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Dry deposition velocity of PM2.5 ammonium sulfate particles to a Norway spruce forest on the basis of S- and N-balance estimations
ARTICLE IN PRESS AE International – Europe Atmospheric Environment 37 (2003) 4419–4424
Dry deposition velocity of PM2.5 ammonium sulfate particles to a Norway spruce forest on the basis of S- and N-balance estimations La! szlo! Horva! th Hungarian Meteorological Service, Gilice t!er 39, Budapest 1181, Hungary Received 28 February 2003; received in revised form 25 June 2003; accepted 3 July 2003
Abstract In this paper we make an attempt to estimate the dry deposition velocity of ammonium sulfate particles using the results of sulfur and nitrogen balance determined between the atmosphere and a forest ecosystem for the years of 1996– 1998. Results of these measurements and of a campaign conducted during summer of 2001 demonstrate that ammonium and sulfate ions exist in nearly equivalent ratio in particle phase. According to the size distribution measurements majority ð92%Þ of ammonium sulfate can be found in the fraction of PM2.5. For the balance calculation, results of throughfall, stemflow and wet-only deposition measurements have been used, together with dry deposition measurements of gaseous sulfur dioxide and ammonia. Dry deposition velocity of ammonium sulfate particles determined from sulfur and nitrogen balance were vX0:8270:25 and ¼ 0:8470:25 cm s1 ; respectively. The two figures determined by different ways are in good agreement and they are in accordance with other experimental results found in the literature. The results suggest the necessity of the revision of the models applied during the theoretical calculation of dry deposition velocity of PM2.5 particles. r 2003 Elsevier Ltd. All rights reserved. Keywords: PM2.5 particles; N-balance; S-balance; Wet deposition; Dry deposition; Throughfall deposition
1. Introduction The knowledge of the rate of dry deposition velocity of ammonium sulfate particles is necessary for the estimation of N and S load or nitrogen and sulfur balance between the atmosphere and forest ecosystems. For aerosol particles, limited field studies (Erisman et al., 1995; Wyers et al., 1995) have obtained systematically higher deposition velocities than there were determined by theoretical calculations and wind tunnel experiments (Ruijgork et al., 1993; Borrell et al., 1997). There are still substantial differences between the experimentally determined and theoretically calculated dry deposition velocity figures. The deposition velocities estimated from calculations and laboratory wind tunnel E-mail address: [email protected] (L. Horv!ath).
measurements lies in the order of 0:1 cm s1 ; while the experimental values from the literature are higher with one order of magnitude ð > 1 cm s1 Þ: Gallagher et al. (2002) also pointed out the discrepancy between the modeled and observed dry deposition figures for fine (0.1–0:2 mm diameter particles). According to some measurements dry deposition velocity to forests are: 2:4 cm s1 for sulfate (S!anchez et al., 1993); 1.2–1:5 cm s1 for ammonium particles over 0:8 mm size (Wyers et al., 1995); 1–2 cm s1 for fine and 5 cm s1 for coarse sulfate and ammonium particles (Erisman et al., 1995). Different research groups agree with the high uncertainty of these figures (Lopez, 1994). Borrell et al. (1997) suggest over 1 cm s1 deposition velocity for particles according to the re-evaluation of theoretical, wind tunnel and field estimations (Ruijgork et al., 1993).
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Sulfur and reduced nitrogen compounds can be deposited either by wet (WD) or dry deposition (DD) processes. There are two gas-phase (SO2 and NH3 ) and two particulate phase (sulfate, ammonium) components which have substantial concentrations in the atmosphere, controlling the N- and S-flux to forest ecosystems (there are no important S-sources from the forest, while ammonia has bi-directional flux). The dry deposited sulfur and reduced nitrogen compounds are leached by the precipitation from the surface of leaves, branches and trunk and can be detected as sulfate and ammonium ions in the throughfall (TF) and stemflow (SF) precipitation samples. The balance of a group of compounds can be written as (see e.g. Ferm and Hultberg, 1999) TF þ SF ¼ WD þ DD þ IC UP;
ð1Þ
where IC is the internal circulation or ion leakage, and UP is the term for the uptake by stomata. 1.1. Sulfur compounds In the case of sulfur compounds IC is negligible. Using Eq. (1) the dry deposition of sulfate particles can be expressed as 2 2 2 DDðSO2 4 Þ ¼ TFðSO4 Þ þ SFðSO4 Þ WDðSO4 Þ DDðSO2 Þ þ UPðSO2 Þ:
ð2Þ
1.2. Nitrogen compounds For reduced nitrogen species the stomatal uptake both of ammonia gas and ammonium are significant, while IC is negligible, in this case Eq. (1) can be written as þ þ þ DDðNHþ 4 Þ ¼ TFðNH4 Þ þ SFðNH4 Þ WDðNH4 Þ DDðNH3 Þ þ UPðNH3 Þ þ UPðNHþ 4 Þ:
ð3Þ In the dry deposition of ammonia two processes are dominant, the uptake by stomata (UP) and the adsorption on wet leaf surface (CU) especially in the presence of high sulfur dioxide concentrations. Hence the dry deposition of ammonia can be written as DDðNH3 Þ ¼ UPðNH3 Þ þ CUðNH3 Þ:
ð4Þ
Theoretically, net ammonia emission by plants cannot be excluded in the case when the net stomatal emission flux (negative UP) exceeds the magnitude of the cuticular deposition. Stomata in the function of the apoplastic pH and ammonium concentration control a compensation point concentration for ammonia. In most of cases the compensation point is lower than atmospheric concentration, i.e. the sign of ammonia flux is generally negative (i.e. deposition occurs). Compensation point is highly depend on the atmospheric nitrogen
load to the forest, in our case it is moderately low: 18 kg N ha1 yr1 (Horv!ath et al., 2002; Horv!ath, 2003). Dry deposition velocity of ammonia determined at our forest site is 1.1–3:7 cm s1 during night and day hours, respectively (Horv!ath et al., 2001). These deposition velocity figures suggest a net ammonia deposition. Eqs. (3) and (4) yield þ þ þ DDðNHþ 4 Þ ¼ TFðNH4 Þ þ SFðNH4 Þ WDðNH4 Þ þ þ UPðNH4 Þ CUðNH3 Þ:
ð5Þ
In our estimation Eq. (5) will be used to calculate the dry deposition rate of ammonium particles. Throughfall (TF) and stemflow (SF) measurements are generally carried out on routine basis. Wet deposition (WD) measurements are also among the basic task of the monitoring networks. Measurement of dry deposition (DD) of sulfur dioxide and ammonia is also well studied, there is a lot of information in the literature for deposition rate of SO2 and for the net flux of NH3 : However, in the estimation of dry deposition of ammonium and sulfate particles there is one order of magnitude of uncertainty between the theoretical, laboratory estimations and experimental results as mentioned above. There is no generally acceptable method for the direct measurement of the dry deposition of fine (PM2.5) aerosol particles. In this paper we aimed to make an attempt to estimate the dry deposition velocity of ammonium sulfate particles, using the results of the sulfur and reduced nitrogen balance over a forest ecosystem based on measurements for the years of 1996–1998 in a Norway spruce stand. For the calculation results of throughfall, stemflow and wet-only precipitation measurements were used, together with dry deposition measurements of sulfur dioxide and ammonia. Another goal of this work is to give a simple methodology to derive the dry deposition velocity figures for particles and encourage to continue the measurements to fill the gap between theoretical and experimental results.
