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Ozone uptake by an evergreen Mediterranean Forest (Quercus ilex) in Italy. Part I: Micrometeorological flux measurements and flux partitioning
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Atmospheric Environment 39 (2005) 3255–3266 www.elsevier.com/locate/atmosenv
Ozone uptake by an evergreen Mediterranean Forest (Quercus ilex) in Italy. Part I: Micrometeorological flux measurements and flux partitioning Giacomo Gerosaa,, Marcello Vitaleb, Angelo Fincoa, Fausto Manesb, Antonio Ballarin Dentia, Stanislaw Cieslikc a
Department of Mathematics and Physics, Universita` Cattolica del S.C., via Musei 41, I-25121 Brescia, Italy b Department of Plant Biology, Universita` ‘‘La Sapienza’’, Piazzale A. Moro 5, I-00185 Roma, Italy c Institute for Environment and Sustainability, Joint Research Centre of the EC, via Fermi, I-21020 Ispra (VA), Italy Received 22 September 2004; received in revised form 13 January 2005; accepted 26 January 2005
Abstract Ozone, water and energy fluxes have been measured over a Mediterranean evergreen forest in Central Italy from August to October 2003 with the eddy-correlation technique in order to evaluate the amount of ozone taken up by plants in dry summer and in mild autumn conditions. The stomatal ozone fluxes have been calculated using the analogy with water vapor fluxes inside the stomata, which are easily measurable. The total ozone dose was obtained by integrating the stomatal fluxes over time. Stomatal flux resulted a minor part (31.5%) of the total ozone flux over the forest ecosystem. The main part of ozone deposition follows non-stomatal pathways. Chemical sink seems to play a relevant role in the morning non-stomatal deposition. Stomatal uptake is enhanced by water availability but, on the average, it does not exceed the 34.4% of the total ozone flux. A comparison between the cumulated stomatal ozone fluxes and the currently used AOT40 exposure index highlighted important distortions introduced by this index. AOT40, which do not take into account plant physiology, lead to substantial overestimation of ozone risk, particularly when water supply is limited, as occurs frequently in Southern European and Mediterranean areas. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ozone deposition; Stomatal uptake; Mediterranean forest ecosystem; Holm oak
1. Introduction Tropospheric ozone is widely recognized as a threat for natural ecosystems because of its toxicity on plants (Fuhrer et al., 1997). Corresponding author. Department of Mathematics and
Physics, Universita` Cattolica del Brescia, c/o via Pertini, 11, I-24035 Curno (BG), Italy. Tel./fax: +39 035 462339. E-mail address: [email protected] (G. Gerosa).
In the Mediterranean area, ozone easily reaches high concentrations in summer due to intense solar radiation, high temperature and very dry conditions which favor atmospheric photochemical activity (Milla´n et al., 1997). As a consequence, Mediterranean natural ecosystems are frequently subject to very high ozone exposure levels (Hjellbrekke, 1998). The exposure level is usually defined as a timeintegrated concentration. However, ozone damage to vegetation occurs through ozone uptake by plant
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.056
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stomata and is not necessarily related to integrated ozone concentrations. The ability of stomata to serve as a sink for atmospheric gases, quantified by stomatal conductance, is, in turn, mainly controlled by water availability (Emberson et al., 2000) and by other environmental factors like air water vapor pressure deficit, solar radiation, wind speed, intensity of turbulence, etc. In the Mediterranean area, water supply is generally limited, and thus also ozone stomatal uptake (Bussotti and Gerosa, 2002). Conversely, an important fraction of ozone deposition (or destruction) is expected to take place on non-transpiring plant surfaces and on soil. This study aims at measuring ozone fluxes over an evergreen Mediterranean forest stand and to quantify stomatal ozone uptake by plants and non-stomatal ozone deposition at ecosystem (canopy) level during 3 months, ranging from summer to autumn, covering different water supply conditions. Direct measurements of ozone deposition fluxes over forest stands using the eddy covariance micrometeorological technique have been made for northern and central European forests (eg. Mikkelsen et al., 2004; Coe et al., 1995; Klemm and Mangold, 2001), but there are few ozone flux data for evergreen Mediterranean forests. Affre et al. (2000) and Lamaud et al. (2002) measured ozone fluxes over an evergreen pine (Pinus pinaster) forest in Southern France. For the same ecosystem, Cieslik (2004) determined the stomatal uptake flux contribution to the total ozone flux. However, the environmental conditions of this forest, as well as the Atlantic climate, greatly differ from the Mediterranean ones. Regarding Southern Europe there are very few information on stomatal ozone uptake, and data have been reported for some crops like wheat, barley and soybean (Gerosa et al., 2003, 2004), Mediterranean shrubs (Cieslik and Labatut, 1997), orange groves and grass (Cieslik, 2004), mainly for short periods. In this context, the data presented here can be used as a basis to test ozone deposition models’ performance for a Southern European evergreen forest, as already made, e.g. for the ODEM (Bassin et al., 2004) or the EMEP models for pasture, crops, Northern Europe evergreen forests and seminatural vegetation (Tuovinen et al., 2004).
2. Measurements and methodology
Fig. 1. Measurement site location (Grotta di Piastra) inside the Castelporziano Presidential Estate near Rome (Italy).
located 0.4 km from the seashore and it is covered by an evergreen Holm oak forest (Quercus ilex L.). Other species, like Pistacia lentiscus L., Phyllirea latifolia and Myrtus communis L. were scarcely present in the forest understorey. The mean height of the forest stand was 13 m; the leaf area index (LAI), measured by a LAI 2000 instrument (Li-Cor, USA), was 4.27 m2leaf m2 ground (Vitale et al., 2005). The soil is sandy and covered by a consistent litter. The whole ecosystem is representative of the original wild coastal rear dune ecosystem and has been part of a protected area, with restricted access, over the last 60 years. The climate is typically Mediterranean with the mean annual rainfall of 753 mm and a summer drought period from May to August (1955–1985 average). The year 2003 was exceptionally dry and hot, with scarce rainfall also during the spring (Table 1 in Vitale et al., 2005). The quantity of ozone taken up per unit area during the measuring period was determined in three steps:
2.1. Sampling site The measurements have been carried out from 6 August to 22 October 2003 inside the Castelporziano Estate (411440 N, 121250 E) located along the Latium coast at 20 km distance from the center of Rome (Italy) (Fig. 1). The sampling site, named Grotta di Piastra, is
measurement of ozone and water vapor fluxes; calculation of fractional stomatal ozone fluxes; determination of the ozone dose. These steps are outlined below.
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2.2. Eddy covariance flux measurements The fluxes of ozone, sensible and latent heat, as well as momentum, were measured by using the micrometeorological approach, which assumes that fluxes are independent from height in the lowest air layers (constant flux hypothesis, see e.g. Stull, 1988). So, a flux measured at several meters above ground level is assumed to be equal to the flux at the air–vegetation interface. The flux was determined by the eddy covariance, method, considered the most reliable one (see e.g. Hicks and Matt, 1988). Fluxes (nmol m2 s1) are equal to the covariances between turbulent fluctuations of the vertical wind vector component and of the scalar entity of interest, such as ozone concentration for ozone fluxes, water vapor for latent heat fluxes, and air temperature for sensible heat fluxes. Calculation of the fluctuations requires averaging over successive periods of time, such as to eliminate non-turbulent, long-term variations. The averaging time was chosen such as to include all turbulent fluctuations (at least 10 min) occurring in the atmospheric surface layer, but it had to be short enough to avoid the synoptic-scale fluctuations (less than 1 h, van der Hoven, 1957). A 30-min averaging time was taken as a compromise. The micrometeorological instrumentation was mounted on the top of an 18-m-high firewatching steel tower located inside the Holm oak forest, exceeding by 5–6 m the mean forest height. The forest canopy was closed but fairly rough. A 17-m-tall Holm oak was present close to the tower in the NNW direction and represented an aerodynamic obstacle for flux measurements when winds blew from that direction (both verses). The fetch, i.e. the distance from the measuring point over which the surface is homogeneous, ranged from a minimum of 400 m toward the seashore line direction (SW) to a maximum of 6 km in the SE direction. The three components of the wind vector and the temperature were measured at high sampling frequency (20 Hz) by means of a USA-1 ultrasonic anemometer (Metek A.G., Germany). Air moisture was recorded by a KH20 fast-response hygrometer (Campbell Sci., USA) and ozone concentrations were measured by a GFAS OS-G-2 fast-response chemiluminescent sensor (Gu¨sten and Heinrich, 1996). Chemiluminescence is obtained with a dye-coated target which has to be replaced every 4 days because of a decrease in sensitivity. The teflon aspiration tube inlet of the fast ozone sensor (1 m length and 2.75 cm of internal diameter) was placed at the sonic anemometer height, at a horizontal distance of 25 cm from it. An aspiration flow rate of 70 l min1 ensured purely turbulent flow through the tube, in order to minimize fluctuation damping (Massman, 1991). Since the sensitivity of the fast ozone sensor decreased regularly in 4 days, it had to be recalibrated
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continuously with a reference slow ozone sensor. For this sake, an API 400A (Advanced Pollution Instruments, USA) slow-response ozone analyzer was used. This analyzer was also equipped with an electronic switch to measure ozone concentrations in the air above and below the canopy (18 and 2 m, respectively) at intervals of 20 min for each height. The analyzer was calibrated before and after the campaign and the data were corrected accordingly. Other relevant parameters were continuously monitored: surface net radiation flux (W m2) by a NR-lite radiometer (Kipp & Zonen, Holland), atmospheric pressure by a PTB101B barometer (Va¨isa¨la¨, Finland) and air temperature and humidity, above and inside the canopy at 18 and 11 m, respectively, by means of two 50Y humitters (Campbell Scientific). The humidity data measured at 18 m were also used as reference for the continuous calibration of the fast humidity sensor. This was necessary because the KH20 krypton hygrometer output shows a drift. 2.3. Data sampling and flux determination The output data of the sonic anemometer and of the fast sensors data were sent every 0.05 s to a personal computer; the other data were transmitted to a CR10 datalogger (Campbell Sci.) every 10 s. An averaging time of 30 min was used for the eddy covariance data processing. The instantaneous fluctuations were calculated by using two methods in parallel. The first one consists in subtracting the actual values of the turbulent variables from a function obtained by passing a mathematical recursive R–C filter through the original time series, as proposed by McMillen (1988). The second method calculates the fluctuations as differences between the instantaneous values and their arithmetic 30-min averages. The reason for using two different methods to calculate the covariances is the following. If the covariances obtained by the two methods are near to equal, the sample is stationary and the deduced flux must be kept as reliable. On the contrary, an important difference between the two obtained covariances means that the sample is nonstationary and thus does not satisfy the basic assumptions of micrometeorological work. The data with a normalized absolute difference between the covariances exceeding 1 (as proposed by Dutaur et al., 1999) have been rejected. At the end of each averaging period, a covariance matrix is calculated and subsequently transformed by two coordinate rotations (McMillen, 1988): the first, to align the wind vector with the mean wind direction; the second, to correct for possible deviations from verticality of the sonic anemometer. The vertical fluxes of ozone, sensible and latent heat (F ; H and lE; respectively) were obtained from elements of the rotated
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covariance matrix between the fluctuations of the corresponding variables (ozone concentration C; temperature T; and specific humidity q; respectively) and the vertical wind w: F ¼ w0 C 0 ,
(1)
H ¼ rcp w0 T 0 ,
(2)
lE ¼ lrw0 q0 .
