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02.O.04 Field measurements of dry deposition and deposition velocity of aerosol particles into a spruce forest ecosystem using a surface wash-off method
J. Aerosol Sci., Vol. 25, Suppl. 1, pp. S9-S10. 1994
Pergamon
02
O
04
Copyright~)1994 Elsevier Science Lid Primed in Great Britain, All right. . . . ed 0021-8502/94 $7.00 + 0.00
FIELD MEASUREMENTS OF DRY DEPOSITION AND DEPOSITION VELOCITY OF AEROSOL PARTICLES INTO A SPRUCE FOREST ECOSYSTEM USING A SURFACE WASH-OFF METHOD
M. Marques, J. Constantin, A. Ibrom, G. Gravenhorst Forest Eco~stem Research Center Institute o f Bioclimatologv, Universi.ty of Gottingen, Bt~sgenweg 1, D - 37077 Gottingen
Keywords: dry deposition, deposition velocity, foliar leaching, aerosol.
1. INTRODUCTION Forest ecosystems are particularly important for the removal of atmospheric aerosols. This process, by which the aerosol is deposited directly onto the vegetation via sedimentation, Brownian diffusion and impaction, is known as dry deposition. It is an important pathway by which pollutants from anthropogenic sources and also nutrients enter sensitive ecosystems, especially forest ecosystems. The forest canopy structure and geometry and the surface roughness properties favour that trees act as an important sink for particles from the atmosphere. In the Soiling Forest (Germany), m situ measurements of dry deposition to living foliage of Norway spruce (Picea abies) was carried out by washing particles from needles. This extraction procedure allows direct measurements of accumulated atmospheric deposition to a "reproducible" natural sampling surface if leaching characteristics can be quantified. Knowing the dry deposition flux and the air concentration for particles without gas phase, the deposition velocity (vd) can directly be determined. For elements with gas phase the gaseous deposition has to be considered separately. 2. METHODS At the beginning of a dry period, the selected branches were cleaned with deionized water. At the end of each dry period, branches were extracted on the tree by enclosing them in a teflon bag containing 250 ml deionized water and shaking the bag for five minutes. This procedure was repeated three times immediately for each branch. The first extract was assumed of containing the total loading, from the sequential extractions a leaching correction term was determined. The atmospheric concentrations of particles were measured in close proximity to the selected branches by a five stage cascade impactor during each sampling period. All samples were analyzed for the soluble parts of AI, Ca, Fe, Mg, Mn, P, Si (with ICP); the heavy metals Cd, Cu, Pb, Zn (with AAS) and CI, K, Na, NH4+ , NO3", SO42- (with IC). The dry deposition rate (DD) onto each branch was computed by subtracting a leaching correction term (medium out of three subsequent leaching extracts: L = I/3 (LI+L2+L3)) from the total loading (C): DD = C - L. 3. RESULTS From the impactor measurements the size distributions for all measured elements were computed. The smallest mass median diameter (MMD) was measured fbr Pb with 0.4 p.m; for Ca and CI $9
M. M^aQUESet al.
SI0
MMDs greater than 3.0 ram were found. Cu and NO 3- showed a bimodal size distribution ranging from 0.7 lam to 1.9 ~tm. With a needle dry mass o f 1774.1 g/m 2 ground surface (Ibrom 1993) the dry deposition flux (including gaseous deposition for NH4 +, NO 3" and SO42") related to the area forest floor, and the deposition velocity (assuming only particle deposition) was computed (Table 1). Table 1: Mean deposition velocities (and standard deviation) in September 1992 for the soluble part of different
trace elements at the Soiling forest: also shown are values from other authors derived from throughfall analysis and a deposition model at the same location. Element
Na
CI
K
4.11
2.1
2.8
±1.0 3.0
NO3-N NH4-N SO4-S
Cd
Cu
Pb
AI
Fe
1.3
0.9
3.61
3.1
vdp(cm/s) Soiling 1992 Soiling 1990/912 Soiling 19803
3.8
1.01
1.0
2.2
± 0 . 3 ±0.6
±2.3
±0.2
~.6
±0.1
3,0
2.3
2,0
1,8
2.2
± 0 . 6 ± 0 . 8 ± 2 . 3 ±2.1 2.4
2.0
1.9
2.5
1.4
1.5
2.2
1.3
2.4
+0.7
+1.5
±0.7
±0.5
~).7
±0,5
±0.9
For the usually most leached element K a realistic deposition velocity of 2.8 (+0.6) cm/s is obtained after the leaching correction. For the super-micron particles (Na, CI, Ca and Mg) large uncertainties due to very low air concentrations were found. Nevertheless, the evaluated dry deposition velocities compare well with former experimental and theoretical values. The main advantage of this method is its direct and easy application at different tree stands. It was therefore employed at two stands in Portugal (Eucalyptus globuhts and Pmus pmea) in September 1993 under different climatic and topographic conditions and with a different composition o f the atmospheric aerosol. The experiments are evaluated at present.
