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Evaluation of SO2 dry deposition over short vegetation in Portugal
Atmospheric Environment 35 (2001) 3633}3643
Evaluation of SO dry deposition over short vegetation in Portugal M.S. Feliciano *, C.A. Pio , A.T. Vermeulen Departamento de Ambiente e Ordenamento, Universidade de Aveiro, Campus de Santiago, 3800 Aveiro, Portugal ECN } Energy Research Foundation, PO Box 1, 1755 ZG Petten, The Netherlands Received 30 June 2000; accepted 16 November 2000
Abstract SO dry deposition was studied over short vegetation, in Portugal, by means of the concentration gradient method. The experimental study involved one "rst phase of long-term measurements carried out in a grassland and, subsequently, a second period of several 1997 intensive "eld campaigns performed in three places representing di!erent climate and surface conditions. Temporal and spatial patterns of dry deposition parameters show that downward #uxes of SO are by some extent a!ected by surface processes. Median R varied from 140 s cm\ to values around 200 s cm\, in a wide range of environmental conditions. Stomatal uptake is an important sink when vegetation is biologically active, but its contribution is e!ectively low when compared with non-stomatal mechanisms, especially when the surface is wet. Under dry conditions R increases by a factor of two, but SO deposition rates then still are signi"cant. The parameterisation of the surface resistance for SO proved to be di$cult, but < derived with the Erisman parameterisation (Erisman et al., Atmos. Environ. 28 (16) (1994) 2595) compared best with measured values, at low time resolution scale and especially under moisture conditions. 2001 Published by Elsevier Science Ltd. Keywords: Sulfur dioxide; Dry deposition; Vegetation; Portugal
1. Introduction Transport of acidic substances, such as sulphur dioxide, from the atmosphere to the earth's surface is one of the major environmental problems of our time. This process, currently called acid deposition, is an undesirable consequence of air pollution and is held responsible for substantial damaging e!ects on vegetation, aquatic life, structural and ornamental materials and, in some instances, on human health (EEA, 1995; Radojevic and Harrison, 1992). In view of these ecological, aesthetic and economical impacts, the member countries of the UNECE have put assessment methodologies into practice with the purpose of reducing emissions of acidifying compounds to avoid exceedances of critical levels and critical loads (Bull,
* Corresponding author. Fax: #351-2-34-429290. E-mail address: [email protected] (M.S. Feliciano).
1991; Metcalfe et al., 1998). The implementation of abatement strategies based on these concepts requires reliable estimates of atmospheric input to ecosystems (wet and dry deposition), in order to determine the exposure or deposition levels below which no adverse e!ects occur (Erisman and Draaijers, 1995; Stedman et al., 1997). Dry deposition of sulphur dioxide is fairly important, since the amounts delivered to the earth's surface through this process represent a substantial part of the total acid deposition (Erisman et al., 1989; Garland, 1978). Estimates of SO dry deposition, on local and regional scales, have been improved with the development of empirical descriptions based on #ux measurements carried out principally in the United States (Hicks and Matt, 1988; Voldner et al., 1986; Wesely, 1989) and northern European regions (Erisman and Wyers, 1993; Fowler, 1978; Garland, 1977). However, as SO dry de position varies with weather conditions and physicochemical and biological properties of the surface, it is important to evaluate the uncertainties arising from the
1352-2310/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 5 3 9 - 2
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generalisation of results from one region to another (Brook et al., 1997; Erisman and Wyers, 1993). Within the framework of the European Union environmental research programme, gradient measurements of SO were carried out over short vegetation in Portu gal, as a part of the SREMP and MEDFLUX projects, with the main purpose of evaluating the applicability of existing dry deposition models to southern European regions. In this part of Europe, climate di!ers substantially from northern European climate and hence the growing cycle of vegetation and other properties of the surface are distinct as well. The large set of data collected in nearly two years of measurements contributes to testing of the current dry deposition schemes for SO under south European conditions, where only a small amount of information existed until now. In this paper our attention will be focused on the evaluation of the principal controlling mechanisms, based on analysis of the temporal and spatial variability of SO dry deposition parameters. A parameterisation scheme is also employed as an evaluation tool and to infer the principal implications of its generalisation for this European region.
2. Experimental description The experimental "eldwork involved two distinct phases. In the "rst phase, long-term measurements were carried out over a grassland ecosystem located in the Aveiro region (Sarrazola), between November 1994 and September 1995. In the second stage, during 1997, measurements were conducted at three di!erent sites through seven intensive "eld campaigns, in a total of approximately 150 sampling days, with the following temporal distribution: at Sarrazola in two periods from 15 January to 26 February and from 30 July to 22 August; at Baldios in four measuring periods: 5}21 March, 5}15 April, 5}22 June and 5}18 July; and at Pancas from 11 to 29 September. Sarrazola is a 40 ha #at area with permanent and temporary meadows, providing a reasonable fetch from almost all directions. The study area is located near the Atlantic coast in the north region of Portugal (40342 20N//833715W). Climate is humid without extreme temperatures and is under the strong in#uence of sea}land breezes. Vegetation starts to grow in early autumn, reaching its maximum activity in late winter or mid-spring. From that time onwards most grasses go into a decline phase reaching the minimum between July and August. Baldios is a typical Mediterranean pseudo-steppe grassland located in the south of Portugal, 80 km southeast from Lisbon (3833358N//831822W). The research area consists of a wide and rolling plain under the in#uence of a hot and semi-arid climate, where grassy vegetation is used mainly for grazing livestock or hay growth.
In contrast with the previous place, vegetation never exhibits a lush state, reaching its maximum height of about 30 cm in June. From June onwards both structural parameters and physiological activity decreased, giving a yellow-brownish colour to the canopy. Pancas is a wide and #at area located in the Natural reserve of the Tejo Estuary, 15}20 km northeast from Lisbon (383517N//835518W). This zone encloses an extensive cultivated area and is dominated by large-scale intensive agriculture (wheat, sun#ower, barley, etc.). Part of the land is used for cattle grazing as well. The undisturbed fetch is higher than 400 m for all directions. During the sampling period, most crops had already been harvested and the area within the likely #ux footprint domain consisted mainly of fallow areas with short sparse grasses, mostly dry, and areas of bare clay soil. Further details on physical and physiological characteristics can be found in Feliciano et al. (1999) and Pio et al. (2000). SO dry deposition was monitored by means of the gradient technique integrated in a monitoring station designed and developed by Netherlands Energy Research Foundation, ECN, which has capabilities for eddy correlation measurements of energy exchange and other gaseous species, as well (Vermeulen, 1998). Gradient #uxes were obtained by coupling an SO monitor (Thermo Environmental Instruments } model 43S), with a 3D ultra-sonic anemometer (Solent Research Gill), mounted vertically on the top of a 5 m mast. The sonic anemometer provides high-frequency measurements of the wind speed vector and sonic temperature, from which turbulence parameters, such as friction velocity, sensible heat and Monin-Obukhov length can be determined directly. High-frequency measurements are processed in real time according to the algorithm implemented by McMillen (McMillen, 1988; Baldocchi et al., 1988). The SO monitor was connected to a Te#on electric valves system, in order to measure at programmed intervals the SO concentration at an alternating height of 0.5 and 5 m. There are advantages and disadvantages in using only two measuring heights. With this con"guration it is not possible to know precisely the vertical concentration pro"le. However, the two point concentration vertical gradients are less a!ected by the temporal variability of the SO concentration, as a result of the shorter sampling cycle routine. In this way, we consider that, when just a single monitor is available, the con"guration implemented in this study is one of the best solutions for gradient measurements. The ambient air was sampled through Te#on tubes, which were heated to prevent water vapour condensation and reaction of SO with the internal tube walls. The SO monitor was calibrated several times in the "eld and in the laboratory with a permeation device. Other micrometeorological variables like global radiation, temperature, relative humidity and precipitation amount were been measured with proper sensors.
