An atmosphere - chemistry model hierarchy

Transcription

1 An atmosphere - chemistry model hierarchy S. E. Bauer, B. Langmann & D. Jacob Max-Planck-Institute for Meteorology Bundesstrafle 55, D Hamburg, Germany EMa.il: susanne.bauer@dkrz.de Abstract To investigate the formation of photo smog in polluted air masses with numerical models a number of atmospheric and chemical processes on various scales have to be taken into account. Here a mesoscale Eulerian model hierarchy consisting of three one-way nested models is presented. The regional model REMO is nested in itself with a higher spatial resolution. Then then the non-hydrostatic model GESIMA is embedded in this simulation to resolve the meso-"/-scale. The atmosphere model GESIMA includes chemistry modules 'on-line', thus meteorological informations are available every model time step to calculate transport, transformation and deposition of the chemical species. First results are presented for the summer smog measurement campaign FLU- MOB, carried out in eastern Germany in The simulation domains are chosen to match the areas covered by the measurements: Europe in 0.5 degree horizontal resolution, Germany in 0.16 degree resolution and Berlin-Brandenburg with a horizontal resolution of 4 km x 4 km. The model results demonstrate the importance of nested chemical simulations and the need of high resolution models for the interpretation of field measurements. 1 Introduction To improve the current understanding of air pollution problems, the complex atmospheric system needs to be investigated using experimental observations as well as numerical simulations. Photochemical air pollution is influenced by chemical and meteorological processes on various time and

2 720 Air Pollution spatial scales. These conditions require numerical tools that treat the atmospheric chemistry together with meteorological processes on different horizontal scales. The complexity of the numerical algorithms together with the size of the model domain limit the grid resolution which can be achieved by current computers. Therefore nested models are useful tools to simulate the large scale atmospheric conditions and to focus on areas of special interest with a higher resolution. Meso-7-scale simulations are important not only to resolve local phenomena, e.g. emission sources and orography, they are also necessary to help bridging the gap between coarse model simulations and point measurements. One system of nested models is the EURAD system (Jakobs et al. 1995) consisting of the meteorological model MM5 and an off-line chemistry-transport model, e.g. the models are run separately. A further development is to use all available meteorological informations for the determination of atmospheric chemistry in an on-line model. That means the chemical reaction mechanism together with all transport processes are embedded in an atmospheric model, so that chemistry and meteorology are calculated together within one model every time step. This leads to a more detailed and realistic simulation of amount and distribution of the chemical species. This paper describes afirstapplication of the model hierarchy with partly on-line chemistry. The model description (chapter 2) is followed by a comparison of a summer smog simulation to measurements (chapter 3). Conclusions are given in chapter (4). 2 The Model Hierarchy and Configuration The model hierarchy consists of the regional model REMO (Jacob and Podzun, 1997) for the meso-a- and /3-scale, with an off-line coupling to a regional chemistry transport model (CTM) (Langmann and Graf, 1997) and the non-hydrostatic meso-7-scale model GESIMA (Kapitza and Eppel, 1992; Eppel et al. 1995). This atmospheric model GESIMA was extended to an on-line atmosphere-chemistry model by embedding the RADM II gasphase mechanism (Stockwell et al. 1990) together with transport processes. Additionally the basic developments of REMO towards an on-line atmosphere-chemistry model for the regional scale have beenfinished.however, the simulations discussed here were nested into off-line REMO-CTM model runs. Two REMO-CTM simulations were carried out, one covering Europe with a horizontal resolution of 0.5 («50 km horizontal grid point distance) and the other one for Germany with 0.16 (w 18 km) horizontal resolution. Analysis data from the German Weather Service were used as initial and boundary meteorologicalfieldsfor the REMO simulation on 0.5 resolution. Chemical concentrations at the boundaries for the coarse

