There is more than just air in the sky. There are also billions of tiny floating particles called aerosols or particulates. In each cubic centimeter of air, there can be hundreds or thousands of aerosols!
Aerosols come in different sizes
Some aerosols are so small that they are made of only a few molecules and can only be seen through an electron microscope. Some aerosols are large enough to be seen with the eye, but are still small enough to be suspended in the air. In general, the smaller and lighter a particle is, the longer it will stay in the atmosphere. Larger particles tend to settle to the ground in a matter of hours, whereas the smallest particles (less than 1 micrometer) can stay in the atmosphere for weeks and are mainly carried back to the Earth's surface through precipitation.
Aerosols are part of air pollution
Some aerosols are a natural part of the atmosphere - coming from erupting volcanoes, sea salt, and wildfires. However, humans add lots of aerosols to the atmosphere by burning fossil fuels such as coal, oil, and gas. Aerosols are a part of air pollution and are dangerous to human health. When we breathe in these tiny particles, they can damage lung tissue and lead to lung diseases. Aerosols can also limit visibility, causing haze in many parts of the world.
Aerosols affect the climate
Aerosols in the atmosphere can change the amount of solar energy reflected away from Earth. Not all aerosols react the same when hit with sunlight. Sea salt particles reflect sunlight back out into space. Black carbon particles from burning wood or fossil fuels absorb most of the sunlight that hits them.
Aerosols help clouds form, and clouds have an impact on climate. The millions of droplets of water that make up a cloud each need a little particle, like an aerosol, to condense upon. More aerosols can create more clouds. Not all clouds have the same effect on climate. This is a topic that scientists are still exploring, but in general, clouds reflect incoming solar radiation back out to space, which can have a cooling effect on the climate.
Over the last century, adding aerosols to the atmosphere has likely reduced the amount of global warming. Today, however, as new technologies have allowed factories, power plants, and automobiles to release less air pollution into the atmosphere, the amount of aerosols has dropped. That's a good thing since air pollution is a problem for human health, but it also means that the pace of global warming is likely to increase.