2. Measurements Measurements were carried out during 1996–1998 in the M!atra-Mountains, NW Hungary, in a Norway spruce forest, planted between 1963 and 1965 in the frame of the IUFRO international project. The average height of the trees was 15–17 m (1996–1998), with a leaf area index of 3.3 (1994). Geographical positions are l ¼ 19 570 E; j ¼ 47 540 N; h ¼ 560 m: The station is jointly operated by the Forest Research Institute and by the Hungarian Meteorological Service. Concentration ðCÞ of sulfur dioxide and ammonia as well as the ammonium and sulfate particles were determined on the basis of 24 h samplings according
ARTICLE IN PRESS ! / Atmospheric Environment 37 (2003) 4419–4424 L. Horvath
to the EMEP (1996) recommendations by the filter pack method as sulfate and ammonium ions in the solution of sampling filters by ion-chromatography and indophenol-blue photometry, respectively. Wet deposition (WD) of sulfate and ammonium ions was determined as WD ¼ Cp; where C is the concentration of the ion measured in the precipitation water, and p is the precipitation amount. Daily precipitation samples were taken by a wet-only collector installed out of the forest canopy. Sulfate and ammonium ions were determined by the methods mentioned above. Throughfall (TF) and stemflow (SF) samples were collected parallel with the precipitation samplings under the canopy by five wet-only collectors and rims installed on the trunk of 10 selected trees, respectively. Concentrations of sulfate and ammonium ions were determined by the same way as mentioned. Beside the concentration and deposition measurements described above aerosol samplings were carried out in the summer of 2001 to determine the amount the ammonium and sulfate in different size ranges. A Ghent-type impactor was used to select the ammonium and sulfate particles in two ranges (do2:5 mm and 2:5 mmodo10 mm). As a total of 12 samplings were carried out.
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ratio of u determined by different methods is 1.16. These figures are below the 730% uncertainty which is indicated in Table 1.
3. Results To determine the distribution and concentration of ammonium sulfate particles in different size ranges result of 12 Ghent-impactor samplings were used. The main measured parameters can be seen in Table 2. As Table 2 shows majority of ammonium and sulfate exist in the range of PM2.5. There is a strong correlation between the ammonium and sulfate particles in the PM2.5 range according to Fig. 1. Ammonium-to-sulfate ratio expressed in equivalents are 93 and 104 nequ m3 i.e. the two ions exist nearly in stochiometric ratio. From Table 3 the corresponding figures for years of 1996–1998 are 79 and 94 nequ m3 : These suggest that the two ions exist nearly in equivalent ratio and can be treated mostly as neutral ammonium sulfate in the PM2.5 range. Using this close stochiometric ratio of ammonium and sulfate one can estimate that dry deposition velocities determined both for ammonium and sulfate ions represent the dry deposition velocity of ammonium sulfate particles.
2.1. Uncertainty analysis 3.1. Sulfur compounds According to international inter-calibrations and parallel samplings the estimated uncertainties of the different terms are compiled in Table 1. Most critical term is the dry deposition that has been determined using results of the gradient flux measurements. Applicability of the gradient method for forests is criticized. Some authors give estimation on the bias. For example, Simpson et al. (1998) demonstrated that the ratio of eddy/gradient fluxes are between 1.1 and 1.24 at approximately 2 times higher measuring height to canopy height (our case). Hargreaves et al. (1996) calculated a ratio of 1.19 for eddy/gradient fluxes. According to our estimation (M!esz!aros et al., 2000) the
In Eq. (2) terms TF, ST, WD and DD were separately determined. Because UP(SO2 ) term remains unknown, only the dry deposition of sulfate minus uptake of sulfur Table 2 Ratio of sulfate to ammonium in different size ranges by 12 cascade impactor samplings in Summer, 2001 Phase
NHþ 4 (nequ m3 )
ð%Þ
SO2 4 (nequ m3 )
ð%Þ
do2:5 mm 2:5 mmodo10 mm
93.3 4.1
96 4
104 9.0
92 8
Table 1 Uncertainty in determination of different terms Uncertainty (%)
NHþ 4 and NO3 concentration measurements in precipitation Precipitation amount Overall uncertainty of WD NH3 and SO2 concentration profile Determination of K Overall uncertainty of DD Uncertainty of TF and SF Overall estimated uncertainty of þ DD(SO2 4 ) and DD(NH4 )
75 710 710 710 725 725 710 730
0.20
sulfate
Parameter
y = 1.0303x + 0.0077 R2= 0.9809
0.15 0.10 0.05 0.00 0.00
0.05
0.10
0.15
0.20
ammonium Fig. 1. Correlation between the concentration of ammonium and sulfate ions in PM2.5 phase (mequ m3 ).
