Imagine a city in summer just before a thunderstorm. The air is thick, heavy, hot and humid. As the traffic builds up and the different smells wander, mix and linger, everything is warm and dense. Each breath weighs in the chest. But on the horizon approach large dark-grey clouds of ominous mass. The city exhales with relief as the sky begins to gloom and the wind picks up. At the first clap of thunder the rain begins to pour down into every crevice and trickle over every edge.
With the rain abating the adjectives of the city invert: what was hot is now cool, humid, relaxed and thick, light. A deep breath invigorates. The air feels cleaner. We put two and two together and sigh: ah, the rain has cleaned the air. But has it really? Do we really breathe in cleaner air after a rainstorm, or is it just wishful thinking?
An Intuitive Understanding
Let’s try and imagine how it would have to happen, if it indeed were to. Before the thunderstorm, the air was full of pollutants in various states: carbon monoxide, carbon dioxide and nitrogen dioxide are all by-products of the combustion of fossil fuels such as oil or natural gas, a process central to the workings of our factories, power stations and cars. Alongside these gases tiny particles of solid matter are also produced, of various elemental compositions generalised under the term particulate matter (PM), or fine dust. These all are present in the air prior to the thunderstorm. In this toy example, we’ll concentrate on how the rain could clean the air of its solid particles, but the same should in theory apply to the gaseous pollutants too.
As the storm breaks, rain starts to fall from the clouds in the form of water droplets. These water droplets vary in size, ranging on average in diameter from 0.5mm to 4mm, before they become too big and break up to form separate droplets. As these droplets descend through the air, they must cut a path through the air. Doing this, some will collide with the pollutants in the air. The falling weight of the droplet and its ability to absorb and coagulate with other materials will drag down the pollutant particle out of the air and onto the ground, where it remains waterlogged and immobile. The first intake of breath after rain really should be cleaner, then. The rain has taken the dirt out of the air down into the gutter.
This all seems rather intuitive, and would explain well why the air feels so clean after it has rained. But whether it actually does is a matter of empirical fact, and cannot be confirmed or denied from our armchairs. For proper confirmation of our intuitive guess we need to turn to the science of the matter.
A Scientific Understanding
It is first gratifying to note that our intuitive understanding of how rain could clean the air of pollutants was indeed correct – the cleaning of the air by rain is termed within the scientific community “scavenging by precipitation”, and the process of collision and sticking together called coagulation, or collection (2015, Dryer, p.9159). The likelihood of a raindrop coagulating with an airborne pollutant (technically known as an “aerosol”) is given by its collection efficiency. If a rain shower had a collection efficiency of 90% relative to the section of air it falls through, then it would remove 90% of the pollutant particles therein found.
But it turns out, however, that this value in practice must be rather low. For a study undertaken in Bogotá, Colombia in 2015 showed that PM2.5 values taken on rainy days which followed dry days were only 11% lower (Blanco-Becerra et al., 2015). The decrease in concentration of PM10 values was slightly better, coming in at 17%. But these numbers fail to match up with what we intuitively thought: that the air felt cleaner after rain.
There are three possible explanations for this: either (a) our senses are fine-grained enough to detect changes in air quality, even though the percentage change was barely over 10%; or (b) our senses are not sensitive to air quality at all, and are being misled into thinking the air is cleaner by some other effect; or finally (c), the air is cleaner, and we can sense it, but the air wasn’t cleaned by the rain.
The correct explanation is likely to be some kind of mix of the three – to generalise over events of great physical complexity inevitably tends towards a skewed, incomplete picture. The best we can hope for is to at least identify and more or less accurately describe the major causal actors involved.
It turns out, however, that we left out a few of these in our intuitive understanding. This could potentially explain the mismatch of our intuition and physical reality. To find out whether this was the case, and to decide which combination of (a), (b) and (c) seems the most likely, we’ll take a look at some of the factors most often considered in the scientific literature: the electric charge of the rain droplets, the rain droplet’s size in comparison with the particulate matter’s, and the prevailing meteorological conditions.
