This project is to evaluate the air-cleaning efficiency of common tree species in Taiwan and to measure the isoprene emission rates of them. Since isoprene may react actively with nitrogen oxides to produce ozone in urban atmosphere, the urban trees with high isoprene emission rate have recently obtained great concerns in Taiwan. A branch enclosure method was used to measure the ozone and nitrogen dioxide absorption rates of 10 tree species in urban parks. Results showed that there was a cleaning efficiency of 2-15% for most of the trees. However this method showed a disadvantage of large variation in wind speed and ambient polluatnt concentration, causing the difficulty in explaining of the results. A PP plastic bag enclosure method was then developed to measure the ozone absorption rates of 20 tree species. Results showed that Taiwan acacia, pudding-pipe tree, camphor tree, Taiwan Zelkova and Chinese pistache have higher ozone absorption rates. A PP plastic bag enclosure method in combination with a GC-FID instrument installed with a Se-30 packing column was used to quickly detect the isoprene emission rates. Totally 33 popular tree species were screened in the first stage in 2003. Among them eight species were found to be isoprene emitters. They are mango, Formosan sweet gum (Liquidambar formosana), sisso tree (Dalbergia sissoo), white bark fig (Ficus benjamina), rubber plant (Ficus elastica), poongaoil (Pongamia pinnata), Ceylon olive (Elaeocarpus serratus), and Hong kong Orhid tree (Bauhinia blakeana). The results suggest that these species should be avoided or limited in urban reforestation, although there is no direct evidence showing that the emitted isoprene is the principal precursor responsible for the raised ozone level in four urban areas in Taiwan.

air-cleaning,tree,bvoc