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3. Computation and analysis of SO2 6uxes
material, etc.:
SO #uxes were computed on the basis of the aerody namic gradient method (Baldocchi et al., 1988) assuming that heat and mass are transported in a similar way within a well-developed surface layer, using the equation F"!u c , H H where u is the friction velocity and H concentration, de"ned according to the siderations presented in Berkowicz and
(1) c is the eddy H theoretical conPrahm (1982):
c "kc/[ln(z /z )! (z /¸)# (z /¸)]. H & &
(2)
c is the 15 min averaged concentration gradient determined through the di!erence between SO concentra tions at levels z and z , k is the Von Karman constant (0.40) and (z/¸) is the integrated stability correction & function for heat de"ned as in Erisman and Draaijers (1995). By convention, F is negative when the #ux is towards the surface and positive when #ux occurs in the opposite direction. Fluxes computed from Eqs. (1) and (2) were then interpreted on the basis of the multiple resistance approach, which describes vertical #ux analogously to Ohm's law, i.e., by considering the existence of several resistances to the transport of the pollutant from the atmosphere to the surface (Garland, 1977; Hicks et al., 1987). Taking into account this conceptual description, F is expressed by C X F"!< C " , X R #R #R
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(3)
where < is the dry deposition velocity, C is the pollu X tant concentration at height z , R is the aerodynamic resistance associated with turbulent transfer across atmospheric boundary layer, R is the quasi-laminar boundary layer resistance, mainly in#uenced by turbulence and molecular di!usivity of the trace constituent, and R is the canopy resistance that depends on physical and chemical interaction between the trace constituent and the deposition surface. By computing directly < , R and R from concentration and meteorological measurements (Erisman et al., 1994a; Pio et al., 2000), the overall canopy resistance R can be derived from Eq. (3) as residual term. Then, time series of calculated R values could be used to obtain a canopy resistance parameterisation or to validate model results. For most purposes and particularly for short vegetation, R is often partitioned into two resistances in parallel, the stomatal resistance (R ), associated with di!usion through the stomata, and the external (or nonstomatal) resistance (R ), which includes chemical destruction of this gaseous pollutant in the various surface elements like leaves cuticle, underlying soil, dead
1 1 1 " # . (4) R R R The absorption of SO via stomata is a well under stood mechanism and in the present study was evaluated through application of the Wesely algorithm (Wesely, 1989):
200 400 R "r 1# G G#0.1 ¹(40!¹)
D & -, (5) D 1where r is the minimum bulk canopy stomatal resistance G for water vapour, reformulated for our environmental conditions, according to the procedure described in Pio et al. (2000), G is the global radiation intensity (W m\) and T is the temperature in degrees Celsius. D / &D ("1.9) is the ratio between molecular di!usion 1coe$cients of water vapour and sulphur dioxide, respectively. In relation to extra stomata mechanisms, the knowledge is still insu$cient to develop proper parameterisations. A useful attempt has been developed by Erisman et al. (1994a), based on measurements performed in the Netherlands over heathland, but already generalised to other Dutch surface types (Erisman, 1994). This author describes the external surface resistance as a function of surface wetness, caused either by precipitation or high relative humidity. During, or just after rainfall, the author assumes R "1 s m\ and under other conditions R is related to relative humidity, RH, as follows:
0.58;10EXP[!0.278RH] if RH'81.3%, R " 25,000EXP[!0.0693RH] if RH(81.3%. (6) 4. Results and discussion 4.1. Data selection and evaluation of SO2 dry deposition A total of 29,814 measuring points (15 min averages), recorded along all experiments, were screened by a set of simple and basic selection criteria. This procedure aimed at studying SO dry deposition from a data set ful"lling the theoretical and technical constraints required for a good performance of the gradient method (Baldocchi et al., 1988; Fowler and Duyzer, 1989). The valid conditions, together with percentage of remaining periods, are shown in Table 1. Basically, the restrictive criterion aims at eliminating or minimising the in#uence of periods associated with adverse meteorological conditions, low precision of measured concentration pro"les, advection and storage episodes and other errors arising both from measurements and mathematical approaches (Baldocchi et al., 1988; Erisman et al., 1994b).
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Table 1 Selection criteria applied to SO dry deposition measurements and percentage of remaining periods after application of each one. The total periods for each site are also given Criteria of acceptance
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Wind speed Taylor hypothesis Friction velocity Drag coe"cient Atmospheric stability Fetch Detection limit Variation coe"cient Temporal variation Limits of V
No. of initial points
Remaining periods for each site (%)
;*1 m s\ ;)0.5; ; *0.5 m s\ H C (0.02 ¸'1 m Wind Dir"f (local) C *3 g m\ 1CV" G /C )0.2 A G C /t)0.2F /z G G < (1.5< +
Sarrazola 95
Sarrazola 97
Baldios 97
Pancas 97
82.61 81.65 69.28 68.22 66.82 66.82 28.84 28.77 17.82 16.04
81.51 80.68 65.29 63.91 61.97 61.97 25.54 18.75 16.57 14.15
95.77 91.75 83.17 80.49 79.35 75.33 19.63 17.33 15.13 12.84
80.87 75.26 60.49 58.71 55.15 55.15 27.77 19.53 15.57 11.48
18,892
5004
4402
1516
Most of the original data were rejected due to the large amount of periods with low concentration. A rejection concentration limit of C (3 g m\ was applied. The 1adoption of 0.5 g m\ as concentration limit ( 3;detection limit of SO monitor) would increase the remain ing data by a factor of about 2, but, in these conditions, gradient measurements were highly a!ected by the SO monitor random noise. The application of 3 g m\ as the minimum limit of data acceptance ensures an interpretation of the dry deposition controlling mechanisms free of artifacts as much as possible, although this could lead to an overestimation of the overall mean #ux. The in#uence of the low concentrations on the reliability of the measuring technique may be inferred from Fig. 1. This "gure, which shows dry deposition #uxes against several SO concentration classes, including peri ods with concentration values higher than 0.5 g m\, proves that upward SO #uxes are a common feature at low concentrations. The observed relationships also suggest that deposition rates increase as concentration rises, with a proportionality coe$cient that is clearly dependent on local speci"c factors. To get better insight into those controlling factors, selected data were grouped into long-term periods, so that representative cause}e!ect relationships might be found. Data collected at Sarrazola were divided into four seasonal periods in the following indicated as: S-Winter 95 (1 November}31 March), S-Winter 97 (15 January}28 February), S-Summer 95 (1 June}31 August) and, "nally, S-Summer 97 (30 July}22 August). Data gathered at Baldios were divided into two classes taking into account the physiological properties of the surface. The "rst three "eld campaigns were grouped into the class, BA1, while the fourth "eld experiment was studied apart as the BA2
period. For Pancas, all data were included in one single period, denominated here as PA. Daytime and nighttime periods have also been distinguished. Daytime was de"ned as the period in which global radiation values are higher than 10 W m\. Once established, statistical values of the dry deposition parameters (< , < and + R ) were estimated for each one. < is the deposition + velocity which should result in the absence of surface resistance, < "1/(R #R ). Main results illustrating + the temporal and spatial variation for daytime and nighttime periods are presented in Fig. 2. In all periods, dry deposition velocities were higher during daytime than at night, independent of the place where they were measured. Daytime median values of < ranged between 0.6 and 0.8 cm s\, exceeding noctur nal values by on average a factor of two. Taking into account the selection procedure, it is worth noting that < values should be interpreted as maximum values, especially at night. In what concerns the seasonality of < , a slight variation is observed, but without any regular pattern. For instance, at Sarrazola, during the "rst measurement period, < decreases gradually from winter (< 0.8 cm s\) to summer (< 0.6 cm s\), while 1997 data, collected at the same site, show a trend in the opposite direction. In terms of spatial variation, SO molecules are removed more rapidly by dry deposition at Sarrazola than in the other two southern places, where < values were similar in magnitude and range. In contrast to the diurnal variation found in < , sur face resistance shows an uncharacteristic daytime/nighttime variation. Daytime R varies between values close to zero and values up to 2 s cm\, with median values predominantly lower than 1 s cm\. Nocturnal median values are somehow higher and exhibit more scattering
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Fig. 1. Median and percentiles of SO exchange rates as a function of SO concentration classes for the di!erent measuring places. These #uxes were computed from selected data, but using C '0.5 g m\ (the "rst category includes only data with 10.5(C )1 g m\). 1-
than those observed during daylight hours. The seasonality in this parameter can be illustrated by observations collected at Sarrazola, where daytime R values under conditions prevailing in winter are lower than those in the other season periods. At night, a seasonal trend is also evident, but while at Sarrazola R nocturnal values decrease from winter to summer, at Baldios the variation was completely di!erent. Di!erences in R observed at Sarrazola, between 1995 and 1997, may be a consequence of the di!erent environmental conditions prevailing throughout these two years or result simply from the distinct representativeness associated to each data set. Another remarkable feature is related to the pronounced contrast between R observed at Sarrazola and R ob tained in the other southern sites. Comparing the variability of < with that of R and < , it follows that dry deposition of SO is a complex + function of atmospheric vertical mixing and surface characteristics, where the in#uence of atmospheric conditions is re#ected principally in the diurnal variation of < , and surface properties a!ect its diurnal, seasonal and spatial patterns. In fact, during daytime and under generally unstable conditions, atmospheric resistance is much smaller than surface resistance, and therefore < is strongly inhibited with increasing R . At night, under
more stable conditions, atmospheric resistance is frequently higher than surface resistance and hence R has a smaller in#uence on < . The magnitude of both dry deposition parameters (< and R ) found here are within the range of the corre sponding values reported in the literature for ecosystems with short vegetation (Erisman and Wyers, 1993; Plantaz, 1998; Sehmmel, 1980; Voldner et al., 1986). The interval of averaged < values, over similar surface cover age, varies from values as low as 0.1 cm s\ to values up to 2.5 cm s\, but nocturnal values are generally in the lower part of the reported range ((1 cm s\). Correspondingly, the mean values of R vary generally be tween values close to zero and 5 s cm\, depending on a large set of surface conditions that determine the strength of the two most important removal mechanisms involved in the SO dry deposition: stomatal uptake and absorption of SO into surface aqueous layers (Chamberlain, 1986; Erisman and Wyers, 1993; Fowler and Unsworth, 1979). Hence, taking into account that our observations comprised the whole spectrum of the most relevant environmental conditions, it is not surprising that measured values of R have a large range and exhibit also some less clear patterns.
