台北市四種行道樹對大氣沉降物的截留比較

Abstract

空氣汙染是日趨嚴重的環境議題，特別是在人口稠密的都市地區，植物透過冠層截留能有效的過濾空氣中的汙染物。本研究藉由收集穿落水及雨水比較榕樹(Ficus microcarpa L.f)、茄苳(Bischofia javanica)、樟樹(Cinnamomum camphora)、台灣欒樹(Koelreuteria elegans)四種台灣常見行道樹對大氣沉降物的截留情形。四個樹種中以榕樹冠層截留的懸浮微粒量最高(667 mg/m^2)，而單位葉面積截留懸浮微粒的能力在四種樹間並無顯著差異。穿落水量與冠層洗出的懸浮微粒重量呈現顯著正相關，且降雨強度大時所洗下的懸浮微粒重量也較高；將葉片採回洗滌的實驗中，樟樹葉片洗出最多的懸浮微粒(423 mg/m^2)，與實際受雨水淋洗的結果不同，其差異應是受洗滌力道所影響；主要陰陽離子的淨變化量也以榕樹較高；懸浮微粒中所含金屬(銅、鉻、鉛、鋁)則是在樹種間沒有顯著差異，並與降雨量、乾沉降累積天數、平均細懸浮微粒濃度的相關性皆非常低，其中鉛在四個樹種穿落水中的含量皆小於雨水，表示冠層對鉛有淨留置的作用。Air pollution is an important environmental issue, especially in heavily populated cities. Tree canopies could effectively filter polluted air through intercepting pollutants. This study compared the interception of atmospheric deposition among four common sidewalk tree species in Taiwan, Ficus microcarpa L.f, Bischofia javanica, Cinnamomum camphora, and Koelreuteria elegans, through collection and analysis of throughfall and rainfall. The canopy of F. microcarpa intercepted the largest amount of suspended particles (667 mg/m^2). However, on a per unit leaf area basis, the efficiency of interception of suspended particles (PM10) was not different among the four species. The amount of PM10 washed off tree canopies positively correlated with throughfall quantity and rainfall intensity. Unlike the result from the wash of rainfall, the amount of suspended particles was highest from leaves of C. camphora’s (423 mg/m^2) in the hand-wash experiment, possibly due to the differences in thoroughness of wash between rainfall and hand-wash. The differences in the content between rainfall and throughfall was higher for F. microcarpa than other species but such differences were not found for metals, Cu, Cr, Pb, and Al. There were no significant differences in net change of metal content between rainfall and throughfall among the four species. Moreover, the changes were either weakly or not significantly related to rainfall intensity, dry period before rainfall, or concentration of particulate matter smaller than 2.5 μm (PM2.5). Unexpectedly, the concentration of Pb is lower in throughfall than in precipitation indicating that tree canopies are a sink of Pb.