Urban Air Pollution

The surface wind is a major factor controlling the distribution and concentration of air pollutants in urban atmospheres. Other factors are the rate of emission of the pollutants and the stability of the lowest part of the troposphere.

Emissions

Most urban pollutants come from either chimneys or automobiles. The former involve relatively slow combustion with plenty of oxygen. The fuels may contain sulphur, in which case the gas sulphur dioxide is created, along with carbon dioxide, water vapour, nitrogen dioxide and particulate matter. Particles less than 10 pm in diameter are referred to as 'PM10', and are dangerous to human health. They are so small as to remain in the air for hours or days, just like cloud droplets. Sulphur dioxide combines with moisture when the RH is over 80 per cent, forming cloud droplets of sulphuric acid. This is one reason why a humid atmosphere is usually hazy, especially in cities. The droplets eventually yield acid rain (Section 10.1). Nitrogen dioxide (NO2) is a poisonous reddish-brown gas and it too forms an acid in combination with water. The carbon dioxide resulting from combustion is implicated in global warming.

Combustion in a vehicle engine is too rapid for complete oxidation of the fuel, so that not all the carbon forms carbon dioxide (CO2) and some makes carbon monoxide (CO) instead, with only one atom of oxygen in each molecule. It is a poisonous gas, which later turns to carbon dioxide. There is also some unburnt hydrocarbon emitted, again depending on the design, speed and condition of the engine, and some monoxide of nitrogen—i.e. 'nitric oxide' (NO)—because nitrogen is the main constituent of air (Table 1.3). The NO converts to NO2 in reaction with the air's oxygen, and the various nitrogen oxides are collectively called 'NOx', pronounced 'nox'. It affects air passages in the body, so that they become sensitive to allergens which cause asthma, for instance. Also, NOx combines with the emitted hydrocarbons to form ozone when temperatures and solar radiation are sufficient, e.g. in summer at the latitude of Sydney (34°S), Santiago (33°S) and Los Angeles at 34°N (Note 14.G). This ozone is referred to as tropospheric ozone, to distinguish it from the desirable ozone in the stratosphere (Section 1.4). Concentrations of tropospheric ozone have been rising in most cities in recent decades, on account of increased automobile traffic (Note 14.G). It harms plants and can cause fatal damage to the human heart and lungs.

Emissions in the open air are diluted in two ways—vertically and horizontally. The first depends on the amount of atmospheric stirring, which increases in faster winds (Section 14.4) and greater instability (Section 7.4). The stirring is confined beneath the lowest appreciable stable layer, whose height depends on the time of day, etc. (Section 7.6). Horizontal dilution is proportional to the surface wind speed (Note 14.G).

Figure 14.20 Air movements during the course of the morning and early afternoon in summer, carrying air pollutants over Sydney. The city's extent of about 35x35 km in 1986 is shown by shading. Initially there is eastwards cold-air drainage from the hills, which picks up effluent from the vehicles and industries along the Parramatta River (1), the pollution becoming increasingly concentrated until the air flows over the Central Business District (3) at the mouth of the river. The sea breeze starts at about this time, gradually becoming a northeasterly which drives the pollutants towards the southwestern suburbs. By now the initial pollutants have reacted in the midday sunshine to form ozone, whose concentration is consequently highest in the south-western suburbs (4). The point marked (2) is Homebush, the site of the Olympic Games in the year 2000.

Figure 14.20 Air movements during the course of the morning and early afternoon in summer, carrying air pollutants over Sydney. The city's extent of about 35x35 km in 1986 is shown by shading. Initially there is eastwards cold-air drainage from the hills, which picks up effluent from the vehicles and industries along the Parramatta River (1), the pollution becoming increasingly concentrated until the air flows over the Central Business District (3) at the mouth of the river. The sea breeze starts at about this time, gradually becoming a northeasterly which drives the pollutants towards the southwestern suburbs. By now the initial pollutants have reacted in the midday sunshine to form ozone, whose concentration is consequently highest in the south-western suburbs (4). The point marked (2) is Homebush, the site of the Olympic Games in the year 2000.

The smoke and gases from vehicles and bush fires, for instance, come effectively from ground level, and vertical dilution depends on the stability of the surface air. The dilution of chimney emissions depends on the temperature of the emergent gases (they rise further if they are hot), on their chimney-top velocity, and on the atmosphere's temperature profile. An inversion layer below the height of the plume actually protects people on the ground from the pollution (Note 7.L). The plume fans out sideways as a thin layer if it is in a stable layer, but loops in vertical eddies downwind if the atmosphere is unstable.

The daily maximum concentration of a pollutant can be often be predicted with useful accuracy by means of numerous previous measurements at the same spot. For instance, today's highest carbon-monoxide measurement (parts per million in the air) might be related to yesterday's values of the maximum concentration (COx) to allow for persistence, the expected wind speed (V) at the time of heaviest traffic, the day of the week D (i.e. the likely traffic density) and the forecast cloudiness (and hence the solar radiation R at that time of year, which governs surface temperature and therefore atmospheric stability). By collecting sets of values for dozens of days, one can derive statistically the appropriate constants a, b, c, d and e in the following equation:

Thereafter, tomorrow's concentration can be predicted by inserting appropriate values for COx, V, D and R. The same sort of equation can be developed for any pollutant.

Ozone Pollution

Maximum ground concentrations of primary pollutants occur just downwind of the sources (Note 14. G). However, ozone is a secondary pollutant, which takes time to form from NOx and hydrocarbons (the 'precursor' ingredients), so that its highest concentrations are found some hours downwind of the precursor sources. Thus, the early morning drainage flow eastwards down the industrialised Parramatta Valley into Sydney (Figure 14.13) collects NOx and hydrocarbons, so that their concentrations increase in the shallow flow to the sea. A sea breeze in the late morning and afternoon then returns the air towards the south-west of the city (Section 14.2), and ozone has formed by that time, given adequate midday temperatures and solar radiation (Figure 14.20). Hence the maximum NOx concentration may occur at 8 a.m., whilst most ozone is measured in the afternoon. Figure 14.20 implies that the worst ozone pollution in Sydney is experienced to the south-west, not where the precursors were emitted, upwind in the western suburbs. This is an example of how an understanding of surface winds is relevant to city planning.

NOTES

14.A The wind profile

14.B Winds and housing

14.C Sea breezes

14.D Density currents

14.E The return period

14.F Dimensions of wind's power density

14.G Air pollution

Part V CLIMATES

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