Aircraft for delivering sub-micron particles into the stratosphere Description of GB2487287 (A)
Title: Atmospheric Delivery System
The present invention relates to an aircraft for dispersing particles into the earth's stratosphere, particularly to achieve a global or local cooling effect.
Background to the invention
Many authors now agree there is a significant possibility of abrupt discontinuities in the earth's climate, e.g. by global warming, through rising atmospheric greenhouse gas levels which in turn may cause significant positive feedback, such as release of is methane from melting arctic tundra or loss of rainforest through changed precipitation patterns. Such discontinuities might lead to a serious impact on ocean levels, and agriculture.
However, emissions of greenhouse gases such as C02, are intimately connected to the economies of the world. An abrupt reduction in the levels of these gases over effective timescales may be difficult to achieve.
Various methods of reducing the levels of solar radiation incident upon the earth have been proposed, for example, space mirrors, cloud seeding and stratospheric sulphate aerosols. The effects of significant stratospheric sulphate aerosol injection have been demonstrated to reduce mean global temperatures by around half a degree centigrade over a period of two years with the eruption of Mount Pinatubo in the Philippines in 1991.
Reducing incident solar radiation to counteract the trapping of heat by rising greenhouse gas emissions leaves ocean acidities rising due to increased levels of absorbed carbon dioxide. However it may reduce rising temperatures generating more greenhouse gas emissions through the melting of arctic permafrost exacerbating the ocean acidity problem.
The particular method of stratospheric sulphate aerosols suffers from a number of drawbacks: the choice of particle diameter is determined by natural processes in the stratosphere if precursor materials such as Sulphur Dioxide or Hydrogen Sulphide are used, and the generation of stable aerosols with defined characteristics presents many difficulties. Furthermore their effect on stratospheric chemistry, particularly the ozone concentration may cause concern. In addition the possibility of generating local shadows' does not arise since it takes a significant time (days at least) after injection of Sulphur Dioxide for Sulphuric Acid or Arnrnonium Sulphate particles to be generated by the natural processes (hydration and reaction with UV atmospheric Nitrous Oxides), by which time any high local concentration will be well dispersed by even moderate stratospheric winds.
There is therefore a need for more effective solutions to the problem of global warming with less alarming side effects.
Summary of the invention
The invention relates to an aircraft, e.g. a plane, balloon or other vehicle, located at a stratospheric location, the aircraft comprising a source of solid sub micron particles, a deagglomeration means and a dispersal means.
If sufficient quantities of particles are dispersed, then a cloud of such particles can form, providing the local, regional or global cooling effect. Additionally, as the particles are solid their size can be controlled to provide optimum results.
The lifetime of such a cloud is largely determined by the rate of depletion of particles from the cloud.
The exact rate of depletion is influenced by several factors, including particularly the size of the particles. For sub micron particles, when the settling rates are very low, the losses from the stratosphere to the troposphere are determined by the relatively small amount of mixing between the stratosphere and troposphere. It is estimated that as many as half of any sub micron particles in the stratosphere may leave the stratosphere per year. In addition, particle-to-particle collision by Brownian motion will cause sub-micron particles to agglomerate and become larger particles, greatly increasing their settling rates.
Depletion rates below the stratosphere will be significantly greater due to the increased turbulence and mixing in the tropopause and the troposphere. Thus, the io elevated location is preferably at an altitude of at least 5,000m, more preferably at least 1O,000m, and most preferably in the stratosphere, in order to minimise particle depletion rates.
For local or regional cooling such a cloud is typified by being able to scatter at least is 10% of incident solar radiation, thus providing a local cooling effect.
In one arrangement the cloud may be generated during daylight hours to provide an effective local or regional cooling effect, but disperses over night.
Thus, the invention can provide local shade effects, which, if over a significant body of water can (a) cool the water surface by a few degrees centigrade thereby reducing the motive force of local tropical storms and (b) allow the creation of onshore winds laden with water to promote irrigation of coastal lands.
