There is always a certain level of particulate matter ambient in our atmosphere, in the air we breathe and the sky above. This isn’t solely due to our industrial activities: there are natural sources of dust pollution, too. Understanding that pollution which we are not responsible for will make clear that pollution which we are responsible for. For where does dust pollution come from, naturally?
This is a very general problem, and providing a general answer will of course leave out a lot of the specifics. For the ratio of natural to anthropogenic dust will vary from region to region, depending on its proximity to natural dust emitting sources and its industrial development and dust control regulations. For instance, in Australia 76% of dust sources were associated with human uses of land, rather than natural sources (Ginoux 2012 as cited in Querol et al., 2019), whereas the human portion of dust emittance in the northern hemispheric part of Africa was as little as 8% (Querol et al., 2019, p.2), leaving 92% coming from natural sources.
But what are they? An EU report from 2012 helpfully detailed the four main natural sources of particulate matter. They are wind-blown desert dust particles, sea spray aerosols, volcanic dust particles, and wild-land fire particles. Let’s go through each in turn, detail the means by which they are made, become airborne, and the different levels of threat they pose.
Deserts are dry, arid regions where rainfall, and consequently vegetation, is rare. While you may have a particular image in mind when you think of a desert, their topography varies a lot more than you might think. For some are still rocky and hard, if some do indeed fulfil the stereotype of masses of sand dunes heaped upon one another; and yet while some are sun-baked hotspots, some are still desolately cold wastelands. For, as mentioned, what really makes a desert a desert is the paucity of water: a widely used definition of a desert is for it to have no more than 25 centimetres of rainfall a year. It is this factor, in combination with certain other subsidiary climatic conditions, which makes deserts one of the four main natural sources of dust pollution.
It’s easy to understand why. Water is the natural way to stop dust pollution, after all. But it is not simply the lack of a dust-binding solute that is the sole problem, but also that the lack of this necessary nutrient means the environment cannot support any form of widespread vegetative life.
In temperate climates, trees, grasses and bushes of sundry kinds all grow with ease. They spread their roots into the ground, anchoring themselves to our blue and green Earth. But in this they anchor not only themselves, but the ground in which they are anchored. As the ballast and tethers of a hot air balloon weigh it down and connect it to terra firma, vegetation sits upon the topsoil and helps to lock it down in the face of other climatic pressures. In the desert, of course, there are no such organic anchors. Cacti have very shallow roots, and sand, when dry, is rather light.
Enter the ever-present eroding force of nature: the wind. Normally, plants and trees, along with regular precipitation, help to keep the soil where it should be: on the ground. But with nothing to anchor it and nothing to slow the wind’s progress, it can run across the landscape at will, picking up great speeds. This aeolian erosion gradually, year on year, trims away the topsoil until there is none left. Clod is smashed against clod, clump against clump, and particle against particle, wearing the matter down to finer and finer gradations. This continues either until all the loose topsoil is blown and transported away, or the terrain itself is ground into sand through a combination of extreme temperatures, lack of water and crushing winds.
But while this process can tend to generate the dusty deserts of our imagination, it is not that which is responsible for the spreading of fine dust, nor even for the creation of it. This is so because of two general facts of natural dust dispersal and creation: (1), the finer grained the particle, the more likely it is to remain airborne for longer times; and (2), fluvial (water based) erosion, and not aeolian erosion, is the more effective creator of fine-grained mineral dust (Pye, 1989, as cited in Prospero et al, 2001, p. 40).
This helps to explain the strange fact that, while desert climates tend to mean that each desert is actually a relatively dusty place, not all deserts spread as much particulate matter pollution as each other. There are three in particular which are of particular renown for their dust spreading: the Saharan Desert, and the Taklaman and Gobi deserts. Saharan dust can end up hazing over the forests of Europe, known as Saharan dust events (SDE), and dust from the Taklaman and Gobi deserts ends up being blown over Eastern Asia each spring, known as Asian dust events (ADE), or Yellow dust events. Examining these deserts and their associated meteorological events will let us in on just how deserts can lead to ‘natural’ dust pollution.