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Fig. 2. Temporal and spatial variation of the SO dry deposition parameters (velocity, maximal velocity and surface resistance), given as daytime and nighttime medians. Whiskers range between the 25th and 75th percentiles.
The evaluation of temporal and spatial trends of R suggests that non-stomatal mechanisms have a meaningful contribution to the SO deposition, when compared with the process of di!usion of SO through stomata. In fact, in the absence of signi"cant non-stomatal mechanisms, R should follow a pattern more consis tent with the diurnal variation of the stomatal opening. For example, over transpiring canopies, daytime R should have much lower values than nocturnal R . Following the same reasoning, the seasonal variation of R should be fairly more pronounced, at least at Sarrazola during 1995, since the vegetation evolved from an actively growing state in winter to an accentuated summertime senescent state. At Baldios, the biological activity of vegetation has also been limited to the "rst period established for this location, but, during this time spell, the grass never reached a luxuriant state, susceptible to promoting accentuated gaseous exchange rates through stomata. In the same way, the spatial contrast should be more pronounced, since SO removal, both at Baldios and Pancas, is believed to occur in response to the physical}chemical interaction of this gaseous pollutant with the external leaf surface and other available elements. The contribution of non-stomatal mechanisms in relation to stomatal uptake has also been evaluated by
parameterising stomatal resistance with the Wesely formulation (Eq. (5)) for a period with biologically active vegetation (part of S-Winter 95), employing a seasonal parameter r of 120. This parameter was calculated for G our conditions from ozone #ux measurements, according to the procedure presented in Pio et al. (2000), in which r is derived by "tting mean (and median) diurnal cycles G of ozone R . The estimated r agrees well with the value G presented in Wesely (1989) for the rangeland type, but is a factor of two higher than that established for the agricultural class with lush vegetation. This procedure led to estimates of minimal bulk R to SO of about 250 s m\, showing that non-stomatal mechanisms are e!ectively more e$cient than destruction of SO molecu les after their di!usion through stomatal opening, even when both mechanisms coexisted at the same time. The estimated R to the SO transfer is fairly acceptable when compared with R to water vapour reported for a wide range of herbaceous annuals and woody perennials, under optimal plant growing conditions (within the range 30}300 s m\) (Erisman et al., 1994a). Since the magnitude of R is mainly determined by non-stomatal mechanisms in the three places, the lower R values observed at Sarrazola seem to be associated with the absorption of SO in surface aqueous layers. In fact, at Sarrazola surface wetness is a common local
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643
feature. In this place measurements were carried out under rather moist conditions, where water vapour content of the atmosphere is high and phenomena like mist, drizzle, fog and rainfall occur frequently. During the night, the atmosphere at Sarrazola is nearly always saturated in relation to water vapour. During daylight hours the relative humidity is slightly lower, ranging from values around 80% in autumn/winter to values of 60}70% in summer. In relation to rainfall, a clear seasonal pattern was also observed, in which the high abundance of rainy events in autumn, winter and part of spring contrasts with the low occurrence of rainfall in summer. In the other places, the presence of water, both in liquid and vapour states, was e!ectively lower and thereby the experimental information was mostly collected under dry conditions. Correspondingly in these locations, where foliage and the underlying soil often remained free of moisture, the a$nity between SO mol ecules and surface was lower, but not low enough to cause a signi"cant decreasing of the SO deposition. Spatial contrasts in what concerns to relative humidity and rainfall are summarised in Fig. 3. The enhancement of SO deposition by surface wet ness has been demonstrated in other "eld experiments
3639
(Erisman and Wyers, 1993; Fowler and Unsworth, 1979). Voldner et al. (1986) reported R values of 4 s cm\ (from a range of 1}9 s cm\) over a mature crop (dry or stubbed) and 2 s cm\ when cropland consists of bare soil and senescent vegetation. Over grassland, throughout an annual cycle, Davies and Mitchell (1983) determined mean R values in the range 0.70}1.65 and 0.8} 1.10 s cm\ for dry and wet surfaces, respectively. In an attempt to provide more complete information on the e!ects of surface wetness on SO dry deposition, non-stomatal resistance (R ), distinguishing night ,12- 1 and daytime periods, was plotted against relative humidity (see Fig. 4). At night, R "R and during day,12- 1 time R is equivalent to R estimated from ,12- 1 Eq. (4), after computing R with Eq. (5), using r values G estimated from ozone deposition measurements (Pio et al., 2000) for each month of the 1995 measuring period and each intensive campaign of 1997. Relative humidity is not a measure of surface wetness, but is certainly a reasonable indicator of surface wetness caused by condensation of atmospheric water vapour or deliquescence of salts at the leaf external surface and, in some instances, caused by precipitation as rainfall, drizzle, fog, mist, etc. Other sources of surface wetness like guttation and soil
Fig. 3. Relative humidity and rainfall registered in the di!erent measuring periods.
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Fig. 4. Non-stomatal resistance against relative humidity given for nighttime (nocturnal R ) and daytime (R ) periods. ,12- 1
distillation, which are particularly relevant at night and in the early morning hours, are also mostly associated with humid conditions. Looking closely at the various relationships, we observe that when relative humidity is high (generally '80%), R values are generally lower and exhibit a narrower variation, as a consequence of the huge probability of the surface being wet. As relative humidity diminishes, R increases substantially, follow ing, in some cases, a linear or an exponential trend. Although the previous analysis seems to demonstrate that SO is e!ectively removed at higher rates under moist conditions, there are also clear indications that dry deposition of SO is not necessarily high in the presence of aqueous layers on the surface. In fact, SO is a reason ably soluble gas in pure water, but the absorption of SO in water layers is associated with a series of reactions strongly dependent on acid and oxidation components (metals, H O , etc.) of the aqueous media (Erisman and Draaijers, 1995; Wesely et al., 1990). Even when the chemical composition initially promotes the SO re moval, as oxidation proceeds, wetness becomes more acidic, SO becomes less soluble and, hence, for this mechanism to be signi"cant the bu!er capacity of surface must be su$ciently high (Fowler and Unsworth, 1979). This dependence leads generally to a large variability in R under "eld conditions. For instance, the almost negli gible R values observed in the Netherlands over grass lands have been ascribed to the humid climate and high
NH levels prevailing in that country (Erisman and Wyers, 1993). These suggestions are also in accordance with ideas discussed in studies of Brimblecombe (1978) and Chameides (1987), who emphasise the importance of the presence of chemicals on wet surfaces. Returning to Fig. 2, we infer that the seasonal behaviour in nocturnal values of R observed at Sarrazola seems to be well correlated with the likely variation in the surface wetness chemistry from winter to summertime. This hypothesis is supported on the seasonal contrast registered in rainfall regime and on the potential relationship between surface wetness chemistry and its origin. When surface wetness is determined by rainfall or other precipitation events, the concentration of H> in water droplets is exceedingly in#uenced by acidity of rainwater, which is generally higher than that resulting from water condensation on the surface. Some "eld studies, mainly those performed in USA, indicate that the surface is a poor sink, when wetted by rain (Hicks et al., 1987; Wesely, 1989). Wesely et al. (1990) demonstrated that, in relation to rain, dew contains larger concentration of SO\, but smaller concentration of H>. Here, the inhibi tor e!ect of rainfall is far from reaching levels compared with those observed in USA, but it is clearly higher than that found in northern European countries, namely in Netherlands, where during or immediately after rain R becomes close to zero or vanishes (Erisman and Wyers, 1993). Hence, although in S-Winter 95 moisture
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643
levels are higher than in S-Summer 95, the chemical composition of liquid water (principally H> content) was more favourable to the chemical destruction of SO in the second period. In summer, surface wetness was caused mainly by water vapour condensation processes, like dew or salts deliquescence. When surface consists practically of dead material or uncovered soil, such as in BA2 and PA, SO removal might occur directly from the gas interaction either with soil particles or dead foliage, and probably is determined mainly by surface alkalinity. The chemical attack of SO to dry surfaces is not well documented, but it is well established that removal rates on dry or acidic surfaces should be much slower than in wet or alkaline receptors (Wesely, 1989). However, while acidic surfaces (pH(3.5) are generally inert to SO , independent of surface moist ure content, Garland (1977) found no appreciable variability of R with surface moisture in a "eld investigation over a chalky soil (pH"8). Apart from nocturnal R values observed in the "rst seasonal period estab lished for Baldios (BA1), the magnitude of R found in these two southern locations was quite similar. This "nding also seems well correlated with the little variability in moisture conditions prevailing in both places. The lowest nocturnal values of R , observed in that "rst period may be a direct consequence of the higher ability of cuticular surface for water retention, due to the presence of green leaves in larger amounts. An experimental study performed by Hove and Adema (1996) supports this hypothesis. The daytime/nighttime variation found
