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| Block 8: Environmental Issues and Public Health - Air Pollution Chapter 4: The Principal Sources Of The Common Air Pollutants; Methods Of Estimating Emission Rates (Continued) |
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The use of fuels such as coal, paraffin, wood and gas for domestic cooking and heating results in significant indoor air pollution, and associated health impacts. An additional hazard associated with the use of these fuels, especially in informal housing situations, is that of accidental fires. Table 4.2 (emission factors for different fuels) shows that the emission rates from different fuels varies greatly, with LPG (liquefied petroleum gas) being the ‘cleanest’ fuel. Caution should be excercised in using published emission factors because actual emission rates vary considerably, especially for coal, wood and paraffin because they are dependent on combustion conditions. Under conditions of poor ventilation carbon monoxide levels may increase to life-threatening concentrations.
The use of unsafe fuels for domestic purposes is strongly dependent on socio-economic status (poverty level). The contribution of domestic fuel use to total ambient pollution levels clearly depends on the fuel used and the total number of household using the fuel. In the townships of Gauteng and surrounding Highveld areas, coal is widely used for domestic cooking and heating, resulting in very high concentrations of particulates and SO2, particularly in winter. Paraffin is the main domestic fuel in poor areas of the Western Cape (due to the high cost of coal in this area).
Emissions from landfill sites (mainly methane and odourous sulphur containing compounds) are regarded as ‘area’ sources. Multiple small sources, such as bakery ovens and small industrial sources are also treated as "area" sources. Agricultural activities may release methane, dust pesticides due to spray drift.
Emissions from motor vehicles may be accounted for as "strip" sources (along the main high traffic routes) or area sources in areas of low traffic density.
An emission inventory is a periodically updated database of pollutants emitted to the atmosphere, usually expressed as total annual emissions. In some countries, for example the USA, Canada, Mexico) the detailed emission inventory is publicly available (on the internet). Citizen’s groups are then able to access the data and to pressurise industries to reduce emissions. (See the annual emission inventory summary reports for the USA and Canada in the Materials folder. Else search for these on the Web.)
An emission inventory (usually expressed as g/s) may be used as input to an air pollution modelling study.
More recently, emission inventories include emissions to all media (air, water and soil), and transfers between media. (Alternate names: Toxic Release Inventory in the US, Pollutant Release and Transfer Registry (PRTR) by UNEP.) At a national level, emissions are estimated based on a common methodology. Total emissions are the sum of emissions from all sources, both stationary and mobile.
A typical emission inventory may include 150 to 200 substances. The list is based on an insight into the activities and processes that are likely to produce specific emissions, and the practicality of estimating and tracking the specific pollutants. Emissions may be estimated using emission factors (literature values, validated whenever possible through direct source emission measurements) and activity rates, with due allowance for emission control devices if applicable.
An appropriate emission factor for each compound may be derived through an analysis of all factors affecting vehicle emissions - fuel composition, vehicle characteristics, driving cycle, a profile of the different vehicles in the fleet, etc., - to obtain a factor that may be expressed as mg Pollutant per litre of fuel consumed, or mg/ km travelled. The factor is then applied to the activity rate (litres per annum or km per annum) to estimate total emissions. If an emission control device is fitted to a vehicle, the emission factor would be adjusted to account for the reducing achieved. Similarly, for stationary emission sources such as a refinery furnace, an appropriate emission factor is applied to the total fuel burnt per annum.
The compilation and maintenance of an emission inventory is a considerable task, although not particularly costly to maintain once the regulatory and administrative infrastructure has been established. Technical (and possibly financial) assistance for the establishment of a PRTR is available from several countries and the United Nations Environmental Program. (See the UNEP website.) The initial inventory may be a first approximation only, but successive annual inventories should reflect a progressive improvement in the quality and completeness of the data.
An emission inventory (or PRTR) cannot be established without regulations for the compulsory provision of input data. The legal framework for such regulations, and the necessary administrative infrastructure, are therefore prerequisites.
Public access to the PRTR data is imperative. The experience in a number of countries that have implemented a PRTR is that public access to the data enables affected communities to exercise their environmental rights, and results in a decrease in emissions, with or without regulatory intervention.
