Particle composition

A baseline scattering system for climate engineering purposes may be implemented with approximately 100-nm diameter spherules comprising at least one (possibly hydrated or carboxylated) oxide of the magnesium-through-titanium group of metals and metalloids; Mg, Al, Si, S, Ca and Ti all appear to be apt prospects. There is little basis in radiative physics for selecting any particular one due to their similar dielectric properties in the UV spectral band; thus convenience and estimated differential side effects constitute the present choice criteria. Metals per se, though of much greater potential scattering efficiency, may raise concerns related to their ozone interactions and respiratory impacts, even when coated, e.g. with a thin durable oxide layer. The use of resonant scatterers, though eminently appealing in terms of mass efficiency, is handicapped by contemporary 'packaging' and deployment technology considerations (Teller et al. 1997).

Among dielectrics, many alternatives have been proposed (e.g. NAS 1992) and all appear to be fundamentally workable. Liquid SO2 (or perhaps SO3) appears to be optimized for mass efficiency, transport convenience and relative non-interference with all known processes of substantial biospheric significance, although fluidized forms of MgO, Al2O3 or SiO2 (e.g. as hydroxides in water) seem competitive in most pertinent respects. Amounts presently considered for stratospheric injection are of the order of 1 per cent of the SO2 annual mass injection into the troposphere by all processes, which are roughly half each of natural and anthropogenic origin, so that the eventual descent of stratospheric sulphate particulates into the troposphere will add negligibly to the globally averaged levels, although somewhat greater fractional increases may be expected at high latitudes. With respect to prospective impact on the ozone layer, Crutzen (2006) estimates the probable effect magnitude of geo-engineering-contemplated stratospheric injections of sulphate particulate to be less than that of the Mount Pinatubo eruption (see also the discussion in Chapter 12, pp. 272-277). Any scheme would need to take into consideration particle aggregation and interaction with water and other compounds found in the stratosphere.

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