(3)
(m2 s1). The displacement height and the roughness 1 length were set to 23 and 10 of the mean canopy height, respectively. The vertical turbulent diffusion coefficient is obtained by the classical Monin-Obukhov similarity relation K H ðzÞ ¼ ku ðz dÞ=FH ðzÞ,
where r; cp are the density and heat capacity of dry air, respectively, and l is the vaporization heat of water; primed quantities refer to fluctuations and overbars represent 30-min averages. The data obtained were screened and kept for the flux partitioning analysis only if they satisfied the following conditions: a single sample duration longer than 20 min, fulfillment of the sample’s stationarity condition as stated above, a positive latent heat flux (leaving the forest ecosystem) and a negative or non-vanishing ozone flux (i.e. only downward fluxes contribute to the ozone dose for the forest plants). 2.4. Ozone flux partitioning: Stomatal and non-stomatal fluxes With the total ozone fluxes F obtained from the measurements, the calculation of the stomatal ozone flux fraction is made in several steps, making use of the potential/resistance analogy, introduced by Chamberlain and Chadwick (1953), where fluxes are supposed to behave like electric currents in a circuit. Considering a very simple electric circuit for the forest canopy (big leaf model), ozone fluxes are expressed by F ¼ ðC m C 0 Þ=ðRa þ Rb þ Rc Þ
(4)
where Ra is the resistance opposed to the flux by the turbulent air layer over the forest canopy, Rb is the canopy quasi-laminar sub-layer (bulk leaf boundary layer) resistance to the ozone flux, Rc is the canopy (bulk leaf) resistance to ozone flux, C m is the ozone concentration at the measurement height and C 0 the ozone concentration in the sub-stomatal cavities. C 0 is assumed to be zero because sub-stomatal cell walls act as perfect ozone sinks, leading to a vanishing ozone concentration (Laisk et al., 1989). The three resistances act opposing to the ozone flux, as if they were mounted in series. The aerodynamic resistance can be calculated by: Z zm 1 Ra ¼ dz, (5) ðzÞ K H dþz0 where z is the height, zm is the height of measurement, z0 is the roughness length, d is the displacement height (the latter four quantities are expressed in meters) and K H ðzÞ is the vertical turbulent diffusion coefficient for scalars
(6)
where k is the dimensionless von Ka´rma´n constant ( ¼ 0.41), u* is the velocity (m s1), obtained directly from the measurements; FH ðzÞ is the Monin-Obukhov similarity function (non-dimensional) and z ¼ ðz dÞ=L with L being the Monin-Obukhov length (meters), deduced from the friction velocity and from the measured sensible heat flux. The analytical formulation of the function FH ðzÞ is taken from Dyer (1974). The quasi-laminar sub-layer resistance Rb depends on the molecular properties of the diffusing substance. It cannot be measured directly in the natural environment, so it must be estimated from theory. Here, a general purpose parameterization proposed by Hicks et al. (1987) has been used . Rb ¼ 2ðSc=PrÞ2=3 ðku Þ (7) where Sc ( ¼ 1.07 for ozone) and Pr ( ¼ 0.72) are the Schmidt and Prandtl numbers, respectively. The canopy resistance Rc is then determined as a residual by Eq. (4), with the measured F fluxes and C m concentrations, and the calculated aerodynamic Ra and laminar sub-layer Rb resistances. This quantity expresses the bulk capacity of the forest ecosystem surfaces to act as a sink for ozone, including all pathways (leaves, branches, trunks, litter, soil). Considering that the flux is constant along the vertical axis between the measurement height and the height of the top of the canopy, we may write F ¼ C m =ðRa þ Rb þ Rc Þ ¼ C c =Rc ,
(8)
where C c is the ozone concentration at the top of the canopy; C c can be easily obtained from this equation. The canopy resistance can be further decomposed in two resistances in parallel, one accounting for stomatal RST and the other for non-stomatal RNS ozone deposition, the latter covering all deposition pathways other than stomatal inside the forest canopy. At canopy-top level, the ozone flux divides into a stomatal contribution F ST and a non-stomatal one F NS F ¼ F ST þ F NS ¼ C c =RST þ C c =RNS .
(9)
The stomatal flux F ST is obtained by combining Eqs. (8) and (9): F ST ¼ C c =RST ¼
Rc Cm. ðRa þ Rb þ Rc ÞRST
(10)
The stomatal resistance for ozone uptake RST is calculated by multiplying the stomatal resistance for water vapor flux RS by 1.65, which is the ratio of the
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molecular diffusion coefficients for water vapor and ozone, respectively, through air. This is justified by assuming that, inside the stomata, diffusion of gases is purely molecular, and thus the resistances against the diffusion of different gases must be inversely proportional to their molecular diffusion constants. The canopy-scale stomatal resistance for water vapor flux RS was obtained from the measured latent heat flux lE (W m2), by using again the potential/resistance analogy applied to water fluxes (Monteith, 1981), and solving it for RS : lE ¼
rcp ½es ðT 0 Þ eðzm Þ gðRa þ Rb þ RS Þ
(11)
Here, cp and r are the air heat capacity (J K1 kg1) and the air density (kg m3), respectively; T 0 is the air temperature (K) at the evaporating (leaf) surface; eðzm Þ is the water vapor pressure (Pa) at the measurement height zm and es ðT 0 Þ is the water vapor pressure (Pa) at the evaporating surface, assumed to be at saturating level in the sub-stomatal cavity; g is the psychrometric constant, equal to 67 Pa K1. In this equation, lE and eðzm Þ are available from the measurements; Ra and Rb are calculated as described above, but Rb here refers to water (i.e. Sc/Pr ¼ 0.9 in Eq. (7)). The evaporating surface temperature T 0 can be estimated from the measured sensible heat flux H and air temperature at measurement height Tðzm Þ by means of the equation: T0 ¼
H ðRa þ Rb Þ þ Tðzm Þ, rcp
(12)
where once again the potential/resistance analogy is applied to the heat flux. The use of this approach is valid only if the whole evaporation process takes place through stomatal transpiration, and thus if no direct evaporation from soil or dew occurs. Regarding the soil evaporation, this condition is practically fulfilled if the soil is covered by a closed canopy, as in our case. Moreover, on the site where our measurements were carried out, the soil was extremely dry in its upper layer and covered by an insulating dense litter. Morning dew was very rare due to the very high temperature and, when it was present, it disappeared in a very short time (less than 30 min). Knowing the ozone canopy resistance Rc and the canopy-scale stomatal resistance RST ; the non-stomatal bulk resistance RNS is then calculated using the resistance composition rules, as RST and RNS are mounted in parallel. For averaging purposes, an upper limit of 10,000 s m1 was set for the values of the resulting resistance; resistances exceeding this limit were considered unrealistic and the corresponding samples were thus rejected.
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2.5. Ozone exposure and dose The ozone dose D was calculated by summing up the ozone stomatal flux F ST over the whole measuring time X (13) D¼ F ST Dt, where Dt are here equal to the chosen eddy covariance averaging period (30 min). The exposure of the forest to ozone was quantified as AOT40 (Ka¨renlampi and Ska¨rby, 1996), using ozone concentrations measured above the canopy C m and concentrations C dþz0 calculated at the displacement height (d+z0), which is the equivalent height within the forest canopy at which ozone molecules appear to be taken up or destroyed (UBA, 1996) X AOT40 ¼ ðC dþz0 40Þ Dt (14) 8½O3 i 440 ppb 8Glob:RadX50 W m2
Here, hourly concentrations are used; Dt is a 1-h interval, such that AOT40 is expressed in ppb h. The concentration C d þ z0 was calculated by solving Eq. (8) for C c (which is identical to C d þ z0), as proposed by Tuovinen (2000), and then rearranging C dþz0 ¼ C m ð1 Ra =Rtot Þ.