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in the other periods demonstrates that, under dry conditions, meteorological variables like global radiation and temperature may a!ect the chemical destruction due to their in#uence on the chemical equilibrium. In spite of these non-stomatal mechanisms having an important contribution in the SO dry deposition, the stomatal uptake seems to also have had a slight in#uence on the spatial pattern and on the seasonal variation prevailing at Sarrazola. The lowest daytime R values observed in winter also result from the higher SO ab sorption via stomata, once vegetation exhibited its maximum biological activity in winter and the "rst half of Spring. From that time onwards, the decline observed in the vegetation growth was accomplished by a gradual decrease in the stomatal activity leading to an increment of the overall SO surface resistance. During these peri ods, in which stomata fail to open, daytime SO uptake is mainly in#uenced by non-stomatal mechanisms, which are more e$cient at night than during the day, due to the likely di!erence in the extension and thickness of surface wetness. Voldner et al. (1986), based on several investigations, reported R values of 1 and 2 s cm\ for the early growing season and the late season, respectively, ascribing this variation to the biological activity. 4.2. Comparison between measured and parameterised R
From the analysis of our observations, it becomes clear that parameterisation of surface resistance to the SO deposition does not always give clear results. The
Table 2 Statistics on the comparison of experimental SO deposition velocities and surface resistance with values computed with Erisman et al. (1994a) parameterisation scheme Mean [STE]
Sarrazola No. of points R (s cm\) Measured Modelled V (cm s\) Measured Modelled Baldios/Pancas No. of points R (s cm\) Measured Modelled V (cm s\) Measured Modelled
Median [Quart. range]
Daytime
Nighttime
Daytime
Nighttime
2528
994
2528
994
1.23 [0.044] 1.56 [0.041]
1.05 [0.049] 0.51 [0.063]
0.72 [1.13] 1.07 [1.52]
0.71 [1.00] 0.06 [0.60]
0.76 [0.0086] 0.66 [0.0085]
0.60 [0.011] 0.76 [0.014]
0.68 [0.52] 0.56 [0.44]
0.53 [0.42] 0.67 [0.51]
335
209
335
209
1.59 [0.14] 2.29 [0.21]
5.14 [0.29] 1.81 [0.17]
0.78 [1.71] 1.24 [2.61]
3.52 [7.25] 0.54 [2.37]
0.70 [0.026] 0.34 [0.016]
0.33 [0.15] 0.38 [0.016]
0.59 [0.53] 0.28 [0.24]
0.23 [0.35] 0.35 [0.37]
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parameterisation scheme, de"ned by Eqs. (5) and (6), was applied to available 15 min time series in order to evaluate the potential errors arising from its generalisation to a di!erent climate region. As already stated, the seasonal parameter, r , in Eq. (6), was adequately adG justed to our environmental conditions. Concerning the R , rainfall was taken into account for 1997 periods only, because the available 1994/95 rainfall values were just registered as daily amounts. In this way the parameterised R for the 1994/95 period is slightly overes timated, especially for those periods in which rainfall was abundant. In Table 2 measured and parameterised values of SO dry deposition parameters (< and R ) are presented as arithmetic mean and median. The standard errors and quartile ranges are given between square brackets. The main results show that the model overpredicted nocturnal < and underpredicted daytime < , with larger deviations under drought conditions. In these drought conditions, the Wesely parameterisation (Wesely, 1989) is expected to give better results than Erisman parameterisation (Erisman et al., 1994a). The agreement between measured and modelled < increases consider ably when daily averages are compared. From the data we can conclude that deposition velocities modelled for long-term periods are quite accurate. For the short-term estimates the agreement between modelled and measured values was much lower.
5. Conclusion SO gradient measurements performed over short veg etation, in Portugal, were used for successfully evaluating the most important mechanisms involved in the dry deposition of this pollutant. Main results show that SO dry deposition is a complex function of aerodynamic factors and surface conditions. The atmospheric processes are particularly relevant in the shape of diurnal pattern, while the large variability of surface mechanisms causes changes in deposition patterns throughout the day, season and space. R values for SO indicate that removal of SO occurs to a large extent through physical and chemical a$nity of this pollutant with the surface. These non-stomatal mechanisms seem more e$cient when surface is wetted, but in the presence of aqueous layers, deposition of SO is not necessarily high, depending on the chemical composition of the water layer. Under dry conditions R is slightly larger, but not large enough to cause an inhibition in the vertical transport of SO . Stomatal uptake is shown to have a low contribution to the mean deposition #ux, mainly when it coexists with environmental conditions favourable to formation of surface water layers, susceptible to promoting rapid and prolonged SO ab sorption/oxidation.
Parameterisation of surface resistance for SO seems to be less successful in describing short-term variations of the most important controlling mechanisms. For low time resolution scales, parameterised SO deposition compares much better with the corresponding measured values. Discrepancies between modelled and measured < arise mainly from the di$culty in parameterising R , which has an important contribution in the SO removal and depends on a large number of variables. Although our study presents some experimental limitations, it allows a better insight into deposition processes over short vegetation and will contribute in the validation of parameterisations of SO dry deposition processes over southern Europe in the future.
Acknowledgements We gratefully acknowledge the European Commission for the "nancial support given through SREMP (contract no. EV5V-CT93316) and MEDFLUX (contract no. ENV4-CT95-0034) projects and the Fundac7 aJ o para a CieL ncia e Tecnologia for the scholarship (PRAXIS/3/ 3.2/AMB/38/94) given to M. Feliciano.
References Baldocchi, D.D., Hicks, B.B., Meyers, T.P., 1988. Measuring biosphere}atmosphere exchange of biologically related gases with micrometeorological methods. Ecology 69 (5), 1331}1340. Berkowicz, R., Prahm, L.P., 1982. Evaluation of the pro"le method for estimation of surface #uxes of momentum and heat. Atmospheric Environment 16 (12), 2809}2819. Brimblecombe, P., 1978. Dew as a sink for sulphur dioxide. Tellus 30, 151}157. Brook, J.R., Di-Giovanni, F., Cakmak, S., Meyers, T.P., 1997. Estimation of dry deposition velocity using inferential models and site-speci"c meteorology } uncertainty due to siting of meteorological towers. Atmospheric Environment 33(23), 3911}3919. Bull, K.R., 1991. The critical loads/levels approach to gaseous pollutants emission control. Environmental Pollution 69, 105}108. Chamberlain, A.C., 1986. Deposition of gases and particles on vegetation and soils. In: Legge, A.H., Krupa, S.V. (Eds.), Air Pollutants and their E!ects on Terrestrial Ecosystem, Advances in Environmental Science and Technology, Vol. 18. Wiley, New York, USA. Chameides, W.L., 1987. Acid dew and the role of chemistry in the dry deposition of reactive gases to wetted surface. Journal of Geophysical Research 92, 11895}11908. Davies, T.D., Mitchell, J.R., 1983. Dry deposition of sulphur dioxide onto grass in rural eastern England with some comparisons with other forms of sulfur deposition. In: Pruppacher, H.R., Semonin, R.G., Slinn, W.G.N. (Eds.), Precipitation Scavenging, Dry Deposition and Resuspension. Elsevier, New York, pp. 795}806.
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643 Erisman, J.W., 1994. Evaluation of a surface resistance parameterization of sulphur dioxide. Atmospheric Environment 28 (16), 2583}2594. Erisman, J.W., Wyers, G.P., 1993. Continuous measurements of surface exchange of SO and NH ; implications for their possible interaction in the deposition process. Atmospheric Environment 27A (13), 1937}1949. Erisman, J.W., van Pul, W.A.J., Wyers, P., 1994a. Parametrization of surface resistance for the quanti"cation of atmospheric deposition of acidifying pollutants and ozone. Atmospheric Environment 28 (16), 2595}2607. Erisman, J.W., van Elzakker, B.G., Mennen, M.G., Hogenkamp, J., Wart, E., van Den Beld, L., RoK mer, F.G., Bobbink, R., Heil, G., Raessen, M., Duyzer, J.H., Verhage, H., Wyers, G.P., Otjes, R.P., MoK ls, J.J., 1994b. The Elspeetsche veld experiment on surface exchange of trace gases: summary of results. Atmospheric Environment 28, 487}496. Erisman, J.W., Draaijers, G.P.J., 1995. Atmospheric Deposition in Relation to Acidi"cation and Eutrophication, Studies in Environmental Science, Vol. 63. Elsevier, Amsterdam. Erisman, J.W., Leeuw, F.A.A.M., Aalst, R.M., 1989. Deposition of the most acidifying components in the Netherlands during the period 1980}1986. Atmospheric Environment 23 (5), 1051}1062. EEA, 1995. In: Stanners, D., Bourdeau, P. (Eds.), Europe's Environment, the Dobris Assessment. European Environment Agency (EEA), Copenhagen, Denmark. Feliciano, M.S., Pio, C.A., Vermeulen, A., 1999. Trace gases and energy exchange over Southwest European grasslands. In: Borrel, P.M., Borrel, P. (Eds.), Proceedings of Eurotrac Symposium '98. WIT Press, Southampton, pp. 208}212. Fowler, D., 1978. Dry deposition of SO on agricultural crops. Atmospheric Environment 12, 369}373. Fowler, D., Unsworth, M.H., 1979. Turbulent transfer of sulphur dioxide to a wheat crop. Quarterly Journal of the Royal Meteorological Society 105, 767}783. Fowler, D., Duyzer, J., 1989. In: Andreae, M.O., Schimel, D.S. (Eds.), Exchange of Trace Gases between Terrestrial Ecosystems and the Atmosphere: Report of the Dahlem Workshop. Wiley, Chichester. Garland, J.A., 1977. The dry deposition of sulphur dioxide to land and water surfaces. Proceedings of the Royal Society of London A 354, 245}268. Garland, J.A., 1978. Dry and wet removal of sulphur from the atmosphere. Atmospheric Environment 12, 349}362.
