| This research is aimed at filling our knowledge gap on the spatio-temporal distribution of ozone (O3), O3 precursors and their roles in O3 production in Hong Kong (HK) of the subtropical Pearl River Delta (PRD) region. The highest average O3 level was found at a rural site in northeastern HK which had increased levels of carbon monoxide (CO) and SO2 (relative to NOx), indicating frequent influences of regional air masses from the inland region. The lowest concentration of O3 was found at the urban site. The southwestern suburban site showed intermediate O3 levels on average, and analysis of CO/NOx and SO2/NOx ratios suggests this site was impacted by both local and regional pollution. The results indicated that it is imperative to consider regional scale emissions and dynamic transport in managing O3 pollution in HK. Volatile organic compounds (VOC), a major group of O3 precursors, were also examined for their spatial distribution, seasonal variation, source-receptor relationships and photochemical reactivity. The roadside site showed the highest VOC level owing to the vehicular emissions and the rural site was characterized by the aged air mass with the lowest VOC level. Ratios of VOC to NOx were about 2--3 in the urban areas and 10 at the rural site, suggesting that the formation of O3 in HK is generally sensitive to VOC. An observation-based model was further employed to study the photochemical O3 production at two suburban/rural sites in HK and the PRD. Photochemical O3 production was calculated to be limited by VOC at both sites. NO tended to suppress the O3 formation and CO was found to be negligible in the O3 formation. Among different VOC, reactive aromatic compounds such as toluene and xylenes dominated the O3 production in most cases, but naturally emitted isoprene could be important on warm days. O3 episodes in HK were indicative of a mixed impact from both local production and long-range transport but the O3 episodes at PRD site were mainly attributable to local production. Photochemistry of two O3 episodes (1-hour O3 levels of 203 and 178 ppbv) was further investigated in detail. |
