Geoengineering with stratospheric sulfate aerosols has been proposed as a means of temporarily cooling the planet, alleviating some of the side effects of anthropogenic CO2 emissions. However, one of ...the known side effects of stratospheric injections of sulfate aerosols under present‐day conditions is a general decrease in ozone concentrations. Here we present the results from two general circulation models and two coupled chemistry‐climate models within the experiments G3 and G4 of the Geoengineering Model Intercomparison Project. On average, the models simulate in G4 an increase in sulfate aerosol surface area density similar to conditions a year after the Mount Pinatubo eruption and a decrease in globally averaged ozone by 1.1−2.1 DU (Dobson unit, 1 DU = 0.001 atm cm) during the central decade of the experiment (2040–2049). Enhanced heterogeneous chemistry on sulfate aerosols leads to an ozone increase in low and middle latitudes, whereas enhanced heterogeneous reactions in polar regions and increased tropical upwelling lead to a reduction of stratospheric ozone. The increase in UV‐B radiation at the surface due to ozone depletion is offset by the screening due to the aerosols in the tropics and midlatitudes, while in polar regions the UV‐B radiation is increased by 5% on average, with 12% peak increases during springtime. The contribution of ozone changes to the tropopause radiative forcing during 2040–2049 is found to be less than −0.1 W m−2. After 2050, because of decreasing ClOx concentrations, the suppression of the NOx cycle becomes more important than destruction of ozone by ClOx, causing an increase in total stratospheric ozone.
Key Points
Different processes affect ozone in stratospheric sulfate aerosol geoengineering
Suppression of NOx cycle becomes more important than ClOx depleting cycle
Polar UV‐B increases by 5% annually and 12% in spring
Ozone fields simulated for the first phase of the Chemistry-Climate Model Initiative (CCMI-1) will be used as forcing data in the 6th Coupled Model Intercomparison Project. Here we assess, using ...reference and sensitivity simulations produced for CCMI-1, the suitability of CCMI-1 model results for this process, investigating the degree of consistency amongst models regarding their responses to variations in individual forcings. We consider the influences of methane, nitrous oxide, a combination of chlorinated or brominated ozone-depleting substances, and a combination of carbon dioxide and other greenhouse gases. We find varying degrees of consistency in the models' responses in ozone to these individual forcings, including some considerable disagreement. In particular, the response of total-column ozone to these forcings is less consistent across the multi-model ensemble than profile comparisons. We analyse how stratospheric age of air, a commonly used diagnostic of stratospheric transport, responds to the forcings. For this diagnostic we find some salient differences in model behaviour, which may explain some of the findings for ozone. The findings imply that the ozone fields derived from CCMI-1 are subject to considerable uncertainties regarding the impacts of these anthropogenic forcings. We offer some thoughts on how to best approach the problem of generating a consensus ozone database from a multi-model ensemble such as CCMI-1.
Aside from the direct surface cooling that sulfate geoengineering (SG) would
produce, investigations of the possible side effects of this method are still
ongoing, such as the exploration of the ...effect that SG may have on upper
tropospheric cirrus cloudiness. The goal of the present study is to better
understand the SG thermodynamical effects on the freezing mechanisms leading
to ice particle formation. This is undertaken by comparing SG model
simulations against a Representative Concentration Pathway 4.5 (RCP4.5)
reference case. In the first case, the aerosol-driven surface cooling is
included and coupled to the stratospheric warming resulting from the aerosol
absorption of terrestrial and solar near-infrared radiation. In a second SG
perturbed case, the surface temperatures are kept unchanged with respect to
the reference RCP4.5 case. When combined, surface cooling and lower
stratospheric warming tend to stabilize the atmosphere, which decreases the
turbulence and updraft velocities (−10 % in our modeling study). The
net effect is an induced cirrus thinning, which may then produce a
significant indirect negative radiative forcing (RF). This RF would go in the
same direction as the direct effect of solar radiation scattering by
aerosols, and would consequently influence the amount of sulfur needed to
counteract the positive RF due to greenhouse gases. In our study, given an
8 Tg-SO2 yr−1 equatorial injection into the lower
stratosphere, an all-sky net tropopause RF of −1.46 W m−2 is
calculated, of which −0.3 W m−2 (20 %) is from the indirect
effect on cirrus thinning (6 % reduction in ice optical depth). When
surface cooling is ignored, the ice optical depth reduction is lowered to
3 %, with an all-sky net tropopause RF of −1.4 W m−2, of which
−0.14 W m−2 (10 %) is from cirrus thinning. Relative to the
clear-sky net tropopause RF due to SG aerosols (−2.1 W m−2), the
cumulative effect of the background clouds and cirrus thinning accounts for
+0.6 W m−2, due to the partial compensation of large positive
shortwave (+1.6 W m−2) and negative longwave adjustments
(−1.0 W m−2). When surface cooling is ignored, the net cloud
adjustment becomes +0.8 W m−2, with the shortwave contribution
(+1.5 W m−2) almost twice as much as that of the longwave
(−0.7 W m−2). This highlights the importance of including all of the
dynamical feedbacks of SG aerosols.
Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulfur gases above the tropopause, causing large increases in stratospheric aerosols. Five major ...volcanic eruptions after 1960 (i.e., Agung, St. Helens, El Chichón, Nevado del Ruiz and Pinatubo) have been considered in a numerical study conducted with a composition-climate coupled model including an aerosol microphysics code for aerosol formation and growth. Model results are compared between an ensemble of numerical simulations including volcanic aerosols and their radiative effects (VE) and a reference simulations ensemble (REF) with no radiative impact of the volcanic aerosols. Differences of VE-REF show enhanced diabatic heating rates; increased stratospheric temperatures and mean zonal westerly winds; increased planetary wave amplitude; and tropical upwelling. The impact on stratospheric upwelling is found to be larger when the volcanically perturbed stratospheric aerosol is confined to the tropics, as tends to be the case for eruptions which were followed by several months with easterly shear of the quasi-biennial oscillation (QBO), e.g., the Pinatubo case. Compared to an eruption followed by a period of westerly QBO, such easterly QBO eruptions are quite different, with meridional transport to mid- and high-latitudes occurring later, and at higher altitude, with a consequent decrease in cross-tropopause removal from the stratosphere, and therefore longer decay timescale. Comparing the model-calculated e-folding time of the volcanic aerosol mass during the first year after the eruptions, an increase is found from 8.1 and 10.3 months for El Chichón and Agung (QBO westerly shear), to 14.6 and 30.7 months for Pinatubo and Ruiz (QBO easterly shear). The corresponding e-folding time of the global-mean radiative flux changes goes from 9.1 and 8.0 months for El Chichón and Agung, to 28.7 and 24.5 months for Pinatubo and Ruiz.
In the recent years, the thermoplastic-based composites are being increasingly used in a variety of industrial applications such as the automotive due to the unique features they exhibit, mainly ...their recyclability and their interesting capacity to be formed and produced in high volume rate. Among the most interesting categories utilized extensively in these sectors, the short fibre composites are an attractive solution to non-critical load carrying structures while the textiles are utilized in more critical components. Nevertheless, all of them are implemented into a varying service temperature range, from sub-zero up to higher temperatures than the ambient. Thus, the definition of their mechanical performance with respect to their material and fabrication characteristics is a crucial procedure. Among them, the shear properties are the most challenging due to the complexity of the standardized mechanical tests. Moreover, these properties are greatly dependent on the matrix quality and, in general, to the overall composite's quality. In the present work, presented is an experimental campaign implementing temperatures from −30 °C up to 80 °C that includes both short and textile thermoplastic based composites, fabricated with different processes. In addition, introduced is a modified testing apparatus based on the standardized one that aims to overcome some of the drawbacks observed so far.
•Introduction of a modified V-Notched Rail Shear Apparatus capable of performing well in wide testing temperature range.•Investigation of the influence of the service temperature on the in-plane shear mechanical properties and behaviour.•Investigation of the effect of the specimen orientation compared to the injection moulding direction.•Assessment of the influence of the specimen orientation with respect to the textile fibre architecture.•Experimental investigation of the combined effect of the specimen orientation, testing temperature and fibre content.
