Block 8: Environmental Issues and Public Health - Air Pollution Chapter 6: Ch 6: Methods of assessing air quality; the use of air quality indices

Introduction

The methods of assessing air quality in a given (urban) area are complicated because air pollution concentrations vary in space (from place to place) and time across the area under consideration. This spatial and temporal variation in ambient air quality is due to several factors, including the location of pollutant emission sources, the height and temperature of release of the pollutants and meteorological conditions. For example, vehicle pollutant concentrations are emitted at ground level, and these concentrations are highest close (within 1 km) to heavy traffic roads. The exposure of the population in a given area is therefore a function of location within the area. Estimates of population exposure should ideally account for both the spatial and temporal variation of air quality and the population distribution across the area.

The most important methods of assessing air quality include:

1. ambient air quality monitoring;
2. modelling;
3. the preparation of emission inventories,

Since each method has limitations, a comprehensive ambient air quality assessment requires the use of all three methods.

Indoor air quality may differ significantly from outdoor (ambient) air quality – concentrations may be lower or higher, and indoor and outdoor pollutants may be significantly different. Accurate assessments of individual exposure may require time-activity data.

It is useful to classify pollutant sources as "natural" (forexample, volcanic eruptions, biogenic emissions from plants and forests) and "anthropogenic" (the result of human activities).

Ambient air quality monitoring:

An air quality monitoring system essentially measures ambient air concentrations at a number of fixed locations, for example across a city or within a region.

Continuous monitors (analysers) are instruments capable of measuring pollutant concentrations (for example, SO₂, NO₂, CO, PM) continuously and more or less instantaneously (in reality, measurements are averaged over very short periods of time). The "instantaneous" values are not in themselves useful for assessing air quality. ("Instantaneous" values are subject to measurement "noise". In addition, in the absence of instantaneous exposure-response relationships, "instantaneous" measurements cannot provide a useful estimate of health impacts.) Thus these values may be averaged over time periods of 10 or 15 minutes, one hour, 3 hours, 8 hours, 24 hours or longer periods. The time-averaged values (time weighted averages) may be compared with air quality standards or guidelines, or may be used, in conjunction with known exposure-response relationships, to estimate the potential health impacts of the air pollutant concentrations.

Automatic analysers that are capable of measuring air pollutant concentrations continuously are comparatively expensive, and have to be housed in a protected and controlled environment, usually at a fixed site or in a mobile station or caravan. (A monitoring station capable of continuously measuring the concentrations of the five common air pollutants - SO2, NO2, ozone, CO and PM - costs about R1.0 million.) A typical monitoring system consists of a limited number of monitoring sites, each measuring a specific set of pollutants. For example, the City of Cape Town has the monitoring network shown on the right:

Figure 1: Cape Town Monitoring Sites

Not all the stations measure all the commons pollutants; additional measurements are done on an ad hoc basis, and a mobile station may be relocated to a new area.

The choice of location of monitoring sites should consider factors such as:

• the need to monitor major pollution sources;
• the need to assess suspected areas of high concentration;
• the need to monitor exposure in areas with high population density and
• the need for data for a ‘control’ area remote from local pollution sources;
• the need for data on ‘background’ pollution levels, outside of the city’s expected area of impact.

Cape Town publishes monthly reports of the results obtained at its monitoring stations. (www.capetown.gov.za/airqual/reports/cmc_reports.asp) Ad hoc reports of ‘air pollution episodes’ - periods of unusually high concentrations – are published as well. The city of Durban (Ethekwini) had a fairly comprehensive monitoring network in the South Durban region. Durban’ system is currently being redesigned and upgraded. (See the available files of examples of monitoring reports for Cape Town and South Durban.)

Active and passive samplers are a lot cheaper than continuous automatic analysers, but they are only capable of periodic sampling over longer time averaging periods (usually several days), and are generally less sensitive than continuous monitors. Passive monitors are useful for initial surveys of air pollutant profiles.

Monitoring sites provide detailed information on concentrations of a particular set of pollutants at a specific site. For example, the Cape Town network provides data on 5 or 6 pollutants, at the 8 sites shown in Figure 1. However, pollutant concentrations vary continuously across the area; measurements at these sites cannot be assumed to represent conditions throughout the metropolitan area (an area of about 2 500 kms2), even if optimally located in relation to pollution sources and the exposed population. There is at present no standardised method of using monitored data to estimate average air quality or population exposure.

Very localised spatial and temporal variations in concentration may occur due to the proximity to point sources, major roads or the effect meteorology and terrain variations due to mountains, hills and buildings. Monitoring on its own does not therefore provide a coherent integrated picture of air quality. Most monitoring networks measure (continuously) the concentration of the five common pollutants. Although remote sensing instruments have been developed for monitoring volatile organic compounds, these instruments are not yet in widespread use.

In general, ambient air monitoring does not give an indication of the source of pollution. For example, sulphur dioxide and nitrogen oxides are both emitted from stationary combustion sources and vehicles, both petrol and diesel driven. Measurements at a particular location cannot readily be apportioned to one or other source on the basis of monitoring data alone. Thus if (health based) standards are exceeded, action cannot easily be taken to manage and control pollution sources.

Nonetheless, in spite of these limitations, monitoring is the most widely used method for assessing ambient air quality. Table 6.1 summarised the available air monitoring methods, together with their approximate costs, advantages and disadvantages.

REFERENCES:

Monitoring ambient air quality for health impact assessment, WHO Regional Publications, European Series No. 85 (1999)