The Sahara Desert
The Sahara has the perfect geological and climatic conditions for the dispersal of airborne dust, and it and the Sahel region to its south “are the most active dust sources in terms of emissions (790/840 million tons/year)” (Querol et al., 2019, p. 2).
Perhaps surprisingly, the main source within the Sahara-Sahel region is not the sand of the desert, but the fine sediment found on the bed of Lake Chad and other dried up waterbeds in the region. (Prospero et al, 2001, p. 23). The sand particles do indeed become airborne, but they usually have too large an aerodynamic diameter to stay afloat for very long. The sediment of the ever-growing dry bed of Lake Chad, in contrast, is of such a fine grain that it can remain suspended in tropospheric air currents indefinitely. It is hypothesised that the coarser sand dune particles work to grind down the finer alluvial deposits on the Lake Chad basin even further, thus exacerbating the area’s dust-spreading potential.
And, as the Sahara is a hot desert, during the day the air becomes incredibly hot, which leads to „strong vertical thermal turbulence, which can reach altitudes of up to 4000 – 5000 metres in [the] summer (Dubief, 1979)“ (EEA, 2012). These currents carry with them the fine grains of mineral dust high up into the air. The freezingly cold nights bring no opposing atmospheric force to send the particles back down to the ground, so that the dust can stay aloft for weeks or even months. If the atmospheric wind currents are of sufficient strength and the right direction, the grains can end up far, far away. Take the early Spring of 2018, where snow in Eastern Europe and the Russian Caucuses turned orange due to Saharan dust.
This is only one of the more intense events in recent memory. Every year, the ambient levels of airborne dust fluctuate, particularly in southern Europe, due to the transport of Saharan dust. It is estimated that Saharan dust effects air quality in the Mediterranean Basin up to 17% to 37% of the time, contributing to that days airborne particulate matter readings from 9% to 43% (Querol et al., 2009 and Pet et al., 2013, as cited in Querol et al., 2019).
Other studies have reported PM10 concentrations reaching “up to 250 μg/m3 in remote sites in Spain and up to 470 μg/m3 in Nicosia” during Saharan dust events (Querol et al., 2019, p.4). The daily limit for PM10 recommended by the EU is 50 μg/m3. Even if these dust events don’t constantly make international news, nor repeated attention due to their aesthetic effects, they still have a large impact on the quality of the air and the consequent respiratory health of the endemic population.
The Gobi and Taklaman Deserts
The story is similar with the Gobi and Taklaman deserts, and their role in ADEs. They count together as the second most active desert dust source, emitting 140-220 million tons of dust per year (Querol et al., 2019, p. 2).
The spread of the “Yellow Dust” across eastern Asian has been a meteorological fixture for millennia, with references to it stretching back into ancient times. The mechanisms for its dispersal are similar to that of the Saharan dust, with a combination of climate and geography creating a dust-dispersal hotspot.
The Taklaman desert is found in the Tarim river basin, surrounded by mountains to the North, South and West. From these mountains considerable amounts of water run through and around the basin as run-off, which provides the fluvial erosion needed for the fine-grained sediment most likely to remain airborne. The mountains allow only one way out for dust-laden winds: eastward.
The Gobi desert and the dry plateaus of Mongolia are similarly situated in the lee of the huge mountain ranges of Tianshan, against which small cyclones form. These function in a similar way to the hot thermal currents of the Sahara, as they drive the dust of the desert below high up into the sky. Winds then drive these floating, hazy dust storms eastwards towards eastern Asia, which deposit themselves over a huge swathe of territory, including the cities of North and South Korea, north-eastern China and Japan. This results in layer upon layer of yellow dust sediment blanketing densely populated, and often already heavily polluted, land. A 2003 study by Mori et al. found that “TSP [total suspended particulate] concentrations reached up to 6700 μg/m3 on an 8 h basis in the Inner Mongolia Autonomous Region (China); 1500 μg/m3 at Beijing (on a 6 h basis, with 93% of the mass concentration being in the 2.1–20 μm fraction); and 230 μg/m3 at a remote island in Japan (on a 24 h basis, with 64% in the 2.1–20 μm fraction).” (Querol et al., 2019, p. 4).
