Near Ground Ozone 1992
Map 03.06.1: Generation – Decomposition
The natural balance in the stratosphere
As presented schematically in Figure 1, the ozone concentration greatly increases above 12 km altitude in the stratosphere and reaches 20 to 30-fold the values in ground proximity. The reason for this is the generation of ozone induced by the effects of energy-rich solar radiation from space (wavelengths < 240 nm) on the uppermost layers of the atmosphere. The plentiful molecular oxygen O2 available there is split into both its atoms which join subsequently with a still intact O2 molecule to form ozone (O3 ) (cf. Fig. 3 ).
This ozone production stands in balance with the natural ozone decomposition which depends on the absorption of less energy-rich radiation (wavelengths by 200 to about 300 nm and weaker). Since ozone has a slighter bonding energy than oxygen, O2 and a single oxygen atom are created when these molecules decompose (cf. Fig. 3). This atom can bond itself again with an oxygen molecule to form O3, so that in the balance the ozone loss remains first of all slight. If one calculates the global ozone distribution alone under consideration these reactions discovered by Chapman in 1930, the actually observed density of the ozone layer in the stratosphere must have about 50 % more ozone and incorrect vertical distribution. Thus there must still be other ozone destroying reactions about which more will be said below.
At altitudes over 30 km, photochemical balance prevails and the atmospheric transport plays hardly a role for the ozone distribution. The highest ozone values are found in the area with the highest amount of radiation. That means at the equatorial regions and decreasing toward the poles.
In layers between 15 and 30 km high the ozone distribution is clearly influenced by the horizontal and vertical transport processes. The average global distribution of the total ozone (cf. Fig. 4), which is determined to over 70 % by the stratospheric ozone in this layer, shows a minimum of around 250 Dobson units (DU) in the equator region and an increase toward the poles.
Since in the tropics through the heavy weather activity an ascending air movement prevails, there low-ozone air climbs from below into the stratosphere. There it is transported in a meridianal direction to the poles and there it sinks again. Because of the high UV-radiation the largest ozone production occurs in the tropical and subtropical stratosphere, so that polewards because of the transport process the ozone values rise to over 400 DU. Since the meridianal air movement is most predominant in the late winter and spring of the respective hemisphere, the ozone maximums are to be observed in the higher latitudes of both hemispheres respectively around this season. In the respective summer, the total ozone values sink and reach their minimum in the late fall. As an example for the annual cycle in our latitudes the average course of the total ozone values (and their standard deviation) over many years at the observatory in Potsdam is presented in Figure 2.
For several years, the spring ozone maximum in the Antarctic, with on average over 340 DU has exhibited a dramatic decline to less than half. In the Antarctic spring 1993 (September/October), the total ozone quantity sank there over a wide area to even under 100 DU. At the beginning of the summer the ozone layer thickness rose again nearly to normal values. This can be traced back to the movement of ozone-rich air from lower latitudes. This is practically suppressed in the winter and up to the beginning of the spring because of the stable wintry polar vortex over the Antarctica. The cause of the rapid ozone decomposition is still to be discussed.
The phenomenon known as the “ozone hole” has no counterpart in the northern hemisphere. There the meridianal transport processes start earlier because the wintry polar vortex over the north pole disappears sooner. Indeed a gradual decomposition of the ozone layer thickness is also to be observed in the northern hemisphere. With 7 to 9 % in the last two winters 1992 and 1993, it had clearly increased – with reference to the value 10 years before, end of the 80s in winter over North America and Europe about 3 % (cf. German Bundestag 1990). In the summer, the decline is significantly lower here as well as in the equatorial regions. This is reflected also in the Potsdam data from 1993 (cf. Fig. 2). Almost throughout the year, the values lay clearly below the long-time average values and occasionally even below the standard deviation.