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Table 3 Mean concentrations of sulfur and reduced nitrogen species (1996–1998) Species
Concentration ðnequ m3 )
Sulfur dioxide Sulfate Ammonia Ammonium
600760 9479 4474 7978
dioxide DD(SO2 4 Þ UPðSO2 ) can be calculated from Eq. (2). To derive dry deposition velocity figures the 2 2 ½DDðSO2 4 Þ UPðSO2 Þ =CðSO4 Þ ¼ vðSO4 Þ f
ð6Þ
equation was used. In the case of f > 0; i.e. when the stomatal uptake of sulfur dioxide is not negligible the calculated vðSO2 4 Þ f gives the lower limit for sulfate deposition. In the extreme case when stomatal uptake is negligible ðf ¼ 0Þ; vðSO2 4 Þ can be directly calculated from Eqs. (5) and (6). The rate of stomatal uptake in the dry deposition of sulfur dioxide depends among others on the moisture, leaf wetness, temperature and the ambient ammonia level. Stomatal uptake of sulfur dioxide can be important, but in our case it has probably of less importance in comparison to the surface adsorption. Despite of the relatively low, 700–750 mm yr1 rainfall, the humidity inside the canopy is high (in most cases > 60%). Though stomatal sulfur dioxide deposition occurs parallel with surface deposition, several authors pointed out the great importance of deposition on wet leaf surfaces involving ammonia (Flechard et al., 1999). One of the most important factor controlling the canopy resistance to the dry deposition of sulfur dioxide is the ratio of ammonia to sulfur dioxide as described by Fowler and Erisman (2003). For example parallel with the increase of ammonia to sulfur dioxide, the canopy resistance of cereal canopy decreased from 130 to 80 s m1 : As one of the main results of the first stage of the BIATEX project it was demonstrated (Erisman et al., 1993) that in dry cases, when the humidity is lower than 60% canopy resistance for SO2 lies between 500 and 1000 s m1 : In other cases except during fog, with low ammonia level and during negative temperature the canopy resistance is low ð50 s m1 Þ: At our site the average dry deposition velocity is 0:64 cm s1 ; giving 150 s m1 for the bulk (Ra ; Rb and Rc ) resistance. Roughly, taking into account, that Ra and Rb are responsible for the half of the bulk resistance, we are in the range of below 100 s m1 ; indicating that surface deposition is the more important deposition process than stomatal uptake in our forest. Comparing the 500– 1000 s m1 canopy resistance with Ro100 s m1 surface resistance, as a first approximation, we can say, stomatal deposition would give the 10%–20% of the total
deposition on yearly basis. In winter when ammonia to sulfur dioxide ratio is low the estimated surface resistance can be higher resulting in higher share of the stomatal deposition. For the calculation of vðSO2 4 Þ f the TF, SF and WD were measured as described above. The results of wet deposition measurements (WD), throughfall (TF) and stemflow (SF) deposition estimates are compiled in Table 4. Dry deposition fluxes of sulfur dioxide were inferred from the continuously monitored 3-year concentration record ðCÞ and dry deposition velocity ðvÞ as DDðSO2 Þ ¼ vC for different seasons and stratification. Dry deposition velocity was determined on campaign basis in different seasons between 1992 and 1994 by the gradient method described in Horva! th et al. (1996). Dry flux was calculated as F ¼ KH dC=dz; where KH is the turbulent diffusion coefficient for the sensible heat flux, dC=dz is the concentration gradient. Concentration gradient was determined at different heights (28, 23, and 18 m) above the canopy by a HORIBA gas monitor. Diffusion coefficient was calculated according to the Monin–Obukhov’s semi-empirical similarity theory (Weidinger et al., 2000) for the layer between 28 and 18 m: Dry deposition velocity was derived and calculated for different seasons and for different stratification as v ¼ F =C: The bulk mean dry deposition velocity averaged for the whole year is 0:64 cm s1 (variation according to the season is 0.40–0:96 cm s1 ranging 0.09–0.34 and 0.60–1:57 cm s1 during stable and unstable stratification, respectively). Result of dry deposition estimate for sulfur dioxide (DD) can be seen in Table 4. According to Eq. (2) the yearly dry flux of sulfate is 2578 mequ m2 yr1 : Bulk dry deposition velocity for sulfate particles calculated from Eq. (6), using the mean concentration in Table 3 and taking into account the Table 4 Throughfall, stemflow, wet and dry deposition measurements of sulfur and nitrogen compounds and the calculated dry deposition flux for sulfate and ammonium particles (1996–1998) Form of deposition
Deposition rate (mequ m2 yr1 )
TF(SO2 4 ) measured SF(SO2 4 ) measured WD(SO2 4 ) measured DD(SO2 ) measured DD(SO2 4 ) calculated
200720 0.570.05 5676 120730 >2578
TF(NHþ 4 ) measured SF(NHþ 4 ) measured WD(NHþ 4 ) measured DD(NH3 ) measured DD(NHþ 4 ) calculated
5175 5.475 3073 40710 2176
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uncertainty of measurements and estimations is v > 0:8270:25 cm s1 or in extreme case (high ammonium level, high moisture, wet leaves, when stomatal uptake is negligible to the cuticular adsorption) it is v ¼ 0:8270:25 cm s1 : 3.2. Nitrogen compounds TF, SF and WD terms in Eq. (5) can be determined experimentally. The term UP(NHþ 4 Þ CUðNH3 ) cannot be calculated from our measurements. However, it is possible to make a rough estimation for the magnitude of these terms. When ammonium ion is taken up by plants from precipitation the rate of uptake (UP) is limited as þ 0oUPðNHþ 4 ÞoWDðNH4 Þ:
ð7Þ
Wet deposition of ammonium according to Table 4 is 30 mequ m2 yr1 ; therefore the rate of stomatal ammonium uptake is somewhere between 0 and 30 mequ m2 yr1 : Cuticular (surface) adsorption of NH3 is limited by the total measured dry deposition of ammonia, hence 0oCUðNH3 ÞoDDðNH3 Þ:
ð8Þ
Dry deposition fluxes of ammonia were inferred by the monitored concentrations ðCÞ and dry deposition velocities ðvÞ; for different seasons and stratification, as DDðNH3 Þ ¼ vC: Dry deposition velocity was measured on campaign basis by the gradient method (Horv!ath et al., 2001, 2003). Dry flux was determined by the same method as described for sulfur dioxide. The bulk mean dry deposition velocity averaged for the whole year is 2:4 cm s1 (variations are 1.1 and 3:7 cm s1 during stable and unstable stratification, respectively). The calculated net dry flux for ammonia is 40 mequ m2 yr1 therefore the cuticular deposition of ammonia lies between 0 and 40 mequ m2 yr1 : Table 5 shows the calculated dry deposition velocities for ammonium particles using Eqs. (5) and (9), þ þ vðNHþ 4 Þ ¼ DDðNH4 Þ=CðNH4 Þ:
ð9Þ
Because both the stomatal uptake of ammonium and the cuticular adsorption of ammonia is important the most Table 5 Calculated dry flux (DD) and dry deposition velocity ðvÞ of ammonium particles in the cases of different rates of cuticular ammonia adsorption and stomatal uptake of ammonium ðTF þ SF WD ¼ 26 mequ m2 yr1 )
UPðminÞ ¼ 0; CUðminÞ ¼ 0 UPðmaxÞ ¼ 30; CUðmaxÞ ¼ 40 UPð0:5Þ ¼ 15; CUð0:5Þ ¼ 20
DD ðNHþ 4Þ ðmequ m2 yr1 )
v ðNHþ 4Þ ðcm s1 Þ
26 16 21
1.04 0.65 0.84
4423
probable rate of dry deposition, v ¼ 0:8470:25 can be inferred when the rate of stomatal and cuticular dry deposition of ammonia is 1:1 [case of CU(0.5)] and half of ammonium ions is taken up from the precipitation [case of UP(0.5)]. Though, these are rough estimations, the good agreement between deposition velocities from sulfur balance vX0:8270:25 cm s1 and from reduced nitrogen balance v ¼ 0:8470:25 cm s1 suggests that the assumptions for the rate of cuticular adsorption of ammonia and stomatal uptake of ammonium ion are close to the reality.
