Regional climate change and its impact on photooxidant concentrations in southern Germany: Simulations with a coupled regional climate-chemistry model

Authors


Abstract

[1] In order to investigate possible effects of global climate change on the near-surface concentrations of photochemical compounds in southern Germany, nested regional simulations with a coupled climate-chemistry model were carried out. The simulations with a horizontal resolution of 60 km for Europe and 20 km for central Europe were driven by meteorological boundary conditions provided by a long-term simulation of the global climate model ECHAM4. Two time slices of about 10 years were compared, one representing the 1990s and one representing the 2030s. For the region of southern Germany the simulations show an increase of the mean summer temperature by almost 2° along with a decrease of cloud water and ice and a corresponding increase of the photolysis frequencies and the emissions of biogenic hydrocarbons. Under the model assumption of unchanged anthropogenic emissions this leads to an increase of the mean mixing ratios of most photooxidants. Because of the complex topography and the heterogeneous distribution of precursor emissions all parameters show pronounced regional patterns. The average daily maximum ozone concentrations in southern Germany increase for the considered scenario by nearly 10% in the summer months. Depending on the region, the increase of the mean daily maximum ranges between 2 and 6 ppb. As a consequence, the number of days when the 8-hour mean of the ozone concentration exceeds the threshold value of 120 μg m−3 increases by 5 to 12 days per year.

1. Introduction

[2] The change of global climate due to anthropogenic emissions of greenhouse gases is expected to result in higher summer temperatures and less precipitation in Germany. Though the results of global climate simulations show some differences depending on the assumed greenhouse gas emission scenarios and the global climate model used, all modeling studies indicate a similar general trend [Hulme and Sheard, 1999].

[3] Climate change can increase the formation of tropospheric ozone and other photooxidants, which are suspected to have harmful effects on human health, agricultural crops, and natural vegetation. Decreased cloudiness will result in an increase of UV radiation and subsequently in enhanced photochemical smog production. This effect may still be enhanced as higher summer temperatures will lead to higher reaction rate constants. Additionally, higher surface temperatures and increased radiation will result in higher emissions of biogenic ozone precursors such as isoprene.

[4] A decrease in precipitation and a higher number of cloud-free days may indicate an increase of high-pressure situations and a decline in the frequency of cyclones, and a reduced exchange of air masses. Finally, lower precipitation and wet removal of soluble constituents, such as H2O2 or HNO3, may further contribute to an increase of the concentrations of these compounds under future climate conditions.

[5] On the global scale, the development of tropospheric ozone under changed climate conditions has been investigated by several modeling studies [e.g., Roelofs et al., 1998; Dameris et al., 1998; Johnson et al., 1999, 2001; Lelieveld and Dentener, 2000; Hauglustaine and Brasseur, 2001]. The production of photooxidants depends strongly on the regional distributions of precursors. Therefore the coarse horizontal resolution of current global climate-chemistry simulations does not permit an estimate of the effects of climate change on tropospheric photooxidant distributions on the regional scale. Regional climate modeling studies focus mostly on meteorological applications [e.g. Jones et al., 1997; Grell et al., 1998; Giorgi et al., 1998; Giorgi and Mearns, 1999; Giorgi et al., 2001; Räisinen et al., 2004]. So far, only a few investigations were carried out on the potential effect of future global climate change on the distribution of photooxidants on the regional scale [Ehhalt et al., 2001]. Hogrefe et al. [2004] have shown that climate change has a significant impact on the occurrence of high ozone concentrations in the eastern United States by performing scenario simulations with the meteorological model MM5 and an offline chemistry transport model for 5 present-day and future summer seasons. In Europe, numerical studies on the effect of changed climate conditions on tropospheric chemistry on the regional scale have been limited to sensitivity studies or to the simulation of episodes.

[6] In this paper, we investigate the potential effect of climate change on tropospheric photooxidant concentrations in southern Germany. Regional climate-chemistry simulations with the coupled three-dimensional meteorology-chemistry model MCCM [Grell et al., 2000] were carried out for two time slices of about 10 years each, one representing the 1990s and one representing the 2030s. To our knowledge, this is the first time that an online coupled meteorology-chemistry model has been applied for climate effect studies over a period of this length. The simulations form a link between regional climate change and air quality and permit a first quantitative estimate of the effects of changed climate conditions on photochemistry for a region in Europe.

2. Methods

[7] The investigation of the effect of global climate change on regional distributions of meteorological variables and photooxidants was performed by dynamical downscaling using the coupled meteorology-chemistry model MCCM. On the basis of the concept of a one-way nesting strategy, the lateral boundary conditions for the regional limited-area model are derived from the output of a global climate model. With respect to meteorological modeling alone, dynamical downscaling of global climate scenarios with limited area models can nowadays be considered as a standard procedure [Giorgi and Mearns, 1999]. In this study, a limited area model with online coupled photochemistry model was applied in order to account for the development of the concentrations of tropospheric ozone and other compounds of photochemical smog.