The anthropogenic destruction of the ozone in the stratosphere
Since 1974 when the two American scientists Molina and Rowland alarmed the world public with their thesis of the destruction of the ozone layer by human-induced trace gas emissions, it has become even more clear that the complex aerochemical balance in the atmosphere can be easily disturbed through anthropogenic activities. Also trace materials, which due to their chemical inertia endure the long transport from the ground to the stratosphere or are introduced there directly by airplanes and volcanic eruptions, contribute to this effect. Like the atmospheric oxygen, they are also broken down by the energy-rich solar radiation into their elements of which some massively react on the chemical balance at the expense of the ozone.
The best known human-induced trace gases are the chlorofluorocarbons (CFC) and the related halons. These are hydrocarbon compounds in which one or several hydrogen atoms have been replaced by fluorine and chlorine and/or bromine. On account of their chemical inertia, these are industrially versatile materials (coolant, solvent, propellant etc.) Their worldwide output has reached the considerable quantity of 1 mil. t/a. The only known compound from this class of materials with a natural source is methyl chloride (and methyl bromide), which is emitted by the oceans into the atmosphere. It contributes however only 10 to 20 % of the chlorine content of the stratosphere, which is responsible for the ozone decomposition located there.
A further important material class with both natural as well as anthropogenic sources are the nitrogen oxides. They play a role in form of laughing gas (N2O), which is introduced both as consequence of bacteriological processes in the ground as well as through the increased discharge of nitrogen fertilizer in the atmosphere. Also NO as an element of airplane exhaust fumes counts as an ozone-decomposing element.
Water plays likewise a role in the stratosphere as an ozone-decomposing substance. Due to the extremely low temperatures at the lower edge of the stratosphere considerable water quantities arrive however only through the air traffic and volcanic eruptions in the higher atmosphere layer.
Catalytic ozone decomposition
The significant ozone-decomposing effect of these trace materials is without the influence energy-rich solar radiation independent of the time of day. They stands in contrast to the incredibly low concentrations in which these materials occur in the atmosphere. The catalytic decomposition reactions sketched in Figure 3 can mean, for instance, that one CFC molecule in a billion other air particles is responsible for the destruction of several thousand ozone molecules. The ozone-decomposing elements of the trace materials in Figure 3 substitute for the different substances marked X and lie again in their original form namely at the end of a reaction chain. They can perform their destructive work repeatedly before they are removed from circulation in part only after several years by other chemical reactions.
The catalyst thereby attacks two-fold at the expense of the ozone in the chemical balance: On one hand it transforms an ozone molecule into oxygen O2 and bonds with the remaining oxygen atom. On the other hand, individual free oxygen atoms are consumed at this degeneration of ozone in oxygen, which are no longer available for ozone formation (cf. Fig. 3).
Chlorine and bromine which originate predominantly from CFC and/or the halons, as well as NO and OH radicals, work as catalysts. The latter come from steam, methane and hydrogen and originate from predominantly natural sources.
The simplicity of the current depiction belies the intense complexity and non-linearity of the aerochemical processes in the stratosphere. The different reaction cycles are coupled with each other strongly and in different ways. For instance, BrO, NO or OH can be involved in the reverse process of ClO to Cl instead of an O atom as partner. A further important role is played by heterogeneous reactions in which the ozone-decomposing materials are conveyed in harmless substances or conversely from ozone-neutral reservoir compounds and are activated as catalytic substances again. This explains the sudden significant decomposition of the ozone layer in the spring over the Antarctica.
The reasons are chemical reactions in the stratospheric clouds. They are composed of ice and nitric acid particles, on which the chlorine from compounds forms residues which are stored before and withdrawn from the ozone decomposition cycle. The clouds emerge only at temperatures under minus 80 °C and are observed predominantly in the area of the stable wintry polar vortex over Antarctica. Energy-rich solar radiation is also necessary for the reactivation of the chlorine, which is available first at the end the polar night, therefore at the beginning of the Antarctic spring. In this period the chlorine is released in large quantities from the reservoir of substances formed in the winter. Since the supply of ozone-rich air transmitted from the lower latitudes through the polar vortex is suppressed until the spring, the massive ozone loss over the Antarctica takes its course. Through the transport processes which begin again in the summer, the ozone deficit is again largely equalized.
