Aerosol

• 1. Introduction

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  Aerosols surround every living person on Earth, every moment of every day, yet most people have never heard the scientific term. Right now, microscopic particles are drifting through the air you breathe. Some formed over volcanic craters. Some rose from desert sands thousands of kilometres away. Some were exhaled by the person standing nearest to you.

  The word aerosol was coined during World War I by a scientist named Frederick G. Donnan. He needed a term for clouds of microscopic particles suspended in air, and he built it by analogy with "hydrosol," a system where water does the suspending. From that wartime coinage, aerosol science grew into one of the most consequential fields in atmospheric chemistry and public health.

  The questions ahead are not small ones. How do particles smaller than a single micrometre reshape the climate of an entire hemisphere? Why did a shipping regulation passed in 2020 trigger what scientists called an unexpected global geoengineering termination shock? And how does the size of an invisible particle determine whether it lodges in your nose or reaches the deepest chambers of your lungs?

• 2. What Counts as an Aerosol

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  Frederick G. Donnan's wartime definition has held up with surprising precision. An aerosol is not just the floating particles alone; it is the mixture of those particles and the gas that holds them. That distinction matters to scientists in ways it rarely does in everyday conversation.

  Particle size sits at the centre of the definition. The liquid or solid particles in a true aerosol have diameters typically less than 1 micrometre. Larger particles, which settle fast enough to fall noticeably under gravity, tip the mixture into what physicists call a suspension. The boundary between the two categories is not perfectly sharp, but the settling speed is the deciding criterion.

  The particles themselves fall into two broad families. Primary aerosols carry particles introduced directly into the gas, the way a breaking ocean wave hurls salt into the air. Secondary aerosols form through gas-to-particle conversion, where chemical reactions in the atmosphere build new particles out of gaseous precursors. Key groups include sulfates, organic carbon, black carbon, nitrates, mineral dust, and sea salt, and in practice these groups rarely stay separate. They clump together into complex mixtures that drift and transform as they travel.

  Scientists track these mixtures using two main measurements. Mass concentration, expressed in micrograms per cubic metre, tells how much material is packed into a given volume of air. Number concentration, expressed as particles per cubic centimetre, tells how many individual particles are present. The two measures can point in very different directions, because a single coarse dust grain weighs far more than a million ultrafine combustion particles of the same total mass.

• 3. Natural Aerosols and Climate

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  Volcanic eruptions write the most dramatic chapter in natural aerosol history. After an eruption breaks through the stratosphere, droplets of sulfuric acid form high above the weather layer and can persist for up to two years. Those droplets scatter incoming sunlight back toward space, lowering surface temperatures across broad regions of the planet.

  Desert dust operates by a different mechanism. Mineral particles blown to high altitudes absorb heat rather than scattering it, and there is evidence they may inhibit the formation of storm clouds. Sea salt lifted by breaking waves, biological particles like pollen and spores, and smoke from wildfires complete the inventory of natural sources. Each type has its own optical properties, its own lifetime in the atmosphere, and its own set of downstream effects.

  The indirect effects on climate reach further than simple heating or cooling. Aerosols can modify the size of cloud droplets in the lower atmosphere, changing the way clouds reflect and absorb light. A cloud seeded with many small particles tends to be brighter and longer-lived than one seeded with fewer, larger drops. Because clouds cover a vast fraction of Earth's surface at any given moment, even modest changes in their properties feed back into the global energy budget in ways that researchers are still working to quantify.

  The Northern Hemisphere carries a heavier load of both natural and human-made aerosols than the Southern Hemisphere. Evidence suggests this extra aerosol burden has slowed surface warming in the North relative to the South, though scientists note that ocean currents transporting warmer water from south to north will eventually redistribute that stored heat.

• 4. Human-Made Aerosols

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  Burning oil and coal releases sulfate aerosols on a scale no natural source can match over a sustained period. These human-made sulfates affect cloud behaviour and, by scattering sunlight, have partially offset the warming caused by greenhouse gases. The offset is real, but it does not cancel the underlying warming; on a global scale, aerosol cooling reduces greenhouse-gas-induced heating without eliminating it.

  Ship tracks offer one of the most visually striking demonstrations of human influence on cloud formation. When a vessel crosses still ocean air, its exhaust feeds tiny aerosol particles into the atmosphere. Water molecules collect around each particle, building what atmospheric scientists call a cloud seed. As more water accumulates, the seed grows into a visible droplet, and the resulting clouds stretch into long, narrow strings tracing the ship's route across the ocean.

  In 2020, international fuel regulations slashed sulfur dioxide emissions from shipping by approximately 80%. The reduction was intended to protect human health, and it succeeded. But removing that much aerosol from the marine atmosphere so abruptly also removed a significant source of the cooling that those ship-track clouds had been providing. Scientists described the outcome as an unexpected global geoengineering termination shock, a sudden loss of an unintentional climate intervention that had been operating for decades without anyone fully accounting for it.

