Titanium dioxide (TiO2) fine particle capture and BVOC emissions of Betula pendula and Betula pubescens at different wind speeds

Abstract

Trees are known to affect air quality by capturing a remarkable amount of particles from the atmosphere. However, the significance of trees in removing very fine particles (diameter less than 0.5 μm) is not well understood. We determined particle capture efficiency (Cp) of two birch species: Betula pendula and Betula pubescens by using inert titanium dioxide fine particles (TiO2, geometric mean diameter 0.270 μm) at three wind speeds (1, 3 and 6 ms−1) in a wind tunnel. Capture efficiencies were determined by measuring densities of TiO2 particles on leaf surfaces by scanning electron microscopy. In addition, the particle intake into an inner structure of leaves was studied by transmission electron microscopy. The effects of fine particle exposure and wind speed on emission rates of biogenic volatile organic compounds (BVOCs) were measured. Particles were captured (Cp) equally efficiently on foliage of B. pendula (0.0026 ± 0.0005) % and B. pubescens (0.0025 ± 0.0006) %. Increasing wind speed significantly decreased Cp. Increasing wind speed increased deposition velocity (Vg) on B. pendula but not on B. pubescens. Particles were deposited more efficiently on the underside of B. pendula leaves, whereas deposition was similar on the upper and under sides of B. pubescens leaves. TiO2 particles were found inside three of five B. pendula leaves exposed to particles at a wind speed of 1 ms−1 indicating that particles can penetrate into the plant structure. Emission rates of several mono-, homo- and sesquiterpenes were highest at a wind speed of 3 ms−1 in B. pendula. In B. pubescens, emission rates of a few monoterpenes and nonanal decreased linearly with wind speed, but emission rates of sesquiterpenes were lowest at 3 ms−1 and increased at 6 ms−1. Emission rates of a few green leaf volatile compounds increased with increasing wind speed in both species. The results of this study suggest that the surface structure of trees is less important for capturing particles with a diameter of ca 0.3 μm than for larger particles. Airborne fine particles penetrated into the intercellular space of the leaf via stomata, and this mechanism should be studied further for a better understanding of nanomaterial accumulation in nature. Wind can affect BVOC emissions and composition.