2.1. Global Climate Scenario

[8] The time-dependent meteorological boundary conditions for the regional simulations for Europe as well as sea surface temperatures were derived from the output of a 240 years run of the global climate model ECHAM4 [Roeckner et al., 1996] coupled with the ocean general circulation model OPYC [Oberhuber, 1993]. The ECHAM4/OPYC model run is based on historic greenhouse gas concentrations between 1860 and 1990 and on the Intergovernmental Panel on Climate Change (IPCC) emission scenario IS92a [Intergovernmental Panel on Climate Change (IPCC), 2001] between 1990 and 2100.

[9] The horizontal resolution of the global simulation used for the present study is T42. The output is available for the corresponding Gaussian grid with 128 grid points in east-west direction and 64 grid points in south-north direction and for a vertical resolution of 19 layers.

[10] For the regional simulations two time slices are selected, 1991–2000 (denoted as “present”) and 2031–2039 (denoted as “future”). The time slice from 2031–2039 has been chosen since the investigation of a scenario for the near future was intended. On the other hand, it is far enough in the future to show a significant climate signal.

[11] Until 2035 the course of the CO2 emissions for the “business as usual” scenario IS92a runs in the middle between the emissions described by the more recent scenarios A2 and B2. According to the scenario IS92a the atmospheric CO2 volume mixing ratio increases by about 100 ppm between 1990 and 2030, which corresponds to an increase by 28%. The resulting temperature increase simulated by ECHAM4 over southern Germany is about 2°.

2.2. Regional Climate-Chemistry Model MCCM

[12] The coupled meteorology-chemistry model MCCM [Grell et al., 2000] is based on the NCAR/Penn State University meteorological model MM5 [Grell et al., 1994]. In addition to the meteorological variables predicted by MM5, MCCM includes prognostic equations for the concentrations of gas phase pollutants and offers the option to apply different gas phase chemistry mechanisms. The simulation of atmospheric chemistry with an online coupled model can be regarded as more consistent than an offline treatment, as the chemistry part of the model receives all necessary meteorological information directly from the meteorological part of the model at each time step. Although the advantages of online coupled meteorology-chemistry simulations against an off-line treatment are most effective for fine horizontal resolutions, effects already become significant at horizontal resolutions of around 30 km [Grell et al., 2004].

[13] Similar to MM5, MCCM permits the choice between different physics and chemistry options. The following options were applied for the simulations: ice phase cloud microphysics according to Reisner et al. [1998], vertical turbulent transport according Burk and Thompson [1989], Grell cumulus parameterization [Grell et al., 1994], and simple radiation schemes with explicit treatment of clouds for short-wave and long-wave radiative transfer. Further details of the physics options are given in the MM5 description [Grell et al., 1994]. The soil-vegetation-snow processes are described by a multilayer model of Smirnova et al. [1997]. This model solves prognostic equations for soil temperature and water content, interception storage on vegetation, and the mass of a snow layer.

[14] The chemical transformations were calculated with the RADM2 gas phase chemistry mechanism [Stockwell et al., 1990]. Evaluation of the RADM2 against numerous smog chamber experiments has shown that the RADM2 yields maximum ozone concentrations which are generally in good agreement with the experimental data [Dodge, 2000]. This widely used mechanism was chosen as it can be applied to a very broad range of chemical scenarios. It contains 38 predicted and 22 diagnosed species and 157 chemical reactions. With respect to the computational effort this number of constituents and reactions is near the upper limit for the simulation of atmospheric chemistry with a 3-D model for a whole decade. Interactions with the particulate phase, i.e., aerosols, cloud, and rainwater are not considered in the present study except for a parameterization of the conversion of N2O5 to HNO3 on particle surfaces.

[15] The photolysis frequencies necessary for the computation of the 21 photolytic reactions of the RADM2 mechanism are computed according to Madronich [1987] at each grid point of the model. The simulated photolysis frequencies depend on the stratospheric ozone column, which is prescribed, and on the profiles of temperature, tropospheric ozone, and the mixing ratios of cloud liquid water and ice, which are provided by the model.

[16] Biogenic hydrocarbons, particularly isoprene are important precursors for tropospheric ozone. As described by Grell et al. [2000], the emission rates of these species and the emission of NO from soils are computed online depending on the land use class assigned to each grid point, and on the actual simulated values of temperature and incoming solar radiation [Simpson et al., 1995; Guenther et al., 1993]. The emission factors for different forest types were adopted from Guenther et al. [1994] and are summarized by Grell et al. [2000, Table 1]. Anthropogenic emissions of primary pollutants, like NOx, SO2, and hydrocarbons have to be supplied at hourly intervals.

[17] Various validation studies have shown that MCCM is able to reproduce observed courses of meteorological variables and pollutant concentrations for different conditions and regions of the earth [Grell et al., 1998; Grell et al., 2000; Jazcilevich et al., 2003; Forkel et al., 2004; Suppan and Schädler, 2004; E. Haas et al., Application and intercomparison of the RADM2 and RACM atmospheric chemistry mechanism including a new isoprene degradation scheme within the online-coupled regional meteorology chemistry model MCCM, submitted to International Journal of Environment and Pollution, 2006, hereinafter referred to as Haas et al., submitted manuscript, 2006; D. Kim and W. R. Stockwell, Regional scale complex terrain effects on the transport and transformation of air pollutants, submitted to Atmospheric Environment, 2006]. Furthermore, MCCM has been successfully applied to regional weather forecasts (http://imk-ifu.fzk.de/de/wetter/index_wetter.htm) for several years.

