How can sulphate aerosols be controlled

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The various models that make up the principal component loadings are indicated. Statistics of yr trends of the modeled AITG indices from twentieth-century climate and b preindustrial simulations. Also shown is the ensemble mean slope long dash line , slope for each of the observed AITG indices short solid lines , and the averaged observed slope long solid line.

Averaged AITG index from 7 single-forcing experiment runs: b Greenhouse gases; c sulfate aerosols; d volcanic aerosols; and e solar insolation. The number of ensemble numbers making up the ensemble means varies: 16 runs for a ; 7 runs for b — e. Since EOF1 of Fig.

The simulated optical depth is for the visible band. As in Fig. Statistics of yr trends of the modeled AITG indices from twentieth-century climate upper panels and from preindustrial lower panels simulations.

IMFs 4 and 5 from sensitivity tests, varying the end year of the data. The tropical Atlantic interhemispheric gradient in sea surface temperature significantly influences the rainfall climate of the tropical Atlantic sector, including droughts over West Africa and Northeast Brazil.

This gradient exhibits a secular trend from the beginning of the twentieth century until the s, with stronger warming in the south relative to the north.

This trend behavior is on top of a multidecadal variation associated with the Atlantic multidecadal oscillation. A similar long-term forced trend is found in a multimodel ensemble of forced twentieth-century climate simulations.

Through examining the distribution of the trend slopes in the multimodel twentieth-century and preindustrial models, the authors conclude that the observed trend in the gradient is unlikely to arise purely from natural variations; this study suggests that at least half the observed trend is a forced response to twentieth-century climate forcings.

Further analysis using twentieth-century single-forcing runs indicates that sulfate aerosol forcing is the predominant cause of the multimodel trend. The authors conclude that anthropogenic sulfate aerosol emissions, originating predominantly from the Northern Hemisphere, may have significantly altered the tropical Atlantic rainfall climate over the twentieth century.

The Tropical Atlantic interhemispheric gradient in sea surface temperature SST significantly influences the climate of the tropical Atlantic sector, including West Africa Giannini et al. While the warming of tropical North Atlantic temperatures has been the focus of Atlantic climate changes studies Mann and Emanuel ; Evan et al.

This is due to the strong control that the interhemispheric SST gradient has on the intertropical convergence zone ITCZ Hastenrath and Greischar and its subsequent effect on tropical Atlantic circulation and climate Moura and Shukla Forced changes in tropical Atlantic climate—whether from natural or anthropogenic causes— potentially manifest themselves in a similar way; indeed, climate forcings such as anthropogenic aerosols have hemispheric asymmetric distributions and may directly or indirectly drive an interhemispheric response Williams et al.

The magnitude and distribution of diffusivity will thus depend on the specific details of the engine and aircraft configuration. To capture the effects of such variations, the three direct H 2 SO 4 injection scenarios considered below have been developed using different options for engine plume injection. As will be discussed in Section 3 , the scale of the intended mission justifies the development of a specialised turbofan engine. Turbofan engines have two different volume flows, the first associated with the warm, high-velocity engine core and the second associated with the cool, low-velocity outer bypass.

For the first scenario, we assume that H 2 SO 4 is injected into the core flow only. This is chosen mainly because it is most consistent with plume models applied in literature Pierce et al.

This is the most conservative of the scenarios considered, as it ignores the potential gains obtainable by also injecting into the bypass flow. This is more conservative than the value for the equal diffusivity and initial concentration used by Pierce et al. The outer bypass flow of the proposed turbofan engine can be expected to have relatively low values of diffusivity, owing to its relatively low velocities. However, as its volume flow is 7.

Thus, in the second scenario, injection into the full engine flow is considered. It is assumed that exhaust mixers are used to achieve a well-mixed and uniformly seeded initial plume. These are commonly employed in low-bypass turbofans Larkin and Blatt ; Holzman et al.

The use of such mixers is a relatively conservative assumption, as lighter, but less proven alternatives exist, such as the use of overexpanded bypass flows. These show the potential for providing uniform mixing in the early plume at virtually no thrust penalty Debiasi et al. Both conventional mixers and overexpanded bypass flows can be expected to have the additional advantage of reducing jet noise Mundt and Lieser Well-mixed early plumes have been found to display slightly higher diffusivities than core-only flows Debiasi et al.