(15)
3. Results The total number of 30-min samples recorded during the observing period was 3124; 21.0% of them were rejected because did not fulfill the stationarity condition; a further 8.0% was discarded because the wind came from the direction of the previously mentioned obstacle (high tree). The data remaining after rejection on basis of unrealistic resistance values, were 50.7% of the total, mostly corresponding to the daytime samples. 3.1. Ozone concentrations and environmental parameters The overall average ozone concentration at the top of the canopy for the sampling period was 47.3 ppb; the maximum value was 116 ppb. The vicinity of the seashore and of the city of Rome affects the wind and ozone regimes (Fig. 2). Ozone concentrations are higher when breezes blow from the sea and lower when winds come from the land, particularly in the direction of the city. In the latter direction, the ozone concentration mean can be 40 ppb lower than in the seashore direction, due to ozone destruction by reaction with nitrogen oxides and volatile organic compounds present in the city plume. Ozone recirculation phenomena from sea to land also play a role.
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N
12%
70
10%
60
8%
50 40
6%
30
4%
20
2% W
(a)
0%
N
10
E
Diurnal Nocturnal
S
W
E
0
Diurnal Nocturnal
S
(b)
Fig. 2. Wind directions and ozone concentration at the top of the canopy. (a) Wind rose with the wind direction and the frequencies. (b) Mean top canopy ozone concentrations (ppb) for each wind direction. Diurnal refers to 8:00–20:00 h.
above canopy
below canopy
70 Ozone concentrations (ppb)
Ozone concentrations below the forest canopy were lower (35.7 ppb on average), particularly at dawn and during nighttime. Fig. 3 reports the mean daily course of ozone concentrations measured above and below the forest canopy during the sampling period. The ozone exposure in the upper part of the forest canopy, expressed as AOT40 for daytime hours, was 16,547 ppb h. This value, corresponding to a period of less than 3 months, is well above the ozone exposure critical level established for a 6 months period for forests by UN/ECE (Ka¨renlampi and Ska¨rby, 1996), and recently adopted by the European Union (EU Directive 03/02/EC) and by the Italian government. For the sake of interpretation, the sampling period has been divided into two parts, the first corresponding to a warmer and dryer period, the second to a wetter period. Table 1 summarizes the values of the relevant environmental parameters.
60 50 40 30 20 10 0 1
3
5
7
9 11 13 15 17 19 21 23 time (UTC+1)
Fig. 3. Mean daily course of ozone concentrations recorded above and below the forest canopy, for heights above ground of 18 and 2 m, respectively.
3.2. Ozone and energy fluxes The ozone fluxes measured over the forest ecosystem are reported on Fig. 4a. Non-stationary data, as well as those with wind directions corresponding to the already mentioned obstacle have been eliminated. The ozone fluxes were slightly higher in the second (wet) period than in the first (dry) one, with a daytime average of 8.612 and 6.924 nmol m2 s1, respectively (Table 2). The maximum observed ozone flux was 51.061 nmol m2 s1. Ozone fluxes are usually higher in the morning; they then decrease during the rest of the day (Fig. 5). Energy fluxes reflect the warm and dry climatic conditions, even for the wetter period. Latent heat fluxes lE were generally low due to the reduced plant transpiration: occasionally negative lE fluxes were
recorded when humid air masses blew from the sea. The sensible heat fluxes H; were correspondingly higher (Fig. 5). During the wetter period, the sensible heat fluxes were lower, while the latent heat fluxes were slightly higher, and their behavior was more regular and less scattered than in the dryer period (Table 1). 3.3. Stomatal uptake and non-stomatal ozone deposition For the whole observing period, the flux partitioning analysis gave a stomatal to total ozone flux ratio F ST =F of 31.5% for daytime (Table 2). The remaining fraction of the ozone flux, referred to as non-stomatal deposition, includes all other deposition pathways.
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Table 1 Values of the relevant environmental parameters and energy fluxes T 1C
Parameter Unit
RH %
Period 06/08/2003–13/09/2003 Mean 28.2 68.2 Max 35.1 98.0 Min 11.9 16.2 Total
VPD kPa
1.277 4.585 0.082
H W m2
LE W m2
40.7 108.8 2.0
92.5 551.2 96.5
24.5 604.5 273.0
41.8 103.1 2.2
31.3 97.2 2.0
36.3 383.1 127.9
30.4 603.2 193.8
47.3 116.1 2.2
35.7 108.8 2.0
66.5 551.2 127.9
27.3 604.5 273.0
O3 above canopy ppb
O3 below canopy ppb
53.5 116.1 2.2 66.4
14/09/2003–21/10/2003 Mean 19.3 71.5 Max 28.0 98.0 Min 9.7 23.6 Total
0.810 3.054 0.008
Whole period Mean 23.1 Max 35.1 Min 9.8 Total
1.061 4.585 0.008
69.1 98.0 16.2
Rain (*) mm
142.4
208.8
O3 flux (nmol m-2s-1)
10 0 -10 -20 -30 -40 -50 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 06/08/2003 13/08/2003 20/08/2003 27/08/2003 03/09/2003 10/09/2003 17/09/2003 24/09/2003 01/10/2003 08/10/2003 15/10/2003
(b)
Date and time 800
120
600
100
400
80
200
60
0
40
-200
20
-400
0
Rain (mm)
H and LE fluxes (Wm-2)
(a)
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 06/08/2003 13/08/2003 20/08/2003 27/08/2003 03/09/2003 10/09/2003 17/09/2003 24/09/2003 01/10/2003 08/10/2003 15/10/2003
Date and time Fig. 4. Ozone and energy fluxes over the forest ecoystem. (a) total ozone fluxes; (b) sensible (gray line) and latent (black line) heat fluxes. The lower bars indicate rainfall.
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Table 2 Diurnal ozone fluxes summary (h 8–18 UTC+1). Stomatal, non stomatal and canopy conductance for ozone, as well as ozone exposures and doses (total) are reported Parameter
6.026 49.395 0.002 8.461
0.285 0.990 0.0001 11645
14/09/2003–21/10/2003 Mean Max Min Total (mmol m2) Exposure
8.612 37.372 0.121 11.781
Whole period Mean Max Min Total (mmol m2) Exposure
7.789 51.061 0.121 21.590
(*) downward flux only.
1.993 15.465 0.0003 2.798
2.649 12.672 0.001 3.624
6.680 35.211 0.012 9.139
6.369 49.395 0.002 17.654
Gst (mm s1)
Gns (mm s1)
4.142 57.572 0.060
1.146 36.362 0.001
4.020 55.693 0.001
6.710 178.540 0.052
1.800 24.8 0.001
5.647 178.509 0.005
5.446 178.540 0.0052
1.491 36.362 0.001
4.871 178.509 0.001
2479
0.315 0.990 0.0001 16547
Gc (mm s1)
5472
0.344 0.990 0.0001 4902
2.339 15.465 0.0003 6.484
AOT40 d+z0 (ppb h)
7952
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6.924 51.061 0.136 9.722
AOT40 top canopy (ppb h)
G. Gerosa et al. / Atmospheric Environment 39 (2005) 3255–3266
Period 06/08/2003–13/09/2003 Mean Max Min Total (mmol m2) Exposure
Fst Fns Fst/Fc (ratio) F (*) (nmol m2 s1) (nmol m2 s1) (nmol m2 s1)
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LE
10
H
250
-2
F
12
200
8
150
6
100
4
50
2
0 -50
0 0
2
4
6
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4. Discussion
300
14
H and LE fluxes (Wm )
Total ozone flux F (nmol m-2 s-1)
G. Gerosa et al. / Atmospheric Environment 39 (2005) 3255–3266
8 10 12 14 16 18 20 22 hours (UTC+1)
Fig. 5. Mean daily course of sensible and latent heat fluxes in the whole sampling period. Total ozone flux mean course F is also reported.
The two observing periods show differences in the absolute values of the total and non-stomatal fluxes, but the shapes of the daily courses are nearly the same (Fig. 6). In both cases, the fluxes reach their maximum values in the first part of the morning and decreases afterwards. The stomatal fluxes are slightly lower in the first observing period than in the second, and the shapes of the daily courses in the two periods are quite different. During the first period, the irregular rise of the stomatal fluxes in the morning is followed by a rapid increase in the middle of the day, followed by an afternoon depression and a subsequent slow decrease. In the second period, the stomatal uptake is, on the contrary, characterized by a well defined bell-shaped daily course with a maximum of about 4 nmol m2 s1 around noon. The daily course of the ratio F ST =F ranges from 10% in the early morning to 50–60% in the early afternoon, during both periods. However, the average daytime stomatal fraction is slightly higher in the wetter period (34.4%) than in the dryer one (28.5%). Canopy level stomatal, non-stomatal and total surface conductances are shown in Fig. 6. The canopy scale stomatal conductance had a mean daytime value of 1.5 mm s1 with a mean daily maximum of 2.1 mm s1, occurring between noon and 2 p.m. During nighttime the averaged stomatal conductance was low (0.51 mm s1). After sunrise, the stomatal conductance increases rapidly; it remains then nearly constant until 2 p.m.; it then decreases continuously. The non-stomatal conductance shows a maximum of about 16 mm s1 in the early morning and then gradually decreases. During the whole observing period the stomatal ozone dose received by the Holm oak plants was 6484 mmol O3 m2 at ground level, whereas the ozone exposure at displacement height was 7952 ppb h. The latter value is about 50% of the AOT40 index, calculated for the top of the canopy.