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Hicks, B.B., Matt, D.R., 1988. Combining biology, chemistry and meteorology in modelling and measuring dry deposition. Journal of Atmospheric Chemistry 6, 117}131. Hicks, B.B., Baldocchi, D.D., Meyers, T.P., Hosker Jr., R.P., Matt, D.R., 1987. A preliminary multiple resistance routine for deriving dry deposition velocities from measured quantities. Water, Air and Soil Pollution 36, 311}330. Hove, L.A.W., Adema, E.H., 1996. The e!ective thickness of water "lms on leaves. Atmospheric Environment 30 (16), 2933}2936. McMillen, R.T., 1988. An eddy correlation technique with extended applicability to non-simple terrain. Boundary-Layer Meteorology 43, 231}245. Metcalfe, S.E., Derwent, R.G., Whyatt, J.D., Dyke, H., 1998. Nitrogen deposition and strategy for the control of acidi"cation and eutrophication across Great Britain. Water, Air, and Soil Pollution 107 (1}4), 121}145. Pio, C.A., Feliciano, M.S., Vermeulen, A.T., Sousa, E.C., 2000. Seasonal variability of ozone dry deposition under southern European climate conditions, in Portugal. Atmospheric Environment 34, 195}205. Plantaz, M.A.H.G., 1998. Surface/atmosphere exchange of ammonia over grazed pasture. Ph.D. Thesis, Netherlands. Radojevic, M., Harrison, R.M., 1992. Atmospheric Acidity: Sources, Consequences and Abatement. Environmental Management Series. Elsevier, London. Sehmmel, G.A., 1980. Particle and gas dry deposition: a review. Atmospheric Environment 14, 983}1011. Setdman, J.R., Campbell, G.W., Vincent, K.J., 1997. Estimated high resolution maps of background air pollutant concentrations in the UK in 1994. National Environmental Technology Centre, United Kingdom. Vermeulen, A.T., 1998. Development of an automatic deposition monitoring system. MEDFLUX Report, Netherlands Energy Research Foundation (ECN), 28pp. Voldner, E.C., Barrie, L.A., Sirois, A., 1986. A literature review of dry deposition of oxides of sulphur and nitrogen with emphasis on long-range transport modelling in North America. Atmospheric Environment 20 (11), 2101}2123. Wesely, M.L., 1989. Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models. Atmospheric Environment 23 (6), 1293}1304. Wesely, M.L., Sisterson, D.L., Jastrow, J.D., 1990. Observations of the chemical properties of dew on vegetation that a!ect the dry deposition of SO . Journal of Geophysical Research 95 (D6), 7501}7514.
Evaluation of SO dry deposition over short vegetation in Portugal M.S. Feliciano *, C.A. Pio , A.T. Vermeulen Departamento de Ambiente e Ordenamento, Universidade de Aveiro, Campus de Santiago, 3800 Aveiro, Portugal ECN } Energy Research Foundation, PO Box 1, 1755 ZG Petten, The Netherlands Received 30 June 2000; accepted 16 November 2000
Abstract SO dry deposition was studied over short vegetation, in Portugal, by means of the concentration gradient method. The experimental study involved one "rst phase of long-term measurements carried out in a grassland and, subsequently, a second period of several 1997 intensive "eld campaigns performed in three places representing di!erent climate and surface conditions. Temporal and spatial patterns of dry deposition parameters show that downward #uxes of SO are by some extent a!ected by surface processes. Median R varied from 140 s cm\ to values around 200 s cm\, in a wide range of environmental conditions. Stomatal uptake is an important sink when vegetation is biologically active, but its contribution is e!ectively low when compared with non-stomatal mechanisms, especially when the surface is wet. Under dry conditions R increases by a factor of two, but SO deposition rates then still are signi"cant. The parameterisation of the surface resistance for SO proved to be di$cult, but < derived with the Erisman parameterisation (Erisman et al., Atmos. Environ. 28 (16) (1994) 2595) compared best with measured values, at low time resolution scale and especially under moisture conditions. 2001 Published by Elsevier Science Ltd. Keywords: Sulfur dioxide; Dry deposition; Vegetation; Portugal
1. Introduction Transport of acidic substances, such as sulphur dioxide, from the atmosphere to the earth's surface is one of the major environmental problems of our time. This process, currently called acid deposition, is an undesirable consequence of air pollution and is held responsible for substantial damaging e!ects on vegetation, aquatic life, structural and ornamental materials and, in some instances, on human health (EEA, 1995; Radojevic and Harrison, 1992). In view of these ecological, aesthetic and economical impacts, the member countries of the UNECE have put assessment methodologies into practice with the purpose of reducing emissions of acidifying compounds to avoid exceedances of critical levels and critical loads (Bull,
* Corresponding author. Fax: #351-2-34-429290. E-mail address: [email protected] (M.S. Feliciano).
1991; Metcalfe et al., 1998). The implementation of abatement strategies based on these concepts requires reliable estimates of atmospheric input to ecosystems (wet and dry deposition), in order to determine the exposure or deposition levels below which no adverse e!ects occur (Erisman and Draaijers, 1995; Stedman et al., 1997). Dry deposition of sulphur dioxide is fairly important, since the amounts delivered to the earth's surface through this process represent a substantial part of the total acid deposition (Erisman et al., 1989; Garland, 1978). Estimates of SO dry deposition, on local and regional scales, have been improved with the development of empirical descriptions based on #ux measurements carried out principally in the United States (Hicks and Matt, 1988; Voldner et al., 1986; Wesely, 1989) and northern European regions (Erisman and Wyers, 1993; Fowler, 1978; Garland, 1977). However, as SO dry de position varies with weather conditions and physicochemical and biological properties of the surface, it is important to evaluate the uncertainties arising from the
1352-2310/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 5 3 9 - 2
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generalisation of results from one region to another (Brook et al., 1997; Erisman and Wyers, 1993). Within the framework of the European Union environmental research programme, gradient measurements of SO were carried out over short vegetation in Portu gal, as a part of the SREMP and MEDFLUX projects, with the main purpose of evaluating the applicability of existing dry deposition models to southern European regions. In this part of Europe, climate di!ers substantially from northern European climate and hence the growing cycle of vegetation and other properties of the surface are distinct as well. The large set of data collected in nearly two years of measurements contributes to testing of the current dry deposition schemes for SO under south European conditions, where only a small amount of information existed until now. In this paper our attention will be focused on the evaluation of the principal controlling mechanisms, based on analysis of the temporal and spatial variability of SO dry deposition parameters. A parameterisation scheme is also employed as an evaluation tool and to infer the principal implications of its generalisation for this European region.
2. Experimental description The experimental "eldwork involved two distinct phases. In the "rst phase, long-term measurements were carried out over a grassland ecosystem located in the Aveiro region (Sarrazola), between November 1994 and September 1995. In the second stage, during 1997, measurements were conducted at three di!erent sites through seven intensive "eld campaigns, in a total of approximately 150 sampling days, with the following temporal distribution: at Sarrazola in two periods from 15 January to 26 February and from 30 July to 22 August; at Baldios in four measuring periods: 5}21 March, 5}15 April, 5}22 June and 5}18 July; and at Pancas from 11 to 29 September. Sarrazola is a 40 ha #at area with permanent and temporary meadows, providing a reasonable fetch from almost all directions. The study area is located near the Atlantic coast in the north region of Portugal (40342 20N//833715W). Climate is humid without extreme temperatures and is under the strong in#uence of sea}land breezes. Vegetation starts to grow in early autumn, reaching its maximum activity in late winter or mid-spring. From that time onwards most grasses go into a decline phase reaching the minimum between July and August. Baldios is a typical Mediterranean pseudo-steppe grassland located in the south of Portugal, 80 km southeast from Lisbon (3833358N//831822W). The research area consists of a wide and rolling plain under the in#uence of a hot and semi-arid climate, where grassy vegetation is used mainly for grazing livestock or hay growth.
