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Aerosol deposition to coastal forests: a wind tunnel approach
Linnaeus University, Faculty of Science and Engineering, School of Natural Sciences. (Naturresurshushållning)
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 [en]
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: urn:nbn:se:lnu:diva-11999ISBN: 978-91-86491-71-0OAI: oai:DiVA.org:lnu-11999DiVA: diva2:419978
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
List of papers
1. Aerosol Deposition at the Sea/Land Interface: Pilot Experiments Using a Wind Tunnel
Open this publication in new window or tab >>Aerosol Deposition at the Sea/Land Interface: Pilot Experiments Using a Wind Tunnel
2004 (English)In: The ESS Bulletin, ISSN 1651-5382, Vol. 2, no 1, 47-73 p.Article in journal (Refereed) Published
Abstract [en]

Climate change in terms of meteorological shift or change affects the atmospheric aerosol behavior that in turn affects the ecosystems by changing their biogeochemistry in rather unpredictable ways. These linkages therefore pose risks to natural resources and thus to society.

 There are strong needs to advance the understanding of coastal-forest ecosystems, in particular with respect to sea-forest linkages and atmosphere/canopy exchange.

 This paper addresses a wind-water tunnel approach as one of the ways to study meteorological and biogeochemical effects of atmospheric aerosols. The aim of the paper is to advance the understanding of atmospheric aerosol physical properties (e.g. particle size distributions) in an experimental situation and to study basic characteristics of the wind tunnel such as aerodynamics and wind patterns.

During the experimental runs it was observed that at the start-up stage particle concentration tends to decline. After start of the aerosol generator the concentration curves begin to increase and grow towards equilibrium. In concentration equilibrium (steady state), the aerosol source strength balances the aerosol sink strength. It was observed that the fine particles (0.3-0.5 μm) and the coarse particles (3-5 μm and larger than 5 μm) behave differently in some respects. In particular, coarse particles show more noise in the concentration variations. This is because coarse particles deposited onto tunnel walls or propeller blades might occasionally become re-suspended into the tunnel air, resulting in momentarily high values in the coarse particle Laser Particle Counter (LPC) counting channel. Since the particle size determines a wide range of physical and chemical characteristics and behavior, including the distances across which particles can be transported, and also deposition rates of substances carried by particles of given sizes, information about particle size distributions is fundamental. The experiments provide valuable information about the performance of the wind tunnel and some essential aerosol properties within the wind tunnel.

Place, publisher, year, edition, pages
Kalmar: ESS, Kalmar University, 2004
Keyword
aerosols, dry deposition, coastal zone forest ecosystem, edge effect, wind tunnel
National Category
Natural Sciences
Research subject
Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-11852 (URN)
Available from: 2011-05-23 Created: 2011-05-23 Last updated: 2011-06-09Bibliographically approved
2. An introductory wind tunnel study of aerosol-borne chloride capture by Quercus robur leaves.
Open this publication in new window or tab >>An introductory wind tunnel study of aerosol-borne chloride capture by Quercus robur leaves.
(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
NaCl aerosol, capture efficiency, wind tunnel, oak
National Category
Natural Sciences
Research subject
Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-11933 (URN)
Available from: 2011-05-26 Created: 2011-05-26 Last updated: 2012-01-03Bibliographically approved
3. Dry deposition of NaCl aerosols: theory and method for a modified leaf-washing technique
Open this publication in new window or tab >>Dry deposition of NaCl aerosols: theory and method for a modified leaf-washing technique
2010 (English)In: Atmospheric Measurement Techniques Discussions, ISSN 1867-8610, E-ISSN 1867-8610, Vol. 3, no 4, 3851-3876 p.Article in journal (Refereed) Published
Abstract [en]

