Abstract. 1. Introduction

Transcription

1 Evaluation of air pollution deposition in Venice lagoon T. Tirabassi,* P. Martino,* G. Catenacci,' C. Cavicchioli' "Institute offisbatofc.n.r., via Gobetti 101, Bologna, Italy *Institute oflsiata ofc.n.r., viaarnesano, Lecce, Italy ^CISE, via Reggio Emilia 39, Segrate, Milan, Italy Abstract The work presents the results of a monitoring program designed to estimate dry deposition rates from field observations of the atmospheric concentrations of chemical species and elementary meteorological data near the ground. In particular we have measured wind and air temperature at 5.5 meters over the water level, air temperature, humidity, global and net radiation at 1.8 meters, and water temperature. At the same time, SO 2 concentrations were recorded and the dry deposition of SO 2 from the atmosphere to water in the Venice lagoon was evaluated. 1. Introduction The dry deposition of gases and particulates are processes of great importance that affect both the dynamics of air pollutant concentrations and their mass balances. Moreover it is recognized that the the atmospheric input into the sea has a marked effect on the marine pollution balance. Over recent years, it has been established that fundamental parameters for describing the characteristics of the atmospheric surface layer and the exchange between air and ground can be evaluated using ground level measurements. Moreover, because there is no simple device capable of directly measuring the dry deposition rates of small particles and trace gases directly, current activity has been focused on the use of inferential technique, where measurements of atmospheric concentrations of selected chemical species are coupled with evaluations of appropriate deposition velocity to obtain estimates of dry deposition rate from their product. While dry deposition is a consequence of the same atmospheric exchange mechanisms responsible for the surface fluxes of heat, moisture and momentum, it is also strongly influenced by surface properties. In the study of gas exchange between air and water, the interface

2 78 Air Pollution Engineering and Management between the two phases is considered as a two-layer film system. The idea of such discontinuity close to the interface is unrealistic and the layer thicknesses will vary both spatially and temporally. In spite of such difficulties, predictions based on the film approach often show little or no difference from those derived using more sophisticated models [1]. The purpose of this study is to evaluate the exchange throught the air-sea interface of SO 2 into the Venice lagoon. 2. The experimental set up In order to obtain the data with which to calculate the fluxes of heat, momentum and deposition into the lagoon, two measurement campaigns of one week's duration were performed in 1994: one in July, one in December. The measurements were performed in a part of the Venice lagoon, known as Laguna della Rose. It consists of a circular area of water of approximately 2 km diameter. Meteorological data were collected from the centre of the Laguna delle Rose, where our team erected a pole of six metres height (above the surface height of the water at high tide). At half hourly intervals data were recorded for water temperature and at a height of 1.8 m. for: rain, pressure, humidity, global radiation and net radiation. At a height of 5.5 m, data was collected for wind speed and direction and standard deviation of the wind direction, while air temperature was measured both at 1.8 m and 5.5 m. In addition, again on a halfhourly basis, data was acquired for SO2, NO^., NO, NO2, CO, O,, although only the results relating to SO 2 will be discussed in the present work. 3. Dry deposition! evaluation There is not simple divice capable of directly measuring the dry deposition rates of air pollutants; thus, current activity is focused on the use of an inferential technique. Measurements of air pollutants concentrations (C) are coupled with estimates of appropiate depoition velocity (V^ ), in order to obtain estimates of deposition flux (F) from their product: F = V, C (1) In our work, deposition velocity V^ was calculated using the resistance model approach (Hicks et al. [2], Slinn et al. [3]): V/' = - [In (z/z, ) - i/ (z/l) + C. ( v./d. )*"] + - -L [In (zjz,j + (vjd) } (2) a ku^,

