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Urban Air Quality (continued)

Meteorology that exacerbates air pollution [A Indicator 3.11]

Air pollution depends on an interplay of factors. The two most important are the nature and quantity of the pollutants that are emitted, and the meteorological conditions. Pollutants are emitted every day at much the same rate in metropolitan airsheds, but air pollution is serious only on a few occasions.

Meteorology can improve air quality or it can exacerbate air pollution. Rain washes pollutants out of the air so there is improved air quality after prolonged rain. Wind disperses air pollutants: this can be good or bad. Temperature inversions, when the normal decrease of temperature with height is reduced or even reversed within the lower parts of the atmosphere, make air pollution worse from surface sources by trapping it close to the ground.

Geographical setting is another factor. Locations along the coast will experience sea breezes, especially in summer, which carry pollution inland with the sea breeze front. Valleys are susceptible to winter local temperature inversions which can worsen air pollution.

Significant air pollution can be caused by subsidence inversions. These occur as a result of a high pressure cell situated over south-east Australia. There is a slow descent of air in such a cell that causes a temperature inversion. There are few clouds associated with a high pressure cell, so that the incoming solar radiation enhances photochemical reactions. This produces smog, provided that there are sufficient precursor emissions.

Newton (1997) has shown that the archived long-term records of the Bureau of Meteorology can be used to produce an indicative measure of smog pollution propensity. Photochemical smog occurs when there are high temperatures, clear skies and light winds. The Bureau of Meteorology climate averages provide monthly data on:

• the number of days on which the maximum temperature exceeds 30C
• the number of clear days.

Adding these two points together gives an indicative measure of smog pollution propensity (Table 17).

Table 17: Smog pollution propensity of some Australian capital cities
Values are based on the sum of both the mean number of days with maximum temperature greater than 30 C and the mean number of clear days.

	Jan.	Feb.	March	April	May	June	July	Aug.	Sept.	Oct.	Nov.	Dec.
Melbourne	12	12	9	4	2	2	3	2	2	3	5	9
Perth	31	29	24	12	7	5	5	7	8	10	14	22
Brisbane	13	10	7	4	3	4	5	5	5	6	8	13
Hobart	6	6	4	3	3	4	3	3	3	3	2	3
Sydney	9	8	8	8	8	8	11	12	9	8	8	9
Adelaide	22	23	18	10	4	3	4	4	5	8	13	17

Source: Newton (1997).

This indicative measure has excluded wind speed, which limits its utility. On the basis of the values in Table 17, the major coastal cities of Australia have varying degrees of assimilative capacity for photochemical smog. Perth seems to have the most adverse conditions, but the potential smog problem there is mitigated by the high winds in the afternoon sea breeze, and a relatively low, albeit growing, population.

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