'Levitating Nanoparticles may make for better Geoengineering'.....
A recent analysis of geoengineering options indicated that, while pumping chemicals into the upper atmosphere would work, the approach comes with significant risks. Since the chemicals would gradually come back out of the atmosphere, these schemes would require constant input to remain effective. A paper released in PNAS this week suggests there might be a lower-effort alternative: nanoparticles that are structured in a way that helps them control their altitude.
Even if CO2 emissions are eventually reduced, we may have put enough in the atmosphere by then to ensure that some form of geoengineering may be needed to lower the global temperature. Aerosols in the upper atmosphere increase the Earth’s albedo (the overall reflectivity of the planet), reducing global temperatures by reflecting more sunlight back into space. Chemical aerosols are popular geoengineering candidates, but they have their own problems; for example, sulfates in the lower stratosphere can accelerate removal of ozone, and we’d have to constantly pump more into the upper atmosphere.
The main advantage of the proposed nano-structures over chemical areosols is their ability to control their altitude. The proposed nano-structures do this by taking advantage of what are called photophoretic forces. Photophoretic forces arise from a temperature difference between the nano-structure and the surrounding atmosphere. If a cold gas particle strikes a warmer nano-structure, then the gas particle will recoil off with more energy. That results in a force on the particle. The amount of energy transferred in this process is a property of the material and a function of the temperature, and is described by something called the accommodation coefficient (α). By constructing nanostructures with different values of α, one can change the net force on the nanostructure.
The authors propose constructing a disk five microns in diameter and 50 nanometers thick. The disk is primarily made up of two layers, metallic aluminum and barium titanate. The aluminum layer reflects sunlight, but is transparent to thermal infrared. The barium titanate layer has a higher density than the aluminum layer and interacts with the atmosphere’s electric field to orient the disk with the reflective side up. The difference in α between aluminum and barium titanate also produces a net photophoretic force, levitating the disk with a force up to three times its weight.
In their simulations, the authors were able to show that their devices would tend to settle at two altitudes: 40 kilometers (just below the stratospause) or 100 kilometers (in the mesopause) Both locations would avoid possible ozone interactions in the lower stratosphere.
The authors also found that the addition of a 0.5 micron magnetite particle would enable interactions with the earth’s magnetic field, causing the particles to drift towards the poles. The ability to concentrate reflective materials at high latitudes may help lessen the impact of the loss of polar ice. The loss of ice lowers the albedo at the poles, increasing the temperature and melting more ice. These nanoparticles might provide a means to short-circuit this feedback.
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