A synoptic climatology of tropospheric ozone episodes in Sydney, Australia

Concentrations of tropospheric ozone often exceed Australian air quality goals in Sydney during summer. However, features in the occurrence of ozone in Sydney are yet to be fully explained. Meteorological conditions associated with ozone episodes in Sydney are caused by complex interactions between synoptic and meso-scale processes. This paper discusses the meteorological influences behind ozone pollution episodes in Sydney. A synoptic climatology of ozone episodes in Sydney was generated using multivariate statistical techniques, including principal component analysis (PCA) and a two-stage cluster analysis, to classify days into meteorologically homogeneous synoptic categories. Surface and upper air meteorological data for warm months (Oct-Mar) over a 10-year period were used as input into the statistical analyses. Eleven synoptic categories were identified in Sydney during the warm season and ozone concentrations associated with each of the synoptic categories were investigated. One synoptic category was found to be associated almost exclusively 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 northwesterly gradient winds, an afternoon sea breeze, high afternoon temperatures, a shallow mixing height at the coast and warming aloft during the day. Over 90% of all days exceeding current air quality goals for ozone in Sydney fell within the synoptic category associated with the highest ozone concentrations. 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 siting of future ozone precursor sources in Sydney and surrounding regions.