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Oak leaves as aerosol collectors: relationships with wind velocity and particle size distribution. Experimental results and their implications
University of Kalmar, School of Pure and Applied Natural Sciences. (Naturresurshushållning)
University of Kalmar, School of Pure and Applied Natural Sciences. (Naturresurshushållning)
Lund University.
University of Kalmar, School of Pure and Applied Natural Sciences.
2009 (English)In: Trees, ISSN 0931-1890, E-ISSN 1432-2285, Vol. 23, no 6, p. 1263-1274Article in journal (Refereed) Published
Abstract [en]

Advancing the understanding of the aerosol-capture efficiencies of forest components such as leaves and needles, and of the mechanisms that underpin these efficiencies, is essential to the many related issues of forest turnover of nutrients and pollutants. For idealized collectors (such as artificial plates or cylinders) aerosol-mechanics offers a means for calculating capture efficiencies. For living collectors, in particular deciduous leaves, experimental investigations become necessary to assist in formulating the sub-models of capture efficiency that are fundamental to the modelling of fluxes of aerosol-borne substances to forests. We here present wind-tunnel based methods and results for leaves of Quercus robur L. exposed to an aerosol whose mass versus aerodynamic particle size distribution is characterised by a geometric mean aerodynamic particle diameter around 1.2 mu m and a geometric standard deviation around 1.8. With respect to that distribution, and founded on a specially designed leaf wash-off method, we obtained average oak-leaf capture efficiencies ranging from 0.006% of the approaching aerosol mass flux at wind-speed 2 ms(-1) to 0.012% of the flux at wind-speeds 10 ms(-1), respectively. These values can be translated into deposition velocities (V (d) ) for a leaf ensemble with a given leaf area index (LAI). With LAI in the range 2-5 (commonly found in the field) and for wind-speeds 2, 5 and 10 ms(-1), resulting V (d) -values would be 0.02-0.05, 0.05-0.13, and 0.2-0.6 cm/s, respectively. To the extent comparisons are possible, our capture efficiency values are at the low end of the range of values reported by other researchers. The strong wind-speed sensitivity of V (d) has implications for the deposition of aerosol-borne substances to forests for which wind regimes may shift as a result of climatic and land-use changes.

Place, publisher, year, edition, pages
2009. Vol. 23, no 6, p. 1263-1274
National Category
Environmental Sciences
Research subject
Environmental Science, Natural Resources Management; Natural Science, Physics; Natural Science, Environmental Science
Identifiers
URN: urn:nbn:se:lnu:diva-2104DOI: 10.1007/s00468-009-0366-4ISI: 000271504800013OAI: oai:DiVA.org:lnu-2104DiVA, id: diva2:309154
Available from: 2010-04-06 Created: 2010-04-06 Last updated: 2017-12-12Bibliographically 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. p. 190
Series
Linnaeus University Dissertations ; 43/2011
Keywords
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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Reinap, AusraWiman, Bo L. B.Gunnarsson, Sara

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