THE TRANSPORT OF ACID POLLUTION

Most states east of the Mississippi River emit as much sulfur as they receive in acid rain (Table 2). In principle, these states could cause their own acid rain. However, closer inspection of Table 2 shows that for some states, such as Vermont and Arkansas, the amount of sulfur deposited by acid rain far exceeds the sulfur emitted within the state. Acid rain in these states clearly cannot come from local sources but instead must be imported from polluters upwind. Furthermore, acid rain falls far out over the Atlantic — where there are no significant marine sources of sulfur — and gradually tapers off hundreds of miles at sea. Clearly, this pollution is transported from the East Coast. On Bermuda, six hundred miles offshore, storms coming from the East Coast bring highly acid rain, while storms coming from the open Atlantic to the east do not.  

Satellite photos reveal that acid haze can travel a thousand miles or more/ It has been argued that satellite photos do not rule out the possibility that a blob of haze may be continually renewed as it moves across the country, depositing and picking up pollution as it travels. However, acid sulfate haze is known from field and laboratory studies to be able to remain aloft for days, ample time to be transported hundreds of miles, and the haze remains intact even when it travels over areas with low emissions (see Plate n). The photos are strong direct evidence that air pollutants can be transported great distances. 

The chemical state of the pollutants also affects the distance they are transported Acid sulfate is transported further than SO2 because it is less strongly adsorbed by the ground thus atmos- 

Three techniques have been used to determine how much sulfur pollution is transported from one region to another computer simulation, back-trajectory analysis, and trace-element analysis. Each technique has confirmed the importance of longdistance transport. 

Computer simulation The distance that air pollutants are carried downwind can be estimated on a computer using data for the location of the emitters, the amount of pollution emitted and the direction and speed of the wind. (More sophisticated computer programs also include the height of the smokestack, since wind speed varies with altitude.) The calculations also require estimates for how fast the pollution returns to earth. This will depend on how strongly SO2 and acid sulfate interact with the ground, and will involve estimates for the relative abundance of SO2 and sulfate in the air, which in turn depends on variables such as the amount of sunlight, the presence of other pollutants, the humidity, etc. (see Chapter 3). As a result of these uncertainties, computer simulations only provide estimates, not exact results. Nevertheless, the results from several computer simulations have agreed well with the experimental observations from back-trajectory analysis and other techniques (discussed below). 

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