In the northern hemisphere, a comparably large and long existing polar vortex does not exist, so that the low temperatures necessary are reached only rarely or short-term and meridianal air currents occur more frequently. The polar clouds necessary for heterogeneous reactions are available therefore in very much slighter density and the ozone decomposition is less dramatic. The balancing out of thin places in the ozone layer through south-north transports of ozone-rich air is more likely. Therefore decomposition comparable to the Antarctic ozone hole in the ozone layer over the northern polar region is not to be expected. Indeed the question was put lately, whether through the increasing greenhouse effect a cooling of the stratosphere is provided and supports the strengthened formation by stratospheric clouds also in the northern hemisphere. This would lead to a heightened decomposition of the ozone through heterogeneous reactions.
Further influential factors
As mentioned at the outset, volcanic eruptions also influence the thermal structure and the chemistry in the upper atmosphere. Due to the high intensity of the energy released at an outbreak, material and gases are shot into the stratosphere, whose chemical composition draws from that of the early earth’s crust and differ therefore very greatly from the present composition of the atmosphere. It is suspected that they directly or indirectly favor the origin of the above described polar stratospheric clouds. Besides the ozone-decomposing chlorine is brought directly in the stratosphere in the form of hydrochloric acid. As measurement results from a large laid out research program 1993 showed, over the northern polar region, an increasing amount of dust particles was found which had originated from the Pinatubo eruption in 1991 and spread out over wide parts of the hemisphere. Indeed there was no spatial coincidence between the incidence of volcano dusts and the stratospheric clouds which appeared in the area of the northern polar vortex in the winter 1992/93. The thinning of the ozone layer at this time can not be explained directly as a result of volcanic eruption. In contrast to the relatively rare emissions through volcanic eruptions, the ozone layer is not in the position from which to recover from the decimation of continuous CFC emissions.
This also applies to the possible increase in the supersonic air traffic in the stratosphere, through which nitrogen oxides and water vapor, which contribute directly and indirectly to ozone decomposition, are directly emitted.
Which consequences a further increase of the trace gas emissions on the ozone layer will have in future, can only be estimated with the help of mathematical model calculations because of the numerous influence factors. Most of the scenario calculations which have been performed up to now consider only the effect of the catalytic gas phase reactions and with it only a part the ozone-decomposing effects.
The results of these calculations show that the insufficient measures of the 1987 Montreal Conference (only 50 % emission decrease by 1999 with numerous exceptions) allow the chlorine content in the stratosphere to quadruple by the middle of the next century, which would lead to an ozone depletion in the stratosphere of around 30 %. As a consequence, the subsequent penetration of the atmosphere by UV radiation into troposphere is calculated to produce a robust rise of the ozone concentration of around 20 %. Thereby the weakening of the UV-filter effect of the stratospheric ozone layer is partially compensated, so that the model calculations of an increase assumes an increase in the cell damaging UV radiation at the ground of 4 to 10 %. Indeed a generally higher ozone level in the lower atmosphere would allow the frequency of injuriously high ozone concentrations to climb there still further, where because of the photochemical formation of ozone an excessively high burden already prevails.
If unhindered CFC emissions are considered in the long-term consequence assessment along with the most important heterogeneous reactions which lead to creation of the Antarctic ozone hole, then the increase of the UV radiation at the ground in the global average can reach 20 to 25 %.
Damage to the ecosystem
Since the area marked as UV-B short-wave radiation (290 to 330 nm) has a cell damaging effect, a significant increase in intensity has direct negative effects on the animals and flora and also on humans.