[18] The meteorological part of MCCM, which is almost identical with MM5, has been used for downscaling of all 15 years of the ERA15 reanalysis and has been extensively evaluated against observations [Kotlarski et al., 2005]. For Germany a bias of the mean temperature of less than −0.5° was found. The simulated precipitation was 12% lower than observed. A detailed discussion of climate simulations with MCCM with special regard to hydrological implications is given by Kunstmann et al. [2004].

[19] Daily ozone forecasts have been performed continuously during the summer of 2005. Overall, the regional patterns of the maximum temperature and the daily maximum ozone concentrations measured by the environmental agencies of the German States and the Federal Environmental Agency (http://www.env-it.de/luftdaten/start.fwd?setLanguage = en) are reproduced very well. However, high ozone concentrations above 90 ppb are frequently underestimated by up to 10%.

[20] A comparison of the results of a continuous simulation without FDDA for two consecutive months of the summer of 2001 (Haas et al., submitted manuscript, 2006) with observed diurnal cycles at different sites in southern Bavaria has shown that the model can capture the variability of the synoptic situation and the duration of photosmog episodes. Correlation coefficients around 0.85 were found for temperature, 0.5–0.6 for ozone, and 0.4–0.5 for NO2. Especially in the case of NO2, it must be considered that the model results represent values for a whole grid cell, whereas the data from the measurement network, in particular data from the urban stations, are subject to local influences. A restriction of the analysis to the observed high-ozone episodes resulted in correlation coefficients around 0.75 for ozone.

2.3. Scenarios for Stratospheric Ozone and Anthropogenic Emissions

[21] For the computation of UV radiation and photolysis frequencies the depth of the stratospheric ozone layer is an important input parameter. Monthly values of the average zonal mean for the years 1985–1997 [Hein et al., 2001] were used for the simulation of present-day conditions. The change in the ozone layer depths between the 1990s and the 2030s was estimated on the basis of the scenario “PROB2050” given by Reuder et al. [2001] that was derived from a global scenario simulation for the development of stratospheric ozone [Dameris et al., 1998]. According to this scenario the ozone layer depths in summer over southern Germany can be expected to increase by 10 to 15 Dobson units.

[22] Anthropogenic emission data of primary pollutants based on an emission inventory for 1998 [Friedrich et al., 2000] were available in hourly intervals for the summer and the winter season with a horizontal resolution of 20 km. The 7-day emission data sets were repeated in a weekly cycle during each of the seasons. In order to isolate the effect of climate change on the formation of photooxidants, the same anthropogenic emissions were applied for the present-day and the future time slice.

2.4. Setup of the Regional Climate-Chemistry Simulations

[23] To obtain the desired horizontal resolution for the region of southern Germany the climate-chemistry simulations with MCCM were carried in two consecutive nesting steps. In the first step, the modeling area (domain D1, 66 × 59 grid points) has a horizontal resolution of 60 km and covers entire Europe (Figure 1). In a second step, a model domain with 20 km resolution (domain D2, 64 × 64 grid points) covering the Alps and surrounding regions was nested into the first domain. In the vertical the atmosphere between the surface and the 100 hPa level is resolved in 25 layers, the lowest being about 14 m thick. The lower boundary of the five5-layer soil model is located at a depth of 3 m.

Figure 1.

Map of the two model domains D1 and D2. The white rectangle within D2 indicates the study area of southern Germany.

[24] For domain D1 the lateral boundary conditions for the meteorological variables were derived from the 12-hourly output of the ECHAM/OPYC run. As the global climate simulations do not include chemistry, no consistent boundary conditions for the chemical constituents were available. Therefore time constant boundary conditions for the chemical compounds were applied. These boundary values were derived from observed background concentrations and from results of global chemistry simulations published in the literature [e.g., von Kuhlmann, 2001; von Kuhlmann et al., 2004]. Boundary values near the surface and above the boundary layer are summarized in Table 1 for selected chemical compounds. Identical boundary conditions for the chemical constituents were used for present and future conditions.

Table 1. Time-Invariant Boundary Values for Chemical Species With Significant Mixing Ratios
 Mixing Ratio, ppb
z = 14 mz = 5000 m
O34045
NO0.10.001
NO20.10.001
CO150100
HCHO0.30.1
H2O20.30.4
HNO30.30.3
PAN0.50.5

[25] The choice of the chemical boundary conditions is subject to some uncertainty and can affect the results of a limited-area chemistry transport model. This holds particularly for the choice of the ozone mixing ratio. Ozone levels in Europe can be affected by pollutant transport from regions outside of Europe, especially from North America. As shown by Li et al. [2002] North American emissions enhance summertime surface ozone in Europe by 2–4 ppb on average and contribute mostly to ozone mixing ratios in the range of 40–60 ppb. This average transport is implicitly included by the choice of the boundary conditions for the chemical constituents. Singular trans-Atlantic transport events, which can increase European ozone mixing ratios by 5–10 ppb are not considered. These events are associated with low-pressure systems near on the North Atlantic and westerly winds and hardly contribute to high ozone levels, which are usually associated with stagnant high-pressure systems. During high-pressure episodes, transport of pollutants from outside Europe is only low and pollutant levels in Europe are mostly determined by emissions within the regional model domain [Jonson et al., 2001].

