Abstract. Introduction

Transcription

1 Ozone depletion in a semiurban site under the inversion layer in Tenerife, Canary Islands J.C. Guerra, J.P. Diaz, A. Diaz, F. Exposito Physical Atmosphere and Environmental Group, Department of Physics, University ofla Laguna, La Laguna, S/C Tenerife, Canary Islands, Spain Abstract We report thefirstsimultaneous measurements of Og and NOx under the inversion layer in the Canary Islands, where the local pollution affects the level of ozone and parents substances. The composition of the mixture in this layer is characterized by a lower ozone concentration than the background level, due to the destruction process from precursor constituents. On average, this destruction process can produce a sink of Og greater than 6 ppbv in the daily average ozone concentration. This ozone drop has a seasonal variation, which is attributed to the different strengths of the photochemical mechanism of Og production and destruction can have during the year. This last hypothesis is according to the calculated correlation with NOx, which is responsible for this ozone depression. Introduction The only station in the North Atlantic Region where trace gases are measured in background conditions, is on the island of Tenerife, in the Canary Islands (28 2' N, 16 3' W), at 2367 meters above sea level. Different air masses arrive at this station, some from the relatively clean air from the North Atlantic and some carrying polluted air from Europe [1]. In Tenerife a strong trade wind inversion layer is usually present below 1800 meters a.s.l. (Fig.l), resulting in two very distinct air masses. The lower one is cooler and moist, while the upper one is warmer and drier. This subsidence

2 548 Air Pollution Monitoring, Simulation and Control inversion layer prevents the arrival at IZO (Izana Observatory) of polluted air masses from the lower layer because the height of the inversion top is normally below the Station altitude. However, polluted air masses generated in the lower layer can reach IZO under some special conditions. This situation is produced when the inversion layer is broken, when its top is closed to the surface (i.e. invasion of Saharan dust, etc.), or when inversion is over the station. Therefore, it is necessary to determine the pollution level under the inversion layer. The problem at stake is to investigate the main sources of anthropogenic emissions and determine the main photochemical characteristics of the air in this layer. In this way we can know the characteristics of the mixture of both the lower and upper layers together Teide Peak (3718 m) SIC Tenerife (a.s.l) Figure 1: Vertical scaled view of Tenerife Island and location of monitoring sites. We have taken the surface O% and related components to study the atmosphere under the thermal inversion. This component is very sensitive to change in the atmospheric composition, and takes part in the more important photochemical process, both in polluted air and in the free troposphere. Tropospheric O% is also a precursor of reactive radical such as OH, which participates in the removal of many chemical species [2]. Both in the urban atmosphere and in rural zones, the main gases that can have influence over the ozone concentration are NOx and VOCs, according to a scheme of reaction which have not been fully identified. Instruments and Site Description The concentration of tropospheric trace gases used in this paper were measured in a small town, La Laguna, situated under the inversion layer, at an altitude of

3 Air Pollution Monitoring, Simulation and Control 549 approximately 550 meter at sea level. La Laguna is located on a valley opened northward (Valle de Aguere). So, "clean" trade winds arrive directly to LLO (La Laguna Observatory). Nevertheless, local pollution must be noted, because the station is placed on a building roof, close to a highway with heavy traffic at peak hours (principally at about 08 and 20 local official time on working days). In the La Laguna university station a UV spectrophotometer DASIBI #1108 has been used since November This instrument was calibrated against the USA-NBS by S. Oltmans [S. J Oltmans, private communication]. Periodically we compare and calibrate our spectrophotometer with the instruments at Izana [3] whose calibration is tied to the U.S. National Institute of Standards Technology reference Og photometer. This analyzer has been completely automated [4] by using the RS-232C interface incorporated in the Dasibi #1108. The automation routine is composed of an autocalibration sequence, daily check of the instrument response (Zero and Span) and storage of averages calculated in magnetic support. Result and Discussion The Og and NOx concentrations measured at LLO, show the direct influence of local pollution sources. The main source of pollution affecting the ozone in the lower layer in Tenerife is due to vehicular traffic. Other possible sources are less important because of the limited number of industries in Tenerife. However, the high traffic density on this island, can release large amount of precursors elements that directly influence the ozone. Some recent studies give values of 55% for VOCs and a 31% for NOx traffic emissions [5] in this zone. The influence of the traffic emissions over the Og in La Laguna appear clearly in the daily variation. Standard diurnal variation, that is to say the mean monthly values for each hour of the day, were computed for weekdays and weekends, due to the different behavior of the primary components sources. The results show minimums at 08 hours during the weekdays and everyday at 20 hours (Fig.2). This behavior is closely related to road traffic density. Our study found that the typical daily pattern of road traffic and NOx for weekdays has two important maximums at 8 and 20 local time. Also we can see that the daily pattern is different on the weekends, which is characterized by only a broad maximum at 20 local time [6], Other evidence about this minimum origin, appear clearly in the representations (Fig.2b). We can see that the bottom of the valley (specifically the first minimum) is not fixed at the same time. We observed that during some months (April-September) the minimum is an hour before with respect to the rest of the