In contrast with the previous place, vegetation never exhibits a lush state, reaching its maximum height of about 30 cm in June. From June onwards both structural parameters and physiological activity decreased, giving a yellow-brownish colour to the canopy. Pancas is a wide and #at area located in the Natural reserve of the Tejo Estuary, 15}20 km northeast from Lisbon (383517N//835518W). This zone encloses an extensive cultivated area and is dominated by large-scale intensive agriculture (wheat, sun#ower, barley, etc.). Part of the land is used for cattle grazing as well. The undisturbed fetch is higher than 400 m for all directions. During the sampling period, most crops had already been harvested and the area within the likely #ux footprint domain consisted mainly of fallow areas with short sparse grasses, mostly dry, and areas of bare clay soil. Further details on physical and physiological characteristics can be found in Feliciano et al. (1999) and Pio et al. (2000). SO dry deposition was monitored by means of the gradient technique integrated in a monitoring station designed and developed by Netherlands Energy Research Foundation, ECN, which has capabilities for eddy correlation measurements of energy exchange and other gaseous species, as well (Vermeulen, 1998). Gradient #uxes were obtained by coupling an SO monitor (Thermo Environmental Instruments } model 43S), with a 3D ultra-sonic anemometer (Solent Research Gill), mounted vertically on the top of a 5 m mast. The sonic anemometer provides high-frequency measurements of the wind speed vector and sonic temperature, from which turbulence parameters, such as friction velocity, sensible heat and Monin-Obukhov length can be determined directly. High-frequency measurements are processed in real time according to the algorithm implemented by McMillen (McMillen, 1988; Baldocchi et al., 1988). The SO monitor was connected to a Te#on electric valves system, in order to measure at programmed intervals the SO concentration at an alternating height of 0.5 and 5 m. There are advantages and disadvantages in using only two measuring heights. With this con"guration it is not possible to know precisely the vertical concentration pro"le. However, the two point concentration vertical gradients are less a!ected by the temporal variability of the SO concentration, as a result of the shorter sampling cycle routine. In this way, we consider that, when just a single monitor is available, the con"guration implemented in this study is one of the best solutions for gradient measurements. The ambient air was sampled through Te#on tubes, which were heated to prevent water vapour condensation and reaction of SO with the internal tube walls. The SO monitor was calibrated several times in the "eld and in the laboratory with a permeation device. Other micrometeorological variables like global radiation, temperature, relative humidity and precipitation amount were been measured with proper sensors.
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3. Computation and analysis of SO2 6uxes
material, etc.:
SO #uxes were computed on the basis of the aerody namic gradient method (Baldocchi et al., 1988) assuming that heat and mass are transported in a similar way within a well-developed surface layer, using the equation F"!u c , H H where u is the friction velocity and H concentration, de"ned according to the siderations presented in Berkowicz and
(1) c is the eddy H theoretical conPrahm (1982):
c "kc/[ln(z /z )! (z /¸)# (z /¸)]. H & &
(2)
c is the 15 min averaged concentration gradient determined through the di!erence between SO concentra tions at levels z and z , k is the Von Karman constant (0.40) and (z/¸) is the integrated stability correction & function for heat de"ned as in Erisman and Draaijers (1995). By convention, F is negative when the #ux is towards the surface and positive when #ux occurs in the opposite direction. Fluxes computed from Eqs. (1) and (2) were then interpreted on the basis of the multiple resistance approach, which describes vertical #ux analogously to Ohm's law, i.e., by considering the existence of several resistances to the transport of the pollutant from the atmosphere to the surface (Garland, 1977; Hicks et al., 1987). Taking into account this conceptual description, F is expressed by C X F"!< C " , X R #R #R
3635
(3)
where < is the dry deposition velocity, C is the pollu X tant concentration at height z , R is the aerodynamic resistance associated with turbulent transfer across atmospheric boundary layer, R is the quasi-laminar boundary layer resistance, mainly in#uenced by turbulence and molecular di!usivity of the trace constituent, and R is the canopy resistance that depends on physical and chemical interaction between the trace constituent and the deposition surface. By computing directly < , R and R from concentration and meteorological measurements (Erisman et al., 1994a; Pio et al., 2000), the overall canopy resistance R can be derived from Eq. (3) as residual term. Then, time series of calculated R values could be used to obtain a canopy resistance parameterisation or to validate model results. For most purposes and particularly for short vegetation, R is often partitioned into two resistances in parallel, the stomatal resistance (R ), associated with di!usion through the stomata, and the external (or nonstomatal) resistance (R ), which includes chemical destruction of this gaseous pollutant in the various surface elements like leaves cuticle, underlying soil, dead
1 1 1 " # . (4) R R R The absorption of SO via stomata is a well under stood mechanism and in the present study was evaluated through application of the Wesely algorithm (Wesely, 1989):
200 400 R "r 1# G G#0.1 ¹(40!¹)
D & -, (5) D 1where r is the minimum bulk canopy stomatal resistance G for water vapour, reformulated for our environmental conditions, according to the procedure described in Pio et al. (2000), G is the global radiation intensity (W m\) and T is the temperature in degrees Celsius. D / &D ("1.9) is the ratio between molecular di!usion 1coe$cients of water vapour and sulphur dioxide, respectively. In relation to extra stomata mechanisms, the knowledge is still insu$cient to develop proper parameterisations. A useful attempt has been developed by Erisman et al. (1994a), based on measurements performed in the Netherlands over heathland, but already generalised to other Dutch surface types (Erisman, 1994). This author describes the external surface resistance as a function of surface wetness, caused either by precipitation or high relative humidity. During, or just after rainfall, the author assumes R "1 s m\ and under other conditions R is related to relative humidity, RH, as follows:
0.58;10EXP[!0.278RH] if RH'81.3%, R " 25,000EXP[!0.0693RH] if RH(81.3%. (6) 4. Results and discussion 4.1. Data selection and evaluation of SO2 dry deposition A total of 29,814 measuring points (15 min averages), recorded along all experiments, were screened by a set of simple and basic selection criteria. This procedure aimed at studying SO dry deposition from a data set ful"lling the theoretical and technical constraints required for a good performance of the gradient method (Baldocchi et al., 1988; Fowler and Duyzer, 1989). The valid conditions, together with percentage of remaining periods, are shown in Table 1. Basically, the restrictive criterion aims at eliminating or minimising the in#uence of periods associated with adverse meteorological conditions, low precision of measured concentration pro"les, advection and storage episodes and other errors arising both from measurements and mathematical approaches (Baldocchi et al., 1988; Erisman et al., 1994b).
