Transactions on Ecology and the Environment vol 6, 1995 WIT Press, ISSN

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1 A deposition experiment of SO^ O^ and NH^ in a suburban area R. San Jose, J. Moreno, M. Angeles San Feliu, B. Moreno Group of Environmental Software and Modelling, Computer Science School, Technical University of Madrid, Montegancedo Campus, Boadilla del Monte , Madrid, Spain Abstract Deposition processes are a very important part of the atmospheric dispersion models. They can be defined as the removal from the atmosphere of the different chemical compounds which are deposited over the different surfaces which are found in a complex environment such us urban centers and surroundings. Mesoscale Transport Models use the deposition modules to remove pollutants from the atmosphere and the quality and importance of the deposition parameterization processes is out of question. The classical approach of deposition processes into the dispersion models has been based on the so-called "big-leaf' approach which is based on a former version of the resistance technique introduced in the 1970's. This is based on the accurate parameterization of the resistances, particularly on the canopy resistance. The characterization of the different land-use types and the quantification of the different parameters for the different chemical compounds is based on field experiments carried out with extreme high sophisticated instrumentation usually based on the eddy-correlation technique. In this contribution, we present first preliminary results on a field deposition experiment based on the eddy-correlation and modified Bowen-ratio technique for ozone (eddy-correlation) and 502 and ammonia (Bowen-ratio). The location of thefieldexperiment is a Military Airport (Cuatro Vientos) located 10 km to the West of the Madrid City center. First results show a good agreement obtained with those carried out in Northern parts of Europe however several up draft fluxes and high deposition velocity values are found because of the dry environmental and surface conditions which are typical for the Southern parts of Europe.

2 102 Air Pollution Engineering and Management Introduction Direct measurements of dry deposition rates using micrometeorological methods have advanced the knowledge of dry deposition processes (Hicks et al.*). However routine implementation of these methods for year-long monitoring of dry deposition rates is difficult and not many references exist in the scientific literature about it. We present, in this contribution, a long-period of measurements of SOi, 03 and NH$ under the basis of routine dry deposition monitoring. The methodology is already in use in different parts of Europe and U.S.A. and it is called the dry deposition inferential measurement (DDIM) technique. In this approach, measured concentrations of pollutants (c) are combined with estimates of the appropriate deposition velocity (v</) for each pollutant such that flux = CtVj. The deposition velocities are estimated by a model using measured meteorological data and the surface characteristics of the site (Hicks et al.\ Wesely^ and Erisman^). Uncertainties in predicting hourly deposition velocities are assessed by comparing model estimates with measured deposition velocities from intensive field experiments (Meyer et al.\ Button et al.^). If hourly estimates of Vfi are combined with hourly measurements of the pollutant concentration, then the weekly integrated flux is the following: 168 F«,edb = 5>rf(0c(0 (1) n=l However, hourly measurement of c for many chemical species requires the implementation of complex chemical analyser that require frequent maintenance, calibration and climate-controlled enclosures. However, this approach is useful if the deposition velocity model has been tested by using complex micrometeorological instrumentation. Eddy-correlation techniques are able to be use on a very few pollutants. We have used this method for the ozone value and the modified Bowen-ratio method for the 502 and NH$. This method allows a very accurate computation of the pollutant fluxes deposited over the soil. In the case of Ammonia, the emission/deposition simultaneous process requires the inclusion of the concept of the "canopy compensation point" which changes slightly the value of the resistances to be applied to the "classical" resistances approach. The objective of this contribution is to show some results to compare the deposition velocities modelled with those measured directly by fast/slow chemical analyzers. The models used take into account several characteristics of the surface and turbulent conditions of the area, such as the micrometeorological turbulent characterization of the surface boundary layer, the meteorological variables and the land use type characterization.

3 Air Pollution Engineering and Management 103 Experimental site and instrumentation The field experiment was conducted at the "Cuatro Vientos" Military Airport of the Madrid Area. The place is located at the west side of the Madrid Urban City center and it belongs to the Madrid Metropolitan Area. The area can be considered highly polluted because of the influence of the city pollution. On the other hand, big open areas are present on the west side of the place which allows some influence of the agricultural areas in the Madrid domain. Figure 1 shows the location of the Military Airport where thefieldexperiment was set-up. The weather conditions at these locations are typical continental which is characterized by dry and hot summer periods and cold winter periods. Rain periods are mainly on October-November and April-May along the year. Fig. 1 Map of the location of "Cuatro Vientos" Military Airport in Spain where thefieldexperiment was conducted. The 03 dry deposition fluxes were evaluated by using eddy correlation method. This method requires fast response and sensitive instrumentation to monitor the small rapid fluctuations of the species of interest. The components of wind velocity were measured at 5 m using a 3-axis sonic anemometer (Solent research).