3 Air Pollution 721 simulation were set to be constant in time. The simplification should be of minor influence on the lower tropospheric chemistry over Central Europe, because for the first simulation a blocking situation was chosen so that the photooxidants were mainly produced inside this area. The second run was driven by initial and boundary conditions from the 0.5 run. GESIMA was one-way-nested into the REMO-CTM simulation on 0.16 degree resolution, which also defines the initial conditions. A one-way nesting technique, which transports informations only from the coarse to the higher resolution and not vice versa, was applied. The meteorological nesting is realized by using a height dependent horizontal homogeneous nudging function, that controls the following meteorological variables: horizontal wind components, potential temperature, specific humidity and the pressure at the model top layer. For the chemical nesting data of all transported chemical species were specified by interpolation to the lateral boundaries of the model domain every hour and a linear time interpolation adapt the data to the time step of the model. The advective transport was calculated after Smolarkeiwicz (1983). For dry deposition the Wesely scheme (Wesely, 1989) is used. Antrophogenic emissions are subscribed as point and area sources and biogenic emissions are parameterized following Guenther et al. (1993). 3 The Summer Smog Experiment: FLUMOB The field experiment FLUMOB (Stark et al. 1995) took place in Berlin- Brandenburg from the 23rd until the 27th of July 1994 to measure ozone and its precursors during a distinct ozone episode. The large scale weather conditions were marked by a low pressure system over the eastern Atlantic and a large high pressure system over the Baltic See, Southern Scandinavia and Poland. This pressure distribution leads to a blocking situation over Central Europe with a long lasting sunny weather period, with temperatures widely above 30 C in Germany The area of Berlin-Brandenburg was chosen for this field experiment, because of itsflattopography. Furthermore the city of Berlin is characterized by high traffic and industrial emissions. Berlin is surrounded by a rural environment with low industrial emissions but frequently passed highways. The characteristic distribution can be seen in the nitrogen dioxide emissions presented in Fig. 1, covering an area of 200 km x 200 km around Berlin. These conditions are optimal to investigate the production and transport of ozone and its precursors in polluted air masses.

4 722 Air Pollution 0:066""6 025" " Longitude [EAST] Figure 1: Emission of NOx at 13 UTC at the 24th of July in the area of Berlin Brandenburg. Countures every 25 t/h. 3.1 Comparison between REMO-CTM and GESIMA Sunday, the 24th of July was selected to illustrate some results from the high resolving REMO-CTM and the GESIMA simulation. During this day the predominant wind direction over Germany was easterly, so that air masses from the east were advected to Germany. Figure 2 illustrates the wind direction and the ozone concentrations at 13 UTC in 30 m height, representing the lowest model layer of the REMO-CTM 0.16 simulation. High ozone concentrations occur in the lee direction of regions with high density of industrial and traffic emission sources. For example in the south of eastern Germany a huge industrial area is initiating the ozone tail over central Germany, with ozone values of 90 ppbv. Two local ozone maxima occur with maximum values above 110 ppbv, due to extremely weak winds in the regions close to Munich and between the French city of Strasbourg and the German city of Stuttgart. High values are also found over the North See coast of the Netherlands. This maximum, with peak values of 120 ppbv is produced by the ozone precursors emitted in the compact settlements of the Netherlands. This air is additionally charged by polluted air masses advected from the industrial center 'Ruhrgebiet' in western Germany. The dashed box printed in Fig. 2 indicates the model domain of the GESIMA simulation. Berlin is located in the center of this box. Figure 3 shows the corresponding results for the surface wind and ozone distribution achivied

5 Air Pollution BOO 700 Figure 2: Distribution of surface ozone and the horizontal wind direction at 13 UTC in 30 m height in the REMO-CTM 0.16 simulation. Contours every 10 ppbv. Figure 3: Distribution of surface ozone and the horizontal wind direction at 13 UTC in 10 m height in the GESIMA simulation. The white line mark the urban area of Berlin. Contours every 5 ppbv.

