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| Block 8: Environmental Issues and Public Health - Air Pollution Chapter 2: An Overview Of The Main Air Pollutants And Their Impacts |
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Table 1 contains some of the more common air pollutants, a characterisation of these pollutants as "primary" or "secondary", and the type of effects that these pollutants may have.
| Pollutant | Primary (S) or Secondary (S) | Effect |
|---|---|---|
| Sulphur dioxide (SO2) | P | Health, vegetation |
| Particulate Matter (PM10, PM2.5) | P and S | Health, visibility impairment |
| Nitrogen oxides (NOx) (NOx = NO + NO2) | P and S | Health, vegetation, Global Warming |
| Volatile Organic Compounds (VOCs) | P and S | Health, ozone formation, smog |
| Ozone | S | Health, vegetation |
| Heavy metals, incl. Lead compounds (PbBr2, PbCl2, oxides) | P | Health |
| Carbon monoxide (CO) | P | Health |
Primary and secondary air pollutants: SO2, CO2, CO, NO, NO2, Particulate Matter (PM) and VOCs are primary pollutants - they are released directly into the atmosphere.
Particulate matter (PM) may be liquid or solid particles. PM is characterised both by its chemical composition and its size range. The term aerosol is used for PM suspended in air, size range less than about 50 µm (micrometers). PM10 refers to the size fraction with a median aerodynamic diameter less than 10 µm; PM2.5, the fine fraction, to that less than 2.5 µm; the coarse fraction to the size range (PM10-PM2.5). |
Primary pollutants are released directly (from the source) into the atmosphere. Secondary pollutants are formed through chemical transformations and physical processes. Aerosols are particulates (liquid or solid) suspended in air. |
Vehicle emissions and emissions from other combustion sources may be significant primary sources of fine fraction (PM2.5) particulate emissions. The combustion of hydrocarbons - fuel oil, diesel and petrol - produces elemental carbon as a primary particulate.
Windblown dust, the main constituent of the coarse fraction, is also a source of primary particulate matter.
Secondary pollutants are formed in the atmosphere through chemical reactions and physical processes. For example, SO2 and NO2 react with ammonia or other alkaline species, atmospheric oxygen and water vapour to form sulphates (ammonium sulphate, ammonium bisulphate and /or sulphuric acid) and nitrates (ammonium nitrate, peroxyacetylnitrate (PAN) and/ or nitric acid). The nuclei that form when these substances condense may grow through the physical processes of deposition of the gaseous component vapour onto the particles, and the agglomeration of smaller particles to form larger particles. Figure 2.1 illustrates the characteristic size distribution of particulate matter.
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| Figure 2.1: Size distribution of airborne articulate matter (adapted from Holgate et al, Figure 5.2) |
Nitric oxide (NO) is mainly produced by through combustion processes. NO is thus present in motor vehicle exhaust gases, stack emissions from stationary combustion sources such as coal, oil and diesel fired boilers and coal fired power stations, and waste incinerators. The negative environmental impacts of NO are not attributed to direct exposure to NO but to the atmospheric transformation products of NO, mainly NO2.
Ozone is a secondary pollutant formed through a complex series of reactions between NOx (NO2 and NO), volatile organic compounds and ultraviolet sunlight. Ozone formation requires several daylight hours, and may lag behind peak precursor NOx/VOC concentrations by several hours or more than 24 hours.
Carbon monoxide is formed due to the incomplete combustion of hydrocarbons - emissions from motor vehicles are the main source of ambient CO.
| Carbon dioxide (CO2) and other ‘Climate Change’ or "greenhouse" gases that contribute to Global Warming. The focus of this module is the health impacts of anthropogenic air pollutants and their impacts on human health. CO2 emissions from fossil fuel (coal, diesel, and petrol) combustion, and the resultant increase in atmospheric CO2 concentrations do not have a direct health impact. Increasing methane and ozone levels also have significant global warming impacts. The (historically) rapid increase in the average earth temperature due to the increase in these "greenhouse gas" concentrations are likely to have major large-scale impacts, including health impacts. These impacts are likely to include an increase in intense precipitation (rainfall) events, higher maximum temperatures and rising sea levels. | Anthropogenic: Originating from human activities. Non-anthropogenic or "Natural" air pollution sources are those that do not arise from human activities. The distinction between the two source types is not always clear-cut. |
