C3–C12 non-methane hydrocarbons in subtropical Hong Kong: spatial–temporal variations, source–receptor relationships and photochemical reactivity

Annual data on C3–C12 non-methane hydrocarbons (NMHCs) from different areas in Hong Kong are analyzed to examine the spatial distribution, seasonal variation, source–receptor relationships, and photochemical reactivity of NMHCs in subtropical Hong Kong. As expected, the highest levels of NMHCs were found at roadsides, and the lowest levels were observed at a rural site. For seasonal variations, the rural site showed the lowest NMHCs levels in summer, but the roadside site gave a different picture, with the highest NMHCs levels in summer. This was believed to be due to the strong evaporation of alkanes in the hot season. With the exception of isoprene, NMHCs levels in Hong Kong were generally low compared to those of other overseas cities. Principal component analysis suggested that while the isoprene at the rural site mainly came from biogenic emissions, vehicular emissions were the major source in the urban areas, especially at roadsides. Ratios of hydrocarbons with different reactivities were also analyzed to evaluate the ages of air masses and emission ratios. A high toluene-to-benzene ratio at roadsides was due to the widespread use of aromatic-rich unleaded fuels in Hong Kong. The ratios of total NMHCs to nitrogen oxides were found to be 2 to 10 (ppbC/ppbv) indicating that the formation of photochemical ozone (O3) in Hong Kong is controlled by the levels of NMHCs. The reactivity of hydroxyl radicals (OH) and O3 formation potential of NMHCs were evaluated using the propene-equivalent concentration and maximum incremental reactivity. Isoprene was found to have the highest OH-reactivity and O3 formation potential at the rural site, while toluene was the most important contributor to the two parameters at the roadside site. These results are valuable for the understanding of O3 pollution in Hong Kong and the formulation of an effective strategy to manage O3.