As discussed, over time the cloud will disperse, and if it was located in the stratosphere, will produce a rarefied cloud without clearly defined boundaries, extending over a significant fraction of the earth's surface. When quantities are sufficient, such a rarefied cloud may only scatter a few percent of incident solar radiation but may provide a global cooling effect.
To maintain such a local or regional cloud in a steady state, emission rates of at least 500 metric tonnes of particles per 12 hours, preferably at least 1000 metric tonnes, more preferably at least 2000 metric tonnes, or more preferably at least 4000 metric tonnes, or most preferably at least 10,000 metric tonnes per 12 hours may be required.
Releasing the solid particles into the stratosphere utilising a dispersal means will result in a cone of particles whose cross-section downstream of the injection point is very broad compared to it's vertical span, and from which a cloud according to the invention results. This is because the turbulent intensity in the vertical direction is less than in the horizontal direction in the stratosphere.
io It has been found that particles having a high refractive index, e.g. in excess of 1.40 are able to effectively scatter incoming solar radiation so that less radiation impinges on the earth.
Preferably the particles have a refractive index of greater than 2.0, more preferably is greater than 2.3. Suitable materials include titanium dioxide, sodium chloride, silicon or mixtures thereof A highly preferred material is titanium dioxide. However, sodium chloride may be preferred as it can be readily obtained from sea water.
Titanium dioxide exists in several material forms, the primary ones used for scattering being rutile and anatase. All of these forms have refractive indices in excess of 2.4 and so make excellent materials for use in the present invention. Additionally titanium dioxide is stable in air and not toxic. Anatase crystal may be preferred as it has relatively lower abrasion properties than the rutile crystal form.
In a preferred embodiment the present invention involves the Mie scattering of incident solar radiation. Mic scattering occurs when the size of the particles is comparable with that of the wavelength of incident light and preferably with the particle diameter being within a factor of 10, more preferably a factor of 3, of the wavelength of the incident light.
Thus, the particles are sub-micron and preferably the particles have a mean particle size in the range of from 0.01 to 1.3 m, more preferably in the range of from 0.02 to 0.5 jim. This size range straddles the ultra violet and visible regions of the
electromagnetic spectrum and therefore will be effective at scattering these dominant regions of the incident solar radiation.
In a particularly preferred embodiment, the particles may have a bimodal size distribution with peaks in the visible and ultra violet wavelengths respectively. For example peaks at from 0.1 to 0.3 jim and at from 0.02 to 0.06 jim are particularly effective.
At such small particle sizes, agglomeration of the particles presents a significant io difficulty. For titanium dioxide the interparticle forces are particularly high because of the high polarisability of oxygen in the particles. Agglomeration will inhibit the ability of the particles to be involved in Mie scattering of the incident radiation due to the increase in effective particle size that results from agglomeration. It is therefore highly desirable to be able to deagglomerate the particles at the stratospheric location.
In a preferred embodiment, the solid particles are coated with one or more materials, different to the material of the particles.
Coatings can be selected from a range of options, for example to reduce chlorine concentrations present in the stratosphere, to reduce the tendency of ice or nitrogen oxides to coat the surface, to reduce ozone depletion, promote ozone generation in the stratosphere, to mitigate the possibility of electrical discharge near the dispersal means. For example, alkaline (metal and earth) coatings involving oxides, hydroxides, aluminates, silicas and combinations thereof can be used to scavenge chlorine from the stratosphere, thus preventing it from destroying ozone.
Hydrophobic coatings can also be chosen to reduce the tendency for water vapour to condense on the particles and form ice. Ice can have a highly deleterious effect on the destruction of ozone, particularly in the polar regions. In a preferred embodiment, the coatings comprise siloxanes or a variety of other hydrophobic coatings.
It is also possible to use a the coating material to produce either a positive or a negative charge on each particle, leading to mutual repulsion of likely charged particles and reduced agglomeration.