Sulfate geoengineering (SG), made by sustained injection of SO2 in the tropical lower stratosphere, may impact the CH4 abundance through several photochemical mechanisms affecting tropospheric OH and ...hence the methane lifetime. (a) The reflection of incoming solar radiation increases the planetary albedo and cools the surface, with a tropospheric H2O decrease. (b) The tropospheric UV budget is upset by the additional aerosol scattering and stratospheric ozone changes: the net effect is meridionally not uniform, with a net decrease in the tropics, thus producing less tropospheric O(1D). (c) The extratropical downwelling motion from the lower stratosphere tends to increase the sulfate aerosol surface area density available for heterogeneous chemical reactions in the mid-to-upper troposphere, thus reducing the amount of NOx and O3 production. (d) The tropical lower stratosphere is warmed by solar and planetary radiation absorption by the aerosols. The heating rate perturbation is highly latitude dependent, producing a stronger meridional component of the Brewer–Dobson circulation. The net effect on tropospheric OH due to the enhanced stratosphere–troposphere exchange may be positive or negative depending on the net result of different superimposed species perturbations (CH4, NOy, O3, SO4) in the extratropical upper troposphere and lower stratosphere (UTLS). In addition, the atmospheric stabilization resulting from the tropospheric cooling and lower stratospheric warming favors an additional decrease of the UTLS extratropical CH4 by lowering the horizontal eddy mixing. Two climate–chemistry coupled models are used to explore the above radiative, chemical and dynamical mechanisms affecting CH4 transport and lifetime (ULAQ-CCM and GEOSCCM). The CH4 lifetime may become significantly longer (by approximately 16 %) with a sustained injection of 8 Tg-SO2 yr−1 starting in the year 2020, which implies an increase of tropospheric CH4 (200 ppbv) and a positive indirect radiative forcing of sulfate geoengineering due to CH4 changes (+0.10 W m−2 in the 2040–2049 decade and +0.15 W m−2 in the 2060–2069 decade).
SO2 and H2S are the two most important gas-phase sulfur species emitted by volcanoes, with a global amount from non-explosive emissions of the order 10 Tg-S/yr. These gases are readily oxidized ...forming SO42− aerosols, which effectively scatter the incoming solar radiation and cool the surface. They also perturb atmospheric chemistry by enhancing the NOx to HNO3 heterogeneous conversion via hydrolysis on the aerosol surface of N2O5 and Br-Cl nitrates. This reduces formation of tropospheric O3 and the OH to HO2 ratio, thus limiting the oxidation of CH4 and increasing its lifetime. In addition to this tropospheric chemistry perturbation, there is also an impact on the NOx heterogeneous chemistry in the lower stratosphere, due to vertical transport of volcanic SO2 up to the tropical tropopause layer. Furthermore, the stratospheric O3 formation and loss, as well as the NOx budget, may be slightly affected by the additional amount of upward diffused solar radiation and consequent increase of photolysis rates. Two multi-decadal time-slice runs of a climate-chemistry-aerosol model have been designed for studying these chemical-radiative effects. A tropopause mean global net radiative flux change (RF) of −0.23 W·m−2 is calculated (including direct and indirect aerosol effects) with a 14% increase of the global mean sulfate aerosol optical depth. A 5–15 ppt NOx decrease is found in the mid-troposphere subtropics and mid-latitudes and also from pole to pole in the lower stratosphere. The tropospheric NOx perturbation triggers a column O3 decrease of 0.5–1.5 DU and a 1.1% increase of the CH4 lifetime. The surface cooling induced by solar radiation scattering by the volcanic aerosols induces a tropospheric stabilization with reduced updraft velocities that produce ice supersaturation conditions in the upper troposphere. A global mean 0.9% decrease of the cirrus ice optical depth is calculated with an indirect RF of −0.08 W·m−2.
