Why damp roads produce no airborne dust

Working out how to stop road dust might seem a hard nut to crack. But the answer was always right before our eyes. Recall those dusty days of yore, take a deep breath, and imagine...

Summer

It’s summer, hot and dry. A quarry truck turns in from the main road onto the quarry’s dirt track access road. In its wake it churns up dust into the air. It becomes a rumbling blur and flickers in the murky foreground.

truck driving on dry road leading to airborne dust
A truck in summer leaves behind a dusty wake.

Winter

It’s winter, cold and wet. A quarry truck turns in from the main road onto the quarry’s dirt track access road. You watch it as it carefully descends the winding path into the quarry with every deviation and jump of its direction discernible.

wintry icy surface with snow in the air
A truck in winter tells a different story.

The differences between

There are a lot of differences between these two scenes. One is in winter, the other summer.  In one, you are warm, potentially overheated, maybe even a little dehydrated. In the other, cold, a little underheated, and your water is colder than you would like. You see different things, too: in one the truck fades out of view almost as soon as it turns onto the dirt path; in the other, it remains in sight until the edge of your gaze.

But a lot of things are the same, as well. Same place, same truck, same activity. But somehow these samenesses create one difference that we want to particularly focus on: in one, the truck churns up dust. In the other, it doesn’t. Why?

You might guess the answer already. If so, you’re on your way to knowing the best way to stop road-dust. But in this article we will carry out a thorough investigation. And even if you’ve guessed the cause already, you might be surprised by the proper physical explanation of its effect.

The investigation take us first into a look at how road-dust is created and sent into the air. Then we can move on to a guess at the effector of the wintry road’s lack of dust, and we can see if your guess was right.

damp gravel road in mountain valley
It doesn’t matter if a road’s surface is the open earth, gravel or asphalt, if its used by wheeled vehicles, there will be airborne dust.

How road dust is created

Well, let’s take the summery scenario, then. The cause of the airborne-dust seems to be quite obvious. It’s the truck. Before it turns into the dirt path, there are dormant dust particles lying on the road’s surface. These can range from the large, coarse particles, to the superfine. These particles, even the biggest, are extremely light, and float in the air readily – all that is needed is a force greater than that of its weight pulling it down.

So as the heavy, forceful truck turns, these particles naturally fly into the air. The truck’s passage exerts a far greater force than the dust particles’ weight. The force it exerts is twofold in nature: there is the concussive force of the truck’s wheels which in their passage exert a downward force upon the road’s surface. This sends vibrations through the road’s surface of sufficient strength to lift the particles into the air. They also grind against the uppermost layer of the road’s surface, pulverising both it and itself into fine dust.

There is secondly the force of the truck’s passage through the air. As it travels through the atmosphere, it sends out a lateral force in front of it and leaves an equivalent ‘sucking’ force behind it. For the former, the truck as a whole pushes the air in front of it out of the way. This displaces the air molecules and sends them whirling about with enough force to conjure up the hitherto dormant dust particles up into the air. Likewise, as the truck carves a path through the air, it leaves a vacuum behind it which the air rushes back in to fill. This rushing back in also serves to stir up and spread dust particles into the air.

vw truck exerts force on road
Even just the passage of a truck along a road sets into motion an endless chain of causal reactions in its environment.

Why not in winter?

So, we have isolated the cause of the dust’s flight: the truck and the various forces it exerts upon its environment while travelling. But, to return to our original question, just why does this not happen on the wintry day? The truck still exerts the same forces – so it must be something to do with the dormant dust particles on the road’s surface. Let’s turn now to an examination of just why that dormant dust is so liable to become airborne. With this in hand we can hazard a guess as to what element of the wintry day restricts this quality of the dust, such that it doesn’t become so easily airborne, despite its tendency to do so.

a damp dirt path with no airborne dust
A damp road produces no airborne dust.

Here’s a clue: we’ve already mentioned it. It is the dust’s lightness, a term we use in English interchangeably for an object’s weight or for its density. Here it is the dust’s lightness in both senses of the word that is the culprit. As the dust’s weight is low, it does not take up much force to fly into the air. And as the dust’s density is low, it can stay floating in the air for a long time (along the same principles by which a helium balloon stays aloft).

But just what is it about the wintry day that could stop the dust being light? Turn your imagination back and picture again our truck’s progress along the road in winter. There must be something added to the dust so that it is no longer light. But what? I can see knowing nods and raised hands: yes, indeed, it was the wetness of the wintry day. Water is what had been added to the dust until it was no longer light, but waterlogged.

Why damp roads produce no airborne dust

It had rained recently in our wintry example. The water fell to the road’s surface and was absorbed by the disparate fine dust particles, forming agglomerates of multiple particles, and seeped into the road’s surface and upper layers as well. As the dust particles were joined up together into agglomerates, the sum of their weights was the force the truck had to contend with. Either the weight of the soggy dust particles won out, or, even if they were sent briefly up into the air, they were so soaked in water that their density was high enough that they sank quickly back down to the ground. From our investigation we can formulate the following fact:

Damp roads produce no airborne dust.

In terms of our day-to-day observations, this fact captures our observations, as exemplified by our wintry example. In terms of the controlling of dust, this fact captures a central tenet of good practice for roadside dust suppression. The simple addition of water transforms a dangerously dusty road into a harmless transit route. Of course, too much water and the road can become a soggy bog. But we have nonetheless unearthed the core controlling factor for roadside dust: its moisture content. Controlling of this factor should be the cornerstone of any dust control plan.

Conclusion

So there you have it. What started from a comparison of the differences between what we see on a wet, wintry day and a dry summery day has in the end yielded an important fact for anyone interested in the controlling of airborne dust. Nature, and natural rain, provides the example.

Our roadside dust control techniques were inspired by this example, – all we had to do was to prevent its negative excesses. But this would be to start another story. For a more in-depth laying out of the principles underlying the proper controlling of road-dust, contact us for our white paper on the subject. For more on dust and its peculiarities, head back to Information.


References

Lundberg, J., (2018). Non-Exhaust PM10 and Road Dust. PhD thesis, KTH Royal Institute of Technology, Stockholm. [Viewed online]. Available from: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-222155

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