Photophoretic levitation of engineered aerosols for Geoengineering
Published June 30, 2010 | Link
Link to Article_Photophoretic levitation of engineered aerosols for geoengineering
Aerosols could be injected into the upper atmosphere to engineer the climate by scattering incident sunlight so as to produce a cooling tendency that may mitigate the risks posed by the accumulation of greenhouse gases. Analysis of climate engineering has focused on sulfate aerosols. Here I examine the possibility that engineered nanoparticles could exploit photophoretic forces, enabling more control over particle distribution and lifetime than is possible with sulfates, perhaps allowing climate engineering to be accomplished with fewer side effects. The use of electrostatic or magnetic materials enables a class of photophoretic forces not found in nature. Photophoretic levitation could loft particles above the stratosphere, reducing their capacity to interfere with ozone chemistry; and, by increasing particle lifetimes, it would reduce the need for continual replenishment of the aerosol. Moreover, particles might be engineered to drift poleward enabling albedo modification to be tailored to counter polar warming while minimizing the impact on equatorial climates.
The use of electrostatic or magnetic materials enables a class of photophoretic forces not found in nature.
atmospheric physics solar radiation management climate change
The possibility of increasing the earth’s albedo to offset CO2-driven warming has been a subject of speculation for decades (1). Over the last few years, more systematic research and debate on the topic has emerged spurred by Crutzen’s (2) call for systematic analysis of geoengineering in response to the continued acceleration of anthropogenic CO2 emissions (3) and the threat of abrupt climate change. Most research has focused on the possibility of injecting sulfur into the stratosphere (2, 4–8), although more elaborately engineered aerosols (9) and space-based solar scattering systems have also been proposed (9, 10). Here I examine the possibility that particles might be engineered to exploit photophoretic forces (11–16), enabling the manipulation of particle distribution and radiative forcing in ways that could not be achieved with sulfate aerosol.
Limitations of Sulfate Aerosols
The salient advantage of sulfate aerosols as a means to modify the earth’s albedo is that nature has already performed relevant experiments in the form of volcanic injections of sulfur, such as the 1991 eruption of Mount Pinatubo, which deposited ∼9 Mt sulfur in the stratosphere creating sulfate aerosols that cooled the earth by ∼0.5 C within a year (17).
As a tool for climate engineering, sulfates are, however, a blunt instrument. Disadvantages of sulfates include the following: First, it is difficult to produce sulfate aerosol with an appropriate size distribution. The mass-specific scattering efficiency of a sulfate aerosol (or similar dielectric sphere) is strongly dependent on its radius. The scattering efficiency peaks at ∼0.3 μm diameter and decreases rapidly for larger or smaller droplets, yet when aerosols are generated by continuous injection of SO2 the resulting size distribution tends to be substantially larger than optimal because most of the added sulfur is deposited on existing particles. This substantially limits the radiative forcing produced by large sulfur injections and can make it difficult to produce a radiative forcing sufficient to offset the radiative effect of a CO2 doubling (4). The problem is compounded if sulfate aerosols reach the warmer temperatures in the upper stratosphere where the vapor pressure of H2SO4 is sufficiently high to enable vapor-phase transfer of mass from smaller to larger particles, compounding the difficulty of maintaining a suitable aerosol size distribution.
The increase in diffuse radiation will in turn tend to whiten the visual appearance of the daytime sky and produce side effects ranging from alteration of ecosystem productivity (19) to the reduction in the output from concentrating solar power systems (20).
Second, a significant fraction of the light scattered by sulfate aerosols is scattered in the forward direction, increasing the ratio of diffuse-to-direct insolation at the surface. The Pinatubo eruption, for example, increased the amount of clear sky diffuse sunlight reaching near surface by more than a factor of 2 (18). The increase in diffuse radiation will in turn tend to whiten the visual appearance of the daytime sky and produce side effects ranging from alteration of ecosystem productivity (19) to the reduction in the output from concentrating solar power systems (20).
Finally, sulfates in the lower stratosphere provide reactive surfaces that can accelerate the catalytic removal of ozone by accelerating the conversion of chlorine from reservoir species to ClO. This effect may be more serious if water vapor concentrations in the lower stratosphere increase with increasing global temperatures (21).
The problems of particle size control, forward scattering, and interference with ozone chemistry would apply, in varying degrees, to other nonsulfate aqueous aerosols in the lower stratosphere (22).
Given the disadvantages of sulfate aerosols as a tool for climate engineering, it is worthwhile to explore the possibility of designing scatters that might enable perturbations in radiative forcing to be more precisely tailored or to be achieved with less severe side effects.” Link