How pollution and radiation spread in the atmosphere

[Roger J Smith]

A guide to how pollution and radiation spread in the atmosphere

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About thirty years ago, I was involved in a project designed to control harmful emissions from industry near Canadian towns and cities, and through this, I learned quite a lot about the way pollution and by extension radiation spread in the lower atmosphere. This guide will give you some basic insights on how this works, what applications can be made to air quality, and potentially, things you might find helpful if you ever have to make quick decisions about escaping from toxic pollution or radiation from terrorist or military attacks.

The first principle of pollution dispersal is atmospheric stability.

The second principle is height of discharge.

The third principle is anticipated linear dispersal rates.

These are all fairly easy concepts to visualize if you have studied meteorology even at the casual, layman's level.

Pollution is defined as any gases or particulates emitted into the air that are not naturally occurring, or at least likely to alter the natural balance of existing elements. Unpolluted air contains roughly 78% nitrogen, 21% oxygen, almost 1% of the inert gas argon, and small amounts of water vapour, carbon dioxide and various other background gases. There is some argument about whether to consider increased carbon dioxide a pollutant or just a free change in the composition of air. But carbon dioxide is also heavier than other air molecules, therefore it tends to sink relative to other gases. The same argument can apply to excess water vapour although the harm done by excess water vapour is in itself a controversial topic, in many climates it may have a beneficial result.

Pollutants can be harmful or even fatal in certain quantities. Sulphur dioxide is perhaps the most toxic pollutant by total volume emitted. Large concentrations of sulphur dioxide can burn away vegetation, oxidize rocks and change the soil chemistry, and have a very harmful effect on human health. Other noxious pollutants include methane, nitrous oxides, metal particulates, and the toxic carbon monoxide.

Industries in most developed countries must develop a regulated emission program of these pollutants. This applies to the overall basic discharge, the height of the stack for dispersal, and in some cases, specific prohibitions to prevent pollution reaching nearby populated or ecologically sensitive areas. In plain language, some industrial plants can be ordered to cut back or shut down in defined atmospheric conditions.

The height of a stack determines to some extent the distance at which the pollution will fall out of the atmosphere to the ground. A mega-stack like the 300 metre stack at INCO Falconbridge in Sudbury Ontario can create wide dispersal that predictably falls to ground 100-300 kilometres from source. The further from source, the more laterally dispersed the "plume" will become. By random variations in wind direction in a given parcel of air, the plume spreads out laterally about 5 to 10 per cent of the distance it travels. The result of building a higher stack at Sudbury was that acid rain began to fall on lakes several hundred kilometres downstream, while the city of Sudbury saw a vast increase in its air quality. This problem has been partly resolved by better smelting practices.

The behaviour of the plume downwind from the source depends largely on atmospheric stability. On a highly unstable day, the plume usually oscillates up and down violently (like an airplane in turbulence) and quickly hits the ground in waves until it is dispersed over a large oval-shaped area downwind, but not that far from the stack. For high stacks this may be 30-60 kms away, but for lower stacks it is likely to be only a few kilometres at most. Thus a low-stack, high-volume emission source must be closely regulated, or it will frequently dump a toxic load on a nearby urban area or farmland, every time the wind direction is right and the air is unstable.

In very stable conditions, the plume is likely to drift a long way trapped in that elevation, so it will only impact the ground if the ground rises, or if the inversion breaks up. However, another factor to be considered is wind speed. At night, in stable conditions and very light winds, the plume is likely to spread around radially and cover a large area. Then when the morning inversion breaks up, the pollution will settle all over the affected area in fairly equal proportions out to a certain drift distance. Individual cases will depend on the wind history overnight. A computer program can usually be developed to predict where a pollution cloud will fall at the time of inversion breakdown, which is usually about 0830 to 1030 hours.

There are of course intermediate cases of moderate instability and most meteorologists work from a table of about six stability types which usually go hand in hand with weather types. A vigorous warm front in winter will generate a moderately stable profile that veers to moderately unstable. Any given plume follows the dynamics of this change, so let's say you're tracking the pollution from some large factory in southwest Birmingham for a day like that, you could expect the plume to impact areas well to the north or northwest at first during the day, then closer to the source and towards the northeast later in the day.