Acknowledgements This work has been supported by the Portuguese Junta Nacionai de Investiga~fioCientifica e Tecaol6gica (JNICT) and by the German Federal Minister of Research and TechnoloD, (BMFT).
References:
Constantin, J. (1993): Stoffeintr~ge in ein FichtenwaldOkosystemdurch Deposition lufigetragcner Partikel und Nebeltr6pfchen. Dissertation. Institut for Bioklimatologie, Universit/RG6ttingen. Draaijers, G. (1993): The variability,of atmospheric deposition to forests: the effects of canopy structure and forest edges. Dissertation. Faculty of Geographical Sciences, University of Utrecht, HOfken, K.-D.; Georgii, H.-W.; Gravenhorst, G. (1981): Untersuchungen fiber die Deposition atmosph~irischer Spurenstoffe an Buchen- und Fichtenwald. Berichte des Instituts ffir Meteorologie und Geophysik. Universit~t Frankfurt am Main; Berieht Nr. 46. Ibrom. A. (1993): Die Deposition und Pflanzenauswaschung (leaching) yon Pflanzenn~ihrstoffen in einer Fichtenflache im Soiling. Dissertation. Institut ffir Bioklimatologie. Universit~t GOttingen. Shanley, J.B. (1989): Field measurements of dry. deposition to spruce foliage and petri dishes in the Black forest F.R.G. Atmospheric Environment, 23, No. 2. p. 403-414.
I Value given without considering leaching. 2 Constantin 1993. 3 H0fken e.a. 1981.
Pergamon
02
O
04
Copyright~)1994 Elsevier Science Lid Primed in Great Britain, All right. . . . ed 0021-8502/94 $7.00 + 0.00
FIELD MEASUREMENTS OF DRY DEPOSITION AND DEPOSITION VELOCITY OF AEROSOL PARTICLES INTO A SPRUCE FOREST ECOSYSTEM USING A SURFACE WASH-OFF METHOD
M. Marques, J. Constantin, A. Ibrom, G. Gravenhorst Forest Eco~stem Research Center Institute o f Bioclimatologv, Universi.ty of Gottingen, Bt~sgenweg 1, D - 37077 Gottingen
Keywords: dry deposition, deposition velocity, foliar leaching, aerosol.
1. INTRODUCTION Forest ecosystems are particularly important for the removal of atmospheric aerosols. This process, by which the aerosol is deposited directly onto the vegetation via sedimentation, Brownian diffusion and impaction, is known as dry deposition. It is an important pathway by which pollutants from anthropogenic sources and also nutrients enter sensitive ecosystems, especially forest ecosystems. The forest canopy structure and geometry and the surface roughness properties favour that trees act as an important sink for particles from the atmosphere. In the Soiling Forest (Germany), m situ measurements of dry deposition to living foliage of Norway spruce (Picea abies) was carried out by washing particles from needles. This extraction procedure allows direct measurements of accumulated atmospheric deposition to a "reproducible" natural sampling surface if leaching characteristics can be quantified. Knowing the dry deposition flux and the air concentration for particles without gas phase, the deposition velocity (vd) can directly be determined. For elements with gas phase the gaseous deposition has to be considered separately. 2. METHODS At the beginning of a dry period, the selected branches were cleaned with deionized water. At the end of each dry period, branches were extracted on the tree by enclosing them in a teflon bag containing 250 ml deionized water and shaking the bag for five minutes. This procedure was repeated three times immediately for each branch. The first extract was assumed of containing the total loading, from the sequential extractions a leaching correction term was determined. The atmospheric concentrations of particles were measured in close proximity to the selected branches by a five stage cascade impactor during each sampling period. All samples were analyzed for the soluble parts of AI, Ca, Fe, Mg, Mn, P, Si (with ICP); the heavy metals Cd, Cu, Pb, Zn (with AAS) and CI, K, Na, NH4+ , NO3", SO42- (with IC). The dry deposition rate (DD) onto each branch was computed by subtracting a leaching correction term (medium out of three subsequent leaching extracts: L = I/3 (LI+L2+L3)) from the total loading (C): DD = C - L. 3. RESULTS From the impactor measurements the size distributions for all measured elements were computed. The smallest mass median diameter (MMD) was measured fbr Pb with 0.4 p.m; for Ca and CI $9