[26] As the impact of stratosphere-troposphere-exchange on tropospheric ozone concentrations is only small in the photochemically most relevant summer months [Lelieveld and Dentener, 2000], its contribution was not explicitly considered in the simulations.

[27] The large extension of domain D1 with horizontal boundaries that are either located over the ocean or over remote regions ensures the development of realistic concentration fields of photochemical pollutants over Europe. A study by Jonson et al. [2001] with a regional model using chemical boundary conditions from a global model has shown that a model domain of the size of D1 is large enough for the investigation of ozone formation in Europe.

[28] For domain D2 the lateral boundary conditions for the meteorological as well as for chemical variables were supplied by the MCCM run for domain D1. Though the main focus of the simulations was on southern Germany, domain D2 extends far toward the south, in order to ensure a free flow around the Alps within D2.

[29] Land use data for the calculation of biogenic emissions and for the provision of soil and surface properties needed for the meteorological part, such as roughness length, were specified on the basis of CORINE land use data. The land use data were interpolated to the model grids of the two domains. Similar to the treatment in MM5, the dominant land use class at each grid point was considered as representative for this grid point. Anthropogenic emissions were also interpolated from the original data set to the grids of the two model domains. Figure 2 displays the distribution of anthropogenic NOx emissions for domain D2.

Figure 2.

Weekly average of anthropogenic NOx emissions in summer for domain D2.

[30] The simulations start in November of the years 1990 and 2030, respectively. The first two months were considered as spin-up time for the meteorological variables and for soil temperature and moisture as well as snow depth. The simulations were carried out continuously until December 2000 and December 2039, yielding 10 and 9 years of model output for the subsequent analysis, respectively.

3. Results and Discussion

[31] Compared to the ECHAM4 outputs the downscaling for domain D1 shows more pronounced regional patterns of pressure, precipitation, and temperature. Still more distinct structures of the meteorological variables as well as the chemical constituents are found for domain D2 due to the better resolution of topography, and anthropogenic and biogenic precursor emissions. As this was the main focus of the research, the presentation of the results will concentrate on the part of D2 covering southern Germany.

3.1. Evaluation of Simulation Results for Present-Day Conditions

[32] As MCCM is driven by meteorological boundary conditions from a global climate simulation rather than a simulation of current weather, the model results cannot be compared with observations on a day-to-day basis, but only with patterns of long-term mean values. The following analysis mainly focuses on an evaluation of the simulated fields of chemical compounds. As already mentioned in section 2.2, the performance of the meteorological part of the regional model has already been asserted in a different study.

[33] Since the global climate simulation with ECHAM4 produces about 1° too high summertime temperatures for Europe [Roeckner et al., 1996], a corresponding effect on the results of the regional simulations can be expected. Summertime precipitation in the midlatitudes is underpredicted by ECHAM4, while good agreement with observations has been found for the solar irradiance over Germany [Liepert and Lohmann, 2001]. Since the bias can partly be considered to be systematic in the model, a smaller bias can be expected when differences between time slices are analyzed. The regional distribution of the near-surface temperature simulated with MCCM for the 1990s was found to be in good agreement with observed climatological mean temperatures in southern Germany. In correspondence with the too low summertime precipitation simulated by ECHAM4, the regional simulation similarly underestimates the observed precipitation [Kunstmann et al., 2004].

[34] Figure 3 displays simulated summertime mean values of ozone and NOx for the time slice 1991–2000 together with observed mean summertime mixing ratios (Lufthygienischer Jahresbericht, 1996–2000, available at http://www.bayern.de/lfu/luft/index.html). The spatial patterns as well as the absolute values of the mean mixing ratios are reproduced rather well. In agreement with the observations mean simulated ozone mixing ratios are high at mountain sites. The observed mixing ratios of 53 ppb (Wank mountain, 1776 m above sea level (asl)) and 43 ppb (Tiefenbach, 750 m asl) near the southern and the eastern border of Germany are slightly underpredicted by the model. This can be attributed to the representation of the topography, which is strongly smoothed for a horizontal resolution of 20 km. The spatial distribution of NOx reflects the location of major NO sources (see also Figure 2) like the city of Munich. There a mean NOx mixing ratio of 71 ppb is observed, which is very well reproduced by the simulations.

Figure 3.

Simulated mean ozone and NOx mixing ratios during summertime (June–August) for the time slice 1991–2000. The numbers show observed mean summertime mixing ratios for the years 1996–2000. The location of the ozone and NOx monitoring sites is indicated by the lower left corner of the numbers.