In one embodiment, the particles are transported to the aircraft via a conduit attached to a substantially ground-level location.
Transporting particles through a conduit to the stratosphere presents a number of engineering challenges. However, delivery of particles into the stratosphere is io believed to be particularly beneficial due to the poor mixing in the region, particularly with the troposphere below, allowing the particles to remain suspended in the stratosphere for extended periods of time.
The particles can be transported through the conduit in a number of different ways.
is For example, the particle transport means could comprise a pneumatic conveying arrangement, wherein containers carrying the particles are conveyed up a conduit to the stratospheric location with the return of emptied containers down a second conduit.
However, preferably the particles are dispersed in a fluid carrier material so that the carrier and particles can be pumped through the conduit in which case the particle transport means comprises a fluid pressurisation means.
Preferably, the carrier fluid is a supereritical fluid or a liquid, to enable the fluid to carry a greater density of particles per unit volume and consequently require a narrower bore of conduit. In a particularly preferred embodiment the particles are present as a slurry in a carrier fluid, at a concentration of from 5 to 50% by volume, more preferably from 10 to 30% by volume, e.g. about 15%.
Carrier fluid temperatures of from -40 to 200°C, preferably from 20 to 200°C at the entry to the conduit are beneficial. Preferred carrier fluids are nitrogen, air, mixtures of nitrogen-rich oxygen streams or water. Most preferred are nitrogen or nitrogen-rich oxygen streams. The carrier fluid pH and composition may be adjusted e.g. to enhance the surface properties of the carried particles, to protect the conduit or to facilitate dispersion at altitude.
In order for a carrier fluid to flow along such a long length of conduft and against gravity, it must be raised to a very high pressure at the entry to the conduft. Pressures at the entry to the conduft in excess of 100 MPa, preferably in excess of 200 MPa, more preferably in excess of 400 MPa and even in excess of 600 MPa might be necessary to achieve acceptable flow rates in the conduft. In order to achieve such high pressures, the supply of carrier fluid may be via a plurality of pressure vessels.
At such elevated temperatures and pressures the carrier liquid will become a supercrftical fluid, which may be desirable, e.g. avoiding the need to de-water the slurry at altitude, for supplying power to the deagglomeration means and/or the dispersal means, as discussed below.
Liquid or supercritical fluid average flow velocities of from 4 to 20 mIs can provide an acceptable balance between keeping the conduit size to a minimum whilst maintaining pressure drop and conduit abrasion during flow to acceptable levels.
As the weight of the conduft added to the weight of the particles and any carrier material must be supported, the conduit is at least 5000 m, preferably at least 10,000 m and typically from 10,000 to 25,000 m long to enable it to reach the elevated location, e.g. the stratosphere, without being excessively long. The weight of the particles and carrier fluid may be partially or fully supported by the pressure provided at the entry to the conduft. The frictional pressure drop may also frilly or partially support the weight of the conduit, thereby reducing the support load for the conduft.
Addftionally, the conduft desirably has a cross-sectional area great enough to allow the flow of a fluid without an unworkable pressure drops but yet not so great that transverse wind loading present structural problems. Thus, the conduit preferably has a cross-sectional area, including the conduit wall, of from 0.002 to 20 m2. NalTower condufts are generally preferred and so cross-sectional areas of from 0.004 to 1.0 m2 are preferred. If the particles are carried by a liquid or supercritical fluid then cross-sectional areas of from 0.004 to 0.05 m2 are preferred.
