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Urban Air Quality – The Role and Importance of Monitoring VOCs

The World Health Organisation estimates poor urban air quality accounts for 7 million deaths per year. Unless poor air quality can be addressed death rates will continue to rise.
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Photochemical and sulphurous smog differ not only in composition but also on the weather conditions that give rise to them. Photochemical smog occurs in hot sunny conditions, and peaks in the afternoon. Sulphurous smog is worse in cold damp atmospheres, peaking at dawn. Pollution may also be carried in the wind into a city, most notably smoke caused by the burning of vegetation, and from power station emissions.

Clean Air

Our atmosphere is dynamic. Biological, physical and chemical processes contribute to ‘clean air’, a gas mix which is remarkably well balanced for life and free of toxic gases and particulates. With the exception of halocarbons and a few inert gases, the concentration of gases in the atmosphere are in constant flux, and are dynamically mediated by living processes. Moreover, they are often at an optimal level for life. Oxygen is balanced at a concentration that enables aerobic organisms such as us to breath easily, whilst not so high as to cause unquenchable forest fires. Carbon dioxide is plentiful enough for plants to grow, both as a source of carbon and in retaining sufficient but not too much warmth from the sun. Elements essential for life such as sulphur and iodine are transported from land to sea in the form of volatile organic compounds (VOCs).

Many gases, which 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. What is more, 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, it would seem to their own and wider advantage. For example, it has been recently established that on being bruised, leaves release VOC messengers (pheromones) that attract predators of leaf eating insects!

acid rain on plants

Photochemical smog is a major environmental pollution culprit, produced when sunlight reacts with nitrogen oxides (NOX) and at least one volatile organic compound (VOC) in the atmosphere.

Human Impact on Air Quality

Gases and particulates arising from human activity face exactly the same processes as those realised naturally: being either photochemically oxidised and-or in forming and condensing on particulates which ultimately fall out as dust or rain. However, the sheer volume of certain ‘primary pollutants’ discharged by human activity can be hazardous, as well as in generating ‘secondary pollutants’ through various reactions. Also, high concentrations of VOCs, for example, when released into the air, may ‘overwhelm’ the very low concentrations of highly reactive atmospheric cleansing agents in air, such as OH and NO3 allowing them to remain unchanged for longer times 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 sulphurous 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 process, fugitive emissions and spillages, but also a feature of urban pollution due to volatilisation of
    solvents in domestic products such as cleaners and polishes.
    Terpenes from forest fires.

Secondary pollutants, resulting from 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).urban air quality
  • A resultant photochemical or ‘Los Angeles’ smog comprising particulates often containing metal oxides, nitric acid, PAN, dissolved VOC’s and SVOC’s.
  • Sulfurous or ‘London’ smog, formed from sulfurous coal burning, comprising water, ash, PAH’s, sulfuric acid and nitric acid.
  • Acid rain is a secondary pollutant, formed by reaction of rain droplets with NOx and SOx.
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“Urban Air Quality – The Role and Importance of Monitoring VOCs”

Urban air pollution is linked with increased levels of stroke, heart disease, lung cancer and chronic respiratory diseases. The world health organisation estimates poor urban air quality accounts for 7 million deaths per year. Currently half of the world’s population live in urban areas and 1.5 million people are added to the global urban population every week. Unless poor air quality can be addressed death rates will continue to rise.

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Monitoring VOCs of Urban Air Quality

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