The statistical probability of various locations receiving pollution has determined a great deal of the economic geography of large cities. It is not a fluke that in most cities, the wealthier residential areas lie to the west of the city and the heavier industries often lie to the east, as well as the run-down lower rent areas. Especially in the century gone by, those areas stood to receive the brunt of pollution and the disease, smell and other unpleasant side effects. This is not so much of an issue in the modern industrial environment (with most factories now being in China, ha ha) but it still remains a foundation of the culture of many cities. The only places it would not apply would be seaside resorts with few industries, for example, Brighton and Hove, but from what I remember of that city, the cultural pattern implanted itself there as well. Perhaps it transfers to the psyche of a culture and even small towns sometimes show the same pattern. In a different climatic zone, however, it should be reversed -- cities in Florida or northern Australia should have their high rent districts in the east end.

Now, this leads to the more practical question, where is it safest or cleanest to live now? This is not nearly such a difficult set of choices as it once was, air quality tends to be closely controlled and without the public being aware of it, most potential sources of pollution are on tight programs that prevent local dumping of excess pollution. However, you are in general wiser to live to the northwest of a given city's industrial concentrations than any other direction.

How about radiation then? There has been much talk in recent years about "dirty bombs" that would amount to nuclear weapons going off at ground level. The military type of nuclear weapons are designed to explode high above the ground, but terrorist strikes would probably occur in warehouses or ships in harbours, or just from briefcases left abandoned. Assuming they were at ground level, the radiation from a small nuclear explosion would act very much like a low-level stack except that a firestorm would be created which would add convective instability. The pollution cloud would at first spread almost in a concentric ring, but would then begin to advect downwind with the complexity of the localized convection. If wind conditions at the time were moderate SW as is most often the case, then the radiation would probably spread more rapidly to the NE than in any other direction. Lethal amounts would probably spread 8-12 kms in all directions from point source, but then only very slowly beyond that except to the northeast, where they might spread as far as 30-40 kms. Thus, if you heard that an explosion had occurred or was imminent and you believed yourself to be in this risk cone, your best course of action would be to drive (or travel by whatever means) at right angles away from the cone's long axis, preferably in the direction of less anticipated traffic congestion and of course towards any known places of refuge.

This could be a practical life-saving consideration for hundreds of thousands, because not everyone exposed to lethal doses of radiation would be exposed in the first half hour. Radiation would spread faster than pollution normally does at first, but as the temperature of the altered air parcel cooled, it would spread slower than before until it reached the normal dispersal speeds. The experience with Chernobyl, hardly on the scale of a nuclear weapon, was that lethal doses of radiation eventually spread out 150-300 kms from source but took a day to 36 hours to do this initially, with subsequent reinforcement as the problems were not immediately contained and some of the radiation eddied behind an initial wave.

There are some atmospheric conditions in which toxic if not lethal levels of radiation could spread the length of a European country before dispersing, although rather slowly, so that even people 200-400 kms downwind of a moderate-sized nuclear blast should consider moving out of the dispersing plume. One should consider the future course of the weather in this calculation. For example, let's say you are in Newcastle and London has been subjected to a terrorist bombing. And let's say a southerly wind flow concerns you that in two days time your air may become toxic. Then to the extent that you are mobile, you would be wise to head southwest to Wales if the second to fourth day of the model run showed a slow trend from S to E winds. At some future point, radiation levels would then probably be higher in Glasgow than in Cardiff or even Birmingham for that matter. Of course other considerations might impact your decision.

Larger nuclear attacks on multiple targets can be survived with intelligent foresight. I mention this because the world we live in has an inherent slight risk of nuclear war, and rather than starting to think about this after the sirens go off, you would be wise to have a plan in your mind. You could draw up your own presumed list of targets, large industrial centres and power stations, military bases etc. It is unlikely that Russia has targetted Stonehenge or the white cliffs of Dover. If this were my problem, I would be making plans to escape to the furthest reachable point on the northwest mainland of Britain. This is the location least likely to receive radiation from the downwind fallout, so the closer you can get to Cape Wrath, the safer you'll be. In my own case, I am not sure why Russia would target large Canadian cities as this is where most left-wingers and Russian emigres live, but supposing that a few of our cities are targetted, I would be heading for a point in central British Columbia, the radiation from targets in the United States would be more of a factor in the overall dispersion, and so I would be choosing a town well to the north of the border.

There would be no guaranteed options in this scenario, but you might want to think about it this weekend if you have some time, because you never know when your planning might come in handy.

If you're in Russia or central Europe, I would just say go to the nearest target and get it over with really fast. There won't be anywhere to hide.