Hungerbühler, K.

Möller, M.

Scheringer, M.

Wegmann, F.

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Are airborne organic pollutants subjectto a ‘‘forest filter effect’’?F. Wegmann, M. Scheringer, M. Mo¨ller, K. Hungerbu¨hlerQuestion 1Our interpretation of the forest filter effect (FFE) is that the FFE includes twoelements, (i) a reduced concentration in the air and (ii) an increased concen-tration in the vegetation-covered soil. For DDT as a test chemical in a five-compartment box model, we find both elements but they do not always occur incombination. The occurrence and magnitude of the concentration changes de-pend on a variety of environmental parameters, substance properties, and theirinterplay. There are also parameter combinations for which a slight increase ofthe atmospheric concentration is observed (shielding effect). Accordingly, vege-tation can alter the mass balance of POPs-type chemicals significantly. Vegetationprovides an additional sink and/or an additional, efficient pathway for masstransfer from the air to the ground. Generally, the influence of vegetation is morepronounced in regional models with large areas covered by vegetation than inglobal models.As Wania and McLachlan (2001) point out in their modeling study on the FFE,degradation is an important factor influencing the FFE. Here, we focus on theeffect of degradation in air and vegetation on the occurrence and extent of theFFE. We use a five-compartment model consisting of tropospheric air, surfaceocean, vegetation, vegetation-covered soil, and bare soil to compare the steady-state results from this model (henceforth called ‘‘VegeZoMo’’) with results from amodel without vegetation and the vegetation-covered soil included into the baresoil compartment (henceforth called ‘‘NoVegeZoMo’’). All processes not affectedby vegetation are identical in both models. 25% of the surface of the model systemare covered by vegetation, 71% by seawater and 4% by bare soil (global averages).The vegetation is described by parameters for foliage–air partitioning, gaseousand particle deposition, leaf fall etc. as reported by Riederer (1990), Horstmannand McLachlan (1998). Values for grassland and deciduous and coniferous forestsare combined in a global average.Stochastic Environmental Research and Risk Assessment 17 (2003) 244–248  Springer-Verlag 2003DOI 10.1007/s00477-003-0139-3A SERRA Journal ForumF. Wegmann, M. Scheringer (&), M. Mo¨ller, K. Hungerbu¨hlerInstitute for Chemical and Bioengineering, Swiss Federal Instituteof Technology, CH-8093 Zu¨rich, SwitzerlandFinancial support by the Swiss Federal Agency for theEnvironment, Forests and Landscape is gratefully acknowledged.244Question 2We use the following DDT properties as the base case for model calculations withvarying degradation rate constants in air and vegetation (variation of the deg-radation rate constants in soils or water does not influence the vegetation effectsignificantly): ks ¼ 2.16 Æ 10)4 d)1, kw ¼ 3.88 Æ 10)3 d)1, ka ¼ 1.72 Æ 10)1 d)1(Howard et al., 1991), log KAW ¼ )2.94, log KOW ¼ 5.98 (Mackay et al., 1985).The base case of the degradation rate constant in vegetation, kveg, is an averagevalue of 1.72 Æ 10)1 d)1 (Mackay et al. 1997; Garrison et al. 2000).The influence of vegetation is expressed in terms of quotients of the steady-state concentrations in VegeZoMo divided by concentrations in NoVegeZoMo. Inall calculations, DDT is released to the air.First, kveg is varied from 1.0 Æ 10)4 d)1 to 1.0 d)1 with all other parameters as inthe base case. Figure 1 shows the ratio of the concentrations in air in the twomodels. For kveg £ 1.0 Æ 10)3 d)1, values slightly above one are obtained. Thisindicates a higher concentration in air in VegeZoMo compared to NoVegeZoMoand represents the shielding effect: the foliage shields the underlying vegetation-covered soil against deposition from the air. The very slow degradation in veg-etation combined with the strong diffusive coupling between the canopy and aircompartments leads to this effect. Degradation in the vegetation accounts for0.2% of the total vegetation outflow, revolatilization into the air for 96.5%, andleaf fall for 3.3%. The slope of the concentration ratio is steepest in the range1.0 Æ 10)3 < kveg < 1.0 d)1; increasing the degradation rate constant in vegetationover this range reduces the air concentration by 11%.Figure 2 shows the variation of ka from 1.72 Æ 10)3 d)1 to 1.72 Æ 101 d)1. Theslower the degradation in air, the stronger the filter effect. The minimum airconcentration quotient is 0.44. The concentration reduction in the air at low ka isdue to the efficient transfer into the foliage with subsequent removal by degra-dation in the foliage (and leaf fall): at kveg ¼ 0.172 d)1, 78% of the mass in thefoliage is removed by degradation and 1% by leaf fall; 21% of the total outflowfrom the foliage volatilizes