[35] Isoprene plays an important role for the formation of tropospheric ozone and is expected to be very sensitive to changed climate conditions. The simulated isoprene mixing ratio over southern Germany shows a patchy pattern that reflects mainly the locations of isoprene emitting plants in the model. Since the dominant land use class at each grid point was regarded as representative for this grid point, isoprene emissions may be overestimated at single grid points. A validation of the simulated present-day isoprene mixing ratio is difficult since measured concentrations are hardly available. For a spruce forest in northeastern Bavaria, Klemm et al. [2006] report mean mixing ratios of 0.5 ppb and 0.27 ppb during the summers of 2001 and 2002, respectively. The simulated mean mixing ratio for the same site is 0.5 ppb. Since isoprene emissions from spruce are comparatively low, higher isoprene concentrations can be expected at grid points covered with mixed forest. A high emission factor resulting in isoprene mixing ratios between 1.5 and 3 ppb was assigned to mixed forest [Guenther et al., 1994; Grell et al., 2000]. However, the composition of mixed forest shows high variability over Germany. Therefore the chosen emission factor probably results in an overprediction of the isoprene mixing ratios at locations where spruce and beech are the predominant species of the mixed forest.

3.2. Regional Climate Change

[36] The occurrence of high summertime concentrations of tropospheric ozone and other photooxidants is strongly determined by meteorological processes within the planetary boundary layer. The well-known fact that high photooxidant concentrations are associated with stagnant high-pressure situations during summer has been confirmed by several studies that show the close relation between ozone concentrations and the meteorological situation [e.g., Vukovich, 1995; Rao et al., 2003, Ordonez et al., 2005]. Meteorological conditions which are known to be associated with high ozone concentrations are little daytime cloud cover, low surface wind speed, high mixing heights [Rao et al., 2003], high solar irradiation, and high temperatures [Ordonez et al., 2005]. Maximum ozone concentrations depend also on the duration of stalled high-pressure systems, which means on the days elapsed since the last frontal passage [Ordonez et al., 2005].

[37] High-level photosmog episodes are usually terminated by synoptic activity, leading to an air mass exchange. Disturbed weather conditions associated with low-pressure systems, fronts, high cloudiness, and precipitation usually prevent the occurrence of high ozone concentrations. Mickley et al. [2004] made an attempt to identify frontal passages by analyzing the change of the sea level pressure and found a decrease from 7.5 to 6.8 events per summer. In the present study, the 3-hour pressure tendency was used as an indication for synoptic activity. The number of days with a maximum pressure tendency above 2 hPa decreases slightly in the future from 5.9 events per summer to 5.6.

[38] For the future conditions, the average number of days with high insolation (which were here defined by a daily integral of solar radiation of at least 70% of the daily clear-sky radiation) increases by 6 to 8 days per summer season in the western part and by about 4 to 7 days in the eastern part of southern Germany. For the development of high maximum ozone concentrations several consecutive days with sunny weather are necessary. The average number of sunny episodes with a minimum length of four days during the months June to August was found to increase by about 2 in southern Germany. In the Black Forest region and in northern Bavaria this corresponds to an increase by 60%. In southeastern Bavaria, where a comparatively high number of sunny episodes is simulated for present-day conditions, the relative increase is only 20%.

[39] Summertime averages over southern Germany of meteorological parameters associated with high ozone levels and the daily maximum mixing ratio of ozone are compiled in Table 2 for present and future climate conditions. Table 2 shows that the coherences, which were identified in the studies mentioned above, also show up when present and future climate conditions are compared.

Table 2. Average Summertime (June–August) Values Over Southern Germany for Meteorological Impact Parameters and Maximum Daily Ozone Mixing Ratios for Present and Future Conditions
 1991–20002031–2039
  • a

    Daily integral of solar radiation >70% of the daily clear-sky solar radiation.

Temperature, °C15.617.5
Vertically integrated cloud water, g m−2119104
Vertically integrated cloud ice, g m−254
Cloud free hours488625
PBL height, m385398
Maximum PBL height, m12521317
Solar radiation, W m−2175192
Number of days with high insolationa28.834.4
Mean duration of high insolation episodes,a days2.252.56
Events with 3-hour pressure tendency >2 hPa5.95.6
Daily maximum ozone mixing ratio, ppb51.054.7

[40] Similar to the results of the global simulation, the regional model calculates a significant rise of the near-surface air temperature between the 1990s and the 2030s for all months of the year. During the photochemically most relevant summer months, June to August, the highest temperature increase in southern Germany, i.e., 2.1° within 40 years, was found in the upper Rhine valley near the border to France (Figure 4a). Toward the east the difference between future and present-day temperature decreases. An increase of only 1.7° was simulated for the southeastern part of Bavaria.

Figure 4.

Difference of (a) temperature and (b) vertically integrated cloud water content between 2031–2039 and 1991–2000 for the months June–August.

