Ozone and heat fluxes over a Mediterranean pseudosteppe

Pergamon PI1: S1352-2310(97)00084-S

Almosphcvk Envin,nmcn~ Vol. 31. No. SI, pp. 117 1x4, I997 fcj IVY1 Elscwcr Saencc Ltd All nghts reserved. Prmted an Great Britain 1352 23lO/Y7 %17.M) + O.(W)

OZONE AND HEAT FLUXES OVER A MEDITERRANEAN PSEUDOSTEPPE S. CIESLIK Joint Research (First

received

I5 March

and A. LABATUT Centre,

1996 und in,finul,jin-m

I-21020 Ispra, Italy 28 September

1996. Published

November

1997)

Abstract-Vertical fluxes of turbulent energy and ozone have been measured in the atmospheric surface layer by the eddy correlation method during two campaigns conducted in June 1993 and in May 1994 over low vegetation, in the frame of the Biogenic Emissions in the Mediterranean Area (BEMA) project. The observed daytime Bowen ratios were very high (2 to 4), and the maximum daytime ozone deposition velocities were low ( _ 0.2 ems-‘), due to the dry state of vegetation and limited stomata1 activity. A resistance analysis was carried out to discriminate between the relative contributions from stomata1 and non-stomata1 processes to ozone deposition. This procedure led to the conclusion that an important part of ozone deposition is not due to stomata1 activity, especially when drought is important. Vertical diffusion coefficients were deduced from the measurements for use in concentration gradient observations made by other teams during the campaign in order to determine emission fluxes of biogenic substances. icj 1997 Elsevier Science Ltd.

I, INTRODUCTION

The release of chemical substances into the atmosphere is best quantified by their vertical fluxes into the atmosphere, expressed in amounts of substance per unit area and time. A vertical flux is the resultant of two different processes, the emission itself and its migration through the atmospheric surface layer. The first phenomenon is, in the case of biogenically emitted compounds, controlled by the physiological activity of the emitting plant, and can be studied using enclosure methods, consisting e.g. in enclosures mounted on single branches. Several articles in this issue deal with this kind of experiments which have been carried out during the BEMA campaigns. The second phenomenon depends on purely atmospheric processes, :n particular on turbulence, which affects the lowest alrmospheric layer, called the surface layer, and which is responsible for the vertical transport of the substances. More generally, the vertical transport of any physical scalar quantity (which can be e.g. enthalpy, water vapor content, concentration), is turbulence driven. The present work is mainly devoted to this aspect. The approach based on the investigation of turbulent processes is generally known as the aerodynamic or micrometeorological method, which complements enclosure studies, and gives a more complete picture of the flux process. Three kinds of micrometeorological methods are currently available: eddy correlation (EC), eddy accumulation (EA) and ve.rtical gradient (VG). The EC method is based on the real-time computation of the covariances of the fluctuations of the vertical

wind component and of the scalar whose flux is searched for. It requires the availability of fastresponse sensors. The EA makes use of a fast (similarly to EC) measurement of the vertical wind component, combined with the sampling of the substance in two reservoirs, depending on the sign of the vertical wind component (accumulation). It is dealt with by Valentini et ~1. (this issue). The VG method is based on the measurement, at lower sampling rate, of the vertical concentration gradient of the substance, and on the application of the Fick-like law for turbulent diffusion. It requires the knowledge of the vertical turbulent diffusion coefficient, which can be deduced, through the Monin-Obukhov similarity theory, from the measurement of the sensible heat flux and of the friction velocity, which permit the calculation of the Monin-Obukhov length and subsequently, of the stability function. The VG measurements made during the BEMA campaigns are described in Ciccioli et al. (this issue). This work deals with a series of measurements of sensible and latent heat fluxes, as well as of ozone fluxes, carried out over the Castelporziano pseudosteppe using the EC method. Complementary observations of the net radiative and soil heat flux are also made in order to obtain all the terms of the surface energy balance. The knowledge of both sensible and latent heat fluxes permits also the derivation, through the use of the Penman-Monteith theory of evaporation, of stomata1 resistances, which, in turn, help

