Measuring the Deposition Velocity of Nitrogen Dioxide on Three Big Trees in Taiwan
Tree leaves can uptake nitrogen dioxide, ozone and other toxic pollutants, thus urban trees are thought as live air cleaners in urban areas. It was on this purpose that Taiwan EPA initiated a project to evaluate the uptake efficiencies of pollutants by various tree species. Three tree species including camphor tree, Formosan michelia (Michelia compressa), and rose wood (Pterocarpus indicus) were tested in this study. A PP plastic branch chamber with size of 45 L was designed for measuring the NO2 uptake rates. When the healthy branch was enclosed, the NO2 gas was immediately introduced into the branch chamber and simultaneously the NO2 concentration was monitored. Depletion rates and deposition velocities were calculated from the net concentration change over the time, the chamber volume and the leaf areas. Results showed that in summer morning time, the old trees of camphor, Formosan michelia, and rose wood, absorbed NO2 for only 52, 6 and 14% that of 3-year-old trees, respectively. The deposition velocity of NO2 to 3-year-old camphor trees in the morning, afternoon, evening and night times were 0.84, 0.77, 0.40 and 0.18 mm/sec, respectively. From theses data, these three species of trees, respectively with height of 10.5, 12.5, and 14 m, were estimated to uptake 549, 33, and 638 mg of NO2 per day in summer time at Taipei. These results suggest that at least camphor tree and rose wood are ideal species for urban areas to uptake more NO2 as well as carbon dioxide.