[41] The meteorological parameter with the highest impact on photochemistry is cloud cover since it controls the amount of solar radiation reaching the lower troposphere. For future climate conditions the simulations yield a 20 to 40% increase of the number of hours with clear-sky conditions. This corresponds to a pronounced decrease of the vertically integrated cloud water content over southern Germany during the summer months. The regional distribution shows the strongest decrease in the western part of southern Germany and lower values in the central part (Figure 4b). Relative to the simulated cloud cover for present-day conditions the reduction corresponds to 5 and 10% in the central area and almost to 20% in the west. The vertically integrated cloud ice content also decreases for future climate conditions. As cloud ice contributes only 5 to 10% to the total cloud water, the main impact on incoming radiation is due to the decrease of the liquid cloud water content.

3.3. Impact on Photolysis Frequencies and Precursor Emissions

[42] Photolysis frequencies and biogenic VOC emissions are quantities which are directly affected by a change in near-surface air temperature and cloudiness. Figure 5 shows the difference between present-day and future climate conditions for the photolysis frequencies image for NO2, and image for the reaction O3 → O(1D) + O2, which can be regarded as the two most important photolysis frequencies for tropospheric photochemistry. Furthermore, image and image reflect the effect of changed cloud water content on incoming solar radiation and on UV radiation, respectively.

Figure 5.

Difference of the photolysis frequencies (a) image and (b) image between 2031–2039 and 1991–2000 for the months June–August.

[43] The regional pattern of the change in image is almost identical with the pattern of the change in incoming solar radiation. The difference of image between future and present-day conditions during the summer months (Figure 5a) follows mainly the pattern of the change in cloud water content. As changes in cloud water content only have a small additional effect for optically very thick clouds, no complete match of the patterns can be expected.

[44] In the case of image (Figure 5b) a decrease is found for large areas, which correspond to the regions where the difference in cloud water content between future and present-day conditions is only small (see also Figure 4b). The expected recovery of the stratospheric ozone layer by 10 to 15 Dobson units until 2030 and, to a minor extent, the increase of tropospheric ozone results in a 6 to 10% lower UV-B radiation (not shown) and image Therefore the effect of decreasing cloud water is partly compensated by the effect of the higher ozone column depth.

[45] The maximum increase of solar radiation in summer is up to 17% for future climate conditions while the maximum increase of UV-B radiation over southern Germany is only 10%. Accordingly the maximum increase of image is 11%, whereas the maximum increase of image is only about 8%.

[46] As the emission of isoprene increases with solar irradiance and temperature, higher emissions are simulated for future climate conditions under the assumption that vegetation remains unchanged. The spatial distribution of the increase of isoprene emissions is superimposed by the distribution of forest areas with high isoprene emissions. Therefore the increase of the isoprene emission shows a patchy pattern that reflects mainly the locations of isoprene emitting plants in the model (Figure 6a). The maximum increase, corresponding to 30–50% of the simulated values for 1991–2000, was found in the Black Forest area and the forested Alpine regions. The increase in the emissions of monoterpenes and other biogenic VOC shows a similar pattern.

Figure 6.

Difference of (a) the isoprene emission and (b) NOx mixing ratio between 2031–2039 and 1991–2000 for the months June–August.

[47] An increase in temperature also results in enhanced NO emissions from the soil. Since the emission of NO is strongest for areas with agricultural land use, it shows the highest values as well as the strongest increase of about 0.01 kg m−2 h−1 in rural areas without isoprene emission. Compared to the total emission of NOx, which is mostly of anthropogenic origin, the higher NO emissions from agricultural soils under future climate condition add only about 1 to 5% to the total emissions of NOx. This increase in NO emissions results in a slightly increased level of NOx for a major part of southern Germany (Figure 6b). The increase is most pronounced in the area along the Danube and in the upper Rhine valley. However, for the urban area around Frankfurt a decrease of the near-surface mixing ratio of NOx was found, which can be ascribed to an increase of mixed-layer height in combination with the absence of significant NO emissions from soils in this region.

3.4. Impact on Regional Distribution of Photooxidants

[48] The concentration of tropospheric ozone depends on the regional distribution of sources of precursors such as NOx and hydrocarbons from anthropogenic and biogenic sources as well as on the meteorological situation. As already mentioned, the future meteorological conditions in southern Germany are more favorable for the formation of tropospheric ozone. Higher isoprene concentrations as a consequence of enhanced isoprene emissions, and, to a minor extent, higher NO emissions from the soil in rural areas are additional factors to promote the formation of tropospheric ozone and other photooxidants.

[49] The increase in isoprene concentrations (not shown) closely follows the pattern of the change in isoprene emission shown in Figure 6a. In the regions with the strongest increase near the southern and western border of Germany the mean isoprene mixing ratio increases by 1–1.5 ppb or 35–45%. Under the model assumption of unchanged anthropogenic emissions the simulated change in the meteorological conditions and the higher isoprene mixing ratios for future climate conditions result in an average increase of the mean daily maximum of near-surface ozone (Figure 7a). The increase ranges between 2 ppb in the northern part of Bavaria and 6 ppb toward the southwest, which corresponds to an increase by 6–10%. As the highest ozone mixing ratios usually do not occur directly in the source regions of precursors but downwind of these regions the increase in the mean ozone maxima reflects only very roughly the areas with high increase of solar radiation and isoprene emissions.