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interpreting the process of emission of biogenic substances, since stomata1 activity is generally admitted as playing an important role during emission. The vertical fluxes of ozone can be measured by EC since the development of a chemiluminescent sensor by Giisten et al. (1990). One of the goals of BEMA is the study of the formation and fate of tropospheric ozone as a possible consequence of the presence of biogenic substances in the Mediterranean area; the determination of ozone surface fluxes is thus necessary as they represent the lower boundary condition in the ozone cycle. It is now well established that the ozone molecules present in the lower troposphere undergo a downward flux through the surface layer, as a consequence of a dry deposition process at the surface. This fact was first described by Regener (1957). Rich et al. (1970) studied the removal of ozone from the air by bean leaves under controlled conditions and found that its rate follows a pattern that is very similar to that of evaporation. Using the resistance analogy, they inferred from these observations that both processes are controlled by the stomata1 aperture. Galbally (1971) and Galbally and Roy (1980) measured ozone deposition rates using enclosure techniques on various surface types, both bare and vegetated. Turner et al. (1974) stated that both bare soils and vegetation are targets for ozone deposition. VG techniques have been used by Duyzer et al. (1983), Broder and Gygax (1985), and Colbeck and Harrison (1985), and led to a number of reliable results that confirmed the role of stomata, but not excluding an ozone destruction process on the soil, however. The development of the EC method greatly helped the understanding of ozone deposition (Wesely et al., 1982; Neumann and den Hartog, 1985; Droppo, 1985; Katen and Hubbe, 1985; Delany et al., 1986, Hicks et al., 1989). The ozone sensors used for these measurements were based on the chemiluminescent reaction with NO or UV absorption. The ozone and heat flux measurements presented here have been made over a surface with almost dry vegetation, showing thus little physiological activity, not totally absent, however. We applied a simple approach, based on a resistance analysis and following a method introduced by Massman (1993) in order to estimate the relative contribution of stomata1 activity to ozone deposition.

the real-time computation of the covariances of the fluctuations of the vertical wind component and of the scalar studied, i.e. the the ?? the for ?? ??

temperature T for the sensible heat flux H, specific humidity q for the latent heat flux AE, ozone mixing ratio or molar fraction X(0,) the ozone flux F(0,).

If the primed symbols denote the fluctuations of the corresponding variables, and the overbars refer to averages over successive time intervals, the fluxes are obtained by H = c,pw’T’.

(1)

AE = Qw’q

(2)

F(0,) = pw’x’(Oa).

(3)

Here the averaging time has been set to 15 min. The fluctuations are calculated using the method developed by MC Millen (1988), where they are obtained by subtraction between the actual instantaneous value and a function obtained by passing a numerical resistor-capacity filter through the time series. This method has two advantages: the computation is faster than in the case of the classical method using the arithmetic mean, and it eliminates (in part) lower frequencies due to horizontal inhomogeneities. The results are close to those of the classical method, however, for homogeneous conditions. Once the fluxes are determined, a further step can be made in order to obtain more information on the mechanisms by which fluxes are generated at the surface. Processes related to stomata1 activity are generally considered as playing an important role in controlling various fluxes. A powerful investigation tool in the study of these phenomena is resistance analysis, based on an analogy between the transfer of a scalar quantity through the surface layer and an electric circuit. In this concept, which was first introduced by Chamberlain and Chadwick (1953), three resistances against the deposition of a given substance are defined: the aerodynamic resistance r,, the laminar sublayer resistance rb, and the surface resistante r,. They have the dimensions a reciprocal velocity (s m - ‘) and are related to the deposition velocity ud by

1 ad = (?, + rb + r,)

(4)

2. METHODOLOGY The mathematical formulation of vertical fluxes through the atmospheric surface layer can be found in any textbook on micrometeorology (e.g. Poggi, 1977; Stull, 1988). Here, we just remind the main features of this formalism insofar they are directly related to the measurements presented. Sensible and latent heat fluxes, as well as ozone fluxes, are obtained using the EC method, based on

where the deposition ozone) by vd(o3)

=

velocity is defined (e.g. for -

F(O,)/X(O,).