The conduit may be made from a wide range of materials, but strong and light materials are preferred such as high strength carbon fibre, an aramid fibre such as Kevlar (TM), Technora (TM), Vectran (TM) or Twaron (TM) and ultra-high molecular weight polyethylenes such as Spectra (TM) and Dyneema (TM) or PBO's such as Zylon (TM). The choice depends particularly on strength, conductivity, operating temperature and abrasion and corrosion resistance. For example, it may be necessary to ensure that the conduit is not conducting, to avoid problems associated with lightning strikes. As particles may be flowing through the conduit, the internal wall of the conduit may be coated with an abrasion-resistant material such as titanium carbide and the like.
is The conduit must be designed to withstand extreme pressures, giving rise to very high hoop stresses and longitudinal stresses. It has been observed that the hoop stress at the base of the conduit will be far in excess of the hoop stress at the top of the conduit due to the drop in fluid pressure as it flows upwards through the conduit. In contrast, the longitudinal tensile stresses will be much greater at the top of the conduit than at the base because more weight of the conduit is supported at the top than at the bottom.
Thus the nature of the stresses can vary along the length of the conduit.
Thus, preferably the conduit comprises reinforcing elongate fibrous material, the fibrous material being braided or wound at an angle to the central axis of the conduit, wherein the angle of winding is greater at one end of the conduit than at the other.
The angle of winding can therefore be adjusted along the length of the conduit to account for the change in the nature of the stress encountered. This results in less material being required, reducing the size and cost of the conduit and supporting dirigible or balloon.
Thus, the end having the greater angle of winding or supply is typically at the substantially ground-level location. This provides greater support for the high hoop stress at the base, whist dealing with the high longitudinal tension forces at the top end.
In a preferred embodiment, the change in angle of winding or supply is gradual or occurs in a plurality of step changes. For example, the greater angle of winding or supply can be from 40 to 900 to a line parallel to the axis of the conduit and the lesser angle of winding can be less than 60° to a line parallel to the axis of the conduit.
Once at the elevated location the particles are typically dispersed into the atmosphere io by the dispersal means. Suitable dispersal means involves transporting the particles into a high velocity air stream and may be provided by a jet engine or one or more fans.
One difficulty with the operation of the dispersal means is providing power to drive it is at the stratospheric location. In one embodiment, power is provided by providing electrical power and/or fuel lines along the conduit.
In another embodiment, fuel gases such as hydrogen and methane can be included in the carrier fluid for the particles. With an appropriate degassing stage, the fuel gases can be extracted for use at the stratospheric location.
tu a preferred embodiment, the apparatus comprises a let-down facility at the stratospheric location, such as a turbine. Such a turbine can then provide power to the dispersal means and/or the deagglomeration means.
The deagglomeration means preferably comprises a microniser, a venturi nozzle or a combination thereof Use of a single or a plurality of micronisers is possible. Such micronisers are preferably used in combination with a supercritical fluid, as described above. This can involve expansion through grinding jets. A suitable dimension for the microniser is in the region of 4 m in diameter. In a preferred embodiment the deagglomerat ion means includes a jet impact mill, preferably before the particles pass to a microniser.
Once the particles have been deagglomerated they are then dispersed into the stratosphere by the dispersal means. As the particles will typically tend to reagglomerate very rapidly, it is preferred that the dispersal means is close-coupled to the deagglomeration means. For example, the exit from the deagglomeration means can be directly in any gas flow generated by the dispersal means.
The dispersal means should encourage the rapid mixing of large quantities of diluent air to increase the distance between individual particles to reduce the chance of the particles colliding and reagglomerating. Such systems typically include one or more io nozzles situated in a fast flowing air stream provided by a combination of the motion of the apparatus, e.g. a fan or jet engine exhaust and/or ejection arrangements provided by the expansion of any carrier fluid vapour.
Such a balloon or dirigible is therefore preferably capable of providing a lifting force is of at least 15 metric tonnes, more preferably at least 100 metric tonnes, and most preferably at least 200 metric tonnes. Large versions for providing a local shade effect might be as much as 2500 metric tonnes. Such lift values can be achieved with a balloon diameter of between 50 and 300m, or alternatively with a dirigible comprising a lighter-than-air chamber with a length of 600 m and a width of 130 m,
The conduit may be equipped with a parachute means to allow release of the conduit from the balloon or dirigible and a more gradual descent of the conduit.