Health impacts of desert-spread dust
The potential health impacts of desert dust are great. While the sand particles are in themselves not toxic, their inhalation at high volumes can lead to a form of pneumoconiosis, otherwise known as ‘desert lung’, where the repeated inhalation of minute dust particles has caused the lungs to scar over time, impairing their proper functioning.
They also are highly likely to entrap and carry with them other anthropogenically sourced particulates, which are in themselves carcinogenic. This is a growing problem as developing nations on the downwind of natural dust clouds industrialise themselves more and more, and add toxicity and superfine combustion particles into the swirling desert dust. This has, in recent years, been the source of a dispute between South Korea and China, as air quality in Korea has dived as Chinese cities upwind of it have industrialised more and more heavily.
Desert regions furthermore often contain fungi and viruses otherwise not commonly found in more cultivated environments. For instance, in the south west of the USA, in the arid soils of states such as Arizona and California, there is a fungi known as coccidioides, which causes a deadly and damaging fungal infection. The incidence rate of the infection has been seen to correlate strongly with the intensity of the particulate matter concentrations sourced from the desert regions. The more arid the area becomes as desertification intensifies, the more likely that foreign contaminants such as these fungal spores become predominant.
The second major natural source of airborne particulate matter makes for a counter-intuitive contrast with the first. The dryness and aridity of the deserts were perfect for the creation and spreading of airborne particulate matter. But so is the sea, the open ocean, where the salty waves crash and churn upon each other and the shore.
The sea produces airborne dust directly at its surface, and is normally termed “sea spray aerosol”, or SSA. This is as the particulate matter, normally inorganic sea salt or organic marine matter, is set free into the air through the creation of ‘spray’ droplets, which eventually evaporate into the air, leaving suspended the minute solid particles they had previously enveloped.
The spray is typically formed through the bursting of bubbles at the sea’s surface. The bubbles are formed through the entrainment of the air particles within the water, normally resulting from some physical disturbance of the water against the air. For instance, when a wave breaks, air is pushed through the water and spread through it in a cloud of bubbles, which subsequently rise to the surface of the water, where they eventually burst (de Leeuw, 2011, p.4). These bursting bubbles make up those white, foamy bits you see at the tip of breaking waves, technically known as “whitecaps”, and the hissing sound as the tide draws back.
This bursting happens in three ways, with each way releasing water droplets into the air in a different manner. The first is a film droplet. When the film of the bubbles breaks, water that had made up the film of the bubble is ejected into the air. These particles are typically the smallest of the sea salt aerosols, with the majority of them measuring in with a diameter of 1-2.5μm.
The second is a jet spray droplet. For jet spray droplets, the bubble’s bursting leaves a cavity behind it which draws up a jet of water into the air. This becomes unstable as it flies upwards, and eventually disperses itself into several larger sized droplets, which generally tend to measure around 10μm. These first two methods of sea salt aerosol production are termed indirect as they lead to aerosol production through the the popping of the whitecap bubbles.
This is in contrast to the third and final direct mechanism by which spume droplets are produced. If the wind gets high enough upon the open sea, the bubbly whitecaps themselves are in part blown into the air, forming spume droplets. These are the largest droplets, with their radii measuring between 20μm and 500μm.
Studies have estimated that, for coastal regions, in comparison with inland areas and the open sea, sea salt aerosol might actually make up the majority of the particulate matter recorded (EEA, p.19). This is as a lot more waves form and break at the coast than upon the open ocean (and a lot more upon the open ocean than on the land, of course). And as typically half of the SSA produced lies in the PM2.5-10 range, its presence will be most keenly felt in the PM10 readings.
This could potentially lead to a confusion with regards to how we understand the quality of our air: a region’s PM10 readings could far exceed the regulatory limits, leading one to perhaps infer heavily polluting anthropogenic sources with insufficient dust control. But in fact it could just be a particularly blustery day on the high seas. If our policy with regards to controlling our industrial practice and the man-made production of dangerously fine aerosols is to be led by our air quality readings, we need to be sure that what we are recording is actually anthropogenic in source.