Indirectly humans are affected as the last link in a long food chain by a possible decline of plant growth. Under these circumstances an important reciprocally strengthening connection rests also itself between ozone decomposition and global temperature increase. The photosynthesis performance of plants is damaged by a higher UV dose, particularly that of the phytoplanktons in the ocean. It draws as much carbon dioxide from the atmosphere through its metabolism as do all terrestrial plants together. If the increasing UV B radiation would deaden for instance 10 % of the plankton, almost so much less carbon would be removed from the atmosphere annually, as the entire mankind discharges through the fossil fuel combustion. The result would be a further reinforcement of the greenhouse effect and with it a further warming of the earth’s atmosphere.
An increasing UV B radiation intensity can become a direct danger since larger radiation quantities, when hitting the skin, induce a carcinogenic effect and can cause over and above this damage to the eye lens (gray star). Since the damage gains with the reduction of the wavelength and with a weakening of the ozone layer the short-wave spectrum of the UV light becomes stronger, a thinning of the ozone layer by 1 % can lead to an increase in skin damage of around 1.7 %. Due to the strong seasonal fluctuations in UV intensity, in the summer four to five-fold the usual winter values with a cloudless sky, the ozone layer thickness in the summer half-year plays a role in the assessment of a possible additional health threat.
Also there is a fair-sized variation of the UV radiation with the geographical latitude because of the different distances for the radiation through the atmosphere. The solar radiation is during summer with cloudless skies around 40 to 100 % more intense in the Tropics than in our latitudes. Therefore the clear rise in skin cancer illnesses can also be traced to the travel fever of an increasing number of persons whose skin is only genetically adjusted to the slighter radiation intensity in higher latitudes and is exposed to the more intense solar radiation in the equatorial vacation areas.
Long-time measurements of the UV B radiation show a significant increase up to now only at stations in New Zealand, South Australia and South America. Presumably the slightly declining filter effect of the stratosphere has been equalized in Europe and the USA through an increase in the ozone and the dust content in the lower atmospheric layers.
Ozone production and destruction in the troposphere
The up to 100 times higher ozone concentrations in the stratosphere (cf. Fig. 1), in comparison with the near ground values, contribute because of the vertical exchange to the fact that ozone occurs naturally also in the near ground air layers. Indeed this contribution is limited, since according to geographical latitude in 8-15 km altitude between both stories of the atmosphere there is a blocking layer. This so-called Tropopause forms the lower edge of one globe encompassing temperature inversion, i.e. the temperature does not drop with altitude but increases. This increase is caused by the warming of the air through the above described filter effect of the ozone layer. It prevents the mixing of the larger mass of warmer and relatively seen, lighter ozone-rich stratospheric air with the colder tropospheric air lying below. The anvil form of high-reaching thunderclouds, which appear to expand at the Tropopause like at an invisible room ceiling, comes about in this way.
The vertical transport of ozone from the stratospheric ozone layer downward is thus only relatively weak. It occurs predominantly in the area of low pressure areas. Its contribution to the increased ozone concentrations, which occur at high pressure weather conditions with high solar radiation, is accordingly very slight. It is at the beginning of the spring the strongest because at this time both the ozone layer in the stratosphere as well as the low pressure activity have reached their average maximum.
As expected, the average year course of the ozone concentration in the southern hemisphere in Figure 5 shows the maximum to be in the spring. Historic ozone measurements from the previous century, taken near Paris, show a qualitatively equal course (cf. Volz-Thomas et al. 1988). The vertical transport of ozone from the stratosphere is in the southern hemisphere, resp. was in the pre-industrial era also in Europe, the main influential factor for the ozone level in ground proximity. The year course registered today in the northern hemisphere, with a maximum in the midsummer, shows that a further very weighty element for the formation is the near ground of ozone has been added. It is caused predominantly by emissions of pollutants originating from anthropogenic combustion processes which are relatively low in the southern hemisphere.
How high the ozone level caused through natural processes in our latitudes is can only be estimated imprecisely since there is hardly a test series which remains unaffected by the anthropogenically formed ozone. On the basis of measurements on mountain stations (cf. Schurath 1984), with aerial ozone sensing and test series from unindustrialized areas, this share lies (cf. Logan 1985) between 50 and 90 µg/m3. Similar values have been obtained on the radio tower in Frohnau in 324 m elevation if fresh, clean polar air masses has reached the Berlin region from the North Sea.
