AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
The Effects of Leaf Area Density Variation on the Collection Efficiency of Ultrafine Particles (UFP)
CHENG-WEI HUANG, Ming-Yeng Lin, Andrey Khlystov, Gabriel Katul, Duke University
Abstract Number: 530 Working Group: Aerosol Physics
Abstract The removal of ultrafine particles (UFP) by vegetated surfaces remains a subject of active research in many applications pertinent to climate, cloud physics, human health and respiratory related diseases. Vegetated surfaces are known to be a sink for UFP, prompting interest in the relationship between leaf area and UFP collection efficiency. A number of models have tackled the UFP collection efficiency by vegetation for an isolated leaf or uniformly distributed leaf area; however, up-scaling these theories to the ecosystem level must confront the problem of how spatial gradients in leaf area density impact the collection efficiency. This question is explored by setting up three scenarios in a wind tunnel: (1) Juniperus Chinensis branches that are uniformly distributed within the test section (uniform LAD); (2) LAD that is linearly increasing with downwind distance and (3) LAD that is decreasing with downwind distance. The total leaf area index (LAI) remains the same in all three cases. Particle concentrations were measured at multiple locations within the vegetated volume for a range of sizes of UFP (12.6-102 nm) using Scanning Mobility Particle Sizer. The measured concentration can be used to evaluate the performance of size-resolving model that couples the turbulent flow field and the collection efficiency for the variable LAD. The model assumes that (i) the mean longitudinal momentum balance is controlled by the interplay between drag force, the pressure gradient and the horizontal advection term, and (ii) the dominant collection mechanism is Brownian diffusion. Hence, other collection mechanisms such as inertial impaction, interception and phoretic effects are ignored. The agreement between the model calculations of the UFP collection efficiency by the vegetation and the wind tunnel measurements for all three cases is discussed for a wide range of wind speeds and particle diameters.