Does rain clean the air? A few forgotten factors
A major factor we omitted in our intuitive description was the electrical charge of the rain droplets. We might well have cited this: surely the crashing of thunder and lightning in electrical storms imparts some electrical force to their surroundings, cauterising and cleaning the air. And it is indeed true that, even in your everyday rain- shower, rain droplets have a small electrical charge, some positive charge, some negative. The pollutants in the air are likewise electrically charged, some positively and some negatively.
Now, you might recall from your school physics classes that opposites attract, and likes repel. This is true of magnets, humans (potentially – studies forthcoming), and, indeed, minute airborne dust and water particles too. Given the right conditions, the falling of a positively charged water particle alongside a negatively charged fine dust particle will result in the collision and collection of the two. This is a process technically known as electro-scavenging.
The airborne interaction of charged particles during rainfall is a process made up of countless individual physical agents, each with their own particular physical properties and relations to their surroundings. A particle by particle understanding is therefore not possible: the process must be understood statistically in terms of probabilities. Take a recent study from the Massachusetts Institute of Technology. They found (see Fig. 7 below), in line with other previous studies, that the higher the electric charge of the water droplet, the higher the likelihood that it would collide and collect with an aerosol (a process technically known as electroscavenging). The higher the water’s charge, the greater pull it exerts upon oppositely charged particles, so the greater the probability that collision will occur.
But while this seems nice and easy to understand, the actuality of the matter is of course much more complex. The MIT scientists also found in the same study that this ability to attract oppositely charged particles fades as the droplet’s radius increase. As the particle’s size increases, other methods of collection, such as simple inertial impaction or interception, begin to become more likely. As their likelihood increases, so does the electric charge’s relevance – the dust particle can only be collected once, after all.
This is relevant for our consideration of the means by which rain could clean the air: the size of the water droplets used in the study, with a diameter of 0,0432 millimetres, were much smaller than our average rain drop, which we estimated at its smallest to be 0.5 millimetres. So while it seems likely that the difference of charge between the water droplets falling in a thunderstorm, say, and a light grey drizzle, should be great, this still should not explain the clear perception of cleaner air – the levels of the droplets’ electric charge should not have a large effect upon how much dust they clean out of the air.
Indeed, what explains well the discrepancy between our intuitions and the scientific reality is the size of the water droplet relative to the dust particle. The same study found that the smaller the droplet, the higher its collection efficiency (p. 9169). This chimes in with what has become the mantra of a plethora of modern dust suppression companies: the so-called “slip-stream effect”. If the water droplet is larger than the dust particle, then it will be more likely to divert the dust particle around it due to the airwaves it pushes out, than for the two to collide directly. Many claims have been made with regards to this effect, and its enaction in practice. A lot less has been said of its activation conditions, and the difficulty of reaching them – consider that the size of a PM2.5 particle is at least 36 times less than a single grain of sand, then how quickly a water droplet of this size would evaporate – the problem becomes clear (Contact us for our white paper on the matter, and our proposed solution).
But in any case, this effect also helps to explain why rain seems to reduce PM10 values a lot more than it does PM2.5 values: the rain is a lot larger than a PM2.5 particle, but is closer in size to a PM10 particle. On average, given the slip-stream effect, the water droplet will more likely collide with the larger PM10 particle than that of the PM2.5. It could well be, then, that the reason the rain doesn’t really clean the air is due to the size of the water droplets, or due to the size of the dust particles on average, which vary with the local environment. But this leaves our intuition unexplained: just why does it seem so much cleaner after rainfall?
