Analysis of a Summer Smog Episode in the Berlin-Brandenburg Region
Ozone production within the Berlin-Brandenburg region
A comparison between observed and modelled concentrations (REMO and GESIMA simulation) (Bauer, 2000) close to the surface and at flight levels had demonstrated that NO2 and urban ozone are well simulated but ozone is systematically underpredicted in the rural environment especially in the afternoon hours. As the measurements include only ozone and NO2 observations, they are insufficient to understand and reconstruct the process of ozone formation which strongly depends on the presence of volatile organic compounds (VOC's). A theoretical study (Bauer, 2000) reveals that the Berlin-Brandenburg area is NOx-saturated with respect to ozone production. In the morning hours the area is definitely VOC-sensitive. At noon and during the afternoon hours the system is in an intermediate state where it is VOC- as well as NOx-sensitive.
Figure 1. Total (broken line), anthropogenically (dark line) and biogenically (light line) emitted hydrocarbons as a function of time of the day (July 23, 1994) as simulated with the GESIMA model. The emissions are sampled over the whole domain of the GESIMA model covering the area of Berlin-Brandenburg.
To further characterise the VOC-composition of air masses in the area of Berlin-Brandenburg the emitted VOC's on July 23 are shown in Figure 1 subdivided into their biogenic and anthropogenic origin. The composition of hydrocarbons in Berlin-Brandenburg during the FLUMOB episode was made up by about 70-80 % of biogenically emitted hydrocarbons, i.e.
monoterpenes and isoprene, and by about 20-30 % of anthropogenically emitted hydrocarbons. Thus, natural emissions released by trees in the widely forested areas of Brandenburg dominate over anthropogenic emissions. This is of special importance for the process of ozone formation in the investigated area because ozone production during FLUMOB was hydrocarbon limited so that their availability define the potential for ozone production and the peak concentrations.
Influence of large scale conditions on ozone during FLUMOB
Simulated species in limited area models, especially longer living species like ozone are sensitive to their vertical distribution at the lateral boundaries (Langmann and Bauer, 2000).
Most trace species show maximum concentrations close to the earth's surface and decrease rapidly with increasing height. Contrarily, ozone concentrations are present in the whole troposphere with relatively high mixing ratios, normally increasing with height. Thus, the lifetime and distribution of ozone is also dependent on the larger scale conditions when investigating the processes of pollutant formation in Brandenburg.
Figure 2 shows the only three ozone soundings available during the FLUMOB episode at Lindenberg. Unusually high ozone concentrations were measured in the free troposphere. To explain the observed high ozone concentrations, backward trajectories, calculated by the German Weather Service, were analysed. They show that at about 700 hPa air masses from south-west Germany are advected to Lindenberg. Following that trajectory the air masses pass highly polluted areas and ozone could have accumulated day by day in the lower troposphere.
Above the planetary boundary layer the observed ozone concentrations originate from the Atlantic ocean four days prior to their arrival in Lindenberg. It is assumed that the observed unusually high mixing ratios of about 100 ppbv throughout the troposphere are the result of mixing of long range transported upper level ozone and upward transported ozone from the planetary boundary layer, formed and accumulated during the summer smog event. Because of the lack of measurements it is difficult to evaluate the assumption that during this summer smog event Europe was additionally influenced by high amounts of long range transported ozone. However, it seems to be the most probable explanation for the large amounts of ozone in the free troposphere. Other ozone balloon soundings, Brussels and Hohenpeissenberg, support this theory. The shape of the European wide ozone soundings exclude also stratospheric ozone intrusion.
Figure 2. Ozone ballon sonde data at Lindenberg during FLUMOB.
Sensitivity of modelled species concentrations on lateral boundary information
Analysing the REMO and GESIMA model simulations and sensitivity studies (Bauer, 2000, Langmann and Bauer, 2000) reveals that
• ozone is underpredicted by the models up to 50 ppbv in the free troposphere,
• in the free troposphere, simulated ozone concentrations are strongly linked to the ozone concentrations provided at the lateral model boundaries and its initial conditions,
• in the planetary boundary layer ozone increases / decreases linearly by the amount of ozone which is added at the lateral inflow boundaries.
Main conclusions Biogenic VOC emissions
Biogenic hydrocarbon emissions dominated significantly over anthropogenically emitted ones during July 1994 in the area of Berlin-Brandenburg where ozone production was hydrocarbon limited. With respect to ozone reduction strategies, the important role of the hydrocarbons has to be taken into account. This would possibly lead to the requirement of considerably stronger NOx emission reduction strategies to reduce the occurrence of ozone episodes in this region than believed until now.
As this conclusion was not drawn by previous investigations of the FLUMOB project, the subsequent ozone measurement campaign in Berlin-Brandenburg, BERLIOZ carried out in summer 1998, did not focus on biogenic hydrocarbons. However, the measurements made during BERLIOZ confirm our conclusions concerning the composition of hydrocarbons in the Berlin-Brandenburg area during summer.
Long range ozone transport and lateral boundary information
In spite of the stagnant high pressure weather conditions during FLUMOB, Brandenburg was influenced by long range transported ozone concentrations. In contrast to previous analyses, this investigation showed that the high ozone concentrations in the free troposphere must originate from outside of Europe. Due to vertical mixing processes these high concentrations influenced the near surface concentrations during the second phase of FLUMOB.
Ozone was dramatically underestimated by the model system in the free troposphere.
Sensitivity studies figured out that above the planetary boundary layer the amount of simulated ozone is almost completely dependent on the concentrations given to the lateral boundaries of the model domain. Even the European wide simulation with REMO was not able to rebuild this situation because climatological concentrations were assumed at the lateral boundaries. Therefore, global information about the three dimensional distribution of longer living chemical trace species and their temporal variability are necessary for mesoscale modelling systems (Langmann and Bauer, 2000).
Aims for the coming year
The FLUMOB case will be investigated again with global chemical information at the boundaries of the mesoscale model system to study the combination of large local ozone production due to the summer smog event and the advection of long range transported ozone concentration. The first step, forcing the European wide REMO simulation with global model results for trace species at the lateral boundaries look promising!
Acknowledgements
The authors acknowledge the helpful support by Ralf Podzun. This work was mainly founded by the Tropospheric Research Program (TFS) of the German Ministry for Education, Science, Research and Technology (BMBF). We also wish to thank Burckhard Wickert, IER Stuttgart, for providing the emission data sets.
References
Bauer, S.E.; Photochemical smog in Berlin-Brandenburg: An investigation with the atmosphere-chemistry model GESIMA, Ph.D. thesis, Examination Report No. 81, Max-Planck-Institute for Meteorology, Hamburg (2000).
Bauer, S.E. and B. Langmann; Summer haze in a polluted atmosphere, J. Aerosol Science 31, Suppl. 1 (2000a) 422-423.
Bauer, S.E. and B. Langmann; An atmosphere-chemistry model on the meso-γ scale. Part I: Model description and evaluation (2000b) Submitted to Atmos. Environ.
Bauer, S.E. and B. Langmann; An atmosphere-chemistry model on the meso-γ scale. Part II: Analysis of a summer smog episode in the Berlin-Brandenburg region (2000c) Submitted to Atmos. Environ.
Langmann, B.; Numerical modelling of regional scale transport and photochemistry directly together with meteorological processes, Atmos. Environ. 34 (2000) 3585-3598.
Langmann, B. and S.E. Bauer; On the importance of reliable initial and boundary concentrations of ozone for regional scale air pollution modelling (2000) submitted to J. Atmos. Chem.