injection scenario for comparison. To accommodate the extreme demands of delivering large payloads to high altitudes, a coupled optimisation procedure is used to design the system. This results in an unmanned aircraft configuration featuring a large, slender, strut-braced wing and four custom turbofan engines. The aircraft is designed to carry high-temperature H2SO4, which is evaporated prior to injection into a single outboard engine plume. Optimised flight profiles are produced for each injection scenario, all involving an initial climb to an outgoing dispersion leg at 20 km altitude, followed by a return dispersion leg at a higher altitude of 20.5 km. All the scenarios considered are found to be technologically and logistically attainable. However, the results demonstrate that achieving high engine plume dispersion rates is of principal importance for containing the scale of SAI delivery systems based on direct H2SO4 injection, and to keep these competitive with systems based on SO2 injection.

Persistent draining of the stratospheric 10Be reservoir after the Samalas volcanic eruption (1257 CE)

<p>More than 2,000 analyses of beryllium‐10 (<sup>10</sup>Be) and sulphate concentrations were performed at a nominal subannual resolution on an ice core covering the last millennium as well as on shorter records from three sites in Antarctica (Dome C, South Pole, and Vostok) to better understand the increase in <sup>10</sup>Be deposition during stratospheric volcanic eruptions.</p><p>A significant increase in <sup>10</sup>Be concentration is observed in 14 of the 26 volcanic events studied. The slope and intercept of the linear regression between <sup>10</sup>Be and sulphate concentrations provide different and complementary information. Slope is an indicator of the efficiency of the draining of <sup>10</sup>Be atoms by volcanic aerosols depending on the amount of sulphur dioxide (SO<sub>2</sub>) released and on the altitude it reaches in the stratosphere. The intercept provides an appreciation of the <sup>10</sup>Be production in the stratospheric reservoir, ultimately depending on solar modulation (Baroni et al., 2019, JGR).</p><p>Among all the identified events, the Samalas event (1257 CE) stands out as the biggest eruption of the last millennium with the lowest positive slope. It released (158 ± 12) Tg of SO<sub>2</sub> up to an altitude of 43 km in the stratosphere (Lavigne et al., 2013, PNAS ; Vidal et al., 2016, Sci. Rep.). We hypothesize that the persistence of volcanic aerosols in the stratosphere after the Samalas eruption has drained the stratospheric <sup>10</sup>Be reservoir for a decade.</p><p>The persistence of Samalas sulphate aerosols might be due to the increase of SO<sub>2</sub> lifetime because of: (i) the exhaustion of the OH reservoir required for sulphate formation (e.g. (Bekki, 1995, GRL; Bekki et al., 1996, GRL; Savarino et al., 2003, JGR); and/or, (ii) the evaporation followed by photolysis of gaseous sulphuric acid back to SO<sub>2</sub> at altitudes higher than 30 km (Delaygue et al., 2015, Tellus; Rinsland et al., 1995, GRL). In addition, the lifetime of air masses increases to 5 years above 30 km altitude compared with 1 year for aerosols and air masses in the lower stratosphere (Delaygue et al., 2015, Tellus). When this high-altitude SO<sub>2</sub> finally returns below the 30 km limit, it could be oxidized back to sulphate and forms new sulphate aerosols. These processes could imply that the <sup>10</sup>Be reservoir is washed out over a long time period following the end of the eruption of Samalas.</p><p>This would run counter to modelling studies that predict the formation of large particle sizes and their rapid fall out due to the large amount of SO<sub>2</sub>, which would limit the climatic impact of Samalas-type eruptions (Pinto et al., 1989, JGR; Timmreck et al., 2010, 2009, GRL).</p>

The multi oxygen isotope analyses on black crust from Sicily highlight the volcanic emission influence from Mount Etna on urban areas

<p>This study reports on measurements of Δ<sup>17</sup>O (derived from the triple oxygen isotopes) in sulphate from black crust sampled in Sicily. Atmospheric oxidants, such as O<sub>3</sub>, H<sub>2</sub>O<sub>2</sub>, OH and O<sub>2</sub> carry specific <sup>17</sup>O-anomalies, which are partly transferred to the sulphate during sulphur gas (e.g. SO<sub>2</sub>) oxidation. Hence, the Δ<sup>17</sup>O in sulphate can be used as a tracer of sulphur oxidation pathways. So far, this method has been mostly applied on sulphate from aerosols, rainwaters, volcanic deposits and ice cores. Here we propose a new approach, that aims to investigate the dominant oxidants of gaseous sulphur precursors into sulphate extracted from black crust material. Black crusts are mostly found on building/monument/sculpture and are the result of the reaction between sulphur compounds (SO<sub>2</sub>, H<sub>2</sub>SO<sub>4</sub>) and carbonate (CaCO<sub>3</sub>) from the substrate, which leads to the formation of gypsum (CaSO<sub>4</sub>, 2H<sub>2</sub>O). Sicilian black crust from sites under different emission influences (anthropogenic, marine and volcanic) were collected. Multi oxygen and sulphur isotope analyses were performed to better assess the origins of black crust sulphate in these different environments. This is crucial for both a better understanding of the sulphur cycle and the preservation of historical monument.</p><p>Multi sulphur isotopes show mostly negative values ranging from -0.4 ‰ to 0.02 ‰ ± 0.01 and from -0.59 ‰ to 0.41‰ ± 0.3 for Δ<sup>33</sup>S and Δ<sup>36</sup>S respectively. This is unique for natural samples and different from sulphate aerosols measured around the world (Δ<sup>33</sup>S > 0‰). This tends to indicate that sulphate from black crust is not generated by the same processes as sulphate aerosols in the atmosphere. Instead of SO<sub>2</sub> oxidation in the atmosphere, dry deposition of SO<sub>2</sub> and its oxidation on the substratum is preferred. The multi oxygen isotopes show a clear dependence with the geographical repartition of the samples. Indeed, black crusts from Palermo (the biggest Sicilian city) show small <sup>17</sup>O-anomalies ranging between -0.16 ‰ to 1.02 ‰ with an average value of 0.45 ‰ ± 0.26 (n=12; 2σ). This is consistent with Δ<sup>17</sup>O values measured in black crust from the Parisian Basin (Genot et al., 2020), which are also formed in an environment influenced by anthropogenic and marine emissions. On the other hand, samples from the eastern part of the Mount Etna region, which are downwind of the volcanic emissions, show the highest <sup>17</sup>O-anomalies ranging from 0.48 ‰ to 3.87 ‰ with an average value of 2.7 ‰ ± 0.6 (n=11; 2σ).</p><p>These results indicate that volcanic emissions influence the oxygen isotopic signature of black crust sulphate. In standard urban areas, SO<sub>2</sub> deposited on the substratum is mostly oxidised by O<sub>2</sub>-TMI and H<sub>2</sub>O<sub>2 </sub>to generate the black crust. Yet, under the influence of volcanic emissions, O<sub>3</sub> may play the main role in the SO<sub>2</sub> oxidation.</p>