4. Conclusions There are great uncertainties in the estimation of dry deposition rate of fine aerosol particles to forest ecosystems. However, the yearly bulk dry deposition velocity of ammonium and sulfate particles can be estimated by a simple way using routine wet deposition, throughfall and stemflow measurements as well as dry deposition measurements of gases. Since ammonium and sulfate mostly exist in PM2.5 range and are nearly in stochiometric ratio dry deposition of ammonium sulfate particles can be generalized for PM2.5 particles. The deposition figures determined either from the Sand N-balance calculations (vX0:8270:25 and ¼ 0:847 0:25 cm s1 ) are in good agreement and they are in accordance with other experimental deposition velocities found in the literature. These results suggest on one hand the necessity of the revision of the models applied during the theoretical calculation of dry deposition velocity for fine particles and on the other, to continue the simple experimental work described here for as many places as possible.
Acknowledgements Investigation were sponsored by: National Committee for Technological Development, 1996–97, PHARE TD&QM No. H-9305–02/1033; National Committee for Technological Development 1996–98, No. 6-97-451047; Ministry for Environment, 1999 No. 063/T; USHungarian Joint Research Fund, 1996–99 No. 608/96; National Committee for Technological Development, 1998–1999, UNDP-HUN/95/002-0119; Ministry for Environment, 2000, No. KAC-20834; Ministry for Environment, 2001, KAC-27822; Ministry for Environment, 2002, KAC-44146; Hungarian Scientific Research Fund, 2000–2003, No. OTKA T-31927.
References Borrell, P., Builtjes, J.H., Grennfelt, P., H^v, O. (Eds.), 1997. Transport and chemical transformation of pollutants in the
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troposphere. Photo-oxidants, Acidification and Tools: policy Applications of EUROTRAC Results, Vol. 10. Springer, Berlin, p. 116. EMEP, 1996. EMEP co-operative programme for monitoring and evaluation of the long-range transmission of air pollutants in Europe. EMEP Manual for sampling and chemical analysis. EMEP/CCC-Report 1/95, NILU, Kjeller, Norway. Erisman, J.W., van Pul, A., Wyers, P., 1993. Parameterization of dry deposition mechanisms for the quantification of atmospheric input to ecosystems. In: Slanina, J., Angeletti, G., Beilke, S. (Eds.), Air Pollution Research Report, Vol. 47. CEC, Brussels, pp. 215–234. Erisman, J.W., Draajers, G., Duyzer, J., Hofschreuder, P., van . Leeuwen, N., Romer, F., Ruijgork, W., Wyers, P., 1995. Particle deposition to forests. Acid rain research: do we have enough answers? In: Heij, G.J., Erisman, J.W. (Eds.), Studies in Environmental Science, Vol. 64. Elsevier, Amsterdam, Lausanne, NY, Oxford, Shannon, Tokyo, p. 115. Ferm, M., Hultberg, H., 1999. Dry deposition and internal circulation of nitrogen, sulphur and base cations into a coniferous forest. Atmospheric Environment 33, 4421–4430. Flechard, C.R., Fowler, D., Sutton, M.A., Cape, J.N., 1999. A dynamic chemical model of bi-directional ammonia exchange between semi-natural vegetation and the atmosphere. Quarterly Journal of the Royal Meteorological Society 125, 2611–2641. Fowler, D., Erisman, J.W., 2003. Biosphere/atmosphere exchange of pollutants. In: Midgley, P., Reuther, M (Eds.), Towards Cleaner Air for Europe. Science, Tools and Applications. Margraph Publishers, Weikersheim, pp. 35–58. Gallagher, M.W., Nemitz, E., Dorsey, J.R., Fowler, D., Sutton, M.A., Flynn, M., Duyzer, J., 2002. Measurements and parameterizations of small aerosol deposition velocities to grassland, arable crops, and forest: influence of surface roughness length on deposition. Journal of Geophysical Research 107, AAC 8–1. Hargreaves, K.J., Wienhold, F.G., Klemedtsson, L., Arah, J.R.M., Beverland, I.J., Fowler, D., Galle, B., Griffith, D.W.T., Skiba, U., Smith, K.A., Welling, M., Harris, G.W., 1996. Measurement of nitrous oxide emission from agricultural land using micrometeorological methods. Atmospheric Environment 30, 1563–1571. Horv!ath, L., 2003. Determination of the nitrogen compound balance between the atmosphere and a Norway Spruce forest ecosystem. Nutrient Cycling in Agroecosystems (under evaluation). Horv!ath, L., Weidinger, T., Nagy, Z., Fuhrer, . E., 1996. Measurement of dry deposition velocity of ozone, sulfur
dioxide and nitrogen oxides above pine forest and low vegetation in different seasons by the gradient method. In: Slanina, J. (Ed.), Biosphere-Atmosphere Exchange of Pollutants and Trace Substances, Vol. 4. Springer, Berlin, Heidelberg. Horv!ath, L., M!esz!aros, R., Pinto, J.P., Weidinger, T., 2001. Estimate of the dry deposition of atmospheric nitrogen and sulfur species to spruce forest. In: Midgley, P.M., Reuther, M., Williams, M. (Eds.), Proceedings of EUROTRAC Symposium 2000 Garmish-Partenkirchen, Germany 27–31 March 2000. Springer, Berlin, Heidelberg. Horv!ath, L., M!esz!aros, R., Weidinger, T., 2002. Measurement of nitrogen compound balance between the atmosphere and a spruce forest ecosystem. In: Midgley, P.M., Reuther, M. (Eds.), Proceedings of the EUROTRAC