Our atmosphere is dynamic with biological, physical and chemical processes contributing to ‘clean air’, a gas mix that is well balanced for life and free of toxic gases and particulates. Oxygen is at as high a concentration as it enables aerobic organisms such as humans to breathe easily, while not too high resulting in unquenchable forest fires.
Carbon dioxide is plentiful enough for plants to grow, both as a source of carbon and in retaining sufficient warmth from the sun. Elements essential for life such as sulfur and iodine are transported from land to sea in the form of volatile organic compounds (VOCs). VOCs are chemical compounds described as volatile because they evaporate easily, releasing molecules into the atmosphere.
Many gases that are toxic or harmful to life are removed by chemical and physical adsorption on solid particles (particulates), which ultimately fall out of the air under gravity as dust or rain. Through a series of chemical reactions, including reactions with sunlight (photochemical reactions), the atmosphere is kept almost completely free of specific VOCs released by plants. For example, it has been recently established that when bruised, leaves release VOC messengers (pheromones) that attract predators of leaf-eating insects.
Gases and particles appearing from human activity face exactly the same processes as those realized naturally: either photochemically oxidized and/or in forming and condensing on particulates which ultimately fall out as dust or rain. However, the volume of certain ‘primary pollutants’ discharged by human activity can be hazardous in itself, as well as in generating ‘secondary pollutants’ through various reactions. Also, high concentrations of VOCs released into the air may ‘overwhelm’ the very low concentrations of highly reactive atmospheric cleansing agents, such as OH and NO3, residing in the air unchanged for longer periods than they would be in cleaner air.
Primary pollutants, released directly into the atmosphere, include:
- NO and NO2 (‘NOx’), chiefly from vehicle exhaust and coal-fired power stations
- SO2 and SO3 (‘SOx’) primarily from sulfurous coal-burning stoves and power stations
- CO from vehicle exhaust, wood and coal burning
- VOCs apart from methane from the following sources:
- Unburnt hydrocarbons from vehicles. This pollutant is eliminated where legislation ensures catalytic conversion of unburnt hydrocarbons.
- Solvent release and spills. These may arise from poorly controlled industrial plant processes, fugitive emissions and spillages, but also a feature of urban pollution due to volatilization of solvents in domestic products such as cleaners and polishes.
- Terpenes from forest fires.
- Particles released directly into the atmosphere such as
- Metal oxide particles generated on vehicular breaking, coal burning, mineral processing.
- Soot from diesel exhaust.
- Tire wear.
- Polyaromatic hydrocarbons (PAHs) ash and soot from forest fires, stubble burning and wood-burning stoves.
Secondary pollutants, resulting from the action of sunlight on NOx and VOCs, are:
- Ozone, O3, which in the lower atmosphere is very harmful to all lifeforms
- Aldehydes such as formaldehyde, a harmful biocide
- Peroxyacyl nitrates (PANs)
- A resultant photochemical or ‘Los Angeles’ smog comprising particulates often containing metal oxides, nitric acid, PAN, dissolved VOCs
- Sulfurous or ‘London’ smog, formed from sulfur coal burning, comprising water, ash, PAH’s, sulfuric acid and nitric acid
- Acid rain is a secondary pollutant, formed by the reaction of rain droplets with NOx and SOx
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