(5)

The aerodynamic resistance is related to purely atmospheric processes; its value is independent from ;he nature of the transported scalar, expressing the Bet that turbulence does not “see” what kind of scalar it is transporting. The aerodynamic resistance may be

Ozone and heat fluxes over a Mediterranean pseudosteppe

or emitted. It cannot be measured directly in the natural environment, so it must be estimated from theory. Here was a parameterization proposed by Hicks et al. (1977) was used:

calculated by the expression: rrr=

2X dz S 10+d KH(Z)

In this expression, KH(z) is the vertical turbulent diffusion coefficient for scalars, z is the height, Z~ is the height of measurement, z,, is the roughness length, and d is the displacement height. The value of K”(z) can be calculated by the classical Monin-Obukhov relation

rb=-

d)/L]

where k is the von Karman constant (= 0.4), u* is the friction velocity, obtained from the measurement, @ is the Monin-Obukhov similarity function and L is the Monin-Obukhov length, also obtained from the measurements. The form of the function @ is taken from Dyer and Hicks (1970). The laminar sublayer resistance depends on the molecular propert:les of the substance being deposited

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where SC and Pr are the Schmidt and Prandtl numbers, respectively. Other parameterizations (see e.g. Jensen and Hummelshoj, 1995), which give quite similar results for low vegetation. Knowing ud from the measurement and the resistances Y, and rb as they are calculated by equations (6) and (7), the surface resistance against ozone deposition r, is readily determined. But the later quantity expresses the bulk capacity of the surface to act as a sink for ozone, including all kinds of sink processes, thus not discriminating between stomata1 and nonstomata1 ones. Formally, the discrimination can be made by stating that r, is a combination of resistances mounted in parallel, the first one, rNs being due to all

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Fig. 1. Terms of the surface energy balance as functions of time, observed at the Castelporziano pseudosteppe, at a lneight of 8 m a.g.l., from 3 June to 10 June 1993 (a), and from 11 May to 18 May 1994 (b). Bold solid lines: sensible heat fluxes. Circles: latent heat fluxes. Thin solid lines: net radiative flux (upper curves) and soil heat flux (lower curve, only for 1993).

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non-stomata1 processes, and the second rsT being due to diffusion through the stomata. This leads to the equation 1 _=‘+&. (9) r, rNS Here a further step can be made by means of a study of evaporation. For a dry soil, water vapor evaporated from a vegetated surface is released to air through the stomata by molecular diffusion. If we admit that the part of ozone deposition due to uptake by the stomata is also controlled by molecular diffusion through the stomata1 cavity, then the ratio between the partial surface resistance for ozone due to stomata1 diffusion (rST) and the resistance against evaporation (denoted by rs) is equal to the ratio between the molecular diffusion coefficients for ozone (Dc3) and water vapor (DH,O), respectively, through air. This leads to rsr -= rs

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Thus, if we can determine rs, discrimination between stomata1 and non-stomata1 processes becomes possible by applying equations (4) and (9). By measuring evaporation or, more precisely, latent heat fluxes,

it is possible to obtain the resistance against water vapor by applying the Penman-Monteith theory (see e.g. Monteith, 1981), where the sensible and latent heat fluxes are related to the resistances by the following equations. j-G)y(l

+&)-2

(1I)

sfy(l +$j (R, - G)s + 2

(12) “E=s+y(l+--& Here, R, and G are the net radiative surface flux and the soil heat flux, respectively; cp and p are the heat capacity and density of air, respectively, 6q, is the water vapor saturation deficit, and s is the derivative of the water vapor saturation function against temperature, y is the psychrometric constant. The measurement of latent and sensible heat fluxes is thus essential, since it gives a way of investigating the importance of stomata1 processes, by permitting the calculation of stomata1 resistances, supposed to be

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Ozone and heat fluxes over a Mediterranean pseudosteppe

time of 10 Hz. Mean values of the various variables were calculated after each 15 min averaging period, as well as their standard deviations and the covariance matrix. Complementary observations of the surface net radiative flux were made with a Schenk radiometer; four PtlOO-type temperature probes were used to record the soil temperature at four depths (1, 10,30, and 60 cm), then converted into soil heat fluxes.

equal to the resistances against evaporation; from stomata1 resistances it is possible to determine the relative contributions of the stomata1 and nonstomata1 processes to dry deposition of ozone.

