Particulate Sodium Chloride - AN Outstanding Candidate for Solar Radiation Management

Teller et al. (1997) suggested deploying fine-grained aluminum oxide (Al2O3, or corundum) as an atmospheric aerosol for Solar Radiation Management (SRM) to offset the anthropogenic contribution to climate change. Unfortunately, corundum is an abrasive (Mohs Hardness = 9) that portends ill health to air-breathing life. We suggest a more promising material: NaCl with anti-caking agent 552 (calcium silicate), a.k.a. Morton® table salt.

We investigated the reflectance properties of both Al2O3 (Nelson et al., 2018) and Morton® salt in well-sorted particle sizes. Our results are in agreement with basic light-scattering theory, which holds that at very small phase angles, particles that are larger than the wavelength λ of incident light are highly reflective (backscattering), and well-dispersed particles that are smaller than λ tend to be forward-scattering.

A comparison between Al2O3 and NaCl indicates that the latter is a far better choice for aerosol dispersal for SRM. NaCl is several times more reflective at small phase angles. As an already abundant, naturally-occurring aerosol it is environmentally benign at the low concentrations required to reduce the solar forcing function by several W/m², the amount estimated by the IPCC to be the anthropogenic contribution to global warming. Furthermore, as a particulate aerosol, NaCl would have short residence time if distributed in the upper troposphere, where it would not interfere with stratospheric ozone.

With great trepidation, we suggest potential use in these areas:

- Temporary regional application to mitigate short-term, life-threatening conditions in areas where extreme temperature events are expected on timescales of days, and

- Global application for immediate relief during a near-term transition period to an atmosphere that is generally free of anthropogenic greenhouse gas.

We offer this as a temporary relief measure and not a solution, somewhat analogous to the application of morphine in a medical situation.

Refs: Teller et al., 1997. UCRL-JC-128715; Nelson et al., 2018. Icarus, 202, 483-498.