Figure 1 shows a schematic ozone profile with a continual decline in the ozone at decreasing elevation is conditioned by the natural decomposition of the ozone on its way to the earth’s surface and at the ground through contact with materials.
In addition, there are also predominantly anthropogenically induced decomposition processes. Ozone reacts as strong oxidant with other substances characterized as pollutants and assumes thereby an important cleaning function in the atmosphere. Sulfur dioxide, for instance, is transformed by ozone into sulfate and into a fine dust which precipitates either on the ground or is washed out as acid rain. Still important in this connection is the reaction of ozone with nitrogen oxides. They are discharged as a final product from nearly all combustion processes as nitrogen monoxide which reacts immediately with ozone. Therefore a lower ozone concentration is to be found in most cases within conurbations and industry regions, in other words where pollutants and particularly NO are emitted. The number of the infringements of the EC notification level of 180 µg/m3 is in many so-called pure air areas and at the edge of the cities similarly high or even higher than in the centers of conurbations.
To reach these frequent infringements of the threshold values, there needs to be an additional formation process for ozone on the ground. The requirement for it is, like for the above described generation in the stratosphere, the availability of free oxygen atoms. Indeed the splitting of oxygen molecules in the lower atmosphere layer is not possible for want of energy rich radiation. Instead the nitrogen dioxide (NO2) functions as supplier of the oxygen atoms. It is the only material, which can become photolyzed already at the lower energy rich radiation in ground proximity and deliver individual oxygen atoms:
NO2 + light (wavelength 300-400 nm) = NO + O
O + O2 = O3
Globally considered, 60 % of the nitrogen oxides discharged into the atmosphere originate from anthropogenic sources. The remaining part is predominantly the result microbacterial processes in the ground (cf. German Bundestag 1990). In highly industrialized Central Europe this part is insignificant in contrast to the nitrogen oxide quantities originating from the numerous combustion processes. Thereby 90 % of nitrogen oxides are emitted however as nitrogen monoxide (NO) which must be transformed first through oxidizing processes into nitrogen dioxide. As mentioned ozone itself also plays an important role since it changes the NO into NO2 and is thereby decomposed:
NO + O3 = NO2 + O2 (misprint in Map 03.06.1)
The most important decomposition reaction for ozone is near nitrogen oxide sources. The decomposition runs within seconds and minutes because of the fast reaction time.
The ozone formation from the photolysis of NO2, occurring in the vicinity of conurbations, is partially compensated through the reaction with nitrogen monoxide. Since however high ozone values sometimes also appear, there have to be additional processes, which convert freshly emitted NO into ozone-forming NO2, without allowing ozone to function as an oxidant and thereby decompose.
The requirement for it is the availability of carbon monoxide (CO) and different reactive hydrocarbon compounds (HCs), which with OH and peroxyradicals (HO2) within more multiple and complicated reaction schemes cause the oxidation of NO to NO2 without ozone consumption and so shift the chemical balance in the direction of ozone formation. Thereby it is just the simultaneous emission of hydrocarbons and nitrogen oxides, which at accordingly high solar radiation and air temperature make possible the formation of high ozone concentrations. CO and hydrocarbons work as fuel for photochemical ozone formation. Radical (OH and HO2) and nitrogen oxides (NO and NO2) play the role of catalysts, without which no ozone is formed (cf. Fig. 6). The necessary spur is provided by the UV radiation up to 400 nm (in Map 03.06.1 it must also be called hv3 in the middle Troposphere).
The speed with which these formation reactions proceed is very different and is intensely non-linear with respect to meteorological conditions and the concentration and composition of the predecessors involved. A cause analysis of high ozone concentrations alone from the measuring courses is therefore quite difficult. Therefore to illuminate the connection between emission, meteorological conditions and ozone concentration model calculations are used in which the chemical processes and atmospheric transport processes are simulated. Using the measuring courses of ozone it can still be ascertained that the formation of ozone proceeds relatively slowly compared with its destruction through NO, with a time scale from several hours up to days.