Stomatal uptake represents a minor part of the total ozone amount received by the forest ecosystem. An important quantity of ozone molecules is destroyed on non-transpiring plant surfaces and on soil, or disappears by gas phase reaction with nitrogen oxides and biogenic volatile organic compounds. This kind of ozone deposition has not always been recognized. Padro (1996) stated that ozone deposition on vegetation is only due to stomatal activity. More recently, the importance of nonstomatal deposition has been emphasized by Tuovinen et al. (1998), Fowler et al. (2001) and Cieslik (2004). The nature of this kind of deposition is not fully understood. Fowler et al. (2001) attributed non-stomatal ozone deposition to thermal decomposition of O3 molecules on reaching plant surfaces; he calculated the activation energy of this process. Generally, this deposition process was considered constant during the day, but this is not the case, as shown, e.g. by Gerosa et al. (2003, 2004) for barley and wheat. On the contrary, it is expected that non-stomatal ozone deposition follows a bell-shaped daily course, because it is driven by atmospheric turbulence, which is more intense when energetic input to the system by solar radiation is higher. Ozone molecules have then a higher probability of undergoing decomposition on plant surfaces or on soil. The non-stomatal ozone deposition maximum observed here in the morning probably has another origin. Even though no NOx measurements were made, this peak is probably due to chemical reaction of ozone molecules with NO trapped in the lowest air layers within the forest. Nitric oxide molecules, emitted continuously by microbial activity in the soil, is stored during nighttime in the forest canopy due to strong atmospheric stability, they then react with ozone molecules. In fact, the nighttime ozone concentrations below the canopy are lower than above the forest canopy (Fig. 3), as also referred by De Santis et al. (2005). In the morning, when the onset of turbulence mixes up the air layers, the poor ozone air trapped within the canopy exits the forest by migrating upwards, while the ozone-richer air above the canopy enters the forest downwards. This causes a net downward ozone flux, which is neither caused by stomatal uptake nor by surface deposition. When the morning non-stomatal flux peaks, the stomatal conductance is, in fact, low, and turbulence intensity is not yet at its maximum. Further NOx flux measurements will allow to confirm this hypothesis. Similar ozone chemical sinks have been observed as a probable cause of non-stomatal ozone fluxes on crops (Gerosa et al., 2004), with minor intensity, however. The total ozone fluxes appear to be dominated by non-stomatal contributions as they follow the daily pattern of the latter, during both dry and wet periods.
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Fig. 6. Ozone flux partition for the two analysis periods: (a) 6 August–13 September 2003, (b) 14 September–22 October 2003. The first two graphs at the top reports the total (F), stomatal (FST) and non-stomatal (Fns) ozone fluxes. The second two graphs in the middle refers to the canopy (Gc), the stomatal (Gst) and the non-stomatal (Gns) bulk conductances to ozone. The last two graphs show the stomatal on total ozone flux ratios (FST/F). The vertical bars indicate the standard error of the means.
Moreover, the non-stomatal fluxes are substantially similar in both periods. The slightly greater total ozone fluxes in the second period are due to the slight increase in stomatal activity, stimulated by greater water availability. Such a high non-stomatal ozone deposition is not surprising in a Mediterranean evergreen forest. It represents a passive defence strategy against high concentrations of photooxidants present during long periods during the Mediterranean summer. The other physiological defence strategy for Mediterranean forest ecosystems is represented by the stomatal resistance increase caused by limited water supply. As a consequence, uptake of gaseous pollutants is limited.
The ozone exposure index AOT40 at the top of the canopy highlights an ozone risk for the forest ecosystem. However, the AOT40 index, if calculated for the zero plane displacement height, is about 50% of the corresponding value for the top of the canopy, and does not exceed the 10,000 ppb h critical level. The two results are ambiguous, and this is because the AOT40 exposure index is sensibly affected by the height of the measurements. Furthermore, the AOT40 values appear as a poor indicator of the ozone dose to vegetation, defined as the time-integrated stomatal ozone flux. In the second period, in fact, although the top canopy ozone exposure was lower than in the first period (4331 ppb h against
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9279 ppb h, respectively) the stomatal dose was higher: 3.624 mmol O3 m2 against 2.798 mmol O3 m2 in the first period (Table 2). This is also the case if the AOT40 index is calculated for the zero plane displacement height. Important errors in risk assessment may thus occur on applying the AOT40 method, in spite of its acceptation by the European Union as a legal basis in air pollution abatement policy making (see EU Directive 03/02/EC). A first comparative look at modeled flux predictions for the Mediterranean area reveals a probable underestimation of ozone fluxes. Emberson et al. (2000) provided a preliminar modeling tool for mapping ozone stomatal fluxes over Europe for wheat and beech. Although our measurements are made on a different plant species, the model predicts for this area a mean stomatal ozone flux for beech ranging from 0.75 to 1 nmol m2 s1 for June, which is three times lower than the mean stomatal flux measured for Holm oak at the same top canopy exposure level (7500–16,000 ppb h). Because Holm oak is a water saving species (Manes et al., 2005) and since the climatic conditions were dry, a significantly lower stomatal uptake in Holm oak than in beech would be expected.
5. Conclusions Ozone flux measurements have been carried out over a Mediterranean evergreen forest for 3 months between August and October. Stomatal flux is a minor part of the total ozone flux over the forest ecosystem. The main part of ozone deposition follows non-stomatal pathways. Chemical sink by NO seems to play a relevant role in the morning non-stomatal deposition. Stomatal uptake is enhanced by water availability but, on the average, it does not exceed the 34.4% of the total ozone flux. Ozone exposure indices AOT40 do not take into account plant physiology, and are thus likely to lead to substantial overestimation of ozone risk, particularly when water supply is limited, as occurs frequently in Southern European and Mediterranean areas. On the contrary, the currently available European model (UBA, 1996) tends to underestimate ozone stomatal fluxes in this area.
Acknowledgements The authors wish to thank Dr. De Michelis, Head of the Castelporziano Estate, and Dr. Tinelli whose assistance was essential in providing logistical coordination and meteorological data, Dr. De Santis of the Montelibretti C.N.R. laboratory for the ozone profile measurements, Dr. Fabi, Dr. Zona and Dr. Marzuoli for their helpful work in field. This research was supported
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by a contract with the Direzione Generale per la Salvaguardia Ambientale—Ministero dell’Ambiente e Tutela del Territorio, and by PRIN (2004057727) Grants.
References Affre, C., Lopez, A., Carrara, A., Druilhet, A., Fontan, J., 2000. The analysis of energy and ozone flux data from the LANDES 94 experiment. Atmospheric Environment 34, 803–821. Bassin, S., Calanca, P., Weidinger, T., Gerosa, G., Fuhrer, J., 2004. Modeling seasonal ozone fluxes to grassland and wheat: model improvement, testing and application. Atmospheric Environment 38 (15), 2349–2359. Bussotti, F., Gerosa, G., 2002. Are the Mediterranean forests in Southern Europe Threatened from ozone? Journal of Mediterranean Ecology 3, 23–34. Chamberlain, A.C., Chadwick, R.C., 1953. Deposition of airborne radioiodine vapour. Nucleonics 11, 22–25. Cieslik, S., 2004. Ozone uptake by various surface types: a comparison between dose and exposure. Atmospheric Environment 38, 2409–2420. Cieslik, S., Labatut, A., 1997. Ozone and heat fluxes over a mediterranean pseudosteppe. Atmospheric Environment 31, 177–184. Coe, H., Gallagher, M.W., Choularton, T.W., Dore, S., 1995. Canopy scale measurements of stomatal and cuticular O3 uptake by Sitka Spruce. Atmospheric Environment 29, 1413–1424. De Santis, F., Zona, D., Bellagotti, R., Vichi, F., Allegrini, I., 2005. Ozone monitoring in a Mediterranean forest using diffusive and continuous sampling. Journal of Environmental Monitoring, in press. Dutaur, L., Cieslik, S., Carrara, A., Lopez, A., 1999. The detection of nonstationarity in the determination of deposition fluxes. Proceedings of EUROTRAC Symposium ’98, vol. 2. WITPress, Southampton. pp. 171–176. Dyer, A.J., 1974. A review of flux-profile relationships. Boundary-Layer Meteorology 7, 363–372. Emberson, L.D., Ashmore, M.R., Cambridge, H.M., Simpson, D., Tuovinen, J.P., 2000. Modelling stomatal ozone flux across Europe. Environmental Pollution 109, 403–413. Fowler, D., Flechard, C., Cape, J.N., Storeton-West, R.L., Coyle, M., 2001. Measurements of ozone deposition to vegetation quantifying the flux, the stomatal and nonstomatal components. Water, Air and Soil Pollution 130, 63–74. Fuhrer, J., Ska¨rby, L., Ashmore, M.R., 1997. Critical levels for ozone effects on vegetation in Europe. Environmental Pollution 97 (1–2), 91–106. Gerosa, G., Cieslik, S., Ballarin-Denti, A., 2003. Micrometeorological determination of time-integrated stomatal ozone fluxes over wheat: a case study in Northern Italy. Atmospheric Environment 37 (6), 777–788. Gerosa, G., Marzuoli, R., Cieslik, S., Ballarin-Denti, A., 2004. Stomatal ozone uptake by barley in Italy. ‘‘Effective exposure’’ as a possible link between concentration- and
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flux-based approaches. Atmospheric Environment 38 (15), 2421–2432. Gu¨sten, H., Heinrich, G., 1996. On-line measurements of ozone surface fluxes: Part I: methodology and instrumentation. Atmospheric Environment 30, 897–909. Hicks, B.B., Matt, D.R., 1988. Combining biology, chemistry and meteorology in modeling and measuring dry deposition. Journal of Atmospheric Chemistry 6, 117–131. Hicks, B.B., Baldocchi, D.D., Meyers, T.P., Hosker, R.P., Matt, D.R., 1987. A Preliminary multiple resistance routine for deriving dry deposition velocities from measured quantities. Water, Air & Soil Pollution 36, 311–330. Hjellbrekke, A.G., 1998. Ozone measurements 1996. Kjeller, Norwegian Institute for Air Research (EMEP/CCC-Report 3/98 O-7727). Ka¨renlampi, L., Ska¨rby, L., (Eds.), 1996. Critical levels for ozone in Europe: testing and finalizing the concepts, UNECE Workshop Report. Department of Ecology and Environmental Science, University of Kuopio. Klemm, O., Mangold, A., 2001. Ozone deposition at a forest site in NE Bavaria. Water, Air, and Soil Pollution: Focus 1, 223–232. Laisk, A., Kull, O., Moldau, H., 1989. Ozone concentration in leaf intercellular air spaces is close to zero. Plant Physiology 90, 1163–1167. Lamaud, E., Carrara, A., Brunet, Y., Lopez, A., Druilhet, A., 2002. Ozone fluxes above and within a pine forest canopy in dry and wet conditions. Atmospheric Environment 36, 77–88. Manes, F., Vitale, M., Donato, E., Giannini, M., Puppi., G., 2005. Different ability of three Mediterranean oak species to tolerate progressive dehydration stress. E´coscience, in press. Massman, W.J., 1991. The attenuation of concentration fluctuations in turbulent flow through a tube. Journal of Geophysical Research 96, 269–273. McMillen, R.T., 1988. An eddy correlation technique with extended applicability to nonsimple terrain. Boundary-layer Meteorology 43, 231–245.
