Air pollution types and sources

Air pollution refers to the mixture of substances in the air that are damaging to human health and the environment.

Pollutants can be gases or particles, and can come from natural or anthropogenic (human) sources (Table 2). Particles can be solid (e.g. soot, dust) or liquid (e.g. water), or a mixture of both. Particles range in size from less than 1 nanometre thick (less than 1 millionth of a millimetre) to hundreds of micrometres. The size and density of particles determine how far they can travel in the atmosphere and how quickly they fall to the ground.

When pollutants come directly from a source, they are called primary pollutants. These include particles such as dust and industrial emissions. Secondary pollutants are created when a primary gas-phase pollutant is oxidised by a chemical reaction. For example, when volatile organic compounds (VOCs) are combined with nitrogen oxides (NO_x), ozone and fine particles such as PM_2.5 may be produced (Figure 6). Chemical reactions can also occur on particle surfaces, and substances can condense onto or dissolve into surface layers.

Air quality is also affected by climate and weather processes (Figure 7). Winds can transport air pollutants to other regions, causing them to settle far from where they were produced. The height of the atmosphere’s boundary layer (the lowest part of the atmosphere) affects how much of a pollutant is found at ground level. During the day, warmer temperatures increase the height of the boundary layer, allowing more room for pollutants to disperse. Cooler temperatures can bring the boundary layer closer to the ground, which can concentrate pollutants. Rain can affect the amount of pollutants in the atmosphere, because water droplets remove particles from the air as they travel through it.

Natural, domestic and industrial sources all contribute to air pollution (Figure 8).

Natural sources of particle emissions include sea salt, windblown dust, bushfires and the condensation of chemically aged biogenic VOCs from vegetation. Biogenic VOCs are types of organic chemicals that turn into gases at ambient temperatures, such as the chemicals that make up the smell of flowers and herbs. Natural sources have a seasonal cycle, and their levels of emissions depend on meteorological variables (e.g. hot summertime temperatures and low soil moisture increase windblown dust; biogenic VOCs also peak in summertime, and summer is the peak bushfire season). Sea salt emissions are dependent on wind speed, and peak during storm periods.

The main human sources of emissions are industrial activities, transport, and commercial and domestic operations. Human-made emissions also vary with time; for example, more emissions are produced from peak-hour traffic during weekdays than on weekends, and emissions from wood heaters are higher in winter. Human-made emissions also vary across regions depending on factors such as local industries, population density and the presence of major roads.

Some human-made emissions do not break down easily and can stay in the environment for many years. Mercury used in gold mining during the mid- to late 1800s is still being cycled through the atmosphere via soil and vegetation processes. Some industries produce long-lived synthetic chemicals (such as persistent organic pollutants and polycyclic aromatic hydrocarbons) that can damage human health.