The first two scenarios assume relatively conservative values of diffusivity. However, improved engine flow mixing technology or more accurate measurements and simulations of plume growth might show that higher values can be attained. When precursors such as SO 2 are injected, aerosol formation does not occur until long after delivery and is thus virtually independent of injection specifics.

In this case, the dispersion rate in flight is not constrained. This means cost-efficient, short, high-payload flights can be employed. However, due to the slow conversion to H 2 SO 4 , average particle sizes increase and scattering efficiency decreases. As a conservative estimate, we assume that approximately twice the amount of sulphur is required to achieve the same radiative forcing with SO 2 with respect to H 2 SO 4 injection, based on radiative forcing estimates from Pierce et al.

To assess the combined effects of the constrained DR but lower annual delivery requirement in H 2 SO 4 injection scenarios, a fourth, SO 2 precursor injection scenario will be examined SO 2. The top half of Table 2 summarises the most important differences between the four scenarios. From the perspective of aircraft design, SAI poses a significant challenge, as it is dominated by the requirement to bring a substantial payload to unusually high altitudes, potentially covering substantial range.

This differs from the combination of take-off, cruise and landing requirements that drive conventional aircraft design.

Furthermore, there are requirements related to the safe operation of the aircraft which must be considered. The impact of these effects and requirements on the design of a specialised SAI aircraft are described below. The total weight is balanced primarily by the lift produced by the wing, L which may be expressed as:. Wing size and shape is also a main contributor to aerodynamic drag D , given by:. Therefore, although D scales with A w , it is beneficial to employ relatively large wing areas, as this reduces the required C L and total D.

The C L -dependent component of C d can also be reduced by using slender wings, with relatively high aspect ratio AR. Operating with slender wings at high speeds is particularly difficult, as these are relatively flexible. As the speed of the aircraft is increased, changes in aerodynamic load lead to increasingly large deflections.

Ultimately, this can lead to dangerous static or dynamic aeroelastic modes, such as control reversal or flutter.

This is particularly true if the aircraft is operating near the speed of sound, where phenomena associated with air compressibility, such as shock waves or shock-induced separation, introduce additional aeroelastic modes. Even with this restriction, excessive values of AR must be avoided. In the current context, however, a relatively thick airfoil is required to maintain sufficient stiffness to support even a moderately large aspect ratio and prevent excessive induced drag.

In spite of the multidisciplinary optimisation, the final estimated operating drag values are still appreciable. D must be balanced by the thrust T produced by the engines. Operating at low air density reduces both engine mass flow and combustion efficiency, yielding a strong thrust lapse with altitude.

Thus, exceptionally powerful engines are required to sustain efficient flight at stratospheric altitudes. This is the driving constraint for the present design and requires turbojet, low-bypass turbofan or high-bypass turbofan engines Torenbeek The former two are generally superior in terms of thrust-to-weight ratio, while the latter generally achieves higher total thrust and lower fuel consumption per unit thrust.

Turbojet engines are widely used in military aircraft, where long-term reliability is less important. For a SAI application, however, the thrust of the most common examples would likely need to be down-rated to maintain sufficient reliability. State-of-the-art low-bypass turbofans are used to power a number of low-weight aircraft to stratospheric altitudes and are therefore suggested for SAI application in Smith and Wagner However, their relatively high fuel consumption diminishes their weight advantage, especially if the aircraft must cover a substantial delivery range.

Hence, high-bypass turbofans might be the more efficient alternative and will be the more environmentally friendly alternative. There are few existing examples of high-altitude high-bypass turbofans. This means that a custom turbofan, developed specifically for a fleet of SAI aircraft at stratospheric altitudes, could be necessary. This would raise development costs. Nevertheless, the potential benefits of a custom engine mandate the inclusion of such a design within the current study.

In Section 5 and in the supplementary material, we quantify the relative benefits of employing existing low-bypass turbofans, existing high-bypass turbofans and custom high-bypass turbofans for the proposed delivery system. Aside from being stiff enough to avoid undesirable aeroelastic modes, the aircraft structure must withstand several distinct load cases.

These include the fully loaded case at take off, where flexibility and high fuel load can result in wing-ground strikes, and the near-empty case approaching the end of the flight, where the low wing weight due to depleted fuel and aerosol combined with the need to support the fuselage results in high wing-root bending moments in gust conditions.