3. SITE AND INSTRUMENTATION

The measuring :site, located about 2 km from the seashore, inside the Castelporziano estate, was a flat area covered with low vegetation including asphodelus, mentha as well as various gramineae species, forming an ecosystem usually called “Mediterranean pseudosteppe”. The micrometeorological measuring system consisted of a Kaijo-Denki Mod. Dat-300 3-D ultrasonic anemometer-thermometer which recorded the three components of the wind vector and the temperature, coupled with a GFAS OSG-2 fast-response ozone analyzer. A Campbell Scientific KH20 krypton hygrometer was used as fast-response moisture sensor. These instruments were mounted on a tower at 8 m a.g.1. An additional sonic anemometer was working at 2 m a.g.1. The data were recorded at a sampling

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4. OBSERVATIONAL

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RESULTS AND DISCUSSION

The shape of the variations of the sensible and latent heat fluxes showed a classical behavior governed by incoming radiation during the two campaigns. The hourly averaged fluxes are shown in Fig. 1, together with the net radiative flux and the soil heat flux (when available), calculated from vertical temperature gradients measured between the surface and a depth of 60 cm; a thermal diffusivity coefficient of 2 m2 s-r was used. The mean daytime Bowen ratio (/I = HIRE) was very high for June 1993, with a value of about 3.8 (calculated including the hours from 8 a.m. to 6 p.m.),

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Daytime ozone deposition velocities as measured at the Castelporziano pseudosteppe (at 8 m a.g.1.) from 3 June to 10 June 1993 (a), and from 11 May to 18 May 1994 (b).

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Time (hours, UTC+2) Fig. 4. Vertical diffusion coefficients at 8 m a.g.1.(bold line) and at 2 m a.g.1. (thin line) over the Castelporziano pseudosteppe from 3 June to 10 June 1993 (a) and from 11 May to 18 May 1994 (b).

due to the drought of both soil and vegetation. In May 1994, the campaign took place earlier in the year, when a significant part of the vegetation was still active; rainfall also occurred during the campaign. As a consequence, the mean Bowen ratio was about 1.0. The energy balance equation proved to be satisfied to a good degree of approximation. A scatter diagram (Fig. 2) shows the sum of nonturbulent terms (R, - G) against that of the turbulent ones (H + LE), for the 1993 campaign. Figure 3 shows the values of the vertical eddy diffusion coefficients (hourly averages) obtained by using equation (7) for the two campaigns, for the two measurement heights of 2 and 8 m. They were used by Ciccioli et al. (this issue) for the calculation of isoprene emission fluxes by the VG (gradient) method. The deposition velocities of ozone show a characteristic daily cycle, with a maximum at noon (Fig. 4). Nighttime values are not trustworthy and thus not reported, since the prevailing conditions are highly non-stationary with weak winds and stable stratifica-

tion. As expected, the daytime values are lower in June 1993 than in May 1994, in relation to the state of vegetation. A resistance analysis, based on equations (4)-(9), was made in order to discriminate which part of ozone deposition was due to stomata1 processes. Figure 5 shows the total surface conductances (equal to the reciprocal resistances) and the stomata1 conductances for ozone deposition. For June 1993, the total conductances were low ( N 0.3 cm s)- ’ and the stomata1 conductances accounted for about 20 to 30% of the total, indicating that the stomata1 contribution was very reduced, due to the drought. In May 1994, however, both total and stomata1 conductances were higher. The total conductances were in the range of 0.4 ems-’ and the stomata1 conductances accounted for about 60% of the total. This indicates that, if stomata1 activity accounts for a part of ozone deposition, its relative importance strongly depends on the state of vegetation. In all cases processes other than the stomata1 ones contribute to ozone deposition. A direct destruction of ozone by contact with the surface elements may occur.

Ozone and heat fluxes over a Mediterranean pseudosteppe

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Fig. 5. Total surface conductances for ozone deposition (bold solid line), and stomata1 conductances for ozone exchange (thin solid line); the circles represent the ratios between the latent heat fluxes and the sum of the latent and the sensible heat fluxes. Frame (a) refers to the June 1993 campaign; frame (b) refers to the May 1994 campaign.

5. CONCLUSIONS

The observations of heat fluxes presented in this work reflect the state of vegetation during the two Castelporziano campaigns, showing reduced, but not vanishing physiological activity. These characteristics are reflected by the high daytime values of the Bowen ratio observed. The results were used to derive the stability parameters and functions which lead to the determination of the vertical diffusion coefficients, which serves as an input for VOC flux measurements by the gradient method. The observation of ozone deposition provide a lower boundar,y condition for the study of the ozone budget. The low values of the deposition velocities are related to the state of vegetation, characterized by a reduced stomata1 activity due to the drought; this is confirmed by the study of the surface energy balance and the subsequent resistance analysis, which permitted to evaluate the relative contribution of stomata1 activity to ozone deposition; it is shown that an important contribution arises from other (non-stomatal) processes. The contribution from stomata1 activity was found to be lower in June 1993 than in May 1994. This is in agreement with the fact that vegetation is more active in May than in June. Ozone deposition seems thus to be a resultant of several processes,, one of which being diffusion through stomata. But an important part seems to occur through other phenomena, especially in case of drought. Little is known about the latter; destruction of ozone by direct contact with the surface elements, similar to a wall effect, is likely to take place.