Symposium, 2002. Margraf Verlag, Weikersheim. Horv!ath, L., Pinto, J., Weidinger, T., 2003. Estimate of the dry deposition of atmospheric nitrogen and sulfur species to + ar!as 107 (3–4) in press. spruce forest. Idoj! Lopez, A., 1994. Biosphere atmosphere exchanges: ozone and aerosol dry deposition velocities over a pine forest. EUROTRAC Annual Report part 4, BIATEX, EUROTRAC ISS, Garmisch-Partenkirchen, p. 80. M!esz!aros, R., Horv!ath, L., Weidinger, T., McMillen, R., Luke, W.T., 2000. Ozone flux measurements over a Central European forest; comparison of the eddy-correlation and the gradient techniques. Geophysical Research Abstracts 2, ST8.05. Ruijgork, W., Nicholson, K.W., Davidson, C.I., 1993. Dry deposition of particles. Models and methods for the quantification of atmospheric input to ecosystems. Nordiske Seminar og Arbejdsrapporter 1993: 573. Nordic Council of Ministers, Copenhagen, pp. 145–161. S!anchez, M.L., Dom!ıngues, J., Sanz, F., Rodr!ıgues, R., 1993. Preliminary study of dry deposition. In: Slanina, J., Angeletti, G., Beilke, S. (Eds.), Air Pollution Research Report 47. CEC, Brussels, p. 65. Simpson, I.J., Thurtell, G.W., Neumann, H.H., Den Hartog, G., Edwards, G.C., 1998. The validity of similarity theory in the roughness sublayer above forests. Boundary-Layer Meteorology 87, 69–99. Weidinger, T., Pinto, J., Horvath, L., 2000. Effects of uncertainties in universal functions, roughness length, and displacement height on the calculation of surface layer fluxes. Meteorologische Zeitschrift 9, 139–154. Wyers, G.P., Veltkamp, A.C., Geusebroek, M., Wayers, A., . J.J., 1995. Deposition of aerosol to coniferous forest. Mols, Acid rain research: do we have enough answers? In: Heij, G.J., Erisman, J.W. (Eds.), Studies in Environmental Science, Vol. 64. Elsevier, Amsterdam, Lausanne, NY, Oxford, Shannon, Tokyo, p. 127.
Dry deposition velocity of PM2.5 ammonium sulfate particles to a Norway spruce forest on the basis of S- and N-balance estimations La! szlo! Horva! th Hungarian Meteorological Service, Gilice t!er 39, Budapest 1181, Hungary Received 28 February 2003; received in revised form 25 June 2003; accepted 3 July 2003
Abstract In this paper we make an attempt to estimate the dry deposition velocity of ammonium sulfate particles using the results of sulfur and nitrogen balance determined between the atmosphere and a forest ecosystem for the years of 1996– 1998. Results of these measurements and of a campaign conducted during summer of 2001 demonstrate that ammonium and sulfate ions exist in nearly equivalent ratio in particle phase. According to the size distribution measurements majority ð92%Þ of ammonium sulfate can be found in the fraction of PM2.5. For the balance calculation, results of throughfall, stemflow and wet-only deposition measurements have been used, together with dry deposition measurements of gaseous sulfur dioxide and ammonia. Dry deposition velocity of ammonium sulfate particles determined from sulfur and nitrogen balance were vX0:8270:25 and ¼ 0:8470:25 cm s1 ; respectively. The two figures determined by different ways are in good agreement and they are in accordance with other experimental results found in the literature. The results suggest the necessity of the revision of the models applied during the theoretical calculation of dry deposition velocity of PM2.5 particles. r 2003 Elsevier Ltd. All rights reserved. Keywords: PM2.5 particles; N-balance; S-balance; Wet deposition; Dry deposition; Throughfall deposition
1. Introduction The knowledge of the rate of dry deposition velocity of ammonium sulfate particles is necessary for the estimation of N and S load or nitrogen and sulfur balance between the atmosphere and forest ecosystems. For aerosol particles, limited field studies (Erisman et al., 1995; Wyers et al., 1995) have obtained systematically higher deposition velocities than there were determined by theoretical calculations and wind tunnel experiments (Ruijgork et al., 1993; Borrell et al., 1997). There are still substantial differences between the experimentally determined and theoretically calculated dry deposition velocity figures. The deposition velocities estimated from calculations and laboratory wind tunnel E-mail address: [email protected] (L. Horv!ath).
measurements lies in the order of 0:1 cm s1 ; while the experimental values from the literature are higher with one order of magnitude ð > 1 cm s1 Þ: Gallagher et al. (2002) also pointed out the discrepancy between the modeled and observed dry deposition figures for fine (0.1–0:2 mm diameter particles). According to some measurements dry deposition velocity to forests are: 2:4 cm s1 for sulfate (S!anchez et al., 1993); 1.2–1:5 cm s1 for ammonium particles over 0:8 mm size (Wyers et al., 1995); 1–2 cm s1 for fine and 5 cm s1 for coarse sulfate and ammonium particles (Erisman et al., 1995). Different research groups agree with the high uncertainty of these figures (Lopez, 1994). Borrell et al. (1997) suggest over 1 cm s1 deposition velocity for particles according to the re-evaluation of theoretical, wind tunnel and field estimations (Ruijgork et al., 1993).
