UV blocking by aerosols
Adding a bit of meat to the bone ...
UV radiation. It used to be all about ozone, but increasingly it’s about clouds and aerosols. Clouds are already the main drivers of day-to-day differences in UV. And aerosols are already responsible for large regional differences. Changes in both will be the main drivers of future changes in UV. I talked about clouds last time. It’s time now for aerosols.
I’ve noticed over the years that word aerosol immediately raises alarm bells with the public who’ve heard that aerosols from spray-cans cause ozone depletion. But that’s a very specific type of aerosol. In older spray-cans, the propellant gases were CFCs that were later found to destroy ozone. They were widely used under pressure to create the ‘aerosol’ sprays because in the lower atmosphere they are inert, so they don’t react with the active ingredients in the can. But the term ‘aerosol’ has a much broader meaning.
To me, an aerosol is any suspension of clusters of molecules in the atmosphere. They may be solid particles like soot or dust, or liquids, or a combination of both. Although each of these clusters may contain thousands or even millions of individual molecules they are still tiny. Usually much less than the thickness of a human hair. Clouds (including fog) are examples of aerosols, with larger suspensions of ice crystals, or droplets of water. These are normally categorised specifically, which is why I discussed them separately.
When I think of aerosols, I think of those layers of pollution you often see when looking down to the Earth’s surface from above, or the whitening of the normally blue sky as viewed from below. The pollutants can be natural, like desert dust or sea-sprays, or man-made emissions including soot.
Maps of aerosol properties are derived from satellite instruments. These instruments usually have long multi-word names, which are often referred to by their acronyms, such as MODIS in the example below. Such data products are now routinely available daily, at least for cloud-free areas.
The contour map shows the annual mean blocking by aerosols at visible wavelengths - expressed as the ‘Aerosol Optical Depth’ (labelled AOD), running from the most pristine conditions (blue) through to the most heavily polluted (red). In a completely aerosol-free atmosphere, the AOD would be zero, and the radiation would be unimpeded. But that never happens. An AOD of 0.1 means that about 10 percent of the incoming light is scattered out of the direct beam and 90 percent is unimpeded. The bigger the AOD the less light is transmitted (though it’s a non-linear scale). For example, an AOD of 1 means that about 37 percent is scattered out of the beam and only 63 percent is unimpeded. An AOD of 2 means that about 90 percent is scattered and 10 percent unimpeded; an AOD of 4 means about 99 percent is scattered and only 1 percent is unimpeded, and so on.
The aerosol optical depth in New Zealand is among the lowest in the world, less than 0.1. This contrasts with the UK where its closer to 0.3, and China where it sometimes exceeds 0.8. In parts of China, you sometimes have difficulty seeing across city streets!
Ground-based measurements in rural New Zealand show much lower values. At Lauder, where I still occasionally ‘go to work’, the mean is closer to 0.02 and it’s sometimes less than 0.01. That’s well below the detection threshold of these satellite instruments. The pristine air there is about 50 times cleaner than the air over parts of China. It’s therefore not surprising that Lauder’s UV levels are much higher.
Because of the complications of ozone absorption and scattering from clean air, it’s difficult for satellite instruments to measure the optical depth at the UV wavelengths we’re interested in, especially for such pristine locations. Two other parameters are also needed:
the ‘Ångström parameter’ (named after a Swedish physicist), which tells us the strength of any wavelength dependence. For example, a value of 1 means that if you halve the wavelength, the scattering doubles. Bigger numbers mean increased effects at UV wavelengths. Typical values are around 1 to 1.4.
the so-called ‘Single Scattering Albedo (SSA)’ , which is just a fancy term to describe the aerosol’s efficiency for scattering radiation without absorbing any of it. Although important, it’s hard to measure, and isn’t generally available daily. The SSA ranges from 0 to 1. For most natural aerosols, it’s close to 1, meaning that they merely scatter radiation without absorbing any of it. At the other extreme is soot, with an SSA close to zero, meaning that hardly any radiation is scattered and just about all is absorbed. These man-made sooty aerosols can lead to big reductions in UV.
Note added 22 Oct. 2024. My colleague Sasha Madronich pointed out that in practise, the SSA of aerosols, even in polluted locations, is rarely less than about 0.5 (but that would still result in big UV reductions). Thanks Sasha.
Both lead to higher extinctions by pollutant aerosols that contain soot and other products of combustion, leading to greater aerosol extinctions in the northern hemisphere which is the home to about 90 percent of the world’s population. As I noted in ‘Saving our Skins’, the peak UVI in rural New Zealand (Lauder) can exceed that at similar latitudes in the USA by 40 percent. At least half of that discrepancy is due to the smaller aerosol extinctions in New Zealand.
Future changes in aerosols represent one of the biggest unknowns about UV levels in the decades ahead. If we successfully move away from fossil-fuel based energy, aerosol extinctions will decrease (and UV will probably increase). If we continue our present path, extinctions by aerosols will increase (and UV will probably decrease).
Unfortunately, satellite measurements still can’t capture the effects of these aerosols, so tend to overestimate UV in polluted places. That probably includes most heavily populated areas and most of the northern hemisphere.
So, we’re left with ground-based measurements. I hope we’ll still have the measurement capability by then to capture these changes. Sadly, the prospects for that don’t look good. More on that later.
You might also be interested in another related chapter from “Saving our Skins” on this important topic.
Sorry if that was all a bit heavy-going. If not, data like these are used in the current version of the GlobalUV app to take account of geographical differences in aerosols. For more, please read my ‘detailed explanation …’. On to lighter things next time ….
