Roadside concentrations of traffic related TAPs is considered crucial for the SUSDEV 21 baseline database, as this is a data gap in EPD's current toxic air pollutants monitoring programme. The general objective of this study is to provide information related to the roadside concentrations of selected TAPs and to incorporate this data into the SUSDEV 21 baseline database. VOCs, carbonyls and PAHs were monitored for one year at the PolyU roadside station by HKPU and the DRI. This study provided an integrated set of data representative of several sources in the urban areas of Hong Kong and bridged the roadside TAP data gap.

From this study, it is evident that the most important source of many TAPs of concern is vehicular exhaust emissions. The concentrations of target air toxic pollutants monitored at the PolyU station were compared with those collected at two of EPD's air quality monitoring stations that presently monitor TAPs (Central/Western & Tsuen Wan. Concentrations of benzene, 1,3-butadiene, anthracene, benzo(a)pyrene and chrysene were highest at PolyU station, most likely due to dominating vehicular emissions.

Monthly variations, seasonal variations and winter/summer ratios at the three stations were also investigated. Monthly trend variation patterns for benzene and 1,3-butadiene were found at the three stations. The concentrations of benzene, methyl chloride and all selected PAHs in winter were higher than in summer at the PolyU station, with the winter to summer ratios being larger than 1. There were many factors that could have affected the seasonal variation. More rainy days in the summertime caused the washout effect of pollutants, especially the pollutants attached to particulates. Chemical removal, especially by hydroxide radicals, is also a very active process during the summertime. The distance to the emission source is another determining main factor for the concentration levels.

Correlations of toxic pollutants were determined and the findings were as follows:

• high correlations were found between five VOCs except 1,3-butadiene;

• the carbonyls formaldehyde and acetaldehyde were well correlated; and

• benzo(a)pyrene and chrysene, two carbonyls and five VOCs (with the exception of methyl chloride) also had high correlation coefficients.