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Sulfur and reduced nitrogen compounds can be deposited either by wet (WD) or dry deposition (DD) processes. There are two gas-phase (SO2 and NH3 ) and two particulate phase (sulfate, ammonium) components which have substantial concentrations in the atmosphere, controlling the N- and S-flux to forest ecosystems (there are no important S-sources from the forest, while ammonia has bi-directional flux). The dry deposited sulfur and reduced nitrogen compounds are leached by the precipitation from the surface of leaves, branches and trunk and can be detected as sulfate and ammonium ions in the throughfall (TF) and stemflow (SF) precipitation samples. The balance of a group of compounds can be written as (see e.g. Ferm and Hultberg, 1999) TF þ SF ¼ WD þ DD þ IC UP;
ð1Þ
where IC is the internal circulation or ion leakage, and UP is the term for the uptake by stomata. 1.1. Sulfur compounds In the case of sulfur compounds IC is negligible. Using Eq. (1) the dry deposition of sulfate particles can be expressed as 2 2 2 DDðSO2 4 Þ ¼ TFðSO4 Þ þ SFðSO4 Þ WDðSO4 Þ DDðSO2 Þ þ UPðSO2 Þ:
ð2Þ
1.2. Nitrogen compounds For reduced nitrogen species the stomatal uptake both of ammonia gas and ammonium are significant, while IC is negligible, in this case Eq. (1) can be written as þ þ þ DDðNHþ 4 Þ ¼ TFðNH4 Þ þ SFðNH4 Þ WDðNH4 Þ DDðNH3 Þ þ UPðNH3 Þ þ UPðNHþ 4 Þ:
ð3Þ In the dry deposition of ammonia two processes are dominant, the uptake by stomata (UP) and the adsorption on wet leaf surface (CU) especially in the presence of high sulfur dioxide concentrations. Hence the dry deposition of ammonia can be written as DDðNH3 Þ ¼ UPðNH3 Þ þ CUðNH3 Þ:
ð4Þ
Theoretically, net ammonia emission by plants cannot be excluded in the case when the net stomatal emission flux (negative UP) exceeds the magnitude of the cuticular deposition. Stomata in the function of the apoplastic pH and ammonium concentration control a compensation point concentration for ammonia. In most of cases the compensation point is lower than atmospheric concentration, i.e. the sign of ammonia flux is generally negative (i.e. deposition occurs). Compensation point is highly depend on the atmospheric nitrogen
load to the forest, in our case it is moderately low: 18 kg N ha1 yr1 (Horv!ath et al., 2002; Horv!ath, 2003). Dry deposition velocity of ammonia determined at our forest site is 1.1–3:7 cm s1 during night and day hours, respectively (Horv!ath et al., 2001). These deposition velocity figures suggest a net ammonia deposition. Eqs. (3) and (4) yield þ þ þ DDðNHþ 4 Þ ¼ TFðNH4 Þ þ SFðNH4 Þ WDðNH4 Þ þ þ UPðNH4 Þ CUðNH3 Þ:
ð5Þ
In our estimation Eq. (5) will be used to calculate the dry deposition rate of ammonium particles. Throughfall (TF) and stemflow (SF) measurements are generally carried out on routine basis. Wet deposition (WD) measurements are also among the basic task of the monitoring networks. Measurement of dry deposition (DD) of sulfur dioxide and ammonia is also well studied, there is a lot of information in the literature for deposition rate of SO2 and for the net flux of NH3 : However, in the estimation of dry deposition of ammonium and sulfate particles there is one order of magnitude of uncertainty between the theoretical, laboratory estimations and experimental results as mentioned above. There is no generally acceptable method for the direct measurement of the dry deposition of fine (PM2.5) aerosol particles. In this paper we aimed to make an attempt to estimate the dry deposition velocity of ammonium sulfate particles, using the results of the sulfur and reduced nitrogen balance over a forest ecosystem based on measurements for the years of 1996–1998 in a Norway spruce stand. For the calculation results of throughfall, stemflow and wet-only precipitation measurements were used, together with dry deposition measurements of sulfur dioxide and ammonia. Another goal of this work is to give a simple methodology to derive the dry deposition velocity figures for particles and encourage to continue the measurements to fill the gap between theoretical and experimental results.
2. Measurements Measurements were carried out during 1996–1998 in the M!atra-Mountains, NW Hungary, in a Norway spruce forest, planted between 1963 and 1965 in the frame of the IUFRO international project. The average height of the trees was 15–17 m (1996–1998), with a leaf area index of 3.3 (1994). Geographical positions are l ¼ 19 570 E; j ¼ 47 540 N; h ¼ 560 m: The station is jointly operated by the Forest Research Institute and by the Hungarian Meteorological Service. Concentration ðCÞ of sulfur dioxide and ammonia as well as the ammonium and sulfate particles were determined on the basis of 24 h samplings according
ARTICLE IN PRESS ! / Atmospheric Environment 37 (2003) 4419–4424 L. Horvath
to the EMEP (1996) recommendations by the filter pack method as sulfate and ammonium ions in the solution of sampling filters by ion-chromatography and indophenol-blue photometry, respectively. Wet deposition (WD) of sulfate and ammonium ions was determined as WD ¼ Cp; where C is the concentration of the ion measured in the precipitation water, and p is the precipitation amount. Daily precipitation samples were taken by a wet-only collector installed out of the forest canopy. Sulfate and ammonium ions were determined by the methods mentioned above. Throughfall (TF) and stemflow (SF) samples were collected parallel with the precipitation samplings under the canopy by five wet-only collectors and rims installed on the trunk of 10 selected trees, respectively. Concentrations of sulfate and ammonium ions were determined by the same way as mentioned. Beside the concentration and deposition measurements described above aerosol samplings were carried out in the summer of 2001 to determine the amount the ammonium and sulfate in different size ranges. A Ghent-type impactor was used to select the ammonium and sulfate particles in two ranges (do2:5 mm and 2:5 mmodo10 mm). As a total of 12 samplings were carried out.
4421
ratio of u determined by different methods is 1.16. These figures are below the 730% uncertainty which is indicated in Table 1.