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Table 1 Selection criteria applied to SO dry deposition measurements and percentage of remaining periods after application of each one. The total periods for each site are also given Criteria of acceptance
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Wind speed Taylor hypothesis Friction velocity Drag coe"cient Atmospheric stability Fetch Detection limit Variation coe"cient Temporal variation Limits of V
No. of initial points
Remaining periods for each site (%)
;*1 m s\ ;)0.5; ; *0.5 m s\ H C (0.02 ¸'1 m Wind Dir"f (local) C *3 g m\ 1CV" G /C )0.2 A G C /t)0.2F /z G G < (1.5< +
Sarrazola 95
Sarrazola 97
Baldios 97
Pancas 97
82.61 81.65 69.28 68.22 66.82 66.82 28.84 28.77 17.82 16.04
81.51 80.68 65.29 63.91 61.97 61.97 25.54 18.75 16.57 14.15
95.77 91.75 83.17 80.49 79.35 75.33 19.63 17.33 15.13 12.84
80.87 75.26 60.49 58.71 55.15 55.15 27.77 19.53 15.57 11.48
18,892
5004
4402
1516
Most of the original data were rejected due to the large amount of periods with low concentration. A rejection concentration limit of C (3 g m\ was applied. The 1adoption of 0.5 g m\ as concentration limit ( 3;detection limit of SO monitor) would increase the remain ing data by a factor of about 2, but, in these conditions, gradient measurements were highly a!ected by the SO monitor random noise. The application of 3 g m\ as the minimum limit of data acceptance ensures an interpretation of the dry deposition controlling mechanisms free of artifacts as much as possible, although this could lead to an overestimation of the overall mean #ux. The in#uence of the low concentrations on the reliability of the measuring technique may be inferred from Fig. 1. This "gure, which shows dry deposition #uxes against several SO concentration classes, including peri ods with concentration values higher than 0.5 g m\, proves that upward SO #uxes are a common feature at low concentrations. The observed relationships also suggest that deposition rates increase as concentration rises, with a proportionality coe$cient that is clearly dependent on local speci"c factors. To get better insight into those controlling factors, selected data were grouped into long-term periods, so that representative cause}e!ect relationships might be found. Data collected at Sarrazola were divided into four seasonal periods in the following indicated as: S-Winter 95 (1 November}31 March), S-Winter 97 (15 January}28 February), S-Summer 95 (1 June}31 August) and, "nally, S-Summer 97 (30 July}22 August). Data gathered at Baldios were divided into two classes taking into account the physiological properties of the surface. The "rst three "eld campaigns were grouped into the class, BA1, while the fourth "eld experiment was studied apart as the BA2
period. For Pancas, all data were included in one single period, denominated here as PA. Daytime and nighttime periods have also been distinguished. Daytime was de"ned as the period in which global radiation values are higher than 10 W m\. Once established, statistical values of the dry deposition parameters (< , < and + R ) were estimated for each one. < is the deposition + velocity which should result in the absence of surface resistance, < "1/(R #R ). Main results illustrating + the temporal and spatial variation for daytime and nighttime periods are presented in Fig. 2. In all periods, dry deposition velocities were higher during daytime than at night, independent of the place where they were measured. Daytime median values of < ranged between 0.6 and 0.8 cm s\, exceeding noctur nal values by on average a factor of two. Taking into account the selection procedure, it is worth noting that < values should be interpreted as maximum values, especially at night. In what concerns the seasonality of < , a slight variation is observed, but without any regular pattern. For instance, at Sarrazola, during the "rst measurement period, < decreases gradually from winter (< 0.8 cm s\) to summer (< 0.6 cm s\), while 1997 data, collected at the same site, show a trend in the opposite direction. In terms of spatial variation, SO molecules are removed more rapidly by dry deposition at Sarrazola than in the other two southern places, where < values were similar in magnitude and range. In contrast to the diurnal variation found in < , sur face resistance shows an uncharacteristic daytime/nighttime variation. Daytime R varies between values close to zero and values up to 2 s cm\, with median values predominantly lower than 1 s cm\. Nocturnal median values are somehow higher and exhibit more scattering
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3637
Fig. 1. Median and percentiles of SO exchange rates as a function of SO concentration classes for the di!erent measuring places. These #uxes were computed from selected data, but using C '0.5 g m\ (the "rst category includes only data with 10.5(C )1 g m\). 1-
than those observed during daylight hours. The seasonality in this parameter can be illustrated by observations collected at Sarrazola, where daytime R values under conditions prevailing in winter are lower than those in the other season periods. At night, a seasonal trend is also evident, but while at Sarrazola R nocturnal values decrease from winter to summer, at Baldios the variation was completely di!erent. Di!erences in R observed at Sarrazola, between 1995 and 1997, may be a consequence of the di!erent environmental conditions prevailing throughout these two years or result simply from the distinct representativeness associated to each data set. Another remarkable feature is related to the pronounced contrast between R observed at Sarrazola and R ob tained in the other southern sites. Comparing the variability of < with that of R and < , it follows that dry deposition of SO is a complex + function of atmospheric vertical mixing and surface characteristics, where the in#uence of atmospheric conditions is re#ected principally in the diurnal variation of < , and surface properties a!ect its diurnal, seasonal and spatial patterns. In fact, during daytime and under generally unstable conditions, atmospheric resistance is much smaller than surface resistance, and therefore < is strongly inhibited with increasing R . At night, under
more stable conditions, atmospheric resistance is frequently higher than surface resistance and hence R has a smaller in#uence on < . The magnitude of both dry deposition parameters (< and R ) found here are within the range of the corre sponding values reported in the literature for ecosystems with short vegetation (Erisman and Wyers, 1993; Plantaz, 1998; Sehmmel, 1980; Voldner et al., 1986). The interval of averaged < values, over similar surface cover age, varies from values as low as 0.1 cm s\ to values up to 2.5 cm s\, but nocturnal values are generally in the lower part of the reported range ((1 cm s\). Correspondingly, the mean values of R vary generally be tween values close to zero and 5 s cm\, depending on a large set of surface conditions that determine the strength of the two most important removal mechanisms involved in the SO dry deposition: stomatal uptake and absorption of SO into surface aqueous layers (Chamberlain, 1986; Erisman and Wyers, 1993; Fowler and Unsworth, 1979). Hence, taking into account that our observations comprised the whole spectrum of the most relevant environmental conditions, it is not surprising that measured values of R have a large range and exhibit also some less clear patterns.
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Fig. 2. Temporal and spatial variation of the SO dry deposition parameters (velocity, maximal velocity and surface resistance), given as daytime and nighttime medians. Whiskers range between the 25th and 75th percentiles.
The evaluation of temporal and spatial trends of R suggests that non-stomatal mechanisms have a meaningful contribution to the SO deposition, when compared with the process of di!usion of SO through stomata. In fact, in the absence of signi"cant non-stomatal mechanisms, R should follow a pattern more consis tent with the diurnal variation of the stomatal opening. For example, over transpiring canopies, daytime R should have much lower values than nocturnal R . Following the same reasoning, the seasonal variation of R should be fairly more pronounced, at least at Sarrazola during 1995, since the vegetation evolved from an actively growing state in winter to an accentuated summertime senescent state. At Baldios, the biological activity of vegetation has also been limited to the "rst period established for this location, but, during this time spell, the grass never reached a luxuriant state, susceptible to promoting accentuated gaseous exchange rates through stomata. In the same way, the spatial contrast should be more pronounced, since SO removal, both at Baldios and Pancas, is believed to occur in response to the physical}chemical interaction of this gaseous pollutant with the external leaf surface and other available elements. The contribution of non-stomatal mechanisms in relation to stomatal uptake has also been evaluated by
parameterising stomatal resistance with the Wesely formulation (Eq. (5)) for a period with biologically active vegetation (part of S-Winter 95), employing a seasonal parameter r of 120. This parameter was calculated for G our conditions from ozone #ux measurements, according to the procedure presented in Pio et al. (2000), in which r is derived by "tting mean (and median) diurnal cycles G of ozone R . The estimated r agrees well with the value G presented in Wesely (1989) for the rangeland type, but is a factor of two higher than that established for the agricultural class with lush vegetation. This procedure led to estimates of minimal bulk R to SO of about 250 s m\, showing that non-stomatal mechanisms are e!ectively more e$cient than destruction of SO molecu les after their di!usion through stomatal opening, even when both mechanisms coexisted at the same time. The estimated R to the SO transfer is fairly acceptable when compared with R to water vapour reported for a wide range of herbaceous annuals and woody perennials, under optimal plant growing conditions (within the range 30}300 s m\) (Erisman et al., 1994a). Since the magnitude of R is mainly determined by non-stomatal mechanisms in the three places, the lower R values observed at Sarrazola seem to be associated with the absorption of SO in surface aqueous layers. In fact, at Sarrazola surface wetness is a common local
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643
feature. In this place measurements were carried out under rather moist conditions, where water vapour content of the atmosphere is high and phenomena like mist, drizzle, fog and rainfall occur frequently. During the night, the atmosphere at Sarrazola is nearly always saturated in relation to water vapour. During daylight hours the relative humidity is slightly lower, ranging from values around 80% in autumn/winter to values of 60}70% in summer. In relation to rainfall, a clear seasonal pattern was also observed, in which the high abundance of rainy events in autumn, winter and part of spring contrasts with the low occurrence of rainfall in summer. In the other places, the presence of water, both in liquid and vapour states, was e!ectively lower and thereby the experimental information was mostly collected under dry conditions. Correspondingly in these locations, where foliage and the underlying soil often remained free of moisture, the a$nity between SO mol ecules and surface was lower, but not low enough to cause a signi"cant decreasing of the SO deposition. Spatial contrasts in what concerns to relative humidity and rainfall are summarised in Fig. 3. The enhancement of SO deposition by surface wet ness has been demonstrated in other "eld experiments
3639
(Erisman and Wyers, 1993; Fowler and Unsworth, 1979). Voldner et al. (1986) reported R values of 4 s cm\ (from a range of 1}9 s cm\) over a mature crop (dry or stubbed) and 2 s cm\ when cropland consists of bare soil and senescent vegetation. Over grassland, throughout an annual cycle, Davies and Mitchell (1983) determined mean R values in the range 0.70}1.65 and 0.8} 1.10 s cm\ for dry and wet surfaces, respectively. In an attempt to provide more complete information on the e!ects of surface wetness on SO dry deposition, non-stomatal resistance (R ), distinguishing night ,12- 1 and daytime periods, was plotted against relative humidity (see Fig. 4). At night, R "R and during day,12- 1 time R is equivalent to R estimated from ,12- 1 Eq. (4), after computing R with Eq. (5), using r values G estimated from ozone deposition measurements (Pio et al., 2000) for each month of the 1995 measuring period and each intensive campaign of 1997. Relative humidity is not a measure of surface wetness, but is certainly a reasonable indicator of surface wetness caused by condensation of atmospheric water vapour or deliquescence of salts at the leaf external surface and, in some instances, caused by precipitation as rainfall, drizzle, fog, mist, etc. Other sources of surface wetness like guttation and soil
Fig. 3. Relative humidity and rainfall registered in the di!erent measuring periods.
