Aerosols have had a PR problem in the past, but in the right place aerosols play a critical role in regulating the climate. Some of us remember a time when aerosol deodorant became an envrinmental nasty and roll-ons were the new norm. In this case, the change was less about the aerosol, and more about the propellent pushing the spray from the can.
CFC (chlorofluorocarbon) propellents and refrigerants were banned under the Montreal agreement after links to degradation of the atmosphere's ozone layer; our first line of defence from UV solar rays, before we slip on a shirt, slop on the sunscreen and slap on a hat. Aerosol sprays are still used, just different propellents, like butane.
What even is an aerosol? It's the sub-micron (under a millionth of a metre) size, droplets mixed in the gas propellent, that makes up the spray from the can.
From armpits to the atmosphere
In the case of the atmosphere sizes fall in three distributions (Aitken 1-5 nm, Accumultion 50-500 nm, Course 500-5000 nm modes) and aerosols form in many natural and manmade processes, from sea spray, dust storms and organic emissions from plant/plankton, to biomass and fossil fuel combustion, demolition dust and transport emissions! Aerosol chemistries include sulphates, nitrates, volatle organics, carbon black (soot), and mineral salts. Before the deep dive, the takehomes:
Aerosols play an important role in regulating global tempertures, with natural levels offsetting around a 1/3 of the 3 Wm-2 radiative forcing of all GHGs.
Sulphur dioxide is precursor to sulphate aerosol and a reduction in emissions from shipping fuel since 2020 has reduced the aerosol offset by 0.2 Wm-2 and added to warming.
The 1991 Mount Pinatubo erupution injected up to 20 Megatonnes of SO2 into the stratosphere and cooled the planet by around 0.5 oC. with an additional 1 Wm-2 of negative forcing over background aerosol.
For the mean atmospheric aerosol particle size of 0.5 microns, calculation shows that 3 Wm-2 or just under 1% of the 340 Wm-2 of solar incident on Earth, is reflected by around 1.7 Mt of SO2 converted to sulphate.
Accumulated CO2 equivalent (CO2e) emissions from GHGs since the industrial revolution has reached around 1800 Gigatonnes (Gt) and contirubtes a near 3 Wm-2 to positive raditaive forcing.
So roughly 1 tonne of SO2 offsets 1000000 tonnes of CO2e GHG in radiative forcing. That's 1 tonne of CO2e is offset by just 1 g of SO2, the equivalent sulphur preservative found in a bag of dried apricots!
Misunderstood by many, these particles play an important role in regulating the climate, and without them the world would be a whole lot warmer than it is. As particles of size, they have volume, so sunlight can reflect and refract at the surface or absorb in the bulk.
Dispersed in the atmosphere, most aerosols increase the albedo of the Earth. Albedo? The fraction of solar radiation bathing the planet that's reflected back to space! So aerosols act to reduce warming from the sun, known as negative radiatiave forcing.
Greenhouse gases (GHGs), carbon dioxide, methane, nitrous oxide and those CFCs from earlier trap heat from Earth, effectively add to solar and thus have positive radiative forcing.
And what are the numbers on these effects? Well the intensity of the sun at the on the top of the Earth's atmosphere is of the order 340 W/m3, the -ve forcing of aerosols, around 1 W/m3 and the +ve forcing of GHGs around 3 W/m3.
Effects of aerosols manifest in many ways. A layer of sulphate high in the stratosphere 15 to 25 km makes the aerosol a parasol, while small particles in the troposphere can seed marine cloud brightening (MCB), notably evidenced by the fluffy trails that track ships using sulphate rich bunker furls.
Indeed a recent uptick in warming rate has been attributed to a 0.2 W/cm2 increase in +ve radiative forcing, following 2020 regulation by the Int. Maritime Organisation (IMO) to reduce sulfur content from 3.5% to 0.5%.[5]
The most dramatic and punctuating effect of aerosol on radiative forcing is from volcanoes, the most recent high profile eruption, Mount Pinatubo, June 15, 1991.
Lessons from nature
The eruption was vast, level VEI-6 on the scary Volcanic Explosivity Index (0-8)[6], with emissions of a size in ranges given in the table below [7-14]. Short, long term, local and global, socio-economic effects of the eruption have been assessed [14, 15] and lessons learned in disaster mitigation and recovery [16, 17].
The most significant and widely reported effect was cooling from sulphate formed from the megatonnes of SO2 dumped in the stratosphere.
Dissolved in water, SO2 forms sulphuric acid H2SO4 (hydrogen sulphate) and evaporation leaves small aerosol particulates, distributed in size between 0.3 - 0.5 microns (0.0000003 m or 50X smaller than the width of a hair!) [18].