3. Results To determine the distribution and concentration of ammonium sulfate particles in different size ranges result of 12 Ghent-impactor samplings were used. The main measured parameters can be seen in Table 2. As Table 2 shows majority of ammonium and sulfate exist in the range of PM2.5. There is a strong correlation between the ammonium and sulfate particles in the PM2.5 range according to Fig. 1. Ammonium-to-sulfate ratio expressed in equivalents are 93 and 104 nequ m3 i.e. the two ions exist nearly in stochiometric ratio. From Table 3 the corresponding figures for years of 1996–1998 are 79 and 94 nequ m3 : These suggest that the two ions exist nearly in equivalent ratio and can be treated mostly as neutral ammonium sulfate in the PM2.5 range. Using this close stochiometric ratio of ammonium and sulfate one can estimate that dry deposition velocities determined both for ammonium and sulfate ions represent the dry deposition velocity of ammonium sulfate particles.
2.1. Uncertainty analysis 3.1. Sulfur compounds According to international inter-calibrations and parallel samplings the estimated uncertainties of the different terms are compiled in Table 1. Most critical term is the dry deposition that has been determined using results of the gradient flux measurements. Applicability of the gradient method for forests is criticized. Some authors give estimation on the bias. For example, Simpson et al. (1998) demonstrated that the ratio of eddy/gradient fluxes are between 1.1 and 1.24 at approximately 2 times higher measuring height to canopy height (our case). Hargreaves et al. (1996) calculated a ratio of 1.19 for eddy/gradient fluxes. According to our estimation (M!esz!aros et al., 2000) the
In Eq. (2) terms TF, ST, WD and DD were separately determined. Because UP(SO2 ) term remains unknown, only the dry deposition of sulfate minus uptake of sulfur Table 2 Ratio of sulfate to ammonium in different size ranges by 12 cascade impactor samplings in Summer, 2001 Phase
NHþ 4 (nequ m3 )
ð%Þ
SO2 4 (nequ m3 )
ð%Þ
do2:5 mm 2:5 mmodo10 mm
93.3 4.1
96 4
104 9.0
92 8
Table 1 Uncertainty in determination of different terms Uncertainty (%)
NHþ 4 and NO3 concentration measurements in precipitation Precipitation amount Overall uncertainty of WD NH3 and SO2 concentration profile Determination of K Overall uncertainty of DD Uncertainty of TF and SF Overall estimated uncertainty of þ DD(SO2 4 ) and DD(NH4 )
75 710 710 710 725 725 710 730
0.20
sulfate
Parameter
y = 1.0303x + 0.0077 R2= 0.9809
0.15 0.10 0.05 0.00 0.00
0.05
0.10
0.15
0.20
ammonium Fig. 1. Correlation between the concentration of ammonium and sulfate ions in PM2.5 phase (mequ m3 ).
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Table 3 Mean concentrations of sulfur and reduced nitrogen species (1996–1998) Species
Concentration ðnequ m3 )
Sulfur dioxide Sulfate Ammonia Ammonium
600760 9479 4474 7978
dioxide DD(SO2 4 Þ UPðSO2 ) can be calculated from Eq. (2). To derive dry deposition velocity figures the 2 2 ½DDðSO2 4 Þ UPðSO2 Þ =CðSO4 Þ ¼ vðSO4 Þ f
ð6Þ
equation was used. In the case of f > 0; i.e. when the stomatal uptake of sulfur dioxide is not negligible the calculated vðSO2 4 Þ f gives the lower limit for sulfate deposition. In the extreme case when stomatal uptake is negligible ðf ¼ 0Þ; vðSO2 4 Þ can be directly calculated from Eqs. (5) and (6). The rate of stomatal uptake in the dry deposition of sulfur dioxide depends among others on the moisture, leaf wetness, temperature and the ambient ammonia level. Stomatal uptake of sulfur dioxide can be important, but in our case it has probably of less importance in comparison to the surface adsorption. Despite of the relatively low, 700–750 mm yr1 rainfall, the humidity inside the canopy is high (in most cases > 60%). Though stomatal sulfur dioxide deposition occurs parallel with surface deposition, several authors pointed out the great importance of deposition on wet leaf surfaces involving ammonia (Flechard et al., 1999). One of the most important factor controlling the canopy resistance to the dry deposition of sulfur dioxide is the ratio of ammonia to sulfur dioxide as described by Fowler and Erisman (2003). For example parallel with the increase of ammonia to sulfur dioxide, the canopy resistance of cereal canopy decreased from 130 to 80 s m1 : As one of the main results of the first stage of the BIATEX project it was demonstrated (Erisman et al., 1993) that in dry cases, when the humidity is lower than 60% canopy resistance for SO2 lies between 500 and 1000 s m1 : In other cases except during fog, with low ammonia level and during negative temperature the canopy resistance is low ð50 s m1 Þ: At our site the average dry deposition velocity is 0:64 cm s1 ; giving 150 s m1 for the bulk (Ra ; Rb and Rc ) resistance. Roughly, taking into account, that Ra and Rb are responsible for the half of the bulk resistance, we are in the range of below 100 s m1 ; indicating that surface deposition is the more important deposition process than stomatal uptake in our forest. Comparing the 500– 1000 s m1 canopy resistance with Ro100 s m1 surface resistance, as a first approximation, we can say, stomatal deposition would give the 10%–20% of the total