Mikkelsen, T.N., Ro-Poulsen, H., Hovmand, M.F., Jensen, N.O., Pilegaard, K., Egeloev, A.H., 2004. Five year measurements of ozone fluxes to a Danish Norway spruce canopy. Atmospheric Environment 38, 2361–2372. Milla´n, M.M., Salvador, R., Mantilla, E., Kallos, G., 1997. Photooxidant dynamics in the Mediterranean basin in summer: results from European research projects. Journal of Geophysical Research 102, 8811–8823. Monteith, J.L., 1981. Evaporation and surface temperature. Quarterly Journal of the Royal Meteorological Society 107, 1–27. Padro, J., 1996. Summary of ozone dry deposition velocity measurements and model estimates over vineyard, cotton, grass and deciduous forest in summer. Atmospheric Environment 30, 2363–2369. Stull, R.D., 1988. An introduction to boundary layer meteorology. Kluwer, Dordrecht. Tuovinen, J.-P., 2000. Assessing vegetation exposure to ozone: properties of the AOT40 index and modifications by deposition modelling. Environmental Pollution 109, 361–372. Tuovinen, J.-P., Aurela, M., Laurila, T., 1998. Resistances to ozone deposition to a flark fen in the northern Aapa Mire zone. Journal of Geophysical Research 103, 16953–16966. Tuovinen, J.-P., Ashmore, M.R., Emberson, L.D., Simpson, D., 2004. Testing and improving the EMEP ozone deposition module. Atmospheric Environment 38, 2373–2385. UBA, 1996. Manual on methodologies and criteria for mapping critical levels/loads and geographical areas where they are exceeded. Texte 71/96. Federal Environmental Agency (Umweltbundesamt, UBA), Berlin, Germany. van der Hoven, I., 1957. Power spectrum of horizontal wind speed in the frequency range from 0.0007 to 900 cycles/h. Journal of Meteorology 14, 160. Vitale, M., Gerosa, G., Ballarin Denti, A., Manes, F., 2005. Ozone uptake by an evergreen mediterranean forest (Quercus ilex) in Italy. Part II: flux modelling. Upscaling leaf to canopy ozone uptake by a process-based model. Atmospheric Environment, in press.
Atmospheric Environment 39 (2005) 3255–3266 www.elsevier.com/locate/atmosenv
Ozone uptake by an evergreen Mediterranean Forest (Quercus ilex) in Italy. Part I: Micrometeorological flux measurements and flux partitioning Giacomo Gerosaa,, Marcello Vitaleb, Angelo Fincoa, Fausto Manesb, Antonio Ballarin Dentia, Stanislaw Cieslikc a
Department of Mathematics and Physics, Universita` Cattolica del S.C., via Musei 41, I-25121 Brescia, Italy b Department of Plant Biology, Universita` ‘‘La Sapienza’’, Piazzale A. Moro 5, I-00185 Roma, Italy c Institute for Environment and Sustainability, Joint Research Centre of the EC, via Fermi, I-21020 Ispra (VA), Italy Received 22 September 2004; received in revised form 13 January 2005; accepted 26 January 2005
Abstract Ozone, water and energy fluxes have been measured over a Mediterranean evergreen forest in Central Italy from August to October 2003 with the eddy-correlation technique in order to evaluate the amount of ozone taken up by plants in dry summer and in mild autumn conditions. The stomatal ozone fluxes have been calculated using the analogy with water vapor fluxes inside the stomata, which are easily measurable. The total ozone dose was obtained by integrating the stomatal fluxes over time. Stomatal flux resulted a minor part (31.5%) of the total ozone flux over the forest ecosystem. The main part of ozone deposition follows non-stomatal pathways. Chemical sink seems to play a relevant role in the morning non-stomatal deposition. Stomatal uptake is enhanced by water availability but, on the average, it does not exceed the 34.4% of the total ozone flux. A comparison between the cumulated stomatal ozone fluxes and the currently used AOT40 exposure index highlighted important distortions introduced by this index. AOT40, which do not take into account plant physiology, lead to substantial overestimation of ozone risk, particularly when water supply is limited, as occurs frequently in Southern European and Mediterranean areas. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ozone deposition; Stomatal uptake; Mediterranean forest ecosystem; Holm oak
1. Introduction Tropospheric ozone is widely recognized as a threat for natural ecosystems because of its toxicity on plants (Fuhrer et al., 1997). Corresponding author. Department of Mathematics and
Physics, Universita` Cattolica del Brescia, c/o via Pertini, 11, I-24035 Curno (BG), Italy. Tel./fax: +39 035 462339. E-mail address: [email protected] (G. Gerosa).
In the Mediterranean area, ozone easily reaches high concentrations in summer due to intense solar radiation, high temperature and very dry conditions which favor atmospheric photochemical activity (Milla´n et al., 1997). As a consequence, Mediterranean natural ecosystems are frequently subject to very high ozone exposure levels (Hjellbrekke, 1998). The exposure level is usually defined as a timeintegrated concentration. However, ozone damage to vegetation occurs through ozone uptake by plant
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.056
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stomata and is not necessarily related to integrated ozone concentrations. The ability of stomata to serve as a sink for atmospheric gases, quantified by stomatal conductance, is, in turn, mainly controlled by water availability (Emberson et al., 2000) and by other environmental factors like air water vapor pressure deficit, solar radiation, wind speed, intensity of turbulence, etc. In the Mediterranean area, water supply is generally limited, and thus also ozone stomatal uptake (Bussotti and Gerosa, 2002). Conversely, an important fraction of ozone deposition (or destruction) is expected to take place on non-transpiring plant surfaces and on soil. This study aims at measuring ozone fluxes over an evergreen Mediterranean forest stand and to quantify stomatal ozone uptake by plants and non-stomatal ozone deposition at ecosystem (canopy) level during 3 months, ranging from summer to autumn, covering different water supply conditions. Direct measurements of ozone deposition fluxes over forest stands using the eddy covariance micrometeorological technique have been made for northern and central European forests (eg. Mikkelsen et al., 2004; Coe et al., 1995; Klemm and Mangold, 2001), but there are few ozone flux data for evergreen Mediterranean forests. Affre et al. (2000) and Lamaud et al. (2002) measured ozone fluxes over an evergreen pine (Pinus pinaster) forest in Southern France. For the same ecosystem, Cieslik (2004) determined the stomatal uptake flux contribution to the total ozone flux. However, the environmental conditions of this forest, as well as the Atlantic climate, greatly differ from the Mediterranean ones. Regarding Southern Europe there are very few information on stomatal ozone uptake, and data have been reported for some crops like wheat, barley and soybean (Gerosa et al., 2003, 2004), Mediterranean shrubs (Cieslik and Labatut, 1997), orange groves and grass (Cieslik, 2004), mainly for short periods. In this context, the data presented here can be used as a basis to test ozone deposition models’ performance for a Southern European evergreen forest, as already made, e.g. for the ODEM (Bassin et al., 2004) or the EMEP models for pasture, crops, Northern Europe evergreen forests and seminatural vegetation (Tuovinen et al., 2004).
2. Measurements and methodology
Fig. 1. Measurement site location (Grotta di Piastra) inside the Castelporziano Presidential Estate near Rome (Italy).
located 0.4 km from the seashore and it is covered by an evergreen Holm oak forest (Quercus ilex L.). Other species, like Pistacia lentiscus L., Phyllirea latifolia and Myrtus communis L. were scarcely present in the forest understorey. The mean height of the forest stand was 13 m; the leaf area index (LAI), measured by a LAI 2000 instrument (Li-Cor, USA), was 4.27 m2leaf m2 ground (Vitale et al., 2005). The soil is sandy and covered by a consistent litter. The whole ecosystem is representative of the original wild coastal rear dune ecosystem and has been part of a protected area, with restricted access, over the last 60 years. The climate is typically Mediterranean with the mean annual rainfall of 753 mm and a summer drought period from May to August (1955–1985 average). The year 2003 was exceptionally dry and hot, with scarce rainfall also during the spring (Table 1 in Vitale et al., 2005). The quantity of ozone taken up per unit area during the measuring period was determined in three steps:
2.1. Sampling site The measurements have been carried out from 6 August to 22 October 2003 inside the Castelporziano Estate (411440 N, 121250 E) located along the Latium coast at 20 km distance from the center of Rome (Italy) (Fig. 1). The sampling site, named Grotta di Piastra, is
measurement of ozone and water vapor fluxes; calculation of fractional stomatal ozone fluxes; determination of the ozone dose. These steps are outlined below.