Wind speeds in the stratosphere are generally much lower than those reached in the troposphere. The balloon or dirigible may therefore be located at a higher altitude than the elevated location to allow it to sit in relatively quiet air. For example, the elevated location could be below the stratosphere whilst the balloon or dirigible sfts in the stratosphere.
In a preferred embodiment, the balloon or dirigible is capable of altering its altitude to alter the effective length of the conduit by altering the altitude of the conduit exit. For example, the dirigible preferably comprises a spool for the conduit to coil onto. Such a spool would need to be large enough to prevent stresses on the conduit wall becoming excessive due to coiling yet not so large that it becomes impractical. Thus a spool with a diameter of from 20 to 100 m is suitable, preferably from 40 to 60 m in diameter.
The spool may have an electric motor driven winding arrangement both to coil the conduit and to provide active damping of significant conduit oscillations.
The apparatus according to the present invention may also comprise an active damping control means, to reduce any unwanted oscillating in the conduit.
The substantially ground-level location is typically at or very near to the land or sea surface of the planet e.g. within 1,000 m, preferably within 100 m of a land or sea surface. In one preferred embodiment, the apparatus comprises a ship, to which is is coupled the base of the conduit. In one possible mode of operation, a plurality of ships may be employed, each with their own conduit and associated equipment. In this way, either one large cloud or a plurality of smaller clouds can be generated.
The invention will now be illustrated, by way of example, and with reference to the following figures, in which: Figure 1 is a diagram showing the effect of a cloud according to the present invention on incoming solar radiation to the earth's surface.
Figure 2 is a schematic representation of an apparatus according to the present invention.
Figure 3 is a schematic representation of the effect of a cloud according to the present invention on local weather conditions.
Figure 4 shows a schematic representation of three apparatuses according to the present invention working together to alter the local weather conditions.
Figure 5 is a schematic representation of the components of an apparatus according to the present invention located at a stratospheric location.
Figure 6 is a schematic representation of the components of an apparatus according to the present invention located at a substantially ground-level location.
Figure 7 is a schematic representation of the conduit construction according to the present invention.
Detailed description of the figures
Figure 1 shows a diagram of the effect of a cloud 100 according to the present invention located in the earth's stratosphere. The cloud 100 comprises titanium dioxide having a unimodal distribution with a peak at 0.25 micrometres or a bimodal is particle size distribution with peaks at 0.15 micrometres and at 0.04 micrometres.
Incoming solar radiation 102 impinges upon the cloud 100. Due to the particle size of the titanium dioxide particles in the cloud, Mie scattering of the incoming solar radiation occurs causing a proportion of the incoming solar radiation 102 to be reflected away from the earth as scattered radiation 104. Thus, a significant proportion of the ultra violet and visible light of the incoming solar radiation 102 is prevented from reaching the earth's surface. A reduced level of solar radiation 106 passes through the cloud to impinge upon the earth's surface.
Infrared radiation 108 is emitted from the earth's surface, which largely passes through the cloud 100 without being scattered due to the wavelength of the infrared radiation being significantly greater than the particle size of the titanium dioxide particles. Thus, the cloud 100 according to the present invention is able to reduce the level of incoming solar radiation without significantly inhibiting the emission of infrared radiation to space, and thus is able to cause a local or regional cooling effect.
Figure 2 shows an apparatus according to the present invention comprising a conduit 210 connecting a substantially ground-level location on a ship 200 to a stratospheric location at a dirigible 220. Dirigible 220 comprises a deagglomeration means (not shown) and a dispersal means (not shown). Also shown is the tropopause 240, signifying the boundary between the troposphere below and stratosphere above.
