We first introduced the term aerosol to you in Session 1's Air Quality and the Atmosphere. To quickly review, the term aerosol specifically refers to any solid or liquid particle suspended in the air with the exception of liquid water and frozen water. Aerosols originate from both anthropogenic and natural sources. Some of the more common aerosols and their sources include: wind blown dust from agricultural processes, salt from sea spray, acidic aerosols from emissions of oxides of nitrogen, oxides of sulfur, and hydrocarbons from combustion in engines and industrial processes, smoke from forest fires, volcanic debris, and pollens. As we will soon discuss, aerosols are central to the processes of cloud droplet and ice formation which ultimately lead to precipitation. However, too much of the wrong aerosols in the atmosphere can be detrimental to the health and safety of humans and other animals, as well as plant life and both natural and man-made structures.

In the next few pages, we will briefly look at how aerosols are categorized, some of their sources, and the global distribution of various aerosol concentrations.

Categorization of Aerosols and Their Sources

Physical size is one way in which aerosols are categorized. The smallest aerosols, which are called Aitken Nuclei, are those that are smaller than 0.2 micrometers in diameter. (One micrometer is equal to 1/1,000,000 of a meter or 1/1,000 of a millimeter. For comparison, a human hair is about 50 micrometers in diameter.) The Aitken nuclei were named after John Aitken who invented the Aitken nucleus counter in the 1800's, a device still in use today for determining the number of aerosols in a sample of air. The Aitken counter essentially grows cloud droplets by quickly expanding a sample of moist air which reduces the pressure and causes the sample to become supersaturated by several hundred percent. This causes the water vapor to condense onto the aerosols present in the sample which produces liquid droplets that can be counted more easily than the aerosols themselves.

The Aitken nuclei are made up of particles that are the by-products of combustion, as well as particles that are products of the gas-to-particle conversion process. The particles that are released as a result of combustion are mainly from anthropogenic sources. However, the gas-to-particle conversion process is a natural source. These particles are produced when trace gases either react with other gases or particles present in the atmosphere or absorb solar radiation and, as a result, undergo photochemical reactions. For instance, sulfur dioxide can convert into various sulfates in the presence of ammonia and moisture. While the reactants may not result from natural sources, the particulates that are produced are by-products of these natural chemical reactions.

The next largest size group of aerosols is called the large aerosols. They range from 0.2 micrometers to 2 micrometers in diameter. This group includes salt from sea spray, small pollens, larger products of combustion, and the coagulation of the smaller Aitken nuclei.

The largest of the aerosols, though still very small, are classified as the giant aerosols. The giant aerosols are all those larger than 2 micrometers. This group includes the wind blown dust, particle by-products of industrial processes, as well as salt from sea spray, and larger pollens.

Much attention has been focused on aerosol size due to the health risk they pose when we inhale them. PM-10's (particles whose diameters are less than or equal to 10 micrometers) and PM-2.5's can travel deep into our lungs and accumulate there where they can eventually cause respiratory illnesses. Many of these small particles have been determined to be carcinogens.

Next, we will look at how various aerosols are distributed across the globe.

On to Global Distribution of Aerosol Concentrations

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