Abstract
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.
Highlights
It is well known that forests contribute to improving the atmospheric environment because they play a crucial role in air purification by the uptake and deposition of air pollutants
Since wind tunnel experiments cannot consider the effect of deposition, their results show the contribution of the effect of trees on air pollutant break
It is deduced that the decrease in the tracer gas concentration among the model trees that appeared in the wind tunnel experiment corresponds to the tree wind break effect, while the concentration gradient appearing in the Ryukoku forest is due to the effects of the plant uptake and deposition of air pollutants
Summary
It is well known that forests contribute to improving the atmospheric environment because they play a crucial role in air purification by the uptake and deposition of air pollutants. Since NO2 is much less reactive than ozone, the observation of NO2 is useful for studying removal mechanism of air pollutants in forests. Gromke et al [10, 11] investigated the effect of trees on the dispersion of pollutants by means of a wind tunnel experiment and a computational fluid dynamics simulation. Wind measurements with a vane anemometer (Vector Instruments W200P) were performed at a height of 5 m using the observation tower in the Ryukoku forest and at a height of 6 m using a mast in the university campus
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