In use, particles typically of sub-micron high refractive index particles, such as titanium dioxide, are transported from the ship 200 by means of a particle transport means, such as a pressurisation of a carrier fluid, so that the particles travel along the conduit 210 to the stratospheric location 220. As the particles of the titanium dioxide are sub-micron, some agglomeration will have occurred during transport through the conduit, and so the particles are passed through a deagglomeration means (not shown) before passing to dispersal means (not shown) such as a fan or jet engine. Once the particles have passed through dispersal means, they begin to form a cloud 230 according to the present invention.
is Figure 3 shows a schematic representation of how local weather patterns may be influenced by the action of a cloud 360 according to the present invention. Incoming solar radiation 370 is scattered by cloud 360, as discussed above, resuhing in a region of cooled water surface 300 in the shadow of the cloud 360. The cooled water surface 300 has the effect of cooling the air above it, which has the effect of inducing a down draught of air 310 which is cooler and therefore denser than adjacent air. The down draught 310 causes an increase in horizontal wind speed at the water surface 320 which has the effect of increasing the water vapour uptake of the air 320 and cooling the water surface, before rising. The rising air column 330 results in the formation of clouds and ultimately precipitation. Rising air 330 is drawn back into the cloud's shadow by moving horizontally 340 and it begins its descent once more.
The formation of the clouds has the effect of further reducing incident radiation to extend the region of cooled water surface 300 to an extended cooled water region 380.
Thus, it can be seen that a cloud according to the present invention can be employed to cool the water surface by a few degrees and allow the creation of onshore winds laden with water to promote irrigation of coastal lands.
Figure 4 shows the effect of using three apparatuses according to the present invention together to generate a more substantial cooling effect. In this embodiment, three ships 400 each with their own conduit 410 attached to dirigible 420 are employed. Three separate clouds of high refractive index particles are generated, having the effect of cooling an extended region of water surface 460.
In the embodiment shown, a significant spacing is left between the clouds, so that circulation patterns between the clouds are generated. Thus, cooled water causes a cooling of the air above resulting in descending air 440 and horizontal wind 430 picking up additional water vapour. Airstreams 430 risc 450 and form clouds and eventually precipitation to subsequently be drawn back into air stream 440. However, it can be advantageous to have multiple sources without a significant spacing.
Figure 5 shows in more detail suitable equipment of an apparatus according to the is present invention located at a stratospheric location, some of which may alternatively be a component of an aircraft according to the present invention. The equipment comprises the upper end of conduit 500 wound around spool 510 which is driven electrically by motor 525 with active damping control to dampen oscillating in the conduit 500, and connected to a pipeline via rotational seal 520. The particles in the conduit are carried by a supercritical water carrier and remain in a supercritical state at the stratospheric location, despite the loss in pressure head and frictional pressure drop during transport from a substantially ground-level location (not shown). The particles carried in this super critical water pass through a turbine and/or reducing valve 530 to flash-off steam and bring the carrier fluid out of a supercritical state.
The particles, now carried in a largely gaseous carrier are then passed to a jet impact mill and microniser 540 via multiple entry points to a microniser of approximately 4 metres in diameter. The jet impact mill and microniser, constituting a deagglomeration means, breaks any agglomerates formed between the particles and ejects the individual particles via nozzle or nozzles 570. Also shown is a fan or jet engine 560 with air inlet 550. The fan or jet engine 560, constituting a dispersal means, generates a significant flow of air. As can be seen, nozzle 570 is located in the flowing gas stream, and the agglomeration means is therefore close-coupled to the dispersal means. This has the effect that the deagglomerated particles do not have an opportunity to reagglomerate and are dispersed into the stratosphere to form a cloud 580 according to the present invention. The particle stream can also be injected into the inlet of the jet, fan or gas turbine to aid dispension but can lead to particle build-up and particle degradation issues. Also shown is valve 525 which can be opened to allow the ingress of air when it is desired to drain the contents of the conduit 500.