Health-impacts of sea-made dust
This potential for confusion is the primary worry in terms of SSAs in relation to the quality of our air, as their direct health effects are actually thought to be negligible. Scientific interest in them is instead primarily aimed towards their as of yet uncertain role in global climate trends. As they are primarily made up of brighter coloured particles, they tend to reflect the sun’s rays, instead of absorbing them like darker, combustion-sourced particulate matter. This cools the earth’s atmosphere. The parsing out and accurate quantification of this effect is vital to a proper modelling of the earth’s climate, and is thus key to the predicting of climate-change and the proper policy towards it.
The third major source of naturally occurring particulate matter is more intuitive, and easily observable to the eye. Underneath the earth’s crust, at certain points in the earth’s tectonic composition, molten rock seethes and boils. Occasionally the conditions are such that this molten rock, termed magma, rises steadily upwards above the denser solid rock until it reaches the open air.
Such an event is termed a volcanic eruption. A volcanic eruption can be explosive, or effusive (non-explosive). The ‚explosive‘ type are those which catch the mind’s eye, and produce the most particulate matter: a cacophonous boom, a terrible heat and a great plume of dust and volcanic rock sent upwards and outwards by the concussive force of the explosion. The classic modern-day example of such an eruption was that of Mount Saint Helens, in 1980.
These types of eruptions happen when the magma is of a higher viscosity (more thick). The further down the magma is found in the earth, the more pressure the earth above exerts downwards onto it. The huge pressure keeps the gases in the magma dissolved. But as the magma rises, the pressure decreases, leading the gases to come out of solution and form bubbles within the magma. These bubbles want naturally to escape the higher density magma into the air – but if the density of the magma is too high, then this becomes difficult. The particles struggle to squeeze a way out of the magma, and instead expand inside it, driving it upwards and upwards along the path of least resistance.
So forms a vicious, viscous and eventually volcanic cycle: as the magma rises, the pressure further decreases, meaning the bubbles multiply and multiply. Yet still they cannot escape effectively, driving the magma quicker and quicker upwards with their fruitless expansion. Eventually the magma gets so bubbly that it is more foam than liquid, and when it becomes over 70% bubbles the foam is accelerating at so fast a pace that it begins to rip itself apart into tiny solid particles. By the time this foamy solid mixture reaches the surface it can have reached speeds of up to 1000 kilometres per hour.
The ejection of the molten rock, gases and volcanic rock (tephra) at such a pace is what makes the eruption explosive. For effusive eruptions, where the gas can escape from the lower viscosity magma more easily, the volcanic matter never reaches such a high speed. This results in the gentle spewing out of the lava, as is more commonly seen on the shield volcanoes of Hawaii.
Unsurprisingly, it is the explosive eruptions which release the greatest amounts of particulate matter. The magma’s upwards pace ejects the tephra, a general term used to cover all the volcanic rock emitted regardless of size or composition, high up into the air, with some eruption plumes reaching up to 45 kilometres into the sky.
The amount of volcanic ash emitted in these plumes can be astronomical. Take the explosive Eyjafjallajökull Volcano eruptions of 2010 which caused the widespread grounding of flights across Europe. The PM10 readings taken on Iceland when the eruption was still ongoing were extremely high:
The 10 min average concentrations reached a maximum of 13157 mg m—3 on 7 May and 12028 mg m—3 on 8 May, with 24 hour averages of 1231 mg m—3 and 718 mg m—3, respectively. The [World Health Organisations] 24 hour health limit of 50 mg m—3 was thus exceeded by factors of 25 and 14, respectively.Thorsteinsson et al, 2012, p. 4.
An explosive volcanic eruption, and the ensuing volcanic dust event, have clear effects on the local and global climate. It also poses in its abundance a serious risk to human health. But the estimated average yearly emissions of volcanic dust pale in the face of the comparable figures thought to originate from deserts, or the sea. This is a result of the irregularity of volcanic eruptions: while year on year the winds reliably blow, and the waves reliably break, a massive dust emitting eruption does not happen very often – the last major eruption to globally affect the earth through its emittance of dust was in 1990, when Mount Pinatubo in the Philippines erupted.
