Measurements of Volatile Organic Compounds (VOC) are analyzed to characterize the sources impacting the Hong Kong area. The ratios of different VOC species, m, p-xylenes-to-ethylbenzene, C6H14-to-toluene and p- xylene-to-total xylenes are used for diagnostic analyses. Photochemical age analysis shows that the sources of reactive aromatics, the most important contributor to the photochemical reactivity, do not appear to be preferentially located in downtown Hong Kong. In addition, they do not appear to be dominated by mobile emissions based on the analyses of speciated VOC data from an earlier study, but related to industrial, waterfront, and fuel-storage activities. The ratios, p-xylene-to-total xylenes and dSO(2)/dNO(y), suggest that the anomalously high pollutant concentrations in western Hong Kong in the early morning hours of two episode days appear to have come from transport of urban-type emissions. Comparison of observed ambient ratios of selected VOC and their ratios in the speciated VOC emission inventories for Hong Kong and adjacent Pearl River Delta (PRD) Region gives mixed results. The observed ratio C6H14-to-toluene is consistent with the speciated version of the VOC emission inventory. The ratios of selected alkanes are not. This may be caused by the ;inaccuracies in the inventory and/or the speciation method.

Data obtained in Hong Kong during the Hong Kong and the Pearl River Delta (PRD) Pilot Air Monitoring Study in autumn 2002 are analyzed to unravel the relationship between ground-level ozone (O-3), pollution precursors, and cross-border transport. Ten ozone episodes, during which the hourly O-3 concentration exceeded 100 ppbv in 9 cases and 90 ppbv in one case, are subject to detailed analysis, including one case with hourly O-3 of 203 ppbv, which is the highest concentration on record to date in Hong Kong. Combined with high-resolution back trajectories, dCO/dNO(y) ( the ratio of enhancement of CO concentration above background to that of NOy) is used to define whether O-3 is locally or regionally produced. Five out of the ten Hong Kong O-3-episodes studied show a "pollution signature" that is indicative of impact from Guangdong Province. Examination of speciated volatile organic compounds (VOCs) shows that the reactivity of VOCs is dominated by anthropogenic VOCs, of which the reactive aromatics dominate, in particular xylenes and toluene. Calculations using a photochemical box model indicate that between 50 - 100% of the O-3 increase observed in Hong Kong during the O-3 episodes can be explained by photochemical generation within the Hong Kong area, provided that nitrous acid (HONO) is present at the concentrations derived from this study. An Observation-Based Model (OBM) is used to calculate the sensitivity of the O-3 production to changes in the concentrations of the precursor compounds. Generally the production of O-3 throughout much of the Hong Kong area is limited by VOCs, while high nitric oxide (NO) concentrations suppress O-3 concentration.