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Aerosols, Chemistry and Climate

Filed under: — gavin @ 12 July 2008

Everyone can probably agree that the climate system is complex. Not only do the vagaries of weather patterns and ocean currents make it hard to see climate changes, but the variability in what are often termed the Earth System components complicates the picture enormously. These components – specifically aerosols (particulates in the air – dust, soot, sulphates, nitrates, pollen etc.) and atmospheric chemistry (ozone, methane) – are both affected by climate and affect climate, since aerosols and ozone can interact, absorb, reflect or scatter solar and thermal radiation. This makes for a rich research environment, but can befuddle the unwary.

I occasionally marvel at the amount of nonsense that is written about climate change in the more excitable parts of the web, and most of the time, I don’t bother to comment. But in relation to the issue of aerosols, chemistry and climate, I read yesterday (h/t Atmoz) probably the most boneheaded article that I have seen in ages (and that’s saying a lot).

The hook for this piece of foolishness were two interesting articles published this week by Ruckstuhl and colleagues and a draft EPA report on the impacts of climate on air quality. First, Ruckstuhl et al found that as aerosols have decreased in Europe over the last few decades (as a result of environmental standards legislation), the amount of solar radiation at the ground has increased while the amount reflected to space has decreased. They hypothesize that this may have helped Europe warm faster in the last few decades than it would have otherwise done. Or equivalently, since the aerosols are anthropogenic, that European temperatures had been subdued due to the cooling effects of the aerosols – and since they are now decreasing, the full effects of the greenhouse gases are starting to be felt. This is just an update to the ‘global brightening‘ story we have touched on before. The EPA report is concerned with the impacts that climate change can have on atmospheric chemistry, and in particular the summertime peaks in urban ground-level ozone which are a well-known and serious health hazard. These are affected by local temperatures, cloudiness, temperature sensitive biogenic emissions and patterns of weather variability. Again, it is a story we have discussed before.

But the NewsBusters article succeeded in getting almost every aspect of these stories wrong. How do I correct thee? Let me count the ways.

  1. Aerosols are not smog:

    First they confuse aerosols with photochemical smog. Both are pollutants, but the first is dominated by sulphate emissions from coal burning power plants, the second from ozone precursors such as NOx, volatile organic compounds, and carbon monoxide mainly emitted from vehicles. (Note that ozone is not directly emitted, but is created by chemical reactions from the precursors with the addition of a bit of photolysis – i.e. sunlight-driven chemistry). The effects on climate are very different: ozone is a greenhouse gas, so increases cause a warming, while sulphate aerosols are reflective, and so increases cause a cooling. The air quality issues in the EPA are almost all focused on ozone.

  2. Europe is not the Globe:

    The next error is to equate changes in temperatures in Europe to the globe. While it would be true that if global aerosol levels declined it would lead to increased global warming, aerosol trends in Asia are increasing strongly, even while those in the US and Europe are dropping. The net effect is possibly a slight drop, but the impact on global temperature is as yet unclear. This regionality matters in both the sulphates case and for ozone. The relevant chemistry is sensitive to water vapour and temperature in varying ways as a function of the pollution level. In remote ocean areas, surface ozone will likely decrease as the globe warms for instance (due to increasing water vapour). In polluted environments increased temperatures and larger temperature-sensitive emissions of isoprene cause enhanced ozone levels.

  3. Surface ozone is not in the stratosphere:

    Next, NewsBusters asserts that the ozone story is confusing because of the

    .. treaty called the Montreal Protocol. This was designed to reduce and eventually eliminate the production and release of a number of substances thought at the time to be depleting ozone.

    Ummm…. those substances (chiefly chlorofluorocarbons – CFCs) are still thought to be depleting the ozone layer – which is in the stratosphere, some 30km above the ground-level ozone that people shouldn’t be breathing. CFCs have no impact on ground-level ozone at all (since their reactive chlorine is only released in the stratosphere).

  4. The final inanity:

    Wouldn’t it be fascinating if such efforts [such as the Montreal Protocol] lead to cleaner air around the world which ended up warming the planet, and that additional warmth is now breaking down the very ozone we thought we could save?

    Every part of this sentence is wrong. The Montreal Protocol had no impact on cleaning the air, it stopped the growth of CFCs which are powerful greenhouse gases (in addition to their role in depleting stratospheric ozone), therefore it slowed global warming, rather than increasing it, and we aren’t trying to save ground-level ozone. Had any of this been true it would indeed have been fascinating.

