Let's get down with pH!

I spent my weekend researching the possible exacerbations geo-engineering may pose to our climate, riveting I know… One that seemed to erode on me was the issue of acid rain as a result of Stratospheric Sulphate Injection. Are we going to have to get down with pH to combat climate change?

Acid rain is the result of an increased presence of Sulphuric or Nitric acids having reacted with water, hydrogen and other molecules which then precipitates as rain, snow, or tiny dry sediments. The concern here is where that precipitation ends after surface processes. Aquatic environments and ecosystems are particularly vulnerable to the effects of acid rain.

The Institute for Public Affairs, an Australian think tank attempted to discuss the topic, ‘Would you swap climate change for acid rain?’ The review lacked any insight to the influence SSI would actually have on global sulphuric deposition; and instead attempted to form an almost sceptical, critical analysis on the discourse of SSI and somehow relate this to previous efforts to prevent acid rain?! I didn’t really get where they were going with it either…

Yes, what goes up, (most of the time), must come down. Whilst the IPA made a feeble attempt to discuss a valid argument, Ben Kravitz did not. A 2008 paper distinguished injection of sulphate aerosols both throughout the tropics and polar regions to study acid circulation and deposition, and as a result where would be most affected. The figure below illustrates the increased load in acid deposition with a peak concentration of 35 mEq m⁻² a⁻¹ due to solely SSI. Injection in tropical latitudes shows increased acid deposition across much of the northern hemisphere whilst this is constrained to mid – latitudes in the southern hemisphere. The regions most effected follow current patterns of areas with greater acid deposition.

Kravitz, B., A. Robock, L. Oman, G. Stenchikov, and A. B. Marquardt (2010), Correction to “Sulfuric acid deposition from stratospheric geoengineering with sulfate aerosols,” J. Geophys. Res., 115, D16119, doi:10.1029/2010JD014579.

Kravitz published two papers, one – concluding the effects of acid deposition were minimal and only in ‘pristine areas’ would the environment not act as a sturdy buffer; and the second – a paper actually correcting himself. Kravitz made a little mistake with his arithmetic and one of the governing equations yet this mistake only progressed his conclusion! (I'm sure he felt very smug and since reading the IPA review I can't help empathising with him). I think one is right in saying Stratospheric Sulphate Injection will influence acid deposition, however, the concentration of Sulphur to be injected is minute compared to that produced by current industrial processes, and realistically the additional loading isn’t a real cause for concern. 

I came across this interview between Alan Robock,  renowned for his research in the field, and Patrick Roddie, an environmental activist. Watching I felt sorry for Alan having to answer what can only be described as a dumb question, 'What are the health effects of SSI (and in turn acid rain)?'' Personally, I feel acid rain is just a popular household phrase one can jump to when discussing harm to the environment, there are much more serious issues surrounding SSI which keep me on the fence. To ask about the 'health effects,' my advice to Roddie would be, drink it and find out.

Initially when writing this post I was curious how SSI would influence acid deposition as I had encountered many contradicting thoughts during lectures and discussions the past few weeks. However, thinking back to the idea of ‘tipping points’ in the Earth system, if SSI can prevent an irreversible moment at the cost of a little extra ‘acid rain’; my answer to the IPA would be, YES, LET’S GET DOWN WITH pH!    

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!