Can asphalt stop dust pollution: the counter-intuitive results of asphalting to control dust
It’s an intuitive, appealing idea for anyone who has ever driven down a dry, unpaved road. Checking the rear-view mirror, you can clearly see the plumes of dust in your wake. It’s not the same on the motorway or in the city. The question naturally comes: why here, and not there? The difference between the two situations quickly becomes clear: here, the road is unpaved open-earth, carved out of the natural environment and compacted by the sporadic traffic. If this road were to be asphalted, the source of the dust would be eliminated. The natural conclusion is that asphalt can stop dust pollution, then.
So goes the natural line of thinking, which is often backed up by environmental bodies; the EPA once recommended asphalting as a dust control method for roads with a long-term use (EPA, 1992).
The line of thought, however, is mistaken. The asphalting of open-earth roads is almost always a bad decision, especially if one’s aim is to reduce dust pollution levels. This is so for a variety of reasons. In the following, we expound upon five of these reasons, which taken together form an argument against the costly investment of asphalting in order to control dust.
1. Moisture Content
The first thing to note is the key factor of the moisture content of a road in relation to its dust-creating potential. In almost every study concerning roadside dust pollution, the moisture content of the respective roads is an important detail to be factored in when calculating the emission rate of a given road with a given amount of traffic (AIRUSE, 2016).
The asphalting of a road necessarily reduces the moisture content of the road. A natural, open-earth road has natural moisture-retaining capillaries stretching down into the soil. An asphalted road has no such capillaries, as it is a smooth, even and dense surface. An asphalted road has no natural dust suppressing capabilities. This means that often an asphalted road becomes dusty very quickly, even after it’s been wettened by rain or artificial means. An open-earth road, in contrast, can stay damp and dust free for several hours, if not the whole day.
2. Mortar and Pestle
Now to the second reason. This is what we call the mortar and pestle principle. A vehicle’s tyres are made of rubber, a relatively soft material. As a vehicle’s tyres turn in transit, they grind against the road’s surface. Even on softer open-earth roads, this motion leads to particulate matter formation. On paved, asphalted roads, the effect is exacerbated tenfold.
The asphalt, a hard, rigid material with little to no give, acts as the perfect mortar to the vehicle’s pestle, grinding up the rubber tyres into smaller and smaller particulate matter. This means that asphalted roads actually increase the overall levels of dust present in the local atmosphere, rather than reducing them.
Thirdly, the asphalt is an alien material in the environment. Whereas an open-earth road has the potential to incorporate loose material back into its structure, through agglomeration and compaction, the asphalt cannot absorb any loose particles into itself. Instead, the loose dust particles form a thin layer which rests on the road’s surface. These are then liable to be shot up into the air with even the gentlest passage of a vehicle.
In an attempt to fix this problem, it is sometimes advised to employ sweeping machines, which go along each asphalted road sweeping the layer of dust away, so that it can’t be jettisoned into the air by normal on-site traffic. Two wrongs don’t make a right, however. This not only increases mechanical and labour costs, but also increases the volume of traffic. The sweeping machine itself will grind its tyres against the asphalt and emit particulate matter from its exhaust. It sweeps the dust ahead while spewing out more from behind.
Asphalting leads to an exacerbation of wind-related dust pollution. The wind on a normal open-earth road can play havoc with any dust present, as it churns any particles present up into the air. This effect is only greater when a road is asphalted. Instead of a craggy, uneven surface against which the wind must spend its force, the asphalt presents a smooth, uniform flat, creating a miniature-wind tunnel. The wind can travel at greater speeds and transport dust particles with greater ease when a road is asphalted.
5. Human behaviour
Finally, the asphalt leads to an almost involuntary change in human behaviour which runs in the face of dust control measures. An asphalted road is easier to drive on, and allows greater speeds. The higher the speed a vehicle travels, the greater the suction force it leaves in its wake. This sucking action again raises up any dormant dust particles and suspends them hanging in the air. As the asphalt promotes greater speeds of travel, it thus also promotes a greater amount of airborne dust particles.
Can asphalt stop dust pollution: the upshot
All in all, asphalting that dusty road is a counterproductive dust control measure. The intuitive line of thought is misguided; we’ve seen the reasons above. Taken together, these reasons coalesce into a simple fact: asphalting roads is not effective at reducing dust pollution. If the only aim of asphalting the road was to reduce dust pollution levels, but it in fact achieves the opposite, the rationale behind asphalting becomes non-existent.
In case you are still unsure, however, there is a further, perhaps even more persuasive argument. Asphalting costs. In terms of labour, in terms of time, and in terms of material. So not only will asphalting not solve your dust problems, it will also hit you in the pocket. We at LAMI® advise you to consider at length all the possible options, before one opts for asphalt.
EPA, 1992. Fugitive Dust Background Document And Technical Information Document For Best Available Control Measures. Available from: https://www.nrc.gov/docs/ML1224/ML12241A395.pdf
Querol, X and Amato, F (eds)., 2015. AIRUSE Guidebook. Available from: http://www.cleanaircities.net/get-guidebook/