Thin water films on coniferous needles

Abstract

Abstract Wetness measurements were conducted directly on the surface of coniferous needles by means of specially designed wetness sensors in different heights of a 40-year-old spruce canopy. During the observation time from May to October, 1992, the wetting by rain amounted to about 28% of the time, not differing significantly with the height of the tree. In addition, during a pproximately 40% of the total time, a strong correlation between relative humidity and electrical conductance on the needles was observed. This suggests and gives evidence for the existence of thin water films present on the spruce needles at relative humidity values as low as 50%, despite the hydrophobic nature of the surfaces. The importance of dry-deposited atmospheric particles for the formation of the films is shown by a comparison between washed and polluted needles and may be explained by a combination of capillary and salt effects (Eiden et al., J. Aerosol Sci., 1994) . During a laboratory measurement in a dew chamber in which the relative humidity was stabilized, constant conductance signals were achieved due to the strong coupling between relative humidity and electrical conductance along the needle surface. After several hours of constant conductance at 70%'relative humidity, the addition of ammonia to the chamber clearly led to an increase of the signal, thus indicating dissolution of NH, and the formation of ammonium ions in the films. This is taken as an indication for the general ability of thin water films to dissolve trace gases as well as for the migration of ions along the needle surface. The implications of these results on the exchange processes between vegetation and the atmosphere are discussed