4 550 Air Pollution Monitoring, Simulation and Control year. This is only a result due to the change in local time (Daylight Savings Time) that occurs every year in Spain, which delays the peak density of traffic Time (U.T.C.) Time (U.T.C.) Figure 2: Isolines representing the standard diurnal variation of concentrations of ozone in ppbv during a) Weekends, b) Weekdays. Between this low ozone concentration appear two maximums. Thefirstis at 4 to 5 o'clock in the morning and the second is around noon. The first maximum is related to very low concentration of ozone precursor, which happens when the density of road traffic is negligible. At this time we can suppose that the conditions are very close to background conditions (cuasi-background

5 Air Pollution Monitoring, Simulation and Control 551 conditions). This last hypothesis is supported by comparison with ozone sounding data [7]. After the first minimum, the increment of ozone concentration can be attributed to photochemical formation and the transport from the higher layer, when the boundary layer is raised. The second maximum is always lower than the nocturnal maximum, which shows little importance of the local production of ozone. Similar behavior can be observed at other points under the inversion layer. The global results point to the existence of the lower sink of ozone in Tenerife. We have made the calculation of the magnitude of this sink from LLO. For this study, we have compared the difference between the maximum nocturnal (cuasi-background conditions) and the daily average, on the calculated daily variations. The results are shown in Fig.3. We can see in this picture that the pollution produces a daily ozone average that is lower than the background condition during whole the year. However there is a seasonal variation in this sink. e o a 7 i I I I _ I I T g- f E. CQ O z o a 9 Figure 3: Differences between the maximum nocturnal (Background conditions) and the daily average during This seasonal variation shows the high values of this sink during the winter time. On the other hand, during the summer months (July to September) the value of the sink shrinks. This behavior is due to the variation of the interaction of solar radiation with the atmosphere between winter and summer. In winter the photochemical reaction that can produce ozone is unfavourable, due to the decrease ofu.v. flux. Also, the low radiation level at this time limits the vertical dispersion of pollutants, allowing an augmentation in the NOx concentration. The destruction process in this form is reinforced. During the summer time, the situation presents some differences. The convective motions allow the vertical transport of air during the day, which is

6 552 Air Pollution Monitoring, Simulation and Control reflected in the low NOx concentration at noon. Also, the increment in the u.v. flux reinforces the photochemical formation of Og All these circumstances produce a displacement of the balance and reduce the observed surface sink. These seasonal variations of the chemical mechanisms of destruction and production of ozone is in agreement with additional calculations. We have made simultaneous measurements of NOx, which is mainly responsible for the ozone sink. We have found that the standard diurnal variation of these precursor components is anticorrelated with standard diurnal variation of ozone. This observed anticorrelation presents an important seasonal variation. The correlation coefficients had been calculated for each month during 1994 and is represented in the Fig.4. We can see in this picture the good anticorrelation in winter time, when the photochemical ozone production is negligible. In summer time the anticorrelation vanishes and in some days we can find good correlation in daily pattern of NOx and ozone , -O.I -O.8-1- O3v.s.NO m03vs N02 m ' m ' ^i " m : ^i ^ m III 1111 i I 111 LMJ I. I AI i I 11 I 11 I: I 11 ~ i I i I i I Ene Feb Mar Abr May Jim Jul Ago Sep Oct Nov Die Figure 4: Coefficients of correlation between daily variation of Og and during NOx References 1. Sancho, P., J. de la Cruz, A. Diaz, F. Martin, E Hernandez, F Valero, and B Albarran, A five-year climatology of back-trajectories from the Izana baseline station, Tenerife, Canary Islands, Atmos. Environm., 26A, , 1992.

7 Air Pollution Monitoring, Simulation and Control Fishman J, Ozone in the Troposphere, Chapter 4, Ozone in the Free Atmosphere, ed. R.C. Whitten & S. S. Prasad, pp.: , Van Nostrand Reinhold Company Inc., Diaz A., J. C. Guerra & E Cuevas, Intercalibracion entre el Espectrofotometro U.V. para la Medida de Ozono Superficial Dasibi #1108 (Univ. de La Laguna) y el Dasibi #1008 (I.N.M. - Estacion Base de Izana). Rev. Acad Canar. Cienc., 1991, Vol. IH(n 2), Guerra JC, et al., Automatization de un Espectrofotometro Ultravioleta para la Medida de Ozono Superficial: Dasibi #1108, Rev. Acad. Canar. Oewc., 1991, %,/. 7//K^, CORINE Program, Data Collection CORINAR Group, C.E.C. DGXI/TF EEA-CORINE 10/ Guerra J C, Primeras Medidas de las Concentraciones de NOx en La Laguna, Rev. Acad. Canar. Cienc., 1993, Vol. V(n 2 & 3) Diaz, J. C Guerra, A. Redondas y E. Cuevas, "Air Pollution Modeling and its Application IX", 1992, (Ed.: H van Dop y G Kallos), Plenum Press, New York, pp.: