Aerosol

An aerosol is a mix of tiny solid pieces or liquid drops floating in air or another gas. These particles are very small, usually smaller than a micrometer. Aerosols can come from nature or human activities. Natural examples include fog, mist, and dust from the earth or living things. Some infections can spread through tiny droplets in a person's breath, called bioaerosols.

Human-made aerosols include things like spray from a spray can, perfume from atomizers, dust from buildings or factories, smoke, mist from irrigation, sprayed pesticides, and air pollution. These tiny particles can affect the climate. For example, volcanic eruptions release droplets of sulfuric acid high in the sky, where they can stay for up to two years and cool the Earth by reflecting sunlight.

Ships also create clouds called ship tracks. When a ship releases exhaust into the air, water molecules gather around the tiny particles from the exhaust, forming cloud seeds. These seeds grow into visible clouds that look like long strings over the ocean. While greenhouse gases warm the planet, human-made aerosols can have a cooling effect. In 2020, rules about fuel use cut down sulfur dioxide emissions from ships by about 80%, changing the Earth's climate in unexpected ways.

Definitions

See also: Particulates

An aerosol is a mix of tiny solid or liquid particles floating in a gas, usually air. These particles can come from nature or human activities. Scientists call the whole mix an aerosol, not just the particles themselves.

Aerosols can be made in different ways. Some start as particles put directly into the air, while others form when gases change into particles. Common types of particles in aerosols include dust, smoke, and sea salt. Scientists measure aerosols in different ways, such as how much weight of particles is in a certain amount of air, or how many particles there are. The size of the particles is also important because it affects how they behave.

Generation and applications

People create aerosols for many useful reasons. For example, they help test scientific instruments, deliver products like deodorants and paints in sprays, support farming, treat breathing problems, and improve fuel systems in cars.

Some tools that make aerosols include aerosol sprays, atomizer nozzles, electrosprays, electronic cigarettes, and vibrating orifice aerosol generators.

In the atmosphere

Main article: Particulates

Aerosols in the air come from many places, both natural and human-made. Natural aerosols include dust from deserts, sea salt, and tiny particles from plants. Human-made aerosols come from burning fuels like oil and coal, which create smoke and other tiny particles.

These tiny particles can change the Earth's climate. For example, volcanic eruptions release gases that form particles high in the sky. These particles can block sunlight, making the Earth a bit cooler. Other particles, like those from cars and factories, can also affect clouds and temperature. Some of these particles can even get into our lungs and affect our health.

Size distribution

See also: Particulates § Size, shape, and solubility matter

For a group of tiny particles all the same size, you only need to know one number—the size of each particle. But when particles are different sizes, we need a way to describe how many are small, medium, or large. One way is to count every single particle, but this is hard when there are millions!

Instead, scientists often group particles into size ranges. They count how many particles fall into each size group and show this information in a chart. This chart helps us see which sizes are most common.

Different patterns can describe how particle sizes are spread out. One common pattern, called the log-normal distribution, works well for many types of particles because it can show a wide range of sizes without any impossible (negative) values. Other patterns are used for special cases, like dust or water droplets in clouds.

Physics

Terminal velocity of a particle in a fluid

Very small particles can float in the air and move slowly because the air pushes back on them. These particles settle at a speed that depends on their size and density. This speed can be calculated using a simple formula that involves the particle's size and the density of the material the particle is made from.

Dynamics

When we look at many particles together, we study how their numbers change over time. Particles can move around due to air currents, bump into each other, or even form new tiny particles. Scientists use special math to describe all these changes.

Coagulation

When particles bump into each other, they can stick together. This makes the particles bigger but reduces the total number of particles. Sometimes, very large particles might break apart, but this is rare for tiny particles we call aerosols.

Dynamics regimes

The way particles move depends on their size compared to the space between air molecules. Very small particles act like tiny balls bouncing off air molecules, while larger ones move more smoothly through the air.

Partitioning

Tiny particles can grow by gathering more material from the air around them. This happens when the air has more of a certain material than it can normally hold, causing it to stick to the particles.

Activation

Water can coat tiny particles in the air, helping them grow. Smaller particles need more moisture in the air to stay coated compared to bigger ones.

Solution to the general dynamic equation

Solving the complex math that describes how particles change over time can be done in several ways, including special methods that break the problem into smaller, easier pieces.

Detection

Aerosols can be studied using special tools placed close to the particles or from a distance. Close measurements use devices like an aerosol mass spectrometer or a condensation particle counter.

From far away, scientists use tools like a sun photometer or lidar to learn about aerosols. Some tools can even tell us what size particles are in the air and where they might end up in our bodies when we breathe. For example, PM₁₀ and PM_2.5 are ways to describe tiny particles in the air that can reach different parts of our lungs.

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