  Beyond shipping, the roster of human-generated aerosols spans a wide range of everyday activities. Perfume released from an atomizer, pesticide sprayed over a field, mist discharged at a hydroelectric dam, and smoke from industrial processes all qualify. Even the spray from a consumer deodorant can is, in the scientific sense, an aerosol delivered by an aerosol.

• 5. Aerosols and Human Health

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  Particle size determines where in the body an aerosol can reach, and that geography is the difference between a mild irritant and a serious hazard. Particles with an effective diameter smaller than 10 micrometres can penetrate into the bronchi. Those smaller than 2.5 micrometres travel deeper still, reaching the gas exchange region of the lungs where oxygen passes into the blood. Damage at that depth is difficult for the body to repair.

  The United States Environmental Protection Agency drew regulatory lines around those thresholds. It replaced older standards based on total suspended particulate with a standard based on PM10 in 1987, then added standards for PM2.5, also called fine particulate matter, in 1997. The ISO defines PM10 as particles passing through a size-selective inlet with a 50% efficiency cut-off at 10 micrometres, and PM2.5 with the cut-off at 2.5 micrometres.

  Researchers studying occupational health work with three overlapping fractions. The inhalable fraction covers every particle that can enter the nose or mouth, and it depends on wind speed, wind direction, and the aerodynamic size distribution of the particles. The thoracic fraction is the share that reaches the chest region. The respirable fraction is the share that penetrates all the way to the alveolar region of the lungs. Measuring that final fraction requires a pre-collector device that mimics the filtering action of the upper airway before a sampling filter catches what remains.

  Aerosols in the 20-micrometre range present a specific indoor hazard because of what researchers call their "jet rider" behaviour. These particles follow air jets in motion but fall out under gravity when the air slows. Air-conditioned rooms create exactly the alternating fast and slow air patterns that keep such particles aloft for unusually long periods. This size range is also the one most effectively deposited in the human nose, which scientists identified as the primary infection site in COVID-19, raising the possibility that jet-rider aerosols contributed to the spread of the pandemic.

• 6. Measuring and Detecting Aerosols

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  Scientists measure aerosols either by sampling the air directly, a method called in situ observation, or by reading signals from a distance, known as remote sensing. Each approach has instruments that excel in particular environments or at particular scales.

  In situ tools include the aerosol mass spectrometer, the differential mobility analyzer, and the condensation particle counter, among others. The aerodynamic particle sizer and the wide range particle spectrometer extend coverage across different portions of the size spectrum. For occupational health surveys, the micro-orifice uniform deposit impactor captures particles at multiple size cuts simultaneously.

  Remote sensing brings a different set of capabilities. Sun photometers measure how much sunlight is absorbed or scattered by the column of atmosphere between the instrument and the sun. Lidar systems fire laser pulses upward and read the backscattered light to build vertical profiles of aerosol layers. Imaging spectroscopy adds the ability to distinguish aerosol types by their optical fingerprints.

  Describing the size distribution of an aerosol population requires mathematics as well as instruments. A monodisperse aerosol, where all particles share the same diameter, can be described with a single number. Real-world aerosols are polydisperse, spanning a wide range of sizes, and their distributions are better captured by a log-normal function than by the ordinary bell curve. The log-normal distribution has no negative values, can stretch across many orders of magnitude, and fits observed atmospheric data reasonably well. For coarsely dispersed dusts and sprays, the Rosin-Rammler distribution often applies instead; for cloud droplets, the Khrgian-Mazin distribution is the standard choice.

• 7. Physics of Suspended Particles

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  A particle drifting through air is subject to forces that vary dramatically with its size. For most aerosol motion, where the Reynolds number stays well below 1, Stokes' law describes the resistance a spherical particle meets from the surrounding fluid. But Stokes' law assumes the gas velocity at the particle surface is zero, an assumption that breaks down for particles smaller than 1 micrometre.

  To correct for this breakdown, scientists apply the Cunningham correction factor, a value always greater than 1 that adjusts the resistance calculation upward for small particles. With that correction in place, a particle's terminal settling velocity under gravity can be calculated from its size, the viscosity of the gas, and the correction factor itself. The terminal velocity is proportional to the square of the aerodynamic diameter.

  Aerodynamic diameter is itself a useful abstraction. It is defined as the diameter of a hypothetical spherical particle with a density of 1000 kilograms per cubic metre that settles at the same speed as the actual, irregular particle. Pharmaceutical companies rely on aerodynamic diameter rather than geometric diameter when characterising particles in inhalable drugs, because aerodynamic diameter predicts where in the respiratory tract a particle will deposit.

  At the smallest scales, particle behaviour depends on how particle size compares to the mean free path of the surrounding gas, a ratio captured by the Knudsen number. When the Knudsen number is much greater than 1, particles are small relative to the gaps between gas molecules and behave almost like gas molecules themselves, diffusing rapidly through Brownian motion. When it is much less than 1, the gas acts as a continuous fluid flowing around the particle. The transition region between these two regimes is described by the Fuchs-Sutugin interpolation formula, a semi-empirical equation that bridges the two extremes.