Estimating an annual-average RSP concentration for Hong Kong using days characteristic of the dominant weather patterns

The air quality modelling system Pollutants in the Atmosphere and their Transport over Hong Kong (PATH) has been designed to provide guidance on policy and emission controls in Hong Kong. One important air quality objective to be addressed is the annual-average concentration of respirable suspended particulates (RSP). In PATH, this is done by suitably weighting the simulation results from seven case-study days, each representative of a major meteorological category. This paper describes the selection of these categories, a procedure which follows Heywood (Technical Memoirs, Hong Kong Observatory, No. 6, 1953) and is based on the dominant meteorological mean sea-level pressure (MSLP) patterns for Hong Kong. A statistical analysis of RSP data from five sites showed that the chosen categories, while stratified by meteorology, are also valid for clustering the daily mean RSP concentrations into groups of similar days. The weighting coefficients (annual frequency) were obtained from an inspection of daily weather charts between 1990 and 1995. For each category, the actual day selected for modelling with PATH was chosen so that the daily-mean RSP value for each site was close to the mean for the category. In this way, the annual mean computed using the modelled days is little different from the observed annual mean at each site.