These and other requirements produce a trend of increasing structural weight with aspect ratio, as Fig. The figure shows how structural weight can be decreased by using composite materials instead of aluminium and an additional strut to support the main wing structure.

In spite of the fact that the aircraft will make use of automated control systems, a minimum level of handling qualities is required in order to ensure that the aircraft is stable and controllable.

This determines the sizes of the vertical and horizontal tail surfaces, as well as those of the rudder, elevator and aileron control surfaces. Finally, the aircraft must be operated with sufficient margin from its altitude ceiling. A useful visualisation of this is a velocity-altitude plot. Figure 1 includes such a plot for the limiting stage of the flight profile discussed in Section 4. The left curve indicates the minimum operating speed, or stall speed, of the aircraft. Below this airspeed, lift is insufficient to sustain flight.

The stall speed increases rapidly with altitude due to the rapid drop in air density. The right curve indicates the maximum operating speed at the drag divergence Mach number and the associated aeroelasticity and thrust boundary.

It is necessary to operate below this speed to avoid excessive thrust requirements and undesirable aeroelastic phenomena as the flight speed approaches the local speed of sound. Due to the extreme operating altitudes required for SAI, flight profile parameters such as total fuel and payload mass, speed, range and altitude strongly influence the aircraft design requirements, while aircraft parameters such as wing geometry and structure strongly influence the achievable total mass, speed and altitude.

Thus, in order to assure both the feasibility and relative efficiency of the aircraft configuration emerging from the design, a simultaneous optimisation of the configuration and flight profile should be performed. The specification of the aircraft configuration is a detailed and nonlinear process, however, and might in principle change for each of the four scenarios described in section 2. Therefore, to simplify the analysis, the optimisation was carried out in two phases.

In the first phase, a coupled aircraft-flight profile design procedure was carried out for a baseline mission akin to the CI scenario, to ensure the feasibility of the entire interval of possible operating scenarios considered here DSE Group 02 In the second phase, the aircraft configuration was held fixed and the flight profile re-optimised for each of the four scenarios in turn. The coupled optimisation procedure is a loop of manually interconnected design tools Lukaczyk et al.

The result of the first optimisation phase was an unusual aircraft configuration capable of carrying a large fuel and payload mass to stratospheric altitudes. This can be expected to be close to the fully optimal configuration for each of the four scenarios, as each scenario is constrained by the same critical flight phase.

In scenarios where dispersion rates are higher, flights can be shorter and less fuel is required. This in turn allows more payload to be carried per flight, reducing the annual number of flights. However, even the lowest number of flights required was found to be substantial, favouring the use of a specially designed aircraft capable of carrying a large fuel and payload mass in all four scenarios.

In summary, the two-phase optimisation approach described above is advantageous in that it allows for a more straightforward comparison of the scenarios. It is also more realistic, in that the ultimate choice of scenario is likely to be made after the aircraft is in service and long-term effects of SAI have been quantified.

This section describes the baseline aircraft configuration which emerged from the first phase of the optimisation procedure. Within this procedure, the remaining systems required for operation—the fuel, hydraulic, electrical, communications, hardware and software and data handling systems—were also developed to the preliminary stage.

These largely mirror implementations in conventional modern aircraft and were not found to significantly influence the main optimisation process. The reader is referred to DSE Group 02 for a more detailed description of these systems. The baseline aircraft configuration is illustrated in Fig. Isometric view and three projection view of the proposed stratospheric aerosol delivery aircraft.

Dimensions are in metres. The unusual aspects of the baseline aircraft configuration are almost entirely the result of high-altitude and high-payload requirements.

It features a large and slender wing, with a supercritical airfoil designed to provide high lift at high Mach numbers. Four custom turbofan engines, each rated to supply 26 kN of thrust at 20 km altitude, are used to overcome the relatively high drag associated with stratospheric operations, while simultaneously capable of supplying up to 2 MW of power to heat and evaporate H 2 SO 4. This number assumes H 2 SO 4 is heated on the ground and maintained at a temperature close to its boiling point until it is evaporated.

Corporation These are practically attainable numbers that are unlikely to influence engine performance. It will thus be assumed that only one outboard engine, constantly operating at a relatively high thrust value, will be used for aerosol injection. This prevents suboptimal aerosol growth conditions by interaction between multiple expanding plumes. In addition, the use of only one outboard plume minimises the risk of corrosive H 2 SO 4 impinging on outer aircraft surfaces. Still, corrosion-resistant coatings are used where necessary on aircraft surfaces and especially engine outlet surfaces.