This question can be answered with a consideration of those factors which usually coincide with a rain-event: not only the rain, but the local temperature, as well as the speed and direction of the wind change during a storm. A comparative study conducted over 10 years of PM2.5 and PM10 data for Europe put the meteorological conditions up against the particulate matter values to see if there was any strong correlation between the two. They found that the higher the wind speed, the lower the particulate matter rates. This was so for both the fine and the coarse particles (2.5 and 2.5-10 microns respectively).
Contrastingly, the PM rates increased as the temperature diverged to extreme cold or heat, with a middle point centring around 10 degrees centigrade (Barmpadimos et al., 2012). For the first correlation the researchers posited the greater physical effect the wind would have on the particulate matter: the quicker and stronger the wind was, the more likely it would be that the dust was simply blown away. For the latter correlation various environmental factors contingent on the relative temperature were invoked: the higher propensity of pollen in warm months as well as a drier soil which promotes a greater uptake of dust particles, and a greater use of wood-burning heating methods in winter.
A storm usually increases windspeeds, and usually brings about mild temperatures. Both of these are correlated with reduced particulate matter levels. But their coincidence with rainfall is not a necessity, and particularly so at levels at which the particulate matter rates are affected. This helps to explain the data from the Bogotá study: rain does have some effect on the particulate matter, and it’s falling often coincides with milder temperatures and higher windspeeds, which also effect the particulate matter’s prevalence. Sometimes a rainstorm really does clean the air, if other meteorological conditions hold. But if these other conditions don’t also hold, it may well be that the dust remains in the air even as it rains, and that only those pollutant particles which neatly match the rain droplets size are filtered out. From this the low average rates of particulate matter reduction in Bogotá follow.
Conclusion
It seems, then that our intuition as to the cleaning effect of rain was a little misguided, if not wholly. Rain really does reduce the amount of pollutant particles in the air, but not as much as we might have thought. Our senses might well be fine-grained enough to detect the rain-based particulate matter reductions. But it is more likely that our intuition of cleaner air is just based off of those days in which rain, high wind speeds and mild temperatures all coincide – yet we mistakenly attribute all the cleanness we feel to the rain, rather than to the combination of factors.
For rain isn’t small enough to effectively collide with the minute pollutant particles, and its largeness negates any possible positive effect from its electric charge. Indeed, the addition of the wind and temperature to our understanding helps to explain our starting intuition pretty well. The air feels ‘cooler’ and ‘fresher’ – two adjectives which can feasibly be attributed to the temperature and wind respectively, rather than the rain. So the air feels cleaner after it rains because it is, really. But it wasn’t just due to the rain, but due to the wind and lowered temperature as well.
If you’ve found this article interesting, feel free to take a look at some of our other articles. If you want to know how to reduce your dust pollution emittance to within safe levels in an eco-friendly, efficient and effective manner, just contact us. 100% water, 0% dust.
References
Ardon-Dryer, K. et al., (2015). Labatory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets on a single-droplet basis. Atmospheric and Chemistry Physics. 15, 9159-9171. [Viewed online]. Available online: https://doi.org/10.5194/acp-15-9159-2015
Barmpadimos, I. et al., (2012). One decade of parallel fine (PM2.5) and coarse (PM10–PM2.5) particulate matter measurements in Europe: trends and variability. Atmospheric and Chemistry Physics. 12, 3189-3203. [Viewed online]. Available online: https://doi.org/10.5194/acp-12-3189-2012
Feng, X.;Wang, S, (2015). Influence of different weather events on concentrations of particulate matter with different
sizes in Lanzhou, China. J. Environ. Sci. 24, 665–674. [Viewed online]. Available online: https://doi.org/10.1016/s1001-0742(11)60807-3
Blanco-Becerra, L. et al., (2015). Influence of precipitation scavencing on the PM2.5/PM10 ratio at the Kennedy locality of Bogotá, Colombia. Revista Facultad de Ingeniería. 76, 58-65. [Viewed online]. Available online: http://dx.doi.org/10.17533/udea.redin.n76a07
Further non-academic reading
You Thought Rain Washes Away Pollution?