REFERENCES

Broder, B. and Gygax, H. A. (1985) The influence of locally induced wind systems on the effectiveness of nocturnal dry deposition of ozone. Atmospheric Environment 19, 1627.

Chamberlain, A. C. and Chadwick, R. C. (1953) Deposition of airborne radioiodine vapor. Nucleon& 11, 22. Ciccioli, P., Fabozzi, C., Cecinato, A., Brancaleoni, E. and Frattoni, M. (1997) The BEMA-project: Biogenic emission from the Mediterranean pseudosteppe ecosystem present in Castelporziano. Atmospheric Environment (this issue). Colbeck, I. and Harrison (1985) Dry deposition of ozone: some measurements of deposition velocity and of vertical profiles up to 100 metres. Atmospheric Environment 19, 1807.

Delany, A. C., Fitzjarrald, D. R., Lenschow, D. H., Pearson Jr., R., Wendel, G. J. and Woodruff, B. (1986) Direct measurement of nitrogen and ozone fluxes over grassland. J. atmos. Chem. 4, 429. Droppo, J. G. (1985) Concurrent measurements of ozone dry deposition using eddy-correlation and profile flux methods. J. geophys. Res. 90, 2111. Duyzer, J. H., Meyer, G. M. and Van Aalst, R. M. (1983) Measurements of dry deposition velocities of NO, NO, and 0, and the influence of chemical reactions. Atmospheric Environment 17,2117.

Galbally, I. E. (1971) Ozone profiles and ozone fluxes in the atmospheric surface layer. Quart. J. R. Met. Sot. 97, 18. Galbally, I. E. and Roy, C. R. (1980) Destruction of ozone at the earth’s surface. Q. JI R. Met.. Sot. 106, 599. Giisten, H., Heinrich, G., Schmidt, R. W. H. and Schurath, U. (1992) A novel ozone sensor for direct flux measurements. In Proc. Ozone Symp. Charlotteville. Hicks. B. B.. Matt. D. R. and McMillen. R. T. (1989) A micrometeorological investigation of surface exchange of O,, SO, and NO,: a case study. Boundary-Layer Met. 47, 321.

Massman, W. J. (1993) Partitioning fluxes to sparse grass and soil and the inferred resistances to dry deposition. Atmospheric Environment 2lA, 167.

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McMillen, R. T. T. (1989) An eddy-correlation technique with a tended applicability to non-simple terrain (1993). Boundar),-Layer Met. 43, 23 1. Monteith, J. L. (1981) Evaporation and surface temperature. Q. J/ R. Met. Sot. 107, 1. Neumann, N. H. and Den Hartog, G. (1985) Eddy-correlation of atmospheric fluxes of ozone, sulfur, and particulates during the Champaign intercomparison study. J. yeophys. Res. 09, 2097. Poggi, A. (1977) Introduction u IN micrometeorologie. Masson, Paris. Regener, V. H. (1957) The vertical flux of ozone. J. yeophJs. Res. 62, 221. Rich, S. E., Waggoner, P. E. and Tomhnson, H. (1957) The vertical flux of ozone. J. qeophys. Res. 62, 221.

Stull, R. B. (1988) An Introduction to Boundary Layer Meteorlogy. Kluwer, Dordrecht. Turner, N. C., Rich, S. and Waggoner, P. S. (1973) Removal of ozone by soil. J. Envir. Qua/. 2, 259. Valentini, R., Greco, S., Seufert, G., Bertin, N., Ciccioli, P., Cecinato, A., Brancaleoni, E. and Frattoni, M. (1997) Fluxes of biogenic VOCs from Mediterranean vegetation by trap enrichment relaxed eddy accumulation. Atmospheric Encironment (this issue). Wesely, M. L., Eastman, J. A., Stedman, D. H. and Yalvac, E. D. (1982) An eddy-correlation measurement of NO, flux to vegetation and comparison to 0, flux. Atmospheric Encironment 16, 815.