6 724 Air Pollution by the GESIMA simulation. However the values are representative for 10 m height above ground. The wind direction is only weakly influenced by the local conditions. Close to the southern border of the model domain the hilly topography influences a weak downhill acceleration, so that locally the wind turns to a south-easterly direction. Figure 3 shows the situation at 13 UTC where the ozone values are close to their daily maximum which occurs at these latitudes between UTC at a sunny day. The ozone concentrations show values from 40 ppbv up to 80 ppbv in this domain. The local ozone maximum downstream of Berlin reaches concentrations of 70 ppbv, whereas the maximum amount of ozone up to 80 ppbv is found downstream of Bitter leld at the south western border of the domain. This ozone is partly advected from outside and partly produced inside of the domain due to local emissions and advected ozone precursors from outside. An interesting feature can be seen close to the highway between Berlin and Bitterfeld. The intense traffic emissions lead to a local ozone minimum, because here the ozone is destroyed by reactions with nitrogen oxide. The comparison of the two simulations, REMO-CTM and GESIMA, shows clearly the dependence of the simulated amounts and distribution of ozone on the spatial model resolution. Due to theflatterrain and the homogeneous weather conditions in this case, the occurring differences are caused mainly by the atmospheric chemistry is influenced by the emissions. Therefore the small scale organized ozone distributions in GESIMA are mainly caused by the detailed emission inventary. 3.2 Comparison between GESIMA and Observations For validation, some GESIMA model results are compared to observational data from the FLUMOB campaign. The radiosonde sounding at the station Kremmen at 15 UTC was chosen to demonstate the quality of the meteorological simulation. In Figure 4a the measured (dashed line) and simulated (solid line) wind directions are compared. The wind is turning from 120 degree clockwise to 250 degree with height. The model results are in good agreement with this observations. Horizontal wind velocities are given in Fig. 4b. The wind velocities in the lower 3000 m are in good agreement, at higher levels stronger wind velocities are observed than simulated. The profiles of the vertical temperature are presented in Fig. 4c. The curves follow mainly the same temperature gradient, but in the lower troposphere the model underestimates the temperature by 3-5 C. First analysis address this problem to the underestimation of surface temperature, which seems to be caused by wrong initialization. Comparisons to other stations show similar agreements and deviations. Figure 5 shows the diurnal cycle of nitrogen dioxide (NO2) and ozone at a ground station in the center of Berlin (Fig. a and b) and in Kyritz (Fig. c and d).

7 Air Pollution b) c) WIND DIRECTION [deg] WIND VELOCITY [m/s] nl ii... i. i. i A TEMPERATURE [C] Figure 4: Comparison of vertical profiles between radiosonde data (dashed line) and the GESIMA simulation (solid line) at the 24th of July 15 UTC at Kremmen: a) wind direction [degree], b) wind speed [m/s], and c) temperature [ C*]. The solid line indicates the simulation and the symbols the measurements. Nitrogen dioxide is distributed over the day with maximum values in the morning and in the evening hours, which are more intense in the city (fig. 5a) then in the small town of Kyritz (Fig. 5c). In the center of Berlin (Fig. 5a) high concentrations of nitrogen dioxide occure at 6 UTC, which may be caused by a local traffic jam. Therefore ozone reduction around 6 UTC (Fig. 5b) can be explained as a local feature, which can not be resolved within a simulation with a horizontal resolution of 4 x4 km^. In general the mean nitrogen dioxide amounts are simulated in good agreement with the measurements, only during evening hours GESIMA overestimates the nitrogen dioxide concentrations (Fig. 5a and c). This is caused by too weak vertical mixing processes during the evening and the night, so that the emissions remain in the surface layer. Again this problem is caused by underestimated surface temperatures during the night, which initialize a wrong temperature gradient close to the surface preventing the right strenght of vertical fluxes. The weak vertical mixing is also responsible for the un-