Health impacts of volcanic dust
While volcanic particulate matter is normally free of toxic material, its inhalation still poses a great threat to human health, just like desert dust. The viscosity of the magma is determined by its silica content; the higher the silica content, the more dense the magma is, and hence the more explosive its eruption will be. The silica particles are razor sharp, and when inhaled lacerate the lungs, causing scar tissue to form and form until it inhibits the proper functioning of the respiratory system. This can, with long-term exposure, develop into the long-term illness silicosis, more often understood as an occupational condition most often contracted by construction workers working without adequate dust protection.
The proliferation of volcanic dust and gases in the atmosphere can have indirect effects on our health, too, forming what is now know as a „vog“ (volcanic fog). Throughout history massive volcanic eruptions have preceded times of increased hardship, and brought with them periods of instability and change.
‚The Year Without A Summer‚
One of these was the „Year without a Summer“ in 1816. It was the coldest summer in the last 250 years, and many a harvest failed. Food shortages were global, and the sun set itself in hazy orange and red hues, as landscape paintings from the time testify. This global event has been linked to the massive eruption of Mount Tambora in Indonesia in 1815, which emitted the equivalent of about 50km3 of tephra into the atmosphere (Brönnimann, p. 8) . This, as well as masses and masses of sulfate oxide, was what caused the hazy sky and the colder summer: they formed a barrier through which the sun’s rays could not penetrate without certain wavelengths being absorbed and diffracted.
During the summer, in hot and dry areas, woodland can suddenly combust. The heat of the sun, or the strike of a lightning bolt, and whoomph – the trees, bushes and whatever else is near goes up in flames.
This is what we call a wildfire – those fires that occur in nature not started with human intent. This is our fourth and final main natural source of dust pollution. There are three conditions that need to be met for a fire to start: oxygen, fuel, and a heat source. These three make up the ‚fire triangle‘: if all three corners of the triangle are met within nature, a wildfire becomes a possibility.
At least two of the three are found in abundance in vegetated areas across the globe. Oxygen is present in the atmosphere, and is a precondition for what makes up the second corner of the wildfire triangle, the vegetation. Be it grasses, shrubs and bushes, or trees, all can, under the right conditions, catch alight. They only need a heat source sufficient to warm up the fuel to its ignition point: the point at which it begins to release some of the volatile, combustible gases it had kept within it into the air, where, if the heat is strong enough, they react with the oxygen. This reaction is what we understand as the burning of a fuel, or the flaming of a fire.
There are generally two natural heat sources that can cause wildfires: hot lightning, and spontaneous combustion. More often than not, however, ’natural‘ phenomena are not the root cause of wildfires. A study from the University of Colorado in 2017 looked into the causes of wildfires in the United States between 1992 and 2012. They found that 84% of them actually had their origins in human activity, rather than in natural causes. Often cited causes are the failure to properly put out campfires, the careless disposal of a cigarette butt, and even arson. This puts the ’natural-ness‘ of wildfire dust into question.
In any case, regardless of its cause, a wildfire will emit dust into the air. But this isn’t just because it’s a fire; in fact, not all burning will produce particulate matter. Take yourself back to your school days in science class, to those exciting days where the Bunsen burners were fetched from the cupboards and the smell of gas, scoldings and muffled yelps drifted through the air. Remember additionally that there was fire then, but no smoke – hopefully.
This was as, when its air vent was fully open and its flame burnt blue, the Bunsen burner combusted its fuel source, natural gas, completely. This means that all of the fuel present reacted with oxygen, forming water and carbon dioxide.
methane + oxygen → carbon dioxide + water vapour
There was nothing left over apart from these two harmless, if not wholly environmentally friendly, compounds. This only happens when the air vent is open, as this is what allows enough oxygen to reach the flame. When the air vent was closed, and not enough oxygen reached the flame, the fuel was combusted incompletely:
methane + oxygen → carbon monoxide+ carbon + water vapour
Not all of the fuel’s elements could find oxygen to react with. This produces the deadly and invisible carbon monoxide, and the equally deadly but all too visible carbon, as well as the water vapour. The carbon soot within the flame glowed as it was heated, giving the flame its yellow colour.
