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An introductory wind tunnel study of aerosol-borne chloride capture by Quercus robur leaves.
Linnaeus University, Faculty of Science and Engineering, School of Natural Sciences. (Naturresurshushållning)
Linnaeus University, Faculty of Science and Engineering, School of Natural Sciences. (Naturresurshushållning)
Lund University. (Department of Physics)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Mathematical models of deposition processes to forests require information on several parameters that need investigation under experimental conditions. Among such parameters is the aerosol capture efficiency of leaves, in particular in terms of the dependency of this efficiency on aerosol type and wind speed. In this contribution, a wind-tunnel approach is used to provide quantitative estimates of capture efficiencies, at various wind velocities, for oak (Quercus robur L.) leaves. For a poly-disperse aerosol carrying a tracer substance (chloride) with ca 80% of its mass borne by particles smaller than ca 3 µm, average capture efficiencies found for Q. robur leaves were 0.0035 at wind speed 2 ms-1, 0.0023 at wind speed 5 ms-1, and 0.0019 at wind speed 9 ms-1. Corresponding values in terms of deposition velocity per unit leaf surface area are 0.7, 1.1, and 1.7 cm s-1. However, the spread in the capture-efficiency values was large. Factors potentially underpinning this spread are discussed, and include variation in the biological material used, and effects of relative humidity on particle size. These results promote possibilities for modelling aerosol-sink processes within various canopy layers (with differing leaf-surface area densities and wind speeds) and can thus assist in estimating aerosol-borne inputs of various chemical substances to forest ecosystems.

Keyword [en]
NaCl aerosol, capture efficiency, wind tunnel, oak
National Category
Natural Sciences
Research subject
Natural Science, Environmental Science
Identifiers
URN: urn:nbn:se:lnu:diva-11933OAI: oai:DiVA.org:lnu-11933DiVA: diva2:419290
Available from: 2011-05-26 Created: 2011-05-26 Last updated: 2012-01-03Bibliographically approved
In thesis
1. Aerosol deposition to coastal forests: a wind tunnel approach
Open this publication in new window or tab >>Aerosol deposition to coastal forests: a wind tunnel approach
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aerodynamically rough surfaces of forests provide for efficient air/ canopy exchange of mass, heat and momentum. In that context, the effects of forest edges come into focus, and therefore, coastal-zone forests constitute aparticular concern. Aerosol-sink modelling is of importance to the global-scalecontext, because sink strengths influence the concentration of aerosol particles in the atmosphere, and that concentration, in turn, influences climate. Dry deposition models are insufficient due to a lack of semi-empirical data and because of difficulties in parameterization of the efficiency (E) with which leaves capture aerosols. Quantifications of such parameters promote possibilities for modelling aerosol-sink processes within various canopy layers. This thesis focuses on studies of sea-salt aerosol dry deposition within models of oak canopies exposed to artificially generated aerosols in a wind tunnel. The overall goal is to advance the understanding of deposition processes in forest ecosystems. Aims are to determine capture efficiencies and deposition velocities (Vd) for oak (Quercus robur L.), to investigate E and Vd dependence on aerosol particle size, wind velocity and vegetation structural elements such as Leaf Area Index (LAI), to explore edge effects on deposition, to relate my results to natural situations in the field, and to address modelling applications. This thesis is a result of five studies. The first study is based on developing awind tunnel approach with a main focus on establishing reference conditions.The next step is to quantify E and provide estimates of how E, with respect toa well defined mass-vs-particle-size distribution, varies with wind speed. To that end, a special wash-off technique is developed. Finally, edge effects ondeposition processes are investigated. Results demonstrate that forest ecosystems would experience substantially increased deposition at edges. The findings suggest that field measurements of deposition in the interior of a forest “island” in an otherwise open landscape would underestimate the deposition to the entire forest. Results clearly indicate needs for further research on the effects of LAI on capture efficiency and deposition velocity. The obtained capture efficiencies can be translated into deposition velocities for trees with a specific leaf area. An increase of Vd with increasing wind speed is found, and is consistent with other studies. Results confirm advantages of the wind tunnel approach, including its ability to enable experiments under controlled conditions. However, several problems require that explicit sub-models be developed of wind-speed dependent effects on leaf posture in the aerosol flow field and that gradients in relative humidity close to leaf surfaces need further attention. The results also propose needs for a range of further experimental investigations regarding aerosol deposition across the complete sea-to-land aerodynamic transition.

Place, publisher, year, edition, pages
Växjö, Kalmar: Linnaeus University Press, 2011. 190 p.
Series
Linnaeus University Dissertations, 43/2011
Keyword
NaCl aerosol, dry deposition, climate change, coastal-zone forest, Quercus robur, wind tunnel, edge effect
National Category
Natural Sciences
Research subject
Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-11999 (URN)978-91-86491-71-0 (ISBN)
Public defence
2011-05-06, A137, Landgången 4, Kalmar, 09:30 (English)
Opponent
Supervisors
Available from: 2011-06-07 Created: 2011-05-30 Last updated: 2011-06-07Bibliographically approved

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