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2.2. Eddy covariance flux measurements The fluxes of ozone, sensible and latent heat, as well as momentum, were measured by using the micrometeorological approach, which assumes that fluxes are independent from height in the lowest air layers (constant flux hypothesis, see e.g. Stull, 1988). So, a flux measured at several meters above ground level is assumed to be equal to the flux at the air–vegetation interface. The flux was determined by the eddy covariance, method, considered the most reliable one (see e.g. Hicks and Matt, 1988). Fluxes (nmol m2 s1) are equal to the covariances between turbulent fluctuations of the vertical wind vector component and of the scalar entity of interest, such as ozone concentration for ozone fluxes, water vapor for latent heat fluxes, and air temperature for sensible heat fluxes. Calculation of the fluctuations requires averaging over successive periods of time, such as to eliminate non-turbulent, long-term variations. The averaging time was chosen such as to include all turbulent fluctuations (at least 10 min) occurring in the atmospheric surface layer, but it had to be short enough to avoid the synoptic-scale fluctuations (less than 1 h, van der Hoven, 1957). A 30-min averaging time was taken as a compromise. The micrometeorological instrumentation was mounted on the top of an 18-m-high firewatching steel tower located inside the Holm oak forest, exceeding by 5–6 m the mean forest height. The forest canopy was closed but fairly rough. A 17-m-tall Holm oak was present close to the tower in the NNW direction and represented an aerodynamic obstacle for flux measurements when winds blew from that direction (both verses). The fetch, i.e. the distance from the measuring point over which the surface is homogeneous, ranged from a minimum of 400 m toward the seashore line direction (SW) to a maximum of 6 km in the SE direction. The three components of the wind vector and the temperature were measured at high sampling frequency (20 Hz) by means of a USA-1 ultrasonic anemometer (Metek A.G., Germany). Air moisture was recorded by a KH20 fast-response hygrometer (Campbell Sci., USA) and ozone concentrations were measured by a GFAS OS-G-2 fast-response chemiluminescent sensor (Gu¨sten and Heinrich, 1996). Chemiluminescence is obtained with a dye-coated target which has to be replaced every 4 days because of a decrease in sensitivity. The teflon aspiration tube inlet of the fast ozone sensor (1 m length and 2.75 cm of internal diameter) was placed at the sonic anemometer height, at a horizontal distance of 25 cm from it. An aspiration flow rate of 70 l min1 ensured purely turbulent flow through the tube, in order to minimize fluctuation damping (Massman, 1991). Since the sensitivity of the fast ozone sensor decreased regularly in 4 days, it had to be recalibrated
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continuously with a reference slow ozone sensor. For this sake, an API 400A (Advanced Pollution Instruments, USA) slow-response ozone analyzer was used. This analyzer was also equipped with an electronic switch to measure ozone concentrations in the air above and below the canopy (18 and 2 m, respectively) at intervals of 20 min for each height. The analyzer was calibrated before and after the campaign and the data were corrected accordingly. Other relevant parameters were continuously monitored: surface net radiation flux (W m2) by a NR-lite radiometer (Kipp & Zonen, Holland), atmospheric pressure by a PTB101B barometer (Va¨isa¨la¨, Finland) and air temperature and humidity, above and inside the canopy at 18 and 11 m, respectively, by means of two 50Y humitters (Campbell Scientific). The humidity data measured at 18 m were also used as reference for the continuous calibration of the fast humidity sensor. This was necessary because the KH20 krypton hygrometer output shows a drift. 2.3. Data sampling and flux determination The output data of the sonic anemometer and of the fast sensors data were sent every 0.05 s to a personal computer; the other data were transmitted to a CR10 datalogger (Campbell Sci.) every 10 s. An averaging time of 30 min was used for the eddy covariance data processing. The instantaneous fluctuations were calculated by using two methods in parallel. The first one consists in subtracting the actual values of the turbulent variables from a function obtained by passing a mathematical recursive R–C filter through the original time series, as proposed by McMillen (1988). The second method calculates the fluctuations as differences between the instantaneous values and their arithmetic 30-min averages. The reason for using two different methods to calculate the covariances is the following. If the covariances obtained by the two methods are near to equal, the sample is stationary and the deduced flux must be kept as reliable. On the contrary, an important difference between the two obtained covariances means that the sample is nonstationary and thus does not satisfy the basic assumptions of micrometeorological work. The data with a normalized absolute difference between the covariances exceeding 1 (as proposed by Dutaur et al., 1999) have been rejected. At the end of each averaging period, a covariance matrix is calculated and subsequently transformed by two coordinate rotations (McMillen, 1988): the first, to align the wind vector with the mean wind direction; the second, to correct for possible deviations from verticality of the sonic anemometer. The vertical fluxes of ozone, sensible and latent heat (F ; H and lE; respectively) were obtained from elements of the rotated
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covariance matrix between the fluctuations of the corresponding variables (ozone concentration C; temperature T; and specific humidity q; respectively) and the vertical wind w: F ¼ w0 C 0 ,
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(m2 s1). The displacement height and the roughness 1 length were set to 23 and 10 of the mean canopy height, respectively. The vertical turbulent diffusion coefficient is obtained by the classical Monin-Obukhov similarity relation K H ðzÞ ¼ ku ðz dÞ=FH ðzÞ,
where r; cp are the density and heat capacity of dry air, respectively, and l is the vaporization heat of water; primed quantities refer to fluctuations and overbars represent 30-min averages. The data obtained were screened and kept for the flux partitioning analysis only if they satisfied the following conditions: a single sample duration longer than 20 min, fulfillment of the sample’s stationarity condition as stated above, a positive latent heat flux (leaving the forest ecosystem) and a negative or non-vanishing ozone flux (i.e. only downward fluxes contribute to the ozone dose for the forest plants). 2.4. Ozone flux partitioning: Stomatal and non-stomatal fluxes With the total ozone fluxes F obtained from the measurements, the calculation of the stomatal ozone flux fraction is made in several steps, making use of the potential/resistance analogy, introduced by Chamberlain and Chadwick (1953), where fluxes are supposed to behave like electric currents in a circuit. Considering a very simple electric circuit for the forest canopy (big leaf model), ozone fluxes are expressed by F ¼ ðC m C 0 Þ=ðRa þ Rb þ Rc Þ
(4)
where Ra is the resistance opposed to the flux by the turbulent air layer over the forest canopy, Rb is the canopy quasi-laminar sub-layer (bulk leaf boundary layer) resistance to the ozone flux, Rc is the canopy (bulk leaf) resistance to ozone flux, C m is the ozone concentration at the measurement height and C 0 the ozone concentration in the sub-stomatal cavities. C 0 is assumed to be zero because sub-stomatal cell walls act as perfect ozone sinks, leading to a vanishing ozone concentration (Laisk et al., 1989). The three resistances act opposing to the ozone flux, as if they were mounted in series. The aerodynamic resistance can be calculated by: Z zm 1 Ra ¼ dz, (5) ðzÞ K H dþz0 where z is the height, zm is the height of measurement, z0 is the roughness length, d is the displacement height (the latter four quantities are expressed in meters) and K H ðzÞ is the vertical turbulent diffusion coefficient for scalars
(6)
where k is the dimensionless von Ka´rma´n constant ( ¼ 0.41), u* is the velocity (m s1), obtained directly from the measurements; FH ðzÞ is the Monin-Obukhov similarity function (non-dimensional) and z ¼ ðz dÞ=L with L being the Monin-Obukhov length (meters), deduced from the friction velocity and from the measured sensible heat flux. The analytical formulation of the function FH ðzÞ is taken from Dyer (1974). The quasi-laminar sub-layer resistance Rb depends on the molecular properties of the diffusing substance. It cannot be measured directly in the natural environment, so it must be estimated from theory. Here, a general purpose parameterization proposed by Hicks et al. (1987) has been used . Rb ¼ 2ðSc=PrÞ2=3 ðku Þ (7) where Sc ( ¼ 1.07 for ozone) and Pr ( ¼ 0.72) are the Schmidt and Prandtl numbers, respectively. The canopy resistance Rc is then determined as a residual by Eq. (4), with the measured F fluxes and C m concentrations, and the calculated aerodynamic Ra and laminar sub-layer Rb resistances. This quantity expresses the bulk capacity of the forest ecosystem surfaces to act as a sink for ozone, including all pathways (leaves, branches, trunks, litter, soil). Considering that the flux is constant along the vertical axis between the measurement height and the height of the top of the canopy, we may write F ¼ C m =ðRa þ Rb þ Rc Þ ¼ C c =Rc ,
(8)
where C c is the ozone concentration at the top of the canopy; C c can be easily obtained from this equation. The canopy resistance can be further decomposed in two resistances in parallel, one accounting for stomatal RST and the other for non-stomatal RNS ozone deposition, the latter covering all deposition pathways other than stomatal inside the forest canopy. At canopy-top level, the ozone flux divides into a stomatal contribution F ST and a non-stomatal one F NS F ¼ F ST þ F NS ¼ C c =RST þ C c =RNS .