The latter reaction is that which occurs during a wildfire. While at some stages and some points complete combustion may well occur, this typically won’t be so for the majority of the fire’s lifetime. Wildfires tend to produce a lot of smoke, a lot of particulate matter, and a lot of dangerous airborne material. The EPA describe the general composition of a wildfire’s emissions to be a
complex mixture of particulate matter, carbon dioxide, water vapor, carbon monoxide, hydrocarbons and other organic chemicals, nitrogen oxides, and trace minerals.(Wildfire Smoke Guide for Public Health Officials p. 12)
Depending on the type of fuel being burnt, and the intensity of the fire, the proportion of these various elements will vary. But dust will be a constant. As the fires usually burn their fuels incompletely, minute solid particles of fuel are spewed up into the air. Their varying chemical composition and rate of production makes the threat of wildfire dust hard to quantify absolutely. But we can look to a recent example of a catastrophic wildfire event, and the dust it brought with it.
Australia’s Black Summer
Starting from around September 2019, and continuing officially until March 2020, wildfires raged in Australia, most widely and viciously in the south-east of the country. They consumed property and nature alike, with estimates of the economic impact putting the price at above the previous highest incurred, 4.4 billion dollars, and the ecological impact at over 17 million hectares of land burnt, with a conservative estimate of over one billion of their mammalian, avian and reptilian inhabitants killed.
The impact the particulate matter emitted from these fires is also huge, and still ongoing. NASA satellites again help us to visualise the sheer size of the amount of smoke emitted, as shown in these two satellite images:
The smoke’s threat lay not only in its huge amount, but in the fineness of the particles which made up the clouds. Wildfire particulate matter is particularly dangerous as it typically lies within the 0.4-0-7 micrometre range, (so within the PM2.5 range) and therefore has a high potential to infiltrate the respiratory system, the lungs, and potentially even the bloodstream and heart.
This proliferation of PM2.5 material in wildfire dust can be seen when turning to the air quality data for Sydney and the surrounding areas from the time. In places, the rates soared more than ten times above the WHO prescribed limits.
This means that, on certain days, the population would have been exposed to an intense concentration of fine particulate matter. This can bring on short-term acute conditions , and worsens existing respiratory conditions and the general health of the vulnerable.
But not only were daily maximums massive, the monthly averages were also dangerously high. This represents long-term exposure to fine particulate matter pollution, something known to worsen general health and lead to the development of long-term chronic respiratory conditions.
The bushfire smoke circumnavigated the whole globe. Glaciers in New Zealand were discoloured, and air quality in South America was reported to have dipped. A wildfire is not only threatening in its heat, but also in its smoke.
The Natural Sources of Dust Pollution
We have reviewed the four main natural sources of particulate matter: the deserts, the ocean waves, explosive volcanic eruptions and forest fires. There are many ways in which small, solid particles end up floating in the air. The erosion of rock by water, and its subsequent journey aloft in the wind; the bursting of salty bubbles at a wave’s crest; the pressure-ful rushing of magma upwards and upwards; and the fierce combustion of vegetative matter after a crack of hot lightning.
Naturally airborne dust isn’t necessarily bad, however. In fact, it is usually a good thing, and plays a vital role in a variety of ecosystems. The Amazon rainforest is seeded and fed mineral nutrients from Saharan winds. Sea salt is a key element in the world’s self-cooling, as wildfires are the forest’s spring-cleaner, removing dead, waste vegetative matter so that new shoots can grow. In Australia there are several species of tree especially evolved to promote the incidence of forest fires. Even volcanic eruptions leave behind fantastically fertile landscapes.
But, as in most things , an excess poses a threat, as would a lack. Combining natural dust emissions with the ever-growing anthropogenic makes for an airborne dust load which is altogether too much to bear. It threatens our environment, our climate, and the current way in which we lead our lives.
This is why we must act to limit our anthropogenic emissions, and those activities which exacerbate the harmful effects of natural dust events, while retaining all of the beneficial effects of normal levels of natural dust.
If you’ve found this article interesting or informative, feel free to check out our other forays into the world of dust here. If you would like help or advice dealing with a dust problem, feel free to contact us here.
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