The first three scenarios described in Section 2 necessitate the use of an evaporation system to ensure the H 2 SO 4 is injected as a condensable gas, which in turns requires engine power. The engines are sized for critical flight phases, as will be outlined in Section 4. Hence, there is sufficient excess engine capacity for aerosol evaporation and temperature control during dispersion segments.

Aerosol evaporation will take place at the aerosol outlets, in order to allow the formed gas to expand. The wings are sufficiently spacious to accommodate the full interval of payloads and fuel considered in this study, allowing a small, slender fuselage.

Compliance with load cases throughout the flight envelope was verified, following the requirements of EASA CS 25 for the certification of large aircraft. The aircraft is operated unmanned, following a programmed mission and when necessary operated remotely from ground stations.

This is mainly due to the very low air densities encountered in the stratospheric parts of its operation, for which maintaining a suitable on-board crew environment imposes a significant weight penalty and reduces the number of feasible aircraft layout choices. In addition, the scale of the mission and necessity for crew redundancy requires a large number of pilots, each with high training and employment costs.

In contrast, one ground operator can simultaneously control several unmanned aircraft, if only one of these is in a critical flight phase.

Unmanned aircraft introduce several additional requirements, however, including the need for complex redundant automated control systems. Ground stations with specialised technical equipment must also be established and maintained. Overall however, a substantial economic benefit can be gained from unmanned operation.

The development and production time required for a fleet of the above aircraft is estimated to be between six to nine years DSE Group 02 This is based on estimates for high-payload transport aircraft and modern airliners Spitz et al.

Modern air traffic regulations specify a minimum distance of 3 nautical miles between consecutive departures of heavy aircraft Rooseleer and Treve At take-off and landing speeds of the proposed aircraft, this results in a minimum delay of slightly less than a minute when appropriate margins are accounted for, allowing slightly over flights per day per airport.

The CI scenario can then be carried out with four airports. For the remaining scenarios which require fewer flights, the use of four airports has also been assumed in order to ensure one airport per hemisphere and one for redundancy.

For a more extreme range of operations, as will be considered in Part 2 of this study, airports are added as needed if more flights per day than in the CI scenario are required. It is assumed that existing airports with a m or longer runway are used. For H 2 SO 4 injection scenarios, the flight lengths needed to achieve the design dispersion rates can be substantial, demanding a dedicated flight profile.

This will be directed along meridional tracks, such that round-trip flights are advantageous compared with transit flights in order to contain the required number of airports and facilitate injection perpendicular to the fastest advection dimension around a specified latitude.

Each of the H 2 SO 4 injection scenarios are thus divided into two legs. The outbound leg is oriented in the local poleward direction south in southern hemisphere, north in northern hemisphere. After climbing to an altitude of 20 km, aerosol delivery is initiated.

At the mid-flight point, the aircraft turns to a reciprocal heading and then climbs to Journal of the Atmospheric Sciences 59 , Myhre, G. Consistency between satellite-derived and modeled estimates of the direct aerosol effect. Modelled radiative forcing of the direct aerosol effect with multi-observation evaluation. Novakov, T. Airborne measurements of carbonaceous aerosols on the East Coast of the United States. Soot and sulfate aerosol particles in the remote marine troposphere.

Putaud, J. A European aerosol phenomenology Physical and chemical characteristics of particulate matter from 60 rural, urban, and kerbside sites across Europe. Atmospheric Environment 44 , Quinn, P. Ramanathan, V. Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze. Remer, L. Journal of Geophysical Research-Atmospheres , D14s07 Schulz, M. Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations.

Stevens, B. Untangling aerosol effects on clouds and precipitation in a buffered system. Storelvmo, T. Global modeling of mixed-phase clouds: The albedo and lifetime effects of aerosols. Trenberth, K. Twomey, S. Influence of pollution on shortwave albedo of clouds. Journal of Atmospheric Sciences 34 , Zhang, Q.

Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes. Geophysical Research Letters 34 , L Modeling Sea Level Rise. The Global Climate System.

Earth's Earliest Climate. Methane Hydrates and Contemporary Climate Change. Citation: Myhre, G. Nature Education Knowledge 4 5 Atmospheric aerosols from human activity influence climate. Uncertainties in the understanding of their effects limit our knowledge about climate change.