4 104 Air Pollution Engineering and Management The 03 fluctuations were measured with fast response chemiluminescence sensor (GFA Ozone sonde OS-G-2) which was installed in the same mast, sampling air trough a Teflon tube as close as possible of the sonic anemometer. The response time of de instrument was 0.1 s. This sensor was used together with an absolute ozone reference monitor (Ultra Violet detector 49 PS Thermo Environmental Ins. Inc.) in order to recalibrate the fast sensor every 15 min., the instrument was placed near the mast sampling air through a 6 m teflon tube. The response time of the instrument is 30 s. The system is able to obtain the absolute value of 03 concentration and its turbulentfluctuation.502 concentrations were sampled by a chemical sensor (Pulsed fluorescent detector 43S Thermo Environmental Ins. Inc.) which was installed near the mast to evaluate 502 concentration at two different levels 5 m and 1 m. The air sampling were conducted through two teflon tubes connected to a pump system placed hi a metallic box near the mast which was controlled by portable computer; the sampling levels were alternated every 2 minutes. The response time of the instrument was 30 s. A system calibration was performed automatically every 24 hours taken air zero from a column coated of active carbon, the value obtained in this process is used to recalibrate the absolute value of 502 concentration during the next period of 24 hours by software. The sonic anemometer was used to parameterize the turbulent surface layer. Slow meteorological sensors were used in order to obtain a complete information of surface boundary layer. A relative humidity and absolute temperature (Rotronic MP300, Campbell Ins.) sensor was placed 3 m in height at the same mast. Absolute temperature was evaluated by using a thermocouple sensor and relative humidity as a function of the conductivity in a polymeric material. His response time was 25 s. Moreover a global solar radiation (Solarimeter Casella), was placed at the top of a near small mast, 2 m in height, his response time was 15 s. Complete instrumentation was controlled by a notebook personal computer where a complete software for data acquisition was implemented. This control unit was designed and developed by ECN (The Netherlands Energy Research Foundation). All instrumentation was calibrated in a previous period following the E.P.A standard. The system has been designed for routine work and maintenance is kept as low as possible. The GFA Sonde had to be changed every seven days and the computer storage capacity limits data acquisition for longer periods. Ammonia concentrations were determined by using a continuous wet annular denuder system described by Slanina et ala This system was located in a separated scaffold where measurements were made at three heights, 0.45 m, 1.25 m and 3.3 m. A denuder tube was located in every level, and only one detector was used to reduce possible bias between denuder s. The denuders sample air at 30 /ram~*. They are rotating continuously and supplied with an acidic stripping solution (HSO^Na, which the collected NH^ was brought (as NHf in the stripping solution) to a common detector for analysis. Analysis of the NHf in this system is by membrane diffusion of NH^ at high ph into a counter flow of dionized water, with subsequent measurement by conductivity. His response time

5 Air Pollution Engineering and Management 105 is 2 min. per level, which 90 s. were used to stabilized the signal and only the last 30 s. were used to perform the measurement. In order to obtain an accurate determination of NH^ concentration this system needs a periodic calibration. It includes calibration of the detector (through three standard calibration solution, 0 ppb, 50 ppb and 500 ppb) and a correct evaluation of the gas flow in every denuder. The system was controlled by a data logger which was able to store values of conductivity, liquid flows and temperature into the detector for period no longer than 20 days. Values of NH^ concentration will be obtained with an external program trough conductivity values as a function of the calibration parameters. Background formulation Fluxes were evaluated by eddy-correlation and Modified Bo wen ratio technique. The eddy correlation technique allows us to measure directly the momentum, sensible heat and ozonefluxes.the flux, F,, of a quantity, a, can be evaluated by measuring the covariances of the product of the vertical wind speed fluctuations and the species concentration fluctuations. The deposition velocity is directly inferred from: w*j = vjc, (2) The indirect method to evaluate fluxes over the canopy is the modified Bo wen ratio technique which is based on the similarity between fluxes ratios and gradient ratios in the following way: ^ - - (3) Fe ~ 60 <3) where FC is the flux of the species, Ftheta is the sensible heat flux and 60 is the temperature difference between the heights Zi and z\. This difference is evaluated by using where 60 is the temperature difference between two heights, L is the Monin- Obukhov length, 0* is the scale temperature of the surface boundary layer. The 03 deposition velocities were determined directly by using the eddy- correlation technique. The SO^ and ammonia deposition velocities where determined by the modified Bowen ratio technique. Additionally, this deposition velocities where parameterized by using the Wesely^, Erisman^ and results from the CIBA experiment in Spain, San Jose et alj. All these parameterizations focus on the generalization of the deposition fluxes over large areas based on meteorological variables, characteristics of the canopy and land-use classification.

