Particle capture efficiency of different-aged needles of Norway spruce under moderate and severe drought

Abstract

Trees can remove particulate matter from the atmosphere, improving air quality and providing ecosystem services. Particle removal capacity is known to differ between tree species, but the influence of environmental factors on the removal capacity is still unclear. In this study, we measured particle capture efficiency (Cp) of Norway spruce (Picea abies (L.) Karst.) in wind tunnel experiments under three watering treatments (well watered, moderate drought, and severe drought) and determined needle characteristics (stomatal conductance and density, wax condition, and needle area) that affect particle uptake. Trees were exposed in the wind tunnel to 0.7 μm (geometric mean diameter) NaCl particles with a mass concentration of 1 mg·m−3, and the Cp of the tree was determined for the current-year (C) and previous-year (C+1) needles. Overall, the Cp was significantly higher for C+1 needles than for C needles for all watering treatments. There was also a trend for higher Cp of C+1 needles of less watered trees, but this was not observed for C needles. We suggest that greater erosion of the wax layer of C+1 needles compared with C needles increases hydrophilicity of the C+1 needle surface and this, in interaction with low stomatal conductance, led to the higher Cp.