Figure 6 shows in more detail suitable equipment of an apparatus according to the present invention located at a substantially ground-level location. Shown is a portion 1° of a slurry pipeline or conduit 600 attached to a spooi 610 to take the tension force from the slurry pipeline 600. The spool 610 has active tensioning arrangements 625 to control pipeline oscillations. The pipeline 600 is secured by a pipe release restraint 690 which is secured by an anchor 695 to a ship or to the ground.
is Also shown is suitable slurry preparation equipment. Separate streams of high refractive index particles and carrier fluid, e.g. water, are passed to slurry mixing vessels 650. Once mixed, the mixture is passed to pumps 640 which increase the pressure of the mixture to very high levels. Once at high pressure, the mixture is passed to pressure surge vessels 630 in order to smooth out any pressure impulses from pumps 640. As shown, a plurality of streams are employed and combined together to increase the pressure of the slurry still further before being passed to pipeline 600 for transmission to the stratosphere.
Additionally, additives may be included via additive feed pump 670 and a pipe dump tank 680 is conveniently to allow draining of the conduit when the dirigible needs to move location independently of the ground location. Typically, heating apparatus (not shown) will also be employed in order for the carrier fluid to become a supercritical fluid.
Figure 7 shows in more detail the conduit construction and arrangement of its structure. Shown is a former 701, surrounded by a composite pipe 702, insulation 703 and surface protection 704. Fluid is pumped from a ship or ground station 709 via the conduit 708 to a dispersal arrangement 710 at an elevated location.
The former 701 is typically steel but can be a strong non-conducting abrasion-resistant plastic, typically from 0.2 to 2.0 mm thick, providing abrasion resistance and abase on which fibre is wound on or which a braid is stretched. An epoxy or other filler would provide some additional stability to the fibre orientation.
Close-up view 706 illustrates how the elongate fibres or braid are wound around the conduit at a very low angle with respect to a line passing through the fibre and parallel to the central axis. This helps to deal with the very high longitudinal tension forces io encountered there due to the weight of the conduit being supported. At the base of the conduit in close-up view 705, it can be seen that the elongate fibres or braid are wound around the conduit at a greater angle with respect to a line passing through the fibre and parallel to the central axis. This is to provide greater protection for the high hoop stress encountered there due to the high fluid pressure. Although not shown, the is angle of winding or braid alters gradually from the top to the bottom to account for the gradually changing nature of the stresses in the conduit. This provides for an efficient use of materials, reducing the size and cost of the conduit.
Claims 1. An aircraft located at a stratosphere location, the aircraft comprising a source of sub micron solid particles, a deagglomeration means and a dispersal means.
2. An aircraft according to any one of the preceding claims, wherein the particles have a refractive index in excess of 1.40, preferably greater than 2.0, more preferably greater than 2.3.
3. An aircraft according to any one of the preceding claims, wherein the particles are particles of titanium dioxide, sodium chloride, silicon or mixtures thereof 4 An aircraft according to claim 3, wherein the particles have a mean particle size in the range of from 0.01 to 1.3 m, more preferably in the range of from 0.02 to 0.5 rim.5. An aircraft according to claim 4, wherein the particles have a bimodal size distribution with peaks in the visible and ultra violet wavelengths respectively.6. An aircraft according to any one of the preceding claims, wherein the particles are coated with one or more materials.7. An aircraft according to claim 6, wherein the coating material is hydrophobic.8. An aircraft according to claim 7, wherein the coating material comprises a siloxane.9. An aircraft according to any one of the preceding claims, wherein the dispersal means involves transporting the particles into a high velocity air stream.10. An aircraft according to any one of the preceding claims, wherein the deagglomeration means comprises a microniser, a venturi nozzle or a combination thereof 11. An aircraft according to any one of the preceding claims, wherein the dispersal means is close-coupled to the deagglomeration means.12. An aircraft according to any one of the preceding claims, wherein the particles are provided via a conduit connecting a substantially ground-level location to the aircraft.