Even on the sunniest of days, many tiny liquid and solid particles are to be found floating in our atmosphere. These tiny atmospheric particles, which we also call atmospheric aerosols, impact our lives in many ways. They contribute to air pollution and its impacts on our health and environment in an as-yet poorly understood way. They add to heart and respiratory disease, in the shape of volcanic ash they are of danger to air traffic, and they form one of the largest uncertainties in our current estimates of climate change.
Atmospheric aerosols exist in great variety – they occur in all kinds of types, shapes and sizes. The smallest aerosols are a few nanometers is size, less than the size of the smallest viruses. The largest aerosols can measure several tens of micrometers, which compares to the diameter of a thin human hair. Examples of aerosols are soot, sea salt, mineral dust or tiny sand particles, and volcanic ash. Most of the atmospheric aerosols, about 90 percent, are of natural origin. Typical natural sources of aerosols are sea-spray, volcanic eruptions, forest fires, and sand storms. The remaining 10 percent of aerosols are anthropogenic, or human-made, and have a wide variety of origins. Anthropogenic aerosol sources include traffic, industry, and biomass burning. Their occurrence depends on many aspects, for example, on the availability of aerosol sources and the local meteorology, and is generally highly variable.