With a size comparable to the peak solar wavelength (500 nm) and a refractive index higher (n ~ 1.45)[19] than water (n ~ 1.33) the sulphate aerosol is efficient in the "extinction" of radiation, via scattering.
Indeed compared to cloud droplets of water (~ 10 microns), "Mie scattering" shows sulphate particles to be over 2X more effective at scattering solar radiation than cloud drops.
Moreover, as fine particles the aerosol is dispersed rapidly across the globe by stratospheric air streams. From Pinatubo, the initial erupta encircled the equator and dispersed 30 oN and 10 oS within 21 days and covered 42% of the Earth within 2 months [8, 9].
Atmospheric optical depth [AOD = ln(incident/transmitted) intensities] showed anomalies 100X over background (May '91<0.001 to Aug '91>0.1) within 3 months and which remained elevated for over 2.5 years.[8] In this case a larger depth = less solar transmission through the atmosphere.
A crude calculation based on the sun-earth radiation balance, with 340 Wm-2 incoming solar and 185 Wm-2 transmitted, gives a typical optical depth ~ ln(340/185) ~ 0.6.
A 0.1 increase in AOD following the Pinatubo eruption corresponds to a reduction in solar transmision to 340/exp(0.7) ~ 169 Wm-2, equivalent to a 16 Wm-2 boost in solar absorption and reflection by the atmosphere.
A chunk of the forcing on the atmosphere is accounted for by a 4K warming of the stratosphere following eruption [8], which computes to a radiative forcing of 𝜎(258(4) ‑ 254(4)) ~ 15 Wm-2, via the Stefan-Boltzmann (SB) equation [see post 21], where 254K is the effective temperature of the Earth (as a blackbody)[22] and 𝜎 is the SB constant.
The net increase in reflection from Mt Pinatubo aerosol was thus of the order 1 Wm-2, seen as the last sharp dip in the "time evolution of effective radiative forcing 1750-2022" [Fig. above ref. 3].
The negative radiative forcing from the sulphate aerosol was sufficient to drop global surface temperatures by 0.5 - 0.6 oC [8, 9, 11], the largest registered cooling event since the trend in global warming from 1950.
Remarkably the -ve forcing more than offset the entire +ve radiative forcing of atmospheric GHGs for more than 2 years [9, 3]. But atmospheric sulphate loadings, optical depths, radiative forcing and temperatures returned to pre-Pinatubo trend within 8 years of the '91 eruption.
And so to the nub! A parasol of aerosol, opened by stratospheric injection with near instant cooling, closed by aerosol removal via natural precipitation processes.
Earth's very own thermostat if you will! Although the sporadic nature of volcanic eruptions is more akin to the couple fighting over the home thermostat!
But these events are nature's experiments from which to learn from, and so it is that stratospheric aerosol injection (SAI) has emerged as a strategy for solar radiation modulation (SRM) to regulate global temperatures [24].
As early as 1955 the great polymath, John Von Neumann predicted "intervention in atmosphere and climate matters, will come in a few decades, and will unfold on a scale difficult to imagine at present."[25].
Recognising the role of aerosols in climate control, Mikhail Budyko was first to propose "One such method is to maintain a certain level of aersol concentration in the atmosphere", often coined the "Budyko Blanket" in honor of his work in climatology [26].
SRM takes many forms [27, 28], including marine cloud brightening (MCB), cirrus cloud thinning (CCT), as well as surface-based albedo enhancement via Arctic ice thickening (AIT), ocean mirror, cool roof tech and passive cooling, and space-bound solutions such as sunshades and even reflective bubbles [29]! Start-up interest and investment is now building to implement some of these SRM strategies [30-35].
Strategies to reduce trapping of infrared (IR) radation from the Earth (technically CCT) or enhance radiative emission through the atmospheric IR window (wavelengths of 8-13 micron), can been classed as Earth Radiation Management (ERM) [28].
International regulation of SRM is tricky due to its global reach, but reports from major governments and organisitions highlight the need for strong cooporation over R&D and policy to avoid unintended consequence [36-42].
All in a bag of dried apricots
Back to the science! Simple calculations can be made to highlight the effictiveness of SAI in cooling. The base argument follows from the offset of radiative forcings.
We're now nearing a gobal surface temperature anomaly of 1.5 oC, corresponding to a positive radiative forcing from GHGs of ~ 3 Wm-2 [Fig 3].
Given incoming solar is around 340 Wm-2, then to offset the forcing, the sun needs dimming by only 3/340 ~ 0.9%.
If we block the sun by reflection from aerosol, then the aerosol must must cover 0.9% of the surface area of the Earth at the height of aerosol injection in the stratosphere.