What should we make of this? Unfortunately one must conclude that no mistake is too dumb for someone, somewhere to make if they think they can spin it into supporting their anti-science agenda. For them complexity is something to be abused rather than a challenge to be understood, underlining quite clearly (again) the difference between science and propaganda.

356 Responses to “Aerosols, Chemistry and Climate”

  1. 351
    Timothy Chase says:

    Guenter Hess wrote in 346:

    1. On a microscopic level:
    Do I understand you correctly that within the atmosphere up to the height up to where LTE holds, collisions are the dominant processes for energy transfer compared to absorption/reemission. The lower the pressure the higher is the percentage of absorption/emission processes.

    Under earth-like conditions, I don’t think that we have to worry about stimulated emission except where LTE has already broken down — and rarely even then. So at this point we can separate emission and absorption, looking at the roles that each plays. But yes, the lower the pressure, the greater the percentage of energy transfer which would be performed by each of the two processes — and of course locally the spectral emissivity will be the same for the same gas at the same wavelength. Emission acts as the source of photons which are emitted by the atmosphere, absorption as one of the two sinks — with space being the other sink.

    Now lets look at emission. Under LTE conditions, collisions have to be occurring at a high enough frequency that molecules which absorb photons rarely have the chance to spontaneously emit before exchanging energy with other molecules. It is the temperature and consequent Maxwell-Boltzmann distribution of kinetic energy of the molecules that determines how many will be in an excited state at any given time. This holds since spontaneous emission is a form of quantum decay that has no memory of how long a molecule has been in an excited state and thus the rate is simply dependent upon the size of population, not the turn-over in this population which will result from some molecules losing energy due to collisions while others gain energy.

    Now lets turn to absorption. What we are concerned with in determining a photon’s chances of making it to space, unlike emission, isn’t simply the local properties of the atmosphere but the path to space. To make it to space, a photon will be passing through different layers of atmosphere where the atmospheric density, temperature and make-up will vary over its path. So what we need to consider is optical depth. According to the Beer-Lambert-Bouguet law, light traveling through a uniform medium will experience exponential decay along its path, such that if it is cut in half over a given distance, it will be cut in half again when it travels twice that distance and so on. Now of course since we are dealing with the earth’s atmosphere in this case, the medium will not be uniform. Nevertheless, it is possible to define a dimensionless optical depth between any two points for any given wavelength — assuming light travels in a straight line between those two points — where the optical depth will be proportional to the logarithm of the probability that a given photon will make it from end-point to end-point, or alternatively, as the logarithm of the fraction of photons of that wavelength that will complete the path. But then of course one also needs to take into account the fact that the photons may leave the atmosphere at an oblique angle — which would mean that the path they take would have a greater optical depth than if they simply travelled perpendicular to the earth’s surface.

    Guenter Hess wrote in 346:

    However, at the escape height which is below the height up to where LTE holds, reemitted photons start to have a significant probability to escape to space.

    The effective radiating height (or altitude) would be the mean height from which photons escape when emitted. This would be a function of where they are emitted, the path of transmission (including the constitution of the atmosphere along that path), the spectral absorptivity of the medium through which they travel, but it would be a function of the energy of the photons themselves — as you would need to weight the wavelength not simply by the number of photons but by the energy of the photons at that wavelength — as you are trying to determine the average height from which thermal energy (in the form of quantized thermal radiation) escapes.

    Needless to say, that is all rather complicated. And actually that isn’t the way that it is normally done.


    The average temperature of the earth is about 14 C. However, given the luminosity of the sun and the earth’s albedo, it is possible to calculate another average temperature — something called the “effective temperature,” or alternatively the “effective radiating temperature.” This is the temperature that the earth would attain — if one were to calculate it simply on the basis of solar irradiance, the earth’s distance from the sun, the earth’s albedo (whatever light is reflected will not contribute to its thermal radiation field) and shape (or cross-section). Or alternatively, the effective temperature is the brightness temperature of the earth, measuring only the thermal radiation emitted by the earth — averaged over the entire spectra. (Brightness temperature itself is normally applied to individual frequencies or wavelengths, but in the case of a black body would remain constant over the entire spectra.) This temperature is approximately -18 C, approximately 33 C below the earth’s actual temperature — with 33 C being essentially a measure of the strength of the earth’s greenhouse effect.