3 Air Pollution Engineering and Management 79 where indices a and w refer to air and water respectively, z is the height, ZQ roughness, u, friction velocity, L the Monin-Obukhov length, v is the cinematic viscosity, D the difrusivity, C an empirical constant, k the von Karman constant (k=0.4) and Y is the integrated diabatic influence function. The first term in Eq.(2) refers to the atmospheric contribution and, in particular, to the aerodynamic and difrusivity boundary resistances. The second term refers to the contribution of the resistance in the water and is very important for those gases which do not form stable compounds in water, for which a concentration equal to zero at the air-water interface cannot be assumed. The factor H/a sets the relative weight of the two terms: for example, for reactive gases, such as SO,, H/a «10~* and therefore deposition is governed by the part in air. The atmospheric stability parameters L and u+ were evaluated using the profile method [4,5,6] on the basis of air temperature, water temperature and dew point temperature of the air. Thus, the flux-profile relations of Dyer and Hicks [7,8,9], for friction velocity, temperature scale (A ) and Monin-Obukhov length, respectively, can be written as follows: u, = k u / [In (z, /z<, ) - ^ (z, /L) + ^ (z, /L)] ( A = k [0(z, ) - 6?(z, ))] / [In (z, / z, ) - p, (z, / L) + p, (z, / L) with: Transactions on Ecology and the Environment vol 6, 1995 WIT Press, ISSN L = pc, Tu//(kgHJ (5) Here, H<> is the surface heatflux,p and C, the air density and specific heat of air, respectively, k is the von Karman constant, u the wind velocity, 0 the potential temperature, z,, z., and z, arbitrary heights in the surface layer, g the acceleration of gravity and T air temperature. ^ functions. For L < 0, they are: ^ = 2 In [1 + x) 12} + In [(1 + x* ) /2] - 2 tg~' (x) + n!2 and ^ are stability p, =21n[(l+x*)/2] (?) where

4 80 Air Pollution Engineering and Management for L > 0: %, =%,, =-5z/L (8) The similarity profiles (3) and (4) are valid typically for zo «z < L [8,10]. For the evaluation of z^ we have used the formula of Charnock [11] where z^ is afunction of wind stress: Zo =*u//g (9) where a is a constant (a = 0.14). Equations (3) and (4) can be solved by iteration with the so-colled profile method [4,5]. 4. Application to the Venice lagoon We have applied the profile method and, subsequently, the equation (2) to the data collected at the centre of the Laguna delle Rose, part of the Venice Lagoon. In this case 0(z%) was the surface temperature of the water. We processed the data adopting the routine FLXSE1 of a software library for the calculation of surface flux over land and sea developped by Beljaars and Holtslag [12,13,14]. Figures 1 and 2 show the evolution of L and u, for the two measurement periods respectively. Using equation (2), solubility variation was evaluated as a function of the temperature fitting empirical data in the literature. The water depth (z^) was evaluated, while the roughness length in water (z^,) was taken to be 1 cm. Friction velocity in water (u*^) was considered to depend exclusively on the atmospheric forcing above the surface (reasonable for the lagoon at low tide) and was thus evaluated by imposing the conservation of the vertical momentum flux across the surface: /, u/ = ^ uj (10) In this way, deposition velocity is correlated to the friction velocity, independent of the characteristic of the gas. Figures 3 and 4 show the evolution in time of the deposition velocity and fluxes for SO, for the two measuremt periods, respectively. In the calculations, the empirical constant C, = 2 [2] was assumed and the second part of equation (2) was neglected since SO, is highly reactive in water. The mean values of deposition velocity and fluxes are 6.6 mm/s and s) for the July data and 3.1 m/s and //g/(m^ s) for those collected

5 Air Pollution Engineering and Management i I i I i I i 0.00 ^r-e ' I ' i ' I ' I ' I ' I ' I r hours Figure 1: Time evolution of 1/L (bold line) and u+. Time origin is midnight July 22, I hours Figure 2: Time evolution of 1/L (bold line) and u*. Time origin is midnight December 14, 1994.