3640
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643
Fig. 4. Non-stomatal resistance against relative humidity given for nighttime (nocturnal R ) and daytime (R ) periods. ,12- 1
distillation, which are particularly relevant at night and in the early morning hours, are also mostly associated with humid conditions. Looking closely at the various relationships, we observe that when relative humidity is high (generally '80%), R values are generally lower and exhibit a narrower variation, as a consequence of the huge probability of the surface being wet. As relative humidity diminishes, R increases substantially, follow ing, in some cases, a linear or an exponential trend. Although the previous analysis seems to demonstrate that SO is e!ectively removed at higher rates under moist conditions, there are also clear indications that dry deposition of SO is not necessarily high in the presence of aqueous layers on the surface. In fact, SO is a reason ably soluble gas in pure water, but the absorption of SO in water layers is associated with a series of reactions strongly dependent on acid and oxidation components (metals, H O , etc.) of the aqueous media (Erisman and Draaijers, 1995; Wesely et al., 1990). Even when the chemical composition initially promotes the SO re moval, as oxidation proceeds, wetness becomes more acidic, SO becomes less soluble and, hence, for this mechanism to be signi"cant the bu!er capacity of surface must be su$ciently high (Fowler and Unsworth, 1979). This dependence leads generally to a large variability in R under "eld conditions. For instance, the almost negli gible R values observed in the Netherlands over grass lands have been ascribed to the humid climate and high
NH levels prevailing in that country (Erisman and Wyers, 1993). These suggestions are also in accordance with ideas discussed in studies of Brimblecombe (1978) and Chameides (1987), who emphasise the importance of the presence of chemicals on wet surfaces. Returning to Fig. 2, we infer that the seasonal behaviour in nocturnal values of R observed at Sarrazola seems to be well correlated with the likely variation in the surface wetness chemistry from winter to summertime. This hypothesis is supported on the seasonal contrast registered in rainfall regime and on the potential relationship between surface wetness chemistry and its origin. When surface wetness is determined by rainfall or other precipitation events, the concentration of H> in water droplets is exceedingly in#uenced by acidity of rainwater, which is generally higher than that resulting from water condensation on the surface. Some "eld studies, mainly those performed in USA, indicate that the surface is a poor sink, when wetted by rain (Hicks et al., 1987; Wesely, 1989). Wesely et al. (1990) demonstrated that, in relation to rain, dew contains larger concentration of SO\, but smaller concentration of H>. Here, the inhibi tor e!ect of rainfall is far from reaching levels compared with those observed in USA, but it is clearly higher than that found in northern European countries, namely in Netherlands, where during or immediately after rain R becomes close to zero or vanishes (Erisman and Wyers, 1993). Hence, although in S-Winter 95 moisture
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643
levels are higher than in S-Summer 95, the chemical composition of liquid water (principally H> content) was more favourable to the chemical destruction of SO in the second period. In summer, surface wetness was caused mainly by water vapour condensation processes, like dew or salts deliquescence. When surface consists practically of dead material or uncovered soil, such as in BA2 and PA, SO removal might occur directly from the gas interaction either with soil particles or dead foliage, and probably is determined mainly by surface alkalinity. The chemical attack of SO to dry surfaces is not well documented, but it is well established that removal rates on dry or acidic surfaces should be much slower than in wet or alkaline receptors (Wesely, 1989). However, while acidic surfaces (pH(3.5) are generally inert to SO , independent of surface moist ure content, Garland (1977) found no appreciable variability of R with surface moisture in a "eld investigation over a chalky soil (pH"8). Apart from nocturnal R values observed in the "rst seasonal period estab lished for Baldios (BA1), the magnitude of R found in these two southern locations was quite similar. This "nding also seems well correlated with the little variability in moisture conditions prevailing in both places. The lowest nocturnal values of R , observed in that "rst period may be a direct consequence of the higher ability of cuticular surface for water retention, due to the presence of green leaves in larger amounts. An experimental study performed by Hove and Adema (1996) supports this hypothesis. The daytime/nighttime variation found
3641
in the other periods demonstrates that, under dry conditions, meteorological variables like global radiation and temperature may a!ect the chemical destruction due to their in#uence on the chemical equilibrium. In spite of these non-stomatal mechanisms having an important contribution in the SO dry deposition, the stomatal uptake seems to also have had a slight in#uence on the spatial pattern and on the seasonal variation prevailing at Sarrazola. The lowest daytime R values observed in winter also result from the higher SO ab sorption via stomata, once vegetation exhibited its maximum biological activity in winter and the "rst half of Spring. From that time onwards, the decline observed in the vegetation growth was accomplished by a gradual decrease in the stomatal activity leading to an increment of the overall SO surface resistance. During these peri ods, in which stomata fail to open, daytime SO uptake is mainly in#uenced by non-stomatal mechanisms, which are more e$cient at night than during the day, due to the likely di!erence in the extension and thickness of surface wetness. Voldner et al. (1986), based on several investigations, reported R values of 1 and 2 s cm\ for the early growing season and the late season, respectively, ascribing this variation to the biological activity. 4.2. Comparison between measured and parameterised R
From the analysis of our observations, it becomes clear that parameterisation of surface resistance to the SO deposition does not always give clear results. The
Table 2 Statistics on the comparison of experimental SO deposition velocities and surface resistance with values computed with Erisman et al. (1994a) parameterisation scheme Mean [STE]
Sarrazola No. of points R (s cm\) Measured Modelled V (cm s\) Measured Modelled Baldios/Pancas No. of points R (s cm\) Measured Modelled V (cm s\) Measured Modelled
Median [Quart. range]
Daytime
Nighttime
Daytime
Nighttime
2528
994
2528
994
1.23 [0.044] 1.56 [0.041]
1.05 [0.049] 0.51 [0.063]
0.72 [1.13] 1.07 [1.52]
0.71 [1.00] 0.06 [0.60]
0.76 [0.0086] 0.66 [0.0085]
0.60 [0.011] 0.76 [0.014]
0.68 [0.52] 0.56 [0.44]
0.53 [0.42] 0.67 [0.51]
335
209
335
209
1.59 [0.14] 2.29 [0.21]
5.14 [0.29] 1.81 [0.17]
0.78 [1.71] 1.24 [2.61]
3.52 [7.25] 0.54 [2.37]
0.70 [0.026] 0.34 [0.016]
0.33 [0.15] 0.38 [0.016]
0.59 [0.53] 0.28 [0.24]
0.23 [0.35] 0.35 [0.37]
3642
M.S. Feliciano et al. / Atmospheric Environment 35 (2001) 3633}3643
parameterisation scheme, de"ned by Eqs. (5) and (6), was applied to available 15 min time series in order to evaluate the potential errors arising from its generalisation to a di!erent climate region. As already stated, the seasonal parameter, r , in Eq. (6), was adequately adG justed to our environmental conditions. Concerning the R , rainfall was taken into account for 1997 periods only, because the available 1994/95 rainfall values were just registered as daily amounts. In this way the parameterised R for the 1994/95 period is slightly overes timated, especially for those periods in which rainfall was abundant. In Table 2 measured and parameterised values of SO dry deposition parameters (< and R ) are presented as arithmetic mean and median. The standard errors and quartile ranges are given between square brackets. The main results show that the model overpredicted nocturnal < and underpredicted daytime < , with larger deviations under drought conditions. In these drought conditions, the Wesely parameterisation (Wesely, 1989) is expected to give better results than Erisman parameterisation (Erisman et al., 1994a). The agreement between measured and modelled < increases consider ably when daily averages are compared. From the data we can conclude that deposition velocities modelled for long-term periods are quite accurate. For the short-term estimates the agreement between modelled and measured values was much lower.
5. Conclusion SO gradient measurements performed over short veg etation, in Portugal, were used for successfully evaluating the most important mechanisms involved in the dry deposition of this pollutant. Main results show that SO dry deposition is a complex function of aerodynamic factors and surface conditions. The atmospheric processes are particularly relevant in the shape of diurnal pattern, while the large variability of surface mechanisms causes changes in deposition patterns throughout the day, season and space. R values for SO indicate that removal of SO occurs to a large extent through physical and chemical a$nity of this pollutant with the surface. These non-stomatal mechanisms seem more e$cient when surface is wetted, but in the presence of aqueous layers, deposition of SO is not necessarily high, depending on the chemical composition of the water layer. Under dry conditions R is slightly larger, but not large enough to cause an inhibition in the vertical transport of SO . Stomatal uptake is shown to have a low contribution to the mean deposition #ux, mainly when it coexists with environmental conditions favourable to formation of surface water layers, susceptible to promoting rapid and prolonged SO ab sorption/oxidation.
Parameterisation of surface resistance for SO seems to be less successful in describing short-term variations of the most important controlling mechanisms. For low time resolution scales, parameterised SO deposition compares much better with the corresponding measured values. Discrepancies between modelled and measured < arise mainly from the di$culty in parameterising R , which has an important contribution in the SO removal and depends on a large number of variables. Although our study presents some experimental limitations, it allows a better insight into deposition processes over short vegetation and will contribute in the validation of parameterisations of SO dry deposition processes over southern Europe in the future.
Acknowledgements We gratefully acknowledge the European Commission for the "nancial support given through SREMP (contract no. EV5V-CT93316) and MEDFLUX (contract no. ENV4-CT95-0034) projects and the Fundac7 aJ o para a CieL ncia e Tecnologia for the scholarship (PRAXIS/3/ 3.2/AMB/38/94) given to M. Feliciano.
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