8 726 Air Pollution derestimation of ozone during the night (Fig. 5b), where the mixing of ozone from higher levels to the ground is too weak. In the afternoon hours the simulated ozone decrease is too strong. Here again the overestimated nitrogen oxide concentrations together with the too weak supply of ozone from higher levels are responsible. Figure 5d shows the ozone amounts at Kyritz, located 100 km north west of Berlin. At that day Kyritz is in the lee direction of Berlin and therefore influenced by polluted air masses of the city. The general underestimation of ozone at Kyritz, reflects the low ozone production in the domain, caused by the underestimated verical mixing processes due to the problems discussed above time [hours] time [hours] 25 _ 100.o CL « b) time fhoursi r a « d) time [hours] Figure 5: Comparison of NO2 and ozone between observations (symbols) and the GESIMA simulation (solid line) in Berlin (a,b) and Kyritz (c,d) at the 24th of July. 4 Conclusion and Outlook First results of a simulation with the mesoscale model hierarchy REMO- GESIMA and comparisons to observations were presented for one day of a summer smog episode in This was thefirstnested simulation with the non-hydrostatic atmosphere-chemistry model GESIMA. As discussed in chapter 3 the model simulates the selected situation with reasonable accuracy. The comparison to observations point out very clearly the current disadvantages, for example the systematic underestimation of temperature. This work is under investigation. The selected atmospheric situation was

9 Air Pollution 727 very suitable to study first results. Furthermore situations with more complex weather conditions will be simulated. As GESIMA is a non-hydrostatic model, it is suitable especially for applications in complex terrain. So, following applications can investigate local meteorological effects together with their influence on the tropospheric chemistry, for example the influence of a sea-breeze on the air quality of a coastal city. In addition to this it is planned to couple the mesoscale model hierarchy to global simulations, so that longer simulation periods with several weather conditions can be realized. The model hierarchy is not resticted to the introduced domains, any areas can be focussed. Acknowledgments. This work is supported by the German Tropospheric Research Program. The emission data are provided by IER, University of Stuttgart. References [1] Eppel, D. P., Kapitza, H., Claussen, M., Jacob, D., Koch, W., Levkov, L., Mengelkamp H. T. & Werrmann, N., The non-hydrostatic mesoscale model GESIMA. Part II: Parameterizations and applications. Contr. Atmosph. Phys. 68, pp , [2] Guenther, A. B., Zimmermann, P. R., Harley, P. C., Monson, R. K. & Fall R., Isoprene and monoprene emission rate variability: Model evaluation and sensitivity analysis. J. geophys. Res. 98, pp , [3] Jacob, D. & Podzun, R., Sensitivity studies with the regional climate model REMO. Meteorol. Atmos. Phys. 63, pp , [4] Jakobs, H. J., Feldmann, H., Hass, H.& Memmersheimer M., The use of nested models for air pollution studies: Application of the EURAD model to a SANA episode. J. Appl Meteor. 34, pp , [5] Kapitza, H. & Eppel, D. P., The non-hydrostatic mesoscale model GESIMA. Part I: Dynamical equations and tests. Contr. Atmosph. Phys. 65, pp , [6] Langmann, B. & Graf, H., The chemistry of the polluted atmosphere over Europe: Simulations and sensitivity studies with a regional chemistry-transport-model. Atmospheric Environment 19, pp , [7] Smolarkiewicz,P. K., A simple positive definite advection scheme with small implicite diffusion. Mon. Wea. Rev., Ill, pp , 1983.

10 728 Air Pollution [8] Stark, B., Abraham, H.-J., Lenschow, P., Lutz, M., Reichenbacher, W., Reiner, E. & Scherer, b., Flugzeug- und Bodenmessungen von Ozon und Verlauferstoffen zur Abschatzung der Wirksamkeit von emissionsmindernden Mafinahmen im Grofiraum Berlin/Brandenburg (FLuMOB- Projekt). Abschlufibericht im Auftrag der Senatsverwaltung fur Stadtentwicklung und Umweltschutz, Berlin und des Ministeriums fiir Umwelt, Naturschutz und Raumordnung, des Landes Brandenburg, Berlin, [9] Stockwell, W. R., Middleton, P., Chang, J. S. & Tang, X., The second generation regional acid deposition model: chemical mechanism for regional air quality modelling. J. geophys. Res. 95,pp , [10] Wesely, M. L., Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models. Atmospheric Environt, 23, pp , 1989.