6 82 Air Pollution Engineering and Management 1000 D o O) i 100 -s 10 -g 1 -g hours Figure 3: Time evolution of the SO; flux and deposition velocity (bold line). Deposition velocity values have to be multiplied by 10~*. Time origin is midnight July, g 10 -^ 1 -i d hours Figure 4: Time evolution of the SO2 flux and deposition velocity (bold line). Deposition velocity values have to be multiplied by 10~*. Time origin is midnight December, 1994.

7 Air Pollution Engineering and Management 83 in December. The results are in agreement with the data in the literature [15,16,17] even if highly variable. 5. Conclusions The deposition of material onto the surface and, in particular, into the sea, is of importance as it gradually reduces atmospheric pollutants, while increasing the input of pollutants in the sea. Thus, an investigation was performed to evaluate the amount of air-borne pollutants deposited in the water of the Venice Lagoon for the purpose of estimating the exchange through the air-sea interface of the principal pollutants of the Venice area. To this end, two measurement campaigns (one in winter and one in summer) were carried out to evaluate, using micrometeorological techniques, the seasonal deposition fluxes from the atmosphere to the lagoon. The preliminary results, limited to SO3 and in agreement with the data presented in scientific literature, have been presented and discussed, and are such as to encourage a continuation of the research undertaken. 6. Acknowledgements This work is a contribution to the project "Sistema Lagunare Venezia", sponsored by MURST (Ministero della Ricerca Scientifica e Tecnologica). References 1. Liss PS & Slater P.O. Flux of gas across the air-sea interface. Nature, 1974,247, Hicks B.B., Boaldocchi D.D., Meyers T.P., Hosker R.P. & Matt D.R. A preliminary multiple resistence routine for deriving dry deposition velocities from measured quantities. Water, Air and Soil Poll, 1987, 36, Slinn W.G.N., Hasse L., Hicks B.B., Hogan AW, Lai D, Liss P.S., Munnich K.O:, Sehmel G.A. & Vittori O. Some aspects of the transfer of atmospheric trace constituents past the air-sea interface. Atmos. Environ., 1978, 12, Nieuwstadt F. The computation of the friction velocity u* and the temperature scale T* from temperature and wind velocity profiles by leastsquare methods, Boundary-layer Meteor., 1978, 14, Berkowicz R & Prahm L.P. Evaluation of the profile method for estimation of surface fluxes of momentum and heat, Atmos. Environ., 1982, 16, Trombetti F.,Tagliazucca M., Tampieri F. & Tirabassi T. Evaluation of similarity scales in the stratified surface layer using wind speed and temperature gradient, Atmos. Environ., 1986, 20, Dyer A.J. & Hicks B.B, Flux-gradient relationships in the constant flux layer. Quart. J. Roy. Meteor. Soc., 1970, 29,

8 84 Air Pollution Engineering and Management 8. Dyer A J A review of flux-profile relationships, Boundary-layer M;ffor.,1974, 7, Paulson, C A The mathematical representation of wind speed and temperature profiles in the unstable atmospheric surface layer, J. Appl Meteor., 1970,9, Businger J.C., Wyngaard J.C., Izumi Y. & Bradley E.F. Flux profile relationships in the atmospheric surface layer. J. Atmos. Sci., 1971, 28, Charnock H. Wind stress on the water surface. Quart. J. Roy. Meteor. Soc., 81, Beljaars ACM & Holtslag A. A.M. A software library for the calculation of surface fluxes over land and sea, Environ. Soft., 1990, 5, Holtslag A.A.M. & van Ulden A.P. A simple scheme for daytime estimation of surface fluxes from routine weather data, J. dim. Appl. Meteor., 1983,22, Van Ulden A.P. & Holstlag A.A.M. Estimation of atmospheric boundary layer parameters for diffusion applications, J. dim. Appl. Meteor., 1985, 24, McMahon T.A. & Denison P.J. Empirical atmospheric deposition parameters - A survey, Atmos. Environ., 1978, 13, Garland J.A. Dry and wet removal of sulphur from the atmosphere, Atmos. Environ., 1978, 12, Sehemel G.A. Particle and gas dry deposition: a review, Atmos. Environ., 1980, 14,