Within the framework of aerosol deposition to vegetation we present a specially designed leaf wash-off method used in a wind-tunnel based study, where leaves of Quercus robur L. were exposed to NaCl aerosols. We summarise the principles and illustrate the method for two types of substances, the chloride ion and the sodium ion, and for two levels of aerosol exposure prior to leaf washing. On the average, in the low-exposure experiments (S1), the 1st (2nd) wash-off step provided 90% (96%) of the amount of Cl− on the leaves. In the high-exposure experiments (S2) the corresponding values were 96% and 99%. For sodium, the general dynamics resembles that of chloride, but the amounts washed off were, in both series, on the average below what would be expected if the equivalent ratio in the tunnel aerosol were to be preserved. Na+ showed adsorption and/or absorption at the leaf surfaces. The difference between the mean values of the amounts of chloride and of sodium washed off in S1 was not statistically significant, the mean Na+ to Cl− difference as a fraction of Cl− being minus 18%±27%; corresponding values for S2 were minus 16%±9%, however (p<0.05). In the latter case, 101±57 μequiv Na+ per m2 of leaf area were missing for the equivalent relationship 1:1 with Cl− to be met. Although uncertainties are thus large, this indicates the magnitude of the Na+-retention. The method is suitable not only for chloride, an inexpensive and easy-to-handle tracer, but also for sodium under exposure at high aerosol concentrations. Our findings will help design further studies of aerosol/forest interactions.

Place, publisher, year, edition, pages
Copernicus Publications on behalf of the European Geosciences Union, 2010
Keyword
aerosol deposition, sodium ion, chloride ion, Quercus robur leaves
National Category
Environmental Sciences
Research subject
Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-11854 (URN)10.5194/amtd-3-3851-2010 (DOI)
Available from: 2011-05-23 Created: 2011-05-23 Last updated: 2016-07-08Bibliographically approved
4. Oak leaves as aerosol collectors: relationships with wind velocity and particle size distribution. Experimental results and their implications
Open this publication in new window or tab >>Oak leaves as aerosol collectors: relationships with wind velocity and particle size distribution. Experimental results and their implications
2009 (English)In: Trees, ISSN 0931-1890, E-ISSN 1432-2285, Vol. 23, no 6, 1263-1274 p.Article 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.

National Category
Environmental Sciences
Research subject
Environmental Science, Natural Resources Management; Natural Science, Physics; Natural Science, Environmental Science
Identifiers
urn:nbn:se:lnu:diva-2104 (URN)10.1007/s00468-009-0366-4 (DOI)000271504800013 ()
Available from: 2010-04-06 Created: 2010-04-06 Last updated: 2014-05-15Bibliographically approved
5. Forest-edge effects on sea-salt aerosol deposition: a wind-tunnel study using living oak leaves
Open this publication in new window or tab >>Forest-edge effects on sea-salt aerosol deposition: a wind-tunnel study using living oak leaves
2012 (English)In: Boreal environment research, ISSN 1239-6095, E-ISSN 1797-2469, Vol. 17, no 3-4, 193-209 p.Article in journal (Refereed) Published
Abstract [en]

Landscape patchiness creates aerodynamic transitions zones that affect the exchange of nutrients and pollutants between the atmosphere and vegetation. Using an artificially generated NaCl aerosol, with its mass-versus-particle-size distribution characterised by an aerodynamic mean particle diameter of 1.6 µm and a geometrical standard deviation of 1.9, we investigate the forest-edge effect on aerosol deposition within a model oak (Quercus robur L.) canopy in a wind tunnel with a smooth up-wind surface, and provide quantitative estimates of deposition rates within the emulated beach-to-forest transition. The deposition rate is maximal around the forest edge with an enhancement factor in relation to the beach deposition of 2.4 for Cl- and 2.0 for Na+. Results are in reasonable agreement with those obtained from deposition models, field studies, and other wind-tunnel based investigations. We find a tendency towards an edge effect also at the downwind forest end, which is in support of model predictions. Estimates of deposition velocities at the edge are 0.06 cms-1 and 0.05 cms-1 for Cl- and Na+, respectively. Because of the edge effect the model forest’s deposition velocity is enhanced, being 1.4 times higher around the edge in comparison with the entire forest. This suggests 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. Our results can help improve estimates of aerosol-borne inputs of nutrients or pollutants to forests that would experience shifts in meteorological regimes due to changes in climate and forestry practices, in particular with respect to deciduous species in coastal environments where forest-edge effects might be substantial.

 

Keyword
Quercus robur, deposition velocity, wind tunnel, sea/land transition, atmosphere/canopy exchange
National Category
Environmental Sciences
Research subject
Environmental Science, Natural Resources Management
Identifiers
urn:nbn:se:lnu:diva-11856 (URN)
Available from: 2011-05-23 Created: 2011-05-23 Last updated: 2016-06-30Bibliographically approved

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