Figure 7.

Difference (a) of the mean daily ozone maximum and (b) of the number of days with exceedances of the threshold value of 120 μg m−3 for the 8-hour mean between 2031–2039 and 1991–2000 for the months June–August.

[50] Figure 7a shows that the highest increase of the maximum ozone concentration seems to occur in regions where a high increase of the isoprene emissions coincides with high NOx emissions (area of Stuttgart (left of the center of Figure 7a)), northern Switzerland, and the Milan region in northern Italy (at the southern margin of Figure 7a). A comparatively small increase of the maximum ozone concentration is found for the high-isoprene regions near the border between Germany and Austria, in the Vosges in eastern France, and some other regions, with low anthropogenic NO emissions. On the other hand, a considerable increase of the maximum ozone concentration is calculated for the polluted upper Rhine valley and a band with low isoprene emissions between the southern border of Germany and the Danube river.

[51] According to the simulations, the target value of 120 μg m−3 (about 60 ppb) for the 8-hour mean of the ozone concentration will be exceeded by additional 5 to 12 days per year for the given scenario (Figure 7b). As the 8-hour mean of the near-surface ozone concentration under present-day conditions is already close to the threshold in the southern part of the study area, a stronger increase in the number of days with threshold exceedance is found there.

[52] The frequency distribution of the simulated daily ozone maxima during summer is shifted toward higher values (Figure 8). This results in a significant increase of the number of days with near-surface ozone concentrations higher than 180 μg m−3 (90 ppb) in southern Germany. For the 232 grid points of the considered region, the average number of events when the threshold of 180 μg m−3 is exceeded increases from 99 to 384 events per summer, i.e., by a factor of 3.9.

Figure 8.

(left) Frequency distribution of the simulated daily ozone maxima averaged over southern Germany during summer (June–August) for the years 1991–2000 and 2031–2039. (right) A zoom of the high-ozone part.

[53] The changed climate conditions and their effect on photolysis frequencies affect all major compounds of photochemical smog. Odd hydrogen radicals (HOx = OH + HO2) play a key role in tropospheric photochemistry. The simulated increase of the HOx mixing ratio (Figure 9a) reflects the pattern of isoprene emissions and concentrations. This can be explained by the fact that regions with high emissions of anthropogenic NO and mostly low HO2 mixing ratios are almost complementary to regions with high biogenic VOC emissions. The latter represent those areas where the volume ratio of VOC/NOx is larger than 1, whereas VOC/NOx is lower in those regions where anthropogenic emissions are dominant.

Figure 9.

Difference of mean volume mixing ratios of (a) odd hydrogen, (b) formaldehyde, (c) nitric acid, and (d) peroxy actylnitrate between 2031–2039 and 1991–2000 for the months June–August.

[54] Carbonyl compounds produced during the oxidation of hydrocarbons, in particular formaldehyde (HCHO), promote additional production of ozone. The formation of formaldehyde strongly depends on the concentration of isoprene, therefore the increase of the HCHO mixing ratio (Figure 9b) matches the pattern of the isoprene emission in Figure 6a. This also holds for the increase of H2O2 and organic acids (not shown). In the areas of maximum change this corresponds to 25% higher values in the case of formaldehyde and to 15–20% for H2O2. As removal of pollutants by precipitation is not included in the model, the simulated increase in the mixing ratios of soluble compounds can be considered as a lower limit. The change of the concentrations of soluble compounds would even be more pronounced if the simulated decrease in precipitation and wet removal were taken into account.

[55] In contrast to HCHO and H2O2, where the highest mean concentrations and the maximum increase are mostly found in rural regions with high isoprene emissions, the increase of nitric acid (HNO3) dominates in the regions with high NOx emissions (see also Figure 2). Consequently, the highest increase of HNO3 (Figure 9c), which is formed to a large extent by the reaction of NO2 with OH, is mainly found in urban regions.

[56] The mixing ratio of PAN (peroxy acetylnitrate) decreases in most parts of southern Germany (Figure 9d). This is mainly due to higher temperatures, which result in an enhanced thermal decomposition of PAN. Apart from the temperature, the mixing ratio of PAN also depends on the NO2/NO ratio. While NO2 promotes the formation of PAN, NO prevents the recombination of NO2 and the acylperoxy radical. As a consequence, NO2/NO ratios around 10 will lead to an “effective lifetime” of PAN which is about 3 times longer than the one for a NO2/NO ratio of 1–2. A ratio of NO2/NO close or above 10 was already found in the mountainous regions for present-day conditions. This study indicates even a further increase under future climate conditions. An increase of PAN mixing ratios between 0.05 and 0.15 ppb (corresponding to 4–10%) was simulated for these mountainous regions. The decrease of PAN mixing ratios is restricted to regions at low altitudes, where NO2/NO ratios between 1 and 3 are usually found. The simulations show that for future climate conditions a further decrease of this ratio will be likely in these areas.