    Now under an (admittedly unrealistic) assumption of equal temperature throughout each equidistant layer of atmosphere one other feature of the atmosphere — a linear falling-off of temperature in the troposphere, one can calculate an effective radiating height.

    Within the troposphere, we have a roughly constant lapse rate where the temperature decreases at a constant rate as a function of altitude, which averaged over the entire globe is roughly 6.5 C / km. Given this, we calculate the effective radiating height (or altitude) as approximately 5.5 km, which is the average height at which the real temperature of the atmosphere is equal to that which it would be in the absence of any greenhouse effect, i.e., the effective radiating temperature. And as adding greenhouse gases to the atmosphere increases the optical thickness or depth of the medium, it raises the height from which photons escape, and assuming a roughly constant lapse rate this will imply a warmer surface. For more on that, please see Tamino’s essay Lapse Rate.


    Guenter Hess wrote in 346:

    2. On a macroscopic level:
    Averaged across the Sphere of the escape height Kirchhoff’s law for extended media holds

    I wouldn’t put it that way. Kirchoff’s law applies locally under local conditions. What are the atmospheric constituents? What are their absorptivities and emissivities, etc.? It isn’t dependent upon the structure of the atmosphere except insofar as LTE conditions obtain locally. All I wished to underscore with the mention of scale is that all of the thermal energy which leaves the earth’s climate system leaves by means of emission (unless you wish to include the minute leakage of hydrogen or the rare bolloid collisions), and the good majority of the energy which the earth radiates comes from photons that were emitted within the LTE region and escaped the atmosphere without absorption.

    Emission is important at that scale for that reason. It makes it possible for the energy entering the system to be balanced by the energy leaving the climate system — once thermal (quasi-)equilibrium is established. But that balance between incoming and outgoing radiation is not a law, but the achievement of an equilibrium due to the temperature of the earth rising or falling until incoming and outgoing radiation balance one-another as a matter of the conservation of energy.

    Anyway, I have to get up in four hours — so I hope you don’t mind if I call it a night.

  2. 352
    Gavin Donehue says:

    I was referred to this calculation website ( , but as I can’t see the programming behind it I am a little dubious. Has anyone seen it before and is it reasonably accurate (I was told it was a USAF program)

  3. 353
    Timothy Chase says:

    Gavin Donehue wrote in 352:

    I was referred to this calculation website ( , but as I can’t see the programming behind it I am a little dubious. Has anyone seen it before and is it reasonably accurate (I was told it was a USAF program)

    It isn’t HiTran (note the prefix of “Mod”), but I believe it is fairly accurate — and certainly better than the LoTran — given its moderate resolution. Moreover, I believe it is along the same lines as what is described here:

    Kirtland Airforce Base: ModTran 4 Software

    In fact, I suspect it is the same program — and you are simply looking at a web interface. However, I believe David Archer (one of the official contributors to this blog) may be in a better position to say.

  4. 354
    Hank Roberts says:

    CO2 — reading the footnotes, I wonder.

    Footnote 8 of the article says:
    “Office buildings exist which are described
    as ‘sick’, in which workers display symptoms of carbon
    dioxide poisoning8.”

    But reading foonote 8, that’s not what it actually says.
    More here:

  5. 355

    Richard Wakefield posts:

    Even if the sun were not chiefly to blame for the past half-century’s warming,

    Solar output has been essentially flat for the last 50 years, so it can’t have caused the sharp upturn in global warming of the last 30 years:

    the IPCC has not demonstrated that, since CO2 occupies only one-ten-thousandth part more of the atmosphere that it did in 1750, it has contributed more than a small fraction of the warming.

    This is a complete non sequitur. The fact that carbon dioxide is a minor component of the atmosphere does not in any way, shape or form mean that it is a minor component of the Earth’s radiative balance. This is just stupid.

    Carbon dioxide is 386 parts per million by volume (ppmv). 0.1 ppmv of fluorine in your air will kill you. Small quantities are not necessarily insignificant quantities.

    Even if carbon dioxide were chiefly responsible for the warming that ceased in 1998

    It didn’t:

    and may not resume until 2015, the distinctive, projected fingerprint of anthropogenic “greenhouse-gas” warming is entirely absent from the observed record.

    The stratosphere is cooling as the troposphere warms, showing that the warming is primarily caused by carbon dioxide. Let me know if you want me to explain why.

    The rest of the denialism snipped for brevity.

  6. 356

    Sorry about that last post — I got my dates mixed up and thought RW’s post was recent. So I’ve answered it twice. My bad.

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