M. M^aQUESet al.
SI0
MMDs greater than 3.0 ram were found. Cu and NO 3- showed a bimodal size distribution ranging from 0.7 lam to 1.9 ~tm. With a needle dry mass o f 1774.1 g/m 2 ground surface (Ibrom 1993) the dry deposition flux (including gaseous deposition for NH4 +, NO 3" and SO42") related to the area forest floor, and the deposition velocity (assuming only particle deposition) was computed (Table 1). Table 1: Mean deposition velocities (and standard deviation) in September 1992 for the soluble part of different
trace elements at the Soiling forest: also shown are values from other authors derived from throughfall analysis and a deposition model at the same location. Element
Na
CI
K
4.11
2.1
2.8
±1.0 3.0
NO3-N NH4-N SO4-S
Cd
Cu
Pb
AI
Fe
1.3
0.9
3.61
3.1
vdp(cm/s) Soiling 1992 Soiling 1990/912 Soiling 19803
3.8
1.01
1.0
2.2
± 0 . 3 ±0.6
±2.3
±0.2
~.6
±0.1
3,0
2.3
2,0
1,8
2.2
± 0 . 6 ± 0 . 8 ± 2 . 3 ±2.1 2.4
2.0
1.9
2.5
1.4
1.5
2.2
1.3
2.4
+0.7
+1.5
±0.7
±0.5
~).7
±0,5
±0.9
For the usually most leached element K a realistic deposition velocity of 2.8 (+0.6) cm/s is obtained after the leaching correction. For the super-micron particles (Na, CI, Ca and Mg) large uncertainties due to very low air concentrations were found. Nevertheless, the evaluated dry deposition velocities compare well with former experimental and theoretical values. The main advantage of this method is its direct and easy application at different tree stands. It was therefore employed at two stands in Portugal (Eucalyptus globuhts and Pmus pmea) in September 1993 under different climatic and topographic conditions and with a different composition o f the atmospheric aerosol. The experiments are evaluated at present.
Acknowledgements This work has been supported by the Portuguese Junta Nacionai de Investiga~fioCientifica e Tecaol6gica (JNICT) and by the German Federal Minister of Research and TechnoloD, (BMFT).
References:
Constantin, J. (1993): Stoffeintr~ge in ein FichtenwaldOkosystemdurch Deposition lufigetragcner Partikel und Nebeltr6pfchen. Dissertation. Institut for Bioklimatologie, Universit/RG6ttingen. Draaijers, G. (1993): The variability,of atmospheric deposition to forests: the effects of canopy structure and forest edges. Dissertation. Faculty of Geographical Sciences, University of Utrecht, HOfken, K.-D.; Georgii, H.-W.; Gravenhorst, G. (1981): Untersuchungen fiber die Deposition atmosph~irischer Spurenstoffe an Buchen- und Fichtenwald. Berichte des Instituts ffir Meteorologie und Geophysik. Universit~t Frankfurt am Main; Berieht Nr. 46. Ibrom. A. (1993): Die Deposition und Pflanzenauswaschung (leaching) yon Pflanzenn~ihrstoffen in einer Fichtenflache im Soiling. Dissertation. Institut ffir Bioklimatologie. Universit~t GOttingen. Shanley, J.B. (1989): Field measurements of dry. deposition to spruce foliage and petri dishes in the Black forest F.R.G. Atmospheric Environment, 23, No. 2. p. 403-414.
I Value given without considering leaching. 2 Constantin 1993. 3 H0fken e.a. 1981.