Despite its avidity, numerous measurements with airplanes, on mountain stations and finally at the Frohnau tower measuring point show (cf. Map 03.06.5) that ozone remains in the free atmosphere over several days. In the course of midsummer weather conditions with strong solar radiation and photochemical ozone formation high ozone concentrations can develop in the near ground air layers. Besides for dynamic reasons the vertical mixing remains in the lower 2,000 m limits in the area of high-pressure areas even in the afternoon. This favors the enrichment of ozone.
At night a temperature inversion develops under clear skies by cooling of the ground which almost completely paralyzes the vertical air exchange. The decline of the ozone values thus occurs only in the lower 100 m to ground proximity. In the layer above it the higher ozone level of the past day almost completely remains. It reacts on the next morning when the sun has warmed the cold air at the ground and the vertical air exchange starts, as ozone reservoir, so that the ozone concentration also rises quickly at the ground.
Since even under fair-weather situations considerable winds can be found at elevations over 300m because of the lack of ground friction, a transport of ozone also over larger areas is to be expected. This is also the reason that increased ozone concentrations are not spatially narrowly limited phenomena, but usually occur, like high air temperatures, over wide areas (cf. Fig. 5 and 7).
Map 03.06.2: Mean Day Course on Summer Days 1992
The map shows the medium day course of the ozone concentration at several stations of the Berlin Air Quality Monitoring Network.
The form of the day course curves at the three ground stations Heiligensee, Mitte and the freeway are to be explained in first approximation by the superimposition of the course of nitrogen oxide emissions through the motor vehicle traffic and the exchange conditions of the atmosphere. The ozone minimum is to be found between 5.00 and 7.00 o’clock in the morning hours. At this time the motor vehicle traffic is already quite heavy, the nightly ground inversion however still pronounced. Hence it is virtually impossible for the ozone-decomposing nitrogen oxides to move upward and/or the ozone-rich air from the top to move downward. The ozone decomposition on the outskirts of town is effective also, because on one hand the pollutants from the city are transported there also, e.g. to the most leeward Heiligensee measuring station. On the other hand a reaction of the ozone with the materials at the ground occurs in the nightly cold air layer also.
The conditions at 324 m elevation, recorded by the gauge at the radio tower in Frohnau, are completely different. There the ozone level remains at summer days on average above the MIK value of 120 µg/m3 (cf. VDI Guideline 2310), because this air layer is isolated at night by ozone-decomposing processes on the ground. This changes in the morning when the sun has warmed the blocking layer on the ground so far that the vertical air exchange starts. Then the tower measuring point is temporarily affected by the polluted air ascending from the ground, in which slighter ozone concentration is present. The minimum at the tower normally appears between 9.00 and 10.00 o’clock in the morning.
At this time the ozone concentration at the other stations has already risen noticeably because ozone from the superimposed storage layer is transported to the ground. The station Heiligensee and the tower measuring point display a roughly parallel course. The further thinning by the ozone-decomposing pollutants and the photochemical processes, urged on through the intense solar radiation, have caused the ozone concentration in the entire lower atmosphere to rise further.
Near nitrogen oxide sources, particularly at the city expressway and somewhat more weakly at the station Mitte, the ozone-decomposing effect of the freshly issued pollutants also remains clearly perceptible in the afternoon. Indeed the increase in traffic during the late afternoon rush-hour hardly has any impact at all. The horizontal and vertical air exchange assure a relatively fast thinning of the ozone-decomposing pollutants. First in the evening, when the wind and also the vertical transport become weaker, the ozone concentration decreases greatly, accelerated by the nitrogen oxide emission of the persistent motor vehicle traffic in the evening. That this quiets down considerably in the second half of the night can be recognized at the light increase of the ozone at the city expressway. The ozone concentration above the near ground cold air layer has remained unaffected. The negligibly decreasing ozone