With a surface area 4𝜋𝑟(2) ~ 5.14 x 10(14) m2, where 𝑟 = 6371 + 25 km, the mean radius of Earth [22] + layer height, the area blocked must be 4.5 x 10(12) m2. Still around 7 million football pitches big!
But here's the skill of the aerosol particle, surface area to volume is also big! So what? Well, we get a large area for a relaltively small volume of particles.
For an effective aerosol radius of 0.5 micron [43], the number of particles needed to match the blocking area is ~ 4.5 x 10(12) / 2𝜋𝑟(2) ~ 2.9 x 10(24), assuming half the particle area faces the incoming solar.
Yep, still massive, but converted to a volume andwe get just 2.9 x 10(24) x 4𝜋𝑟(3)/3 ~ 1500000 m3 of aerosol.
Converting to mass, we finally get to compare with the atmospheric GHG burden. For a sulphate density of 1.7 g/ml (75% H2SO4)[44], aerosol mass comes in at 1.5M m3 x 1.7 g/ml x 1M ml/m3 ~ 2.6 x 10(12) g or 2.6 Tg or just 2.6 Megatonnes (Mt) of sulphate.
For the 1 to 1 sulphur dioxide to suphate reaction SO2 + H2O + 1⁄2O2 → H2SO4 the mass equivalent of gas reduces by the molar mass ratio 64/98 to only ~ 1.7 Megatonnes of SO2.
Boom! So 1.7 million tonnes of SO2 can produce sufficient sulphate aerosol (100% conversion [43]) to generate a -3 Wm-2 radiative forcing, offsetting the entire positive forcing from anthropogenc GHGs since 1750. But how much CO2 equivalent is that?
CO2 has now risen to 420 ppm from pre-inductrial levels of 280 ppm. Including effects of other GHGs, methane, nitrous oxide and CFCs pushes CO2 equivalent levels to 520 ppm [45].
For a total atmospheric mass 5.1 x 10(15) tonnes of mean molecular weight 29 g/mol [22], the mass of constituent CO2 at 44 g/mol is 0.00042 x 5.1 x 10(15) x 44/29 ~ 3280 Gigatonnes (Gt) giving 3298/420 ~ 7.85 Gt/ppm.
The mass of CO2 equivalent (CO2e) contributing to the 3 Wm-2 forcing and 1.5 oC warming since 1750 is this (520 - 280) ppm x 7.85 Gt/ppm ~ 1880 Gt or 1880000 Mt CO2e.
That's near a 1 tonne CO2 offset per 1 g of SO2, or the equivalent sulphur dioxide preservative in around 150 plump dried apricots [46]! Imagine that, SAI becomes Stratospheric Apricots Injection!
Wait, how does this compare with a top down estimate made from Pinatubo? Emissions of 18Mt SO2, gave rise to around -1 Wm-2 forcing and a -0.6 temperature drop. That's 18/0.6 ~ 30 Mt per 1 oC offset compared to 1.7/1.5 ~ 1.1 Mt per 1 oC for the toy model calculation above.
Assumptions of 100% scattering efficiencies in the toy model and the range of SO2 emissions measurements (see table) may account for some discrepancy in tonnage, but a 30X difference, really?
OK, the comparison is over-simplified and the 4 oC stratospheric warming from the eruption and assoicated 15 Wm-2 forcing, should be accounted for. If sulphate aerosol effectively compensates for this, as well as the 1 Wm-2 solar reflection, then the level of SO2 required to effect an overall 0.6 oC cooling is 18 Mt/16 Wm-2 ~ 1.1 Mt per Wm-2.
The effective mass of SO2 associated with Pinatubo cooling then reduces to 1.1/0.6 ~ 1.8 Mt per 1 oC, +/- 0.2 Mt accounting for the range in estimates of stratospheric SO2 emissions from Pinatubo. The "model" falls within a factor 1.8/1.1 ~ 1.6 of the "experiment", which could be accounted for by a rough, but realistic, 1/1.6 ~ 60% scattering efficiency in the toy.
Does the risk-benefit stack up?
So the science works, and despite good alignments between a top down analysis of nature's "experiment" with SAI and a bottom-up "model", uncertainties remain.
No explicit account of the co-emissions from Pinatubo, i.e. ash, CO2, water (see table) is made or the uncertainties associated with the range of values placed on these and the SO2 injections derived from both measurements and simulations in the literature [7-14].
Then there are all the eco, bio, medico, socio, politco, knowns and unknowns [35-42]. Are there just too many adverse knowns, too many unknowns or uncertainties on known benefits too large, that SAI is just too risky? Or does inaction pose greater risk from the slow and delayed cooling via decarbonisation pathways? Over to you, deep dive, digest, decide!
