A Climatology of Photochemical Smog Episodes in Sydney, Australia

Concentrations of ozone often exceed Australian air quality goals in Sydney during summer. There remain features in the occurrence of photochemical smog in Sydney that are yet to be fully explained. Meteorological conditions associated with photochemical smog episodes in Sydney are complex and include both synoptic and meso-scale processes. This paper aims to increase the understanding of the meteorological influences behind photochemical smog formation in Sydney.

A synoptic climatology of photochemical smog episodes in Sydney was undertaken. This analysis involved using multivariate statistical techniques including principal component analysis and a two-stage cluster analysis to classify days into meteorologically homogenous synoptic categories. Surface and upper air meteorological data for warm months (Oct-Mar) over a ten year period were used as input into the statistical analysis. Eleven synoptic categories were found in Sydney during the warm season. Ozone concentrations associated with each of the synoptic categories were investigated. One synoptic category was found to be associated with high pollution concentrations. High ozone concentrations were found to be associated with a high pressure system located in the middle to eastern Tasman Sea producing light north westerly gradient winds, an afternoon sea breeze, high afternoon temperatures, a shallow mixing height and warming aloft.

Of the days in the high ozone cluster, 48% were found to exceed the New South Wales future 1 hour ozone goal (8 pphm), a further 32% were found to exceed the World Health Organization 4 hour ozone goal (6 pphm), while the remaining 20% of days recorded concentrations less than 6 pphm ozone. To further understand the meteorological influences behind photochemical smog episodes in Sydney, the days falling in the high pollution cluster were analysed in greater detail, particularly with respect to meso-scale conditions. This involved investigating the meteorological differences between those days with high ozone concentrations and days with similar synoptic conditions yet relatively low ozone.

Two days falling within the high pollution category were examined in more detail. These consecutive days had almost identical synoptic charts, yet experienced quite different ozone concentrations. An examination of surface and upper air meteorological observations showed that it was the differing and complex meso-scale flows experienced on these days that produced the different pollution regimes. These examples illustrated the complexity and difficulty of predicting some high ozone events in Sydney. A discussion of future investigations to be undertaken to refine this climatology is included.

It is envisaged that results from this research will be useful to Australian regulatory bodies from both a forecast point of view and for the planning of future sources in Sydney and surrounding regions.