(9)
The stomatal flux F ST is obtained by combining Eqs. (8) and (9): F ST ¼ C c =RST ¼
Rc Cm. ðRa þ Rb þ Rc ÞRST
(10)
The stomatal resistance for ozone uptake RST is calculated by multiplying the stomatal resistance for water vapor flux RS by 1.65, which is the ratio of the
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molecular diffusion coefficients for water vapor and ozone, respectively, through air. This is justified by assuming that, inside the stomata, diffusion of gases is purely molecular, and thus the resistances against the diffusion of different gases must be inversely proportional to their molecular diffusion constants. The canopy-scale stomatal resistance for water vapor flux RS was obtained from the measured latent heat flux lE (W m2), by using again the potential/resistance analogy applied to water fluxes (Monteith, 1981), and solving it for RS : lE ¼
rcp ½es ðT 0 Þ eðzm Þ gðRa þ Rb þ RS Þ
(11)
Here, cp and r are the air heat capacity (J K1 kg1) and the air density (kg m3), respectively; T 0 is the air temperature (K) at the evaporating (leaf) surface; eðzm Þ is the water vapor pressure (Pa) at the measurement height zm and es ðT 0 Þ is the water vapor pressure (Pa) at the evaporating surface, assumed to be at saturating level in the sub-stomatal cavity; g is the psychrometric constant, equal to 67 Pa K1. In this equation, lE and eðzm Þ are available from the measurements; Ra and Rb are calculated as described above, but Rb here refers to water (i.e. Sc/Pr ¼ 0.9 in Eq. (7)). The evaporating surface temperature T 0 can be estimated from the measured sensible heat flux H and air temperature at measurement height Tðzm Þ by means of the equation: T0 ¼
H ðRa þ Rb Þ þ Tðzm Þ, rcp
(12)
where once again the potential/resistance analogy is applied to the heat flux. The use of this approach is valid only if the whole evaporation process takes place through stomatal transpiration, and thus if no direct evaporation from soil or dew occurs. Regarding the soil evaporation, this condition is practically fulfilled if the soil is covered by a closed canopy, as in our case. Moreover, on the site where our measurements were carried out, the soil was extremely dry in its upper layer and covered by an insulating dense litter. Morning dew was very rare due to the very high temperature and, when it was present, it disappeared in a very short time (less than 30 min). Knowing the ozone canopy resistance Rc and the canopy-scale stomatal resistance RST ; the non-stomatal bulk resistance RNS is then calculated using the resistance composition rules, as RST and RNS are mounted in parallel. For averaging purposes, an upper limit of 10,000 s m1 was set for the values of the resulting resistance; resistances exceeding this limit were considered unrealistic and the corresponding samples were thus rejected.
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2.5. Ozone exposure and dose The ozone dose D was calculated by summing up the ozone stomatal flux F ST over the whole measuring time X (13) D¼ F ST Dt, where Dt are here equal to the chosen eddy covariance averaging period (30 min). The exposure of the forest to ozone was quantified as AOT40 (Ka¨renlampi and Ska¨rby, 1996), using ozone concentrations measured above the canopy C m and concentrations C dþz0 calculated at the displacement height (d+z0), which is the equivalent height within the forest canopy at which ozone molecules appear to be taken up or destroyed (UBA, 1996) X AOT40 ¼ ðC dþz0 40Þ Dt (14) 8½O3 i 440 ppb 8Glob:RadX50 W m2
Here, hourly concentrations are used; Dt is a 1-h interval, such that AOT40 is expressed in ppb h. The concentration C d þ z0 was calculated by solving Eq. (8) for C c (which is identical to C d þ z0), as proposed by Tuovinen (2000), and then rearranging C dþz0 ¼ C m ð1 Ra =Rtot Þ.
(15)
3. Results The total number of 30-min samples recorded during the observing period was 3124; 21.0% of them were rejected because did not fulfill the stationarity condition; a further 8.0% was discarded because the wind came from the direction of the previously mentioned obstacle (high tree). The data remaining after rejection on basis of unrealistic resistance values, were 50.7% of the total, mostly corresponding to the daytime samples. 3.1. Ozone concentrations and environmental parameters The overall average ozone concentration at the top of the canopy for the sampling period was 47.3 ppb; the maximum value was 116 ppb. The vicinity of the seashore and of the city of Rome affects the wind and ozone regimes (Fig. 2). Ozone concentrations are higher when breezes blow from the sea and lower when winds come from the land, particularly in the direction of the city. In the latter direction, the ozone concentration mean can be 40 ppb lower than in the seashore direction, due to ozone destruction by reaction with nitrogen oxides and volatile organic compounds present in the city plume. Ozone recirculation phenomena from sea to land also play a role.
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N
12%
70
10%
60
8%
50 40
6%
30
4%
20
2% W
(a)
0%
N
10
E
Diurnal Nocturnal
S
W
E
0
Diurnal Nocturnal
S
(b)
Fig. 2. Wind directions and ozone concentration at the top of the canopy. (a) Wind rose with the wind direction and the frequencies. (b) Mean top canopy ozone concentrations (ppb) for each wind direction. Diurnal refers to 8:00–20:00 h.
above canopy
below canopy
70 Ozone concentrations (ppb)
Ozone concentrations below the forest canopy were lower (35.7 ppb on average), particularly at dawn and during nighttime. Fig. 3 reports the mean daily course of ozone concentrations measured above and below the forest canopy during the sampling period. The ozone exposure in the upper part of the forest canopy, expressed as AOT40 for daytime hours, was 16,547 ppb h. This value, corresponding to a period of less than 3 months, is well above the ozone exposure critical level established for a 6 months period for forests by UN/ECE (Ka¨renlampi and Ska¨rby, 1996), and recently adopted by the European Union (EU Directive 03/02/EC) and by the Italian government. For the sake of interpretation, the sampling period has been divided into two parts, the first corresponding to a warmer and dryer period, the second to a wetter period. Table 1 summarizes the values of the relevant environmental parameters.
60 50 40 30 20 10 0 1
3
5
7
9 11 13 15 17 19 21 23 time (UTC+1)
Fig. 3. Mean daily course of ozone concentrations recorded above and below the forest canopy, for heights above ground of 18 and 2 m, respectively.
3.2. Ozone and energy fluxes The ozone fluxes measured over the forest ecosystem are reported on Fig. 4a. Non-stationary data, as well as those with wind directions corresponding to the already mentioned obstacle have been eliminated. The ozone fluxes were slightly higher in the second (wet) period than in the first (dry) one, with a daytime average of 8.612 and 6.924 nmol m2 s1, respectively (Table 2). The maximum observed ozone flux was 51.061 nmol m2 s1. Ozone fluxes are usually higher in the morning; they then decrease during the rest of the day (Fig. 5). Energy fluxes reflect the warm and dry climatic conditions, even for the wetter period. Latent heat fluxes lE were generally low due to the reduced plant transpiration: occasionally negative lE fluxes were
recorded when humid air masses blew from the sea. The sensible heat fluxes H; were correspondingly higher (Fig. 5). During the wetter period, the sensible heat fluxes were lower, while the latent heat fluxes were slightly higher, and their behavior was more regular and less scattered than in the dryer period (Table 1). 3.3. Stomatal uptake and non-stomatal ozone deposition For the whole observing period, the flux partitioning analysis gave a stomatal to total ozone flux ratio F ST =F of 31.5% for daytime (Table 2). The remaining fraction of the ozone flux, referred to as non-stomatal deposition, includes all other deposition pathways.
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Table 1 Values of the relevant environmental parameters and energy fluxes T 1C
Parameter Unit
RH %
Period 06/08/2003–13/09/2003 Mean 28.2 68.2 Max 35.1 98.0 Min 11.9 16.2 Total
VPD kPa
1.277 4.585 0.082
H W m2
LE W m2
40.7 108.8 2.0
92.5 551.2 96.5
24.5 604.5 273.0
41.8 103.1 2.2
31.3 97.2 2.0
36.3 383.1 127.9
30.4 603.2 193.8
47.3 116.1 2.2
35.7 108.8 2.0
66.5 551.2 127.9
27.3 604.5 273.0
O3 above canopy ppb
O3 below canopy ppb
53.5 116.1 2.2 66.4
14/09/2003–21/10/2003 Mean 19.3 71.5 Max 28.0 98.0 Min 9.7 23.6 Total
0.810 3.054 0.008
Whole period Mean 23.1 Max 35.1 Min 9.8 Total
1.061 4.585 0.008
69.1 98.0 16.2
Rain (*) mm
142.4
208.8
O3 flux (nmol m-2s-1)
10 0 -10 -20 -30 -40 -50 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 06/08/2003 13/08/2003 20/08/2003 27/08/2003 03/09/2003 10/09/2003 17/09/2003 24/09/2003 01/10/2003 08/10/2003 15/10/2003
(b)
Date and time 800
120
600
100
400
80
200
60
0
40
-200
20
-400
0
Rain (mm)
H and LE fluxes (Wm-2)
(a)
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 06/08/2003 13/08/2003 20/08/2003 27/08/2003 03/09/2003 10/09/2003 17/09/2003 24/09/2003 01/10/2003 08/10/2003 15/10/2003
Date and time Fig. 4. Ozone and energy fluxes over the forest ecoystem. (a) total ozone fluxes; (b) sensible (gray line) and latent (black line) heat fluxes. The lower bars indicate rainfall.
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Table 2 Diurnal ozone fluxes summary (h 8–18 UTC+1). Stomatal, non stomatal and canopy conductance for ozone, as well as ozone exposures and doses (total) are reported Parameter
6.026 49.395 0.002 8.461
0.285 0.990 0.0001 11645
14/09/2003–21/10/2003 Mean Max Min Total (mmol m2) Exposure
8.612 37.372 0.121 11.781
Whole period Mean Max Min Total (mmol m2) Exposure
7.789 51.061 0.121 21.590
(*) downward flux only.
1.993 15.465 0.0003 2.798
2.649 12.672 0.001 3.624
6.680 35.211 0.012 9.139
6.369 49.395 0.002 17.654
Gst (mm s1)
Gns (mm s1)
4.142 57.572 0.060
1.146 36.362 0.001
4.020 55.693 0.001
6.710 178.540 0.052
1.800 24.8 0.001
5.647 178.509 0.005
5.446 178.540 0.0052
1.491 36.362 0.001
4.871 178.509 0.001
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0.315 0.990 0.0001 16547
Gc (mm s1)
5472
0.344 0.990 0.0001 4902
2.339 15.465 0.0003 6.484
AOT40 d+z0 (ppb h)
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6.924 51.061 0.136 9.722
AOT40 top canopy (ppb h)
G. Gerosa et al. / Atmospheric Environment 39 (2005) 3255–3266
Period 06/08/2003–13/09/2003 Mean Max Min Total (mmol m2) Exposure
Fst Fns Fst/Fc (ratio) F (*) (nmol m2 s1) (nmol m2 s1) (nmol m2 s1)
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LE
10
H
250
-2
F
12
200
8
150
6
100
4
50
2
0 -50
0 0
2
4
6
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4. Discussion
300
14
H and LE fluxes (Wm )
Total ozone flux F (nmol m-2 s-1)
G. Gerosa et al. / Atmospheric Environment 39 (2005) 3255–3266
8 10 12 14 16 18 20 22 hours (UTC+1)
Fig. 5. Mean daily course of sensible and latent heat fluxes in the whole sampling period. Total ozone flux mean course F is also reported.