But some points to ponder. Obviously tight terms and cautious conditions must apply in any rollout, decarbonisation must be first and foremost, and SRM is no excuse for business as usual.
However, deep learning in SRM methods and outcomes, the good, the bad and the ugly is essential for future scenarios. Not only to understand the risk-benefit of SRM as a tool in the fight to cool an overheating planet, but to mitigate effects from aerosol induced winters, from a lively volcano for example, nuclear fallout, a rogue actor in SRM or even a (non-existential) hit from a lump of space rock with Earth's name on it! Eek, bit dark, but best prepared.
Some fear "geoengineering" of this kind, but humans have being doing this throughout its short history. From the dawn of farming, to urbanisation, the industrial revolution, shipping and aviation, all these "engineered" advancements have impacted land, sea and sky. Others fear "termination shock" when the engineering is stopped or reversed. Who knows, the race to net zero may be that shock and we may need SRM yet, to slow the path and soften the blow!
Well that's it and if any of this sticks, then the next time you look-up to a hazy sky, you may remember it has something to do with apricots, just not remember the heck why :))
[5] Yuan, T., Song, H., Oreopoulos, L. et al. Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming. Commun Earth Environ 5, 281 (2024). https://doi.org/10.1038/s43247-024-01442-3
[9] McCormick, M., Thomason, L. & Trepte, C. Atmospheric effects of the Mt Pinatubo eruption. Nature 373, 399–404 (1995). https://doi.org/10.1038/373399a0
[10] Guo, S., G. J. S. Bluth, W. I. Rose, I. M. Watson, and A. J. Prata (2004), Re-evaluation of SO2 release of the 15 June 1991 Pinatubo eruption using ultraviolet and infrared satellite sensors, Geochem. Geophys. Geosyst., 5, Q04001. https://doi.org/10.1029/2003GC000654
[11] Abdelkader, M., Stenchikov, G., Pozzer, A., Tost, H., and Lelieveld, J.: The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud, Atmos. Chem. Phys., 23, 471–500, https://doi.org/10.5194/acp-23-471-2023, 2023.
[13] Guo, S., Rose, W.I., Bluth, G.J.S., I. Watson, I.M., Particles in the great Pinatubo volcanic cloud of June 1991: The role of ice. Geochem. Geophys. Geosyst. (2004). https://doi.org/10.1029/2003GC000655
[18] Murphy, D. M., Froyd, K. D., Bourgeois, I., Brock, C. A., Kupc, A., Peischl, J., Schill, G. P., Thompson, C. R., Williamson, C. J., and Yu, P.: Radiative and chemical implications of the size and composition of aerosol particles in the existing or modified global stratosphere, Atmos. Chem. Phys., 21, 8915–8932, https://doi.org/10.5194/acp-21-8915-2021, 2021.
[20] Figure 7.2 in IPCC, 2021: Chapter 7. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. doi: 10.1017/9781009157896.009
[24] Crutzen, P.J. Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?. Climatic Change 77, 211–220 (2006). https://doi.org/10.1007/s10584-006-9101-y
[25] von Neumann, John. “John von Neumann on Technological Prospects and Global Limits.” Population and Development Review, vol. 12, no. 1, 1986, pp. 117–26. JSTOR, https://doi.org/10.2307/1973354. Accessed 31 Aug. 2024.
[26] Budyko, M.I. (1977). Summary. In Climatic Changes, M.I. Budyko (Ed.). https://doi.org/10.1002/9781118665251.oth02; Budyko, M. I. (1974). Izmeniya Klimata. Gidrometeoizdat, also published as: Budyko, M. I. 1977 Climatic changes (transl. Izmeniia Klimata Leningrad: Gidrometeoizdat, 1974). American Geophysical Union.
[27] https://www.carbonbrief.org/explainer-six-ideas-to-limit-global-warming-with-solar-geoengineering/
[28] Ming, T et al. Fighting global warming by climate engineering: Is the Earth radiation management and the solar radiation management any option for fighting climate change? 31, 792-834 (2014) https://doi.org/10.1016/j.rser.2013.12.032
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[41] Peter J. Irvine et al. Towards a comprehensive climate impacts assessment of solar geoengineering 5, 93-106 (2017). https://doi.org/10.1002/2016EF000389
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[43] Quaglia, I et al. Interactive stratospheric aerosol models' response to different amounts and altitudes of SO2 injection during the 1991 Pinatubo eruption. ACP, 23, 921–948, 2023. https://doi.org/10.5194/acp-23-921-2023
*Links accessed 30/08/24