Effect of weather on air pollution

Conditions of air pollution hazardous to people on the ground arise when there is a coincidence of three factors:

• a high rate of emissions (E) per unit surface area (or from a point or line source);

• an inversion layer at a low height, which limits vertical mixing; in particular, the increase of potential temperature (Dq) in the lowest km above the ground;

• a low wind speed (w), which determines pollutant transport.

The latter two factors are purely meteorological and therefore highly variable, although there are some seasonal trends. Thus, forecasters need to monitor the product E.Dq.w, and determine safety thresholds.

A long-term reduction of the frequency of days of high pollution might be due either to reduced emissions, stronger winds or less common (or weaker) inversions, or any combination of these. Fewer high-pollution days do not necessarily prove better control of emissions (1).

An analysis of days of high concentrations of pollutants in Sydney, Australia, during 1978 - 1992 showed that pollution tends to be particularly bad when there is a light northwesterly wind over the region, and an anticyclone is located just east or southeast of Sydney, over the Tasman Sea. This is most likely in late autumn or winter. Such conditions favouring high pollution were more common in the earlier part of the period, which would account for some of the drop in high-pollution days, though this was not enough to explain all the reduction. In other words, there must have been some reduction of emissions too.

(1) Leighton, R.M. & E. Spark,1997: Relationship between synoptic climatology and pollution events in Sydney. Internat. J. Biometeor., 41, 76 - 89.