The atmospheric impact of aircraft NOx emissions are studied using updated aircraft inventories for the year 2006, in order to estimate the photochemistry-related mitigation potential of shifting ...cruise altitudes higher or lower by 2000 ft. Applying three chemistry-transport models (CTM) and two climate-chemistry models (CCM) in CTM mode, all including detailed tropospheric and stratospheric chemistry, we estimate the short-lived radiative forcing (RF) from O3 to range between 16.4 and 23.5 mW m−2, with a mean value of 19.5 mW m−2. Including the long-lived RF caused by changes in CH4, the total NOx-related RF is estimated to about 5 mW m−2, ranging 1–8 mW m−2. Cruising at 2000 ft higher altitude increases the total RF due to aircraft NOx emissions by 2 ± 1 mW m−2, while cruising at 2000 ft lower altitude reduces RF by 2 ± 1 mW m−2. This change is mainly controlled by short-lived O3 and show that chemical NOx impact of contrail avoiding measures is likely small.
Display omitted
•Multi-model study on the aircraft NOx-related effect on the atmosphere.•New inventory for aircraft NOx emissions, representative for year 2006.•The total RF due to NOx emissions is found to be about 5 mW m2.•Shifting cruise altitudes up or down by 2000 ft changes RF by about 2 mW m2.•Contrail-avoiding measures will likely give small NOx impact.
Three years of measurements of aerosol vertical profiles (2007–2009) made at the lidar station of L'Aquila, a site in central Italy that is part of the European Aerosol Research Lidar Network, are ...studied by means of well‐tested radiative transfer models to analyze the radiative impact of mineral dust aerosols transported from the Sahara desert. Sixteen major episodes of desert dust transport are considered; the radiative analysis is conducted in terms of diurnal averages of the top‐of‐atmosphere radiative flux changes (TOARFC) with respect to a reference “clean” aerosol profile not perturbed by long‐range transported desert particles. The aerosol size distribution, needed as an input parameter for the Mie scattering program to obtain single‐scattering albedo, asymmetry parameter, and extinction scaling over the whole wavelength spectrum, is obtained from simultaneous surface measurements with a multichannel aerosol spectrometer. The calculated average net TOARFC is +2.3 and +3.0 W/m2 in clear‐ and total‐sky conditions, respectively. Solar, planetary components account for −0.42 and +2.7 W/m2 in clear‐sky conditions and +0.93 and +2.1 W/m2 in total‐sky conditions, respectively. The large effective radius of these coarse mode soil dust particles (reff = 1.5 µm) makes the longwave planetary component of the TOARFC dominant over the solar component, at least for typical continental surface albedo values (0.18 on average, at L'Aquila). The solar component, however, shows a pronounced sensitivity to the surface albedo and becomes dominant over the longwave component for both an ocean albedo (0.07) and a typical surface‐snow albedo (0.4), with TOARFC values of −6.3 and +10.6 W/m2, respectively.
Key Points
Desert dust significantly affects the radiative balance in remote regions
Lidar measurements allow a good vertical discrimination of aerosol extinction
On land albedo, the planetary radiative perturbation is larger than the solar
Market strategies have greatly incentivized the use of diesel engines for land transportation. These engines are responsible for a large fraction of black carbon (BC) emissions in the extra-tropical ...Northern Hemisphere, with significant effects on both air quality and global climate. In addition to direct radiative forcing, planetary-scale transport of BC to the Arctic region may significantly impact the surface albedo of this region through wet and dry deposition on ice and snow. A sensitivity study is made with the University of L’Aquila climate-chemistry-aerosol model by eliminating on-road diesel emissions of BC (which represent approximately 50% of BC emissions from land transportation). According to the model and using emission scenarios for the year 2000, this would imply an average change in tropopause direct radiative forcing (RF) of −0.054 W∙m−2 (globally) and −0.074 W∙m−2 over the Arctic region, with a peak of −0.22 W∙m−2 during Arctic springtime months. These RF values increase to −0.064, −0.16 and −0.50 W∙m−2, respectively, when also taking into account the BC snow-albedo forcing. The calculated BC optical thickness decrease (at λ = 0.55 µm) is 0.48 × 10−3 (globally) and 0.74 × 10−3 over the Arctic (i.e., 10.5% and 16.5%, respectively), with a peak of 1.3 × 10−3 during the Arctic springtime.
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