6 1 06 Air Pollution Engineering and Management Results and discussion Fig. 2 shows 502 the resistances r«(aerodynamic), r& (bulk resistance) and canopy resistance for 502- Also the deposition velocity ranges between -0.2 ems'* to 1.2 cms_i. We can seen results corresponding to the gradient and 5 concentrations at 5 m in height SJ C.O 326J 32* : S3 32*.0 326J 32* K J S.O 32*J 32*3 32* J s J 32* J 32*3 m J J S3 ' 'siij ' 32S3 Julian day Fig concentrations, deposition velocities,fluxesand resistances measured by the gradient technique at "Cuatro Vientos" Military Airport in Madrid (Spain). Fig. 3 show results from NH^ concentrations at two levels, 0.45 m and 3.3 m. for the Julian days, Fig. 4 show a comparison between deposition velocities for the two models tested. Results show a higher sensitivity for San Josefs parameterization based on the results of the CffiA experiment in Valladolid (Spain) in 1991 than for Wesely's model.

7 Air Pollution Engineering and Management :00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 L.S.T, Julian days 39, 40 and 41 (1995) Fig. 3 NHz concentrations at two levels at "Cuatro Vientos" Military Airport in Madrid (Spain). Fig. 5 shows a comparison for 502 deposition velocities for the three models tested. In this case, Erisman's model seems to be very sensitivity to concentration fluctuations and San Jose's parameterization seems to match the results in an acceptable agreement. Wesely's parameterization seems to model the results very smoothly and in general underpredicting the deposition velocities specially when high values are found by the Modified Bo wen ratio technique. Eddy correlation ooooo Wesely Modei(L-2) AAAAA CIBA Experiment 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 L.S.T, Julian days 39, 40 and 41 (1995) Fig. 4 Deposition velocities of 0$ measured by eddy-correlations and modelled by Wesely's model and San Jose's model (based on data obtained at CIBA experiment).

8 108 Air Pollution Engineering and Management ooooo Wesely M.B.R. AAAAA Erismon Model(L-2) Model ooooo CIBA Experiment 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:OO 18:00 00:00 L.S.T, Julian days 39, 40 and 41 (1995) Fig. 5 Deposition velocities of 502 measured (indirect method based on the modified Bo wen ratio) and modelled by Wesely's, Erisman's and San Jose's parameterizations. Acknowledgements We would like to thank to Mr. A. T. Venneulen. from the ECN (The Netherlands) for providing the software to data gathering during the field experiment. The Military authorities of the "Cuatro Vientos" Military Airport for the help provided to install the equipment at the Airport. This work has been done under the contracts: EV5V-CT and EV5V-CT from the European Union. References 1.- Hicks B.B., Wesely M.L. Durham J.L. and Brown M.A. "Some direct measurements of atmospheric sulfur fluxes over a pine plantation". Atmospheric Environment, 16, (1982). 2.- Hicks B.B., Baldocchi D.D., Meyers T.P., Hosker R.P. Jr. and Matt D.R. "A preliminary multiple resistance routine for deriving dry deposition velocities from measured quantities". Wat. Air. Soil PoUut. 36, (1987). 3.- Wesely M.L. "Parameterization of surface resistances to gaseous dry deposition in a regional-scale numerical models". Atmospheric Environment, 23, 6, pp (1989). 4.- Erisman J.W. "Evaluation of a surface resistance parameterization of sulphur dioxide". Atmospheric Environment, 28, 16, pp , (1994).

9 Air Pollution Engineering and Management Meyer T.P. and Yuen T.S. "An assessment of averaging strategies associated with day/night sampling of dry depositionfluxesof 502 and Og", J. Geophys. Res. 92, pp , (1987). 6.- Sutton M.A., Fowler D., Hargreaves K.J. and Storeton-West R.L. "Interactions of JVf/3 and SO?, exchange inferred from simultaneous flux measurements over a wheat canopy", Air Pollution Research Report 47, Ed. J. Slanina, G. Angeletti and S. Beilke, (1993). 7.- San Jose R, Bilbao, J., de Miguel A., Cachorro V.E., Viloria R.E., Tricio V., Garcia A., Cancillo, M.L., Casado H., Encinas, D. and Palacios, P. "An experimental and modelling study of 502 and Og dry deposition over Castilla-Leon (Spain)", Air Pollution Research Report 47, Ed. J Slanina, G. Angeletti and S. Beilke, , (1993). 8.- Slanina J., de Wild J. and Wyers G.P. "The application of Denuders systems to the analysis of atmospheric components", Gaseous Pollutants: Characterization and cycling, Ed. J.O. Nriagu, John Wiley and Sons, ISBN (1992).