Examination of the Dilution and Neutralization of Acid Droplets by Tree Leaves Using a pH-Imaging Microscope

Abstract

To clarify the actual conditions necessary for the neutralization of acid rain by tree leaves, an examination of the neutralization effects of artificially-made acid droplets on several kinds of tree leaves, was conducted using a newly-developed pH-imaging microscope. Through these examinations, it was clarified that broad-leaved tree species such as beech, horse chestnut, Japanese zelkova and oak trees could dilute and neutralize acid droplets more effectively than coniferous trees, such as Japanese cedar, Japanese black pine and Japanese red pine. This result is in accordance with our previous observations conducted in the same site that broad-leaved trees were more effective in diluting and neutralizing throughfall than coniferous trees (Kikuchi et al. 2000, 2004a). Furthermore, the pH values of throughfall in September were higher than those in June for all tree species examined. In the broad-leaved trees, the relationship between the lapse of time, and pH values, could be expressed by a logarithmic approximation. The temporal variations in pH values of beech tree leaves, collected from the D-site (a forest near the Shirakami-Sanchi Natural Heritage Area), and E-site (rural, but relatively heavy traflic area) showed very similar trends. Therefore, the neutralization of acidity in these two areas was basically similar in nature, and did not depend on the amounts and constituents of aerosol substances on their leaf surfaces. The capacity to neutralize acid droplets on the abaxial surface was greater than that on the adaxial surface of beech leaves.