4. Conclusions

[57] Online coupled regional climate-chemistry simulations were carried out for two time slices with a length of about one decade each, in order to obtain regional distributions of meteorological parameters and photooxidant concentrations for present-day and future climate conditions. The model results show how a possible climate change according to the greenhouse gas scenario IS92a can affect the meteorological conditions and photochemistry in southern Germany within the next 30 years.

[58] For the summer months the simulations show an increase of the length of clear-sky episodes, an increase in near-surface temperature, and a decrease of cloud water and cloud ice. The decrease in cloudiness results in higher insolation and photolysis frequencies at the surface. For the underlying scenario assuming a recovery of the stratospheric ozone layer within the next 30 years the impact of changed stratospheric ozone on UV radiation and JO3 was found to be small compared to the effect of changed cloud cover due to climate change.

[59] The higher incoming radiation for future climate conditions results in higher biogenic emissions of isoprene, which increase regionally by up to 50%, and in an enhanced formation of ozone and other photooxidants. Assuming unchanged anthropogenic precursor emissions, 10% higher daily maximum values of near-surface ozone concentrations were simulated.

[60] Despite of existing correlations between meteorological parameters and daily maximum concentrations, the regions with the highest increase in maximum ozone concentrations are not identical with the regions with the largest increase in temperature, solar radiation or isoprene emission. This indicates that changes in meteorological variables resulting from climate change cannot be extrapolated in a simple way in order to estimate future changes in ozone concentration.

[61] The number of days when the threshold value of 120 μg m−3 for the 8-hourly mean is exceeded were found to increase by 5 days in northern Bavaria and 12 days in the south. A pronounced increase was also found for the occurrence of high ozone concentrations. Considering the average over southern Germany, the number of events with maximum ozone concentrations above 180 μg m−3 increases by a factor of 3.9, which demonstrates that there is a higher probability for very high ozone concentrations in southern Germany under future climate conditions.

[62] Since identical boundary values for the mixing ratios of the chemical compounds were used for present and future conditions, the simulated effect might even represent a lower limit, as higher boundary values for the photochemical compounds can be expected in the future. As unchanged anthropogenic emissions were assumed for the present study, the increase of maximum ozone concentrations due to the more favorable conditions for photo smog situations could be compensated by regional emission reduction measures. However, the increase in the emission of biogenic compounds under future climate conditions should be considered when mitigation strategies are projected.

[63] For the interpretation and a possible further processing of the results it should always be kept in mind that these simulations are not a forecast, but a scenario simulation. The main assumptions comprehend the future development of greenhouse gas concentrations specified by the greenhouse gas emission scenario IS92a, the choice of the stratospheric ozone scenario, and the assumption of unchanged anthropogenic emissions of tropospheric ozone precursors. Further uncertainties can be attributed to unpredictable internal variations of the climate system or unpredictable events such as volcano eruptions. Additionally, shortcomings of the global and regional model may have an effect on the results. However, the global climate model ECHAM4 represents an internationally accepted standard. An intercomparison of global climate models shows that the extent and the patterns of global warming are slightly different for each model, but all models show agreement for the general trend. The regionalization of global models by regional models is also known as a reliable technique. The regional climate-chemistry model MCCM has proven its ability to regionalize global climate patterns and to reproduce observed pollutant concentration fields.

[64] Uncertainties with respect to atmospheric chemistry can result from uncertainties related with the calculation of biogenic VOC emissions, i.e., from the distribution of land cover, the application of a majority based land use, and the specification of BVOC emission factors. Furthermore, the results depend on the anthropogenic emission scenario and on the choice of the lateral boundary conditions for the chemical constituents for future climate conditions. The latter problem could be overcome, if output of global climate-chemistry simulations were available, which will surely be the case in near future.

[65] With respect to the statistical analysis of climate data, ensembles made up of only one decade are still near the lower limit. Therefore the climatological interpretation of the results can still be limited due to the limited length and the choice of the time slices.

[66] Nevertheless, the results of the MCCM simulations presented here can be regarded as meaningful under the premise of the underlying ECHAM4 scenario simulation and the given precursor emissions. The availability of regional distributions of ozone and other photooxidants as well as primary pollutants for present-day and future conditions offers the possibility for further impact studies with respect to ecological, economical, and biological systems [Kesik et al., 2006], and for the development of mitigation and adaptation strategies. However, for future applications the availability of output from global climate-chemistry models is highly desirable in order to increase the significance of the simulation results.

Acknowledgments

[67] This investigation was funded by the Bavarian Ministry for Environment, Health, and Consumer Protection within the joint project BayForUV. The global climate simulations with ECHAM4 were supplied by the DKRZ (Deutsches Klimarechenzentrum) and the Max Planck Institute for Meteorology in Hamburg. The anthropogenic emission data were supplied by IER, University of Stuttgart. The simulations were carried out on three double-processor AMD 1900+ PCs with LINUX operating system. The authors are particularly indebted to E. Haas and J. Werhahn, who insured the permanent availability of the computers. We thank the anonymous reviewers for helpful comments and suggestions. We would also like to acknowledge helpful comments by several colleagues, in particular, W. R. Stockwell and H. E. Scheel.

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