Let's talk physics!

As far as geo-engineering goes, Stratospheric Sulphate Injection (SSI) is at the forefront of debate amongst the global community concerned with climate change. As so, before we discuss its opportunities and issues, in my opinion it would be wise to ask the question:

What is Stratospheric Sulphate Injection and how does it work?

Let’s first take a step back and depict this (almost scary sounding?) idea.

The Stratosphere – look below and you will see it just above the Troposphere ranging from ~12 – 49Km. The method is to ‘inject’ Sulphur aerosols into this region of the atmosphere and alter the Earth’s energy balance by increasing the opacity of our atmosphere to the Sun’s incoming energy. Aerosols naturally occur within the Earth’s atmospheric system and their physical and chemical processes are well documented via major volcanic eruptions (stay tuned for the fun stuff).

Before I forget: an aerosol – defined as the suspension of particulates (solid or liquid) in a gaseous substrate.

© 2006. Steven C. Wofsy, Abbott Lawrence Rotch Professor of Atmospheric and Environmental Science, lecture notes    

This is important. In the Troposphere the retention time of sulphate aerosols is just mere days and this is due to Earth’s weather. If we inject into the Stratosphere, the aerosols will stay in the Stratosphere! Well for a year or so… 

These Sulphur aerosols have both a direct an indirect effect on our atmosphere. Direct Radiative Forcing - the difference between an increased net irradiance at the Tropopause to a state of radiative equilibria in the Stratosphere. A fancy way of saying the difference between long wave radiation form the Sun at the top of the Atmosphere compared to the radiative norm of the Stratosphere. The Beer – Lambert law describes the interaction between an incoming beam of light and the aerosol.

The extinction co-efficient is a function of the aerosols ability to remove photons from the direct beam and the optical depth, or thickness, represents the aerosol’s power to prevent transmission of light by scattering or absorption. The path of the photon can now be described as a more familiar friend – the single scattering albedo, calculated as the ratio of the scattering optical depth to the total optical depth. 

Put simply, the efficacy of the light hitting the aerosol being scattered compared to becoming extinct. Through this equation it is known sulphate aerosols have a negative direct forcing.

If you are interested in the mathematics, take a look at the further reading page for a more in-depth derivation! (click the link) 

Figure 1. Radiative effects aerosols incur with in the atmosphere directly and indirectly. Black straight lines indicate reflected solar radiation and wavy lines represent terrestrial radiation. Small black dots are aerosols. Taken from http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf as a modification from (Haywood and Boucher, 2000)

Figure. 1 above illustrates the relation aerosols have on clouds and their micro-physical properties. Aerosols may act as cloud condensation nuclei (CCN) – this has two in-direct effects on cloud formation by changing the density and modal size of cloud droplets! Whilst direct radiative properties are well derived these following processes are very difficult to quantify; primarily because the mechanisms between CCN and aerosols are not completely known!

The Twomey effect:

• A positive correlation between an increased concentration of aerosols and albedo
• Increased number of aerosols means greater density of small particles of liquid droplets
• The resultant greater surface area means an increase in the irradiance and subsequently the albedo, thus greater negative radiative forcing!

The Albrecht effect:

• The lifetime of a cloud – a greater concentration of smaller droplets produced by CCN reduces the efficiency of precipitation.
• Smaller droplets take a longer period of time to coalesce to form large enough droplets and cause precipitation 

There we have it! The direct and in-direct effects of aerosols cause a negative radiative forcing, reduce the total energy in the Earth’s atmospheric system, which is directly linked to the temperature at the surface!