deposition on yearly basis. In winter when ammonia to sulfur dioxide ratio is low the estimated surface resistance can be higher resulting in higher share of the stomatal deposition. For the calculation of vðSO2 4 Þ f the TF, SF and WD were measured as described above. The results of wet deposition measurements (WD), throughfall (TF) and stemflow (SF) deposition estimates are compiled in Table 4. Dry deposition fluxes of sulfur dioxide were inferred from the continuously monitored 3-year concentration record ðCÞ and dry deposition velocity ðvÞ as DDðSO2 Þ ¼ vC for different seasons and stratification. Dry deposition velocity was determined on campaign basis in different seasons between 1992 and 1994 by the gradient method described in Horva! th et al. (1996). Dry flux was calculated as F ¼ KH dC=dz; where KH is the turbulent diffusion coefficient for the sensible heat flux, dC=dz is the concentration gradient. Concentration gradient was determined at different heights (28, 23, and 18 m) above the canopy by a HORIBA gas monitor. Diffusion coefficient was calculated according to the Monin–Obukhov’s semi-empirical similarity theory (Weidinger et al., 2000) for the layer between 28 and 18 m: Dry deposition velocity was derived and calculated for different seasons and for different stratification as v ¼ F =C: The bulk mean dry deposition velocity averaged for the whole year is 0:64 cm s1 (variation according to the season is 0.40–0:96 cm s1 ranging 0.09–0.34 and 0.60–1:57 cm s1 during stable and unstable stratification, respectively). Result of dry deposition estimate for sulfur dioxide (DD) can be seen in Table 4. According to Eq. (2) the yearly dry flux of sulfate is 2578 mequ m2 yr1 : Bulk dry deposition velocity for sulfate particles calculated from Eq. (6), using the mean concentration in Table 3 and taking into account the Table 4 Throughfall, stemflow, wet and dry deposition measurements of sulfur and nitrogen compounds and the calculated dry deposition flux for sulfate and ammonium particles (1996–1998) Form of deposition
Deposition rate (mequ m2 yr1 )
TF(SO2 4 ) measured SF(SO2 4 ) measured WD(SO2 4 ) measured DD(SO2 ) measured DD(SO2 4 ) calculated
200720 0.570.05 5676 120730 >2578
TF(NHþ 4 ) measured SF(NHþ 4 ) measured WD(NHþ 4 ) measured DD(NH3 ) measured DD(NHþ 4 ) calculated
5175 5.475 3073 40710 2176
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uncertainty of measurements and estimations is v > 0:8270:25 cm s1 or in extreme case (high ammonium level, high moisture, wet leaves, when stomatal uptake is negligible to the cuticular adsorption) it is v ¼ 0:8270:25 cm s1 : 3.2. Nitrogen compounds TF, SF and WD terms in Eq. (5) can be determined experimentally. The term UP(NHþ 4 Þ CUðNH3 ) cannot be calculated from our measurements. However, it is possible to make a rough estimation for the magnitude of these terms. When ammonium ion is taken up by plants from precipitation the rate of uptake (UP) is limited as þ 0oUPðNHþ 4 ÞoWDðNH4 Þ:
ð7Þ
Wet deposition of ammonium according to Table 4 is 30 mequ m2 yr1 ; therefore the rate of stomatal ammonium uptake is somewhere between 0 and 30 mequ m2 yr1 : Cuticular (surface) adsorption of NH3 is limited by the total measured dry deposition of ammonia, hence 0oCUðNH3 ÞoDDðNH3 Þ:
ð8Þ
Dry deposition fluxes of ammonia were inferred by the monitored concentrations ðCÞ and dry deposition velocities ðvÞ; for different seasons and stratification, as DDðNH3 Þ ¼ vC: Dry deposition velocity was measured on campaign basis by the gradient method (Horv!ath et al., 2001, 2003). Dry flux was determined by the same method as described for sulfur dioxide. The bulk mean dry deposition velocity averaged for the whole year is 2:4 cm s1 (variations are 1.1 and 3:7 cm s1 during stable and unstable stratification, respectively). The calculated net dry flux for ammonia is 40 mequ m2 yr1 therefore the cuticular deposition of ammonia lies between 0 and 40 mequ m2 yr1 : Table 5 shows the calculated dry deposition velocities for ammonium particles using Eqs. (5) and (9), þ þ vðNHþ 4 Þ ¼ DDðNH4 Þ=CðNH4 Þ:
ð9Þ
Because both the stomatal uptake of ammonium and the cuticular adsorption of ammonia is important the most Table 5 Calculated dry flux (DD) and dry deposition velocity ðvÞ of ammonium particles in the cases of different rates of cuticular ammonia adsorption and stomatal uptake of ammonium ðTF þ SF WD ¼ 26 mequ m2 yr1 )
UPðminÞ ¼ 0; CUðminÞ ¼ 0 UPðmaxÞ ¼ 30; CUðmaxÞ ¼ 40 UPð0:5Þ ¼ 15; CUð0:5Þ ¼ 20
DD ðNHþ 4Þ ðmequ m2 yr1 )
v ðNHþ 4Þ ðcm s1 Þ
26 16 21
1.04 0.65 0.84
4423
probable rate of dry deposition, v ¼ 0:8470:25 can be inferred when the rate of stomatal and cuticular dry deposition of ammonia is 1:1 [case of CU(0.5)] and half of ammonium ions is taken up from the precipitation [case of UP(0.5)]. Though, these are rough estimations, the good agreement between deposition velocities from sulfur balance vX0:8270:25 cm s1 and from reduced nitrogen balance v ¼ 0:8470:25 cm s1 suggests that the assumptions for the rate of cuticular adsorption of ammonia and stomatal uptake of ammonium ion are close to the reality.
4. Conclusions There are great uncertainties in the estimation of dry deposition rate of fine aerosol particles to forest ecosystems. However, the yearly bulk dry deposition velocity of ammonium and sulfate particles can be estimated by a simple way using routine wet deposition, throughfall and stemflow measurements as well as dry deposition measurements of gases. Since ammonium and sulfate mostly exist in PM2.5 range and are nearly in stochiometric ratio dry deposition of ammonium sulfate particles can be generalized for PM2.5 particles. The deposition figures determined either from the Sand N-balance calculations (vX0:8270:25 and ¼ 0:847 0:25 cm s1 ) are in good agreement and they are in accordance with other experimental deposition velocities found in the literature. These results suggest on one hand the necessity of the revision of the models applied during the theoretical calculation of dry deposition velocity for fine particles and on the other, to continue the simple experimental work described here for as many places as possible.
Acknowledgements Investigation were sponsored by: National Committee for Technological Development, 1996–97, PHARE TD&QM No. H-9305–02/1033; National Committee for Technological Development 1996–98, No. 6-97-451047; Ministry for Environment, 1999 No. 063/T; USHungarian Joint Research Fund, 1996–99 No. 608/96; National Committee for Technological Development, 1998–1999, UNDP-HUN/95/002-0119; Ministry for Environment, 2000, No. KAC-20834; Ministry for Environment, 2001, KAC-27822; Ministry for Environment, 2002, KAC-44146; Hungarian Scientific Research Fund, 2000–2003, No. OTKA T-31927.
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