Analysis of Air Purification in a Woodland by Field Observation and Wind Tunnel Experiment

As part of a comprehensive study of the harmonious coexistence between man and nature focusing on woodland, we evaluated the role of the Ryukoku forest in air purification. The Ryukoku forest, located at Seta Hill in Shiga Prefecture, Japan, is composed of a mixture of conifer and deciduous trees. There are two mechanisms in purifying forest air: the deposition of air pollutants onto leaf surfaces and the effect of a tree wind break (air pollutant break). We measured the vertical distributions of nitrogen dioxide (NO2) concentration inside and outside the Ryukoku forest using passive samplers attached to a 25-m-tall observation tower and to the neighbouring university campus buildings, respectively. Wind tunnel experiments that treat atmospheric dispersion in model trees but cannot consider the effect of deposition have been conducted to evaluate the effect of a tree wind break. The observed NO2 concentrations in the Ryukoku forest showed a 5 - 20 percent decrease depending on the season towards the forest floor from the top of the canopy. Within the forest canopy layer, the average NO2 concentration was approximately 30 percent lower than that outside the forest. The results of the wind tunnel experiment showed a similar decrease in the concentration of air pollutants inside the forest. However, the decrease in the concentration of air pollutants towards the forest floor from the top of the canopy was not observed in the wind tunnel experiment. It is deduced that the low NO2 concentration inside the forest compared with that outside is mainly due to tree wind break effect and that the variation in NO2 concentration with height from the forest floor is mainly due to the deposition of air pollutants onto leaf surfaces.