back into the air.Fig. 1. Ratio of the concentrations in air in VegeZoMo and NoVegeZoMo vs. degradationrate constant in vegetation, kveg. ka ¼ 1.72 Æ 10)1 d)1245In the third step, the two degradation rate constants in air and vegetation arevaried simultaneously between 1.72 Æ 10)3 d)1 and 17.2 d)1 (all other parametersas in the base case). Figure 3A shows that, with increasing degradation in air andvegetation, the concentrations’ quotient first falls to a minimum of 0.82 atka ¼ kveg ¼ 1.72 Æ 10)2 d)1 and then approaches one if the degradation rateconstants are further increased. At high values of ka and kveg, degradation is theonly relevant process in vegetation (leaf fall: 0.009% of total outflow; revolatil-ization: 0.27%; degradation: 99.7% at kveg = 17.2 d)1). Since ka equals kveg, bothvegetation and air are efficient sinks and the concentration ratio is one. Thediffusive coupling between the air and the vegetation is ineffective because anysubstance transferred into the vegetation is degraded.The minimum of the air concentration ratio can be explained by reducing thedegradation rate constants in air and vegetation and analyzing the mass fluxes inthe vegetation. If the two degradation rate constants are decreased (starting fromthe maximum value, 17.2 d)1), two processes in air change: degradation becomesless important and, correspondingly, the transfer from air into foliage isenhanced. Although kveg is lowered in parallel with ka, the degradation mass fluxwithin vegetation increases because transfer from the air into the canopy isincreasing. At kveg ¼ 1.72 Æ 10)2 d)1, the degradation mass flux within the canopyis maximal and the minimal concentration ratio of 0.82 is observed. At even lowerdegradation rate constants, the degradation mass flux decreases and therevolatilization flux pushes substance back into the air (leaf fall accounts for amass flux of only 3% of the revolatilization mass flux at all values of kveg and ka).At very low kveg, the revolatilisation flux is the dominant pathway out of thecanopy (97%), air and canopy are efficiently coupled by diffusive exchange, andthe concentration ratio again approaches one (not shown in Fig. 3A).Finally, the quotient of the vegetation-covered soil concentration divided bythe bare soil concentration is plotted in Fig. 3B. The vegetation-covered soil has ahigher concentration than the bare soil if degradation is slow because of thehigher input by leaf fall. Leaf fall is responsible for 26% of the total inflow into theFig. 2. Ratio of the concentrations in air in VegeZoMo and NoVegeZoMo vs. degradationrate constant in air, ka. kveg ¼ 1.72 Æ 10)1 d)1246vegetation-covered soil at kveg ¼ 1.72 Æ 10)3 d)1, whereas it contributes only 0.1%of the total inflow at kveg ¼ 17.2 d)1.In conclusion, we generally find a filter effect in our model. However, the effectis not as pronounced as in regional models. Variation of the degradation rateconstants ka and kveg shows the whole range from a shielding effect to a significantfilter effect. Our results indicate a need for reliable degradation rate constants,especially in air and vegetation.ReferencesGarrison AW, Nzengung VA, Avants JK, Jackson Ellington J, Jones WJ, Rennels D, LeeWolfe N (2000) Phytodegradation of p,p-DDT and the enantiomers of o,p-DDT. Environ.Sci. Technol. 34: 1663–1670Fig. 3. A Ratio of the concentrations in air in VegeZoMo and NoVegeZoMo vs. degradationrate constants in vegetation and in air, kveg ¼ ka. B Ratio of the concentrations in vege-tation-covered soil in VegeZoMo and in bare soil in NoVegeZoMo vs. degradation rateconstants in vegetation and in air, kveg ¼ ka247Horstmann M, McLachlan MS (1998) Atmospheric deposition of semivolatile organiccompounds to two forest canopies. Atmos. Environ. 32: 1799–1809Howard PH, Boethling RS, Jarvis WF, Meylan WM, Michalenko EM (1991) Handbook ofEnvironmental Degradation Rates, Lewis Publishers: Boca RatonMackay D, Paterson S, Cheung B, Brock Neely W (1985) Evaluating the environmentalbehavior of chemicals with a level III fugacity model. Chemosphere 14: 335–374Mackay D, Shiu W-Y, Ma KC (1997) Illustrated Handbook of Physicochemical Propertiesand Environmental Fate for Organic Chemicals. Vol. V: Pesticide Chemicals. Lewis Pub-lishers: Boca RatonRiederer M (1990) Estimating partitioning and transport of organic chemicals in the foli-age/atmosphere system: discussion of a fugacity-based model. Environ. Sci. Technol. 24:829–837Wania F, McLachlan MS (2001) Estimating the influence of forests on the overall fate ofsemivolatile organic compounds using a multimedia fate model. Environ. Sci. Technol. 35:582–590248

Are airborne organic pollutants subject to a "forest filter effect”?

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Are airborne organic pollutants subject to a "forest filter effect”?

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