The two observing periods show differences in the absolute values of the total and non-stomatal fluxes, but the shapes of the daily courses are nearly the same (Fig. 6). In both cases, the fluxes reach their maximum values in the first part of the morning and decreases afterwards. The stomatal fluxes are slightly lower in the first observing period than in the second, and the shapes of the daily courses in the two periods are quite different. During the first period, the irregular rise of the stomatal fluxes in the morning is followed by a rapid increase in the middle of the day, followed by an afternoon depression and a subsequent slow decrease. In the second period, the stomatal uptake is, on the contrary, characterized by a well defined bell-shaped daily course with a maximum of about 4 nmol m2 s1 around noon. The daily course of the ratio F ST =F ranges from 10% in the early morning to 50–60% in the early afternoon, during both periods. However, the average daytime stomatal fraction is slightly higher in the wetter period (34.4%) than in the dryer one (28.5%). Canopy level stomatal, non-stomatal and total surface conductances are shown in Fig. 6. The canopy scale stomatal conductance had a mean daytime value of 1.5 mm s1 with a mean daily maximum of 2.1 mm s1, occurring between noon and 2 p.m. During nighttime the averaged stomatal conductance was low (0.51 mm s1). After sunrise, the stomatal conductance increases rapidly; it remains then nearly constant until 2 p.m.; it then decreases continuously. The non-stomatal conductance shows a maximum of about 16 mm s1 in the early morning and then gradually decreases. During the whole observing period the stomatal ozone dose received by the Holm oak plants was 6484 mmol O3 m2 at ground level, whereas the ozone exposure at displacement height was 7952 ppb h. The latter value is about 50% of the AOT40 index, calculated for the top of the canopy.
Stomatal uptake represents a minor part of the total ozone amount received by the forest ecosystem. An important quantity of ozone molecules is destroyed on non-transpiring plant surfaces and on soil, or disappears by gas phase reaction with nitrogen oxides and biogenic volatile organic compounds. This kind of ozone deposition has not always been recognized. Padro (1996) stated that ozone deposition on vegetation is only due to stomatal activity. More recently, the importance of nonstomatal deposition has been emphasized by Tuovinen et al. (1998), Fowler et al. (2001) and Cieslik (2004). The nature of this kind of deposition is not fully understood. Fowler et al. (2001) attributed non-stomatal ozone deposition to thermal decomposition of O3 molecules on reaching plant surfaces; he calculated the activation energy of this process. Generally, this deposition process was considered constant during the day, but this is not the case, as shown, e.g. by Gerosa et al. (2003, 2004) for barley and wheat. On the contrary, it is expected that non-stomatal ozone deposition follows a bell-shaped daily course, because it is driven by atmospheric turbulence, which is more intense when energetic input to the system by solar radiation is higher. Ozone molecules have then a higher probability of undergoing decomposition on plant surfaces or on soil. The non-stomatal ozone deposition maximum observed here in the morning probably has another origin. Even though no NOx measurements were made, this peak is probably due to chemical reaction of ozone molecules with NO trapped in the lowest air layers within the forest. Nitric oxide molecules, emitted continuously by microbial activity in the soil, is stored during nighttime in the forest canopy due to strong atmospheric stability, they then react with ozone molecules. In fact, the nighttime ozone concentrations below the canopy are lower than above the forest canopy (Fig. 3), as also referred by De Santis et al. (2005). In the morning, when the onset of turbulence mixes up the air layers, the poor ozone air trapped within the canopy exits the forest by migrating upwards, while the ozone-richer air above the canopy enters the forest downwards. This causes a net downward ozone flux, which is neither caused by stomatal uptake nor by surface deposition. When the morning non-stomatal flux peaks, the stomatal conductance is, in fact, low, and turbulence intensity is not yet at its maximum. Further NOx flux measurements will allow to confirm this hypothesis. Similar ozone chemical sinks have been observed as a probable cause of non-stomatal ozone fluxes on crops (Gerosa et al., 2004), with minor intensity, however. The total ozone fluxes appear to be dominated by non-stomatal contributions as they follow the daily pattern of the latter, during both dry and wet periods.
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First period
14
10 8 6 4
F Fst Fns
12
nmol m-2 s-1
12
nmol m-2 s-1
Second period
14 F Fst Fns
2
10 8 6 4 2
0
0 0
2
4
6
8
0
10 12 14 16 18 20 22
2
4
6
8
time (UTC+1) 0.025
0.025 Gc Gst Gns
0.015 0.01
Gc Gst Gns
0.02
ms-1
ms-1
0.02
0.005
0.015 0.01 0.005
0
0 0
2
4
6
8
10 12 14 16 18 20 22
0
2
4
6
8
time (UTC+1)
10 12 14 16 18 20 22
time (UTC+1)
0.7
0.7 Fst/F
0.6
Fst/F
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1 0
0 0
(a)
10 12 14 16 18 20 22
time (UTC+1)
2
4
6
8
10 12 14 16 18 20 22
time (UTC+1)
0
(b)
2
4
6
8
10 12 14 16 18 20 22
time (UTC+1)
Fig. 6. Ozone flux partition for the two analysis periods: (a) 6 August–13 September 2003, (b) 14 September–22 October 2003. The first two graphs at the top reports the total (F), stomatal (FST) and non-stomatal (Fns) ozone fluxes. The second two graphs in the middle refers to the canopy (Gc), the stomatal (Gst) and the non-stomatal (Gns) bulk conductances to ozone. The last two graphs show the stomatal on total ozone flux ratios (FST/F). The vertical bars indicate the standard error of the means.
Moreover, the non-stomatal fluxes are substantially similar in both periods. The slightly greater total ozone fluxes in the second period are due to the slight increase in stomatal activity, stimulated by greater water availability. Such a high non-stomatal ozone deposition is not surprising in a Mediterranean evergreen forest. It represents a passive defence strategy against high concentrations of photooxidants present during long periods during the Mediterranean summer. The other physiological defence strategy for Mediterranean forest ecosystems is represented by the stomatal resistance increase caused by limited water supply. As a consequence, uptake of gaseous pollutants is limited.
The ozone exposure index AOT40 at the top of the canopy highlights an ozone risk for the forest ecosystem. However, the AOT40 index, if calculated for the zero plane displacement height, is about 50% of the corresponding value for the top of the canopy, and does not exceed the 10,000 ppb h critical level. The two results are ambiguous, and this is because the AOT40 exposure index is sensibly affected by the height of the measurements. Furthermore, the AOT40 values appear as a poor indicator of the ozone dose to vegetation, defined as the time-integrated stomatal ozone flux. In the second period, in fact, although the top canopy ozone exposure was lower than in the first period (4331 ppb h against
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9279 ppb h, respectively) the stomatal dose was higher: 3.624 mmol O3 m2 against 2.798 mmol O3 m2 in the first period (Table 2). This is also the case if the AOT40 index is calculated for the zero plane displacement height. Important errors in risk assessment may thus occur on applying the AOT40 method, in spite of its acceptation by the European Union as a legal basis in air pollution abatement policy making (see EU Directive 03/02/EC). A first comparative look at modeled flux predictions for the Mediterranean area reveals a probable underestimation of ozone fluxes. Emberson et al. (2000) provided a preliminar modeling tool for mapping ozone stomatal fluxes over Europe for wheat and beech. Although our measurements are made on a different plant species, the model predicts for this area a mean stomatal ozone flux for beech ranging from 0.75 to 1 nmol m2 s1 for June, which is three times lower than the mean stomatal flux measured for Holm oak at the same top canopy exposure level (7500–16,000 ppb h). Because Holm oak is a water saving species (Manes et al., 2005) and since the climatic conditions were dry, a significantly lower stomatal uptake in Holm oak than in beech would be expected.
5. Conclusions Ozone flux measurements have been carried out over a Mediterranean evergreen forest for 3 months between August and October. Stomatal flux is a minor part of the total ozone flux over the forest ecosystem. The main part of ozone deposition follows non-stomatal pathways. Chemical sink by NO seems to play a relevant role in the morning non-stomatal deposition. Stomatal uptake is enhanced by water availability but, on the average, it does not exceed the 34.4% of the total ozone flux. Ozone exposure indices AOT40 do not take into account plant physiology, and are thus likely to lead to substantial overestimation of ozone risk, particularly when water supply is limited, as occurs frequently in Southern European and Mediterranean areas. On the contrary, the currently available European model (UBA, 1996) tends to underestimate ozone stomatal fluxes in this area.
Acknowledgements The authors wish to thank Dr. De Michelis, Head of the Castelporziano Estate, and Dr. Tinelli whose assistance was essential in providing logistical coordination and meteorological data, Dr. De Santis of the Montelibretti C.N.R. laboratory for the ozone profile measurements, Dr. Fabi, Dr. Zona and Dr. Marzuoli for their helpful work in field. This research was supported
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by a contract with the Direzione Generale per la Salvaguardia Ambientale—Ministero dell’Ambiente e Tutela del Territorio, and by PRIN (2004057727) Grants.
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