Aa Aa Aa. What is the source and composition of the aerosols? How are aerosols distributed globally? Aerosol optical depth AOD retrieved by remote sensing from space is highly inhomogeneous, with the largest values in Asia and the tropical regions of Africa see Figure 2. The estimated contributions from different aerosol types in selected regions are shown in pie charts. In general, there is large spatial and temporal variability in global aerosol composition.

Remote sensing from both space and ground together with in situ observations have substantially advanced an understanding of geographical aerosol distribution, but there are still large uncertainties in the chemical composition and the anthropogenic contribution to the AOD Figure 2. Pie charts show how various aerosol types contribute to the total AOD for different regions, as estimated by a global aerosol model Myhre et al. Some aerosol types, e. The contribution from OC is likely underestimated as in most of the global aerosol models Zhang et al.

How do aerosols affect the climate? Why is the uncertainty in the aerosols important for predictions of climate sensitivity? However, the Earth is not in radiative equilibrium, since less thermal radiation is currently emitted to space compared to what is absorbed of solar radiation Hansen et al.

The simple equation above has two key uncertainties. The observed surface temperature change is rather well determined over the industrial era, but the climate sensitivity and the total RF are both highly uncertain. The climate sensitivity is an essential parameter for prediction of future climate change. The total RF through the industrial era is also uncertain, mainly due to lack of quantification of the aerosol effects discussed above. The implication of this uncertainty in the aerosol RF for the quantification of the climate sensitivity can be illustrated as follows:.

Has there been any progress in the understanding of the climate effect of aerosols? There has been a tremendous improvement in the understanding of atmospheric aerosols and their climate effect over the last decades, with some important observational and modelling breakthroughs. Long-term measurements of aerosols e. However, an understanding of the greater complexity of atmospheric aerosols has at the same time limited more robust quantification of their climate effect.

The first estimate of the direct aerosol effect in the early s was limited to sulphate aerosols Charlson et al. Observations have shown that OC is an important aerosol component Novakov et al.

In addition, multi-model studies are performed to understand and reduce uncertainties due to model differences Schulz et al. An example of recent progress is reduced uncertainty in the estimate of the total direct aerosol effect. This estimate was made possible by advances that have occurred on both the modelling and the observational side, and was based on a combination of global aerosol models and observation based methods mostly remotely sensed data.

Initially, observational estimates of RF were up to three times stronger than model based calculations Forster et al. Consistency between these two different approaches has subsequently been reached, and was found to arise from necessary and simplified assumptions of the pre-industrial aerosol composition in the observation-based method Myhre Although the uncertainty in the total direct aerosol effect is reduced, it is still substantial compared to uncertainties associated with greenhouse gases.

In addition the uncertainty in individual RF for several of the aerosol components, such as BC, OC, and nitrate, is large.

Furthermore, they only included the influence of sulphate aerosols on cloud albedo, disregarding any effects on cloud lifetime and extent. With the realization that other aerosol species of anthropogenic origin could also form cloud droplets and that effects on cloud lifetime and extent were also possible, global climate models estimated the aerosol indirect effect to be stronger e.

Some even predicted this cooling effect to be comparable in magnitude to the warming greenhouse effect. Recent publications have later pointed to oversimplifications in model representation of clouds and how their lifetimes are affected by aerosols e.

It is now acknowledged that aerosol effects on cloud lifetime will vary with the cloud type in question, and that complex feedback processes can sometimes complicate the ultimate cloud response to aerosol perturbations. Recent model studies have found that by forming ice in super-cooled liquid clouds, aerosols may in fact shorten cloud lifetime, because of the more efficient precipitation formation when cloud ice is present e.

In summary, whether aerosols are acting as CCN or IN or are simply modifying atmospheric stability by absorbing solar radiation, there is still high uncertainty associated with their effect on cloud lifetime. This uncertainty reflects how challenging it is to represent aerosol-and-cloud processes that occur on microscopic scales in models that have resolutions of tens to hundreds of kilometres.

Although much uncertainty remains, model and satellite estimates of the cloud albedo effect seem to converge on a negative RF that has about half the magnitude of the positive RF attributed to increasing CO 2 concentrations. We appreciate useful reviewer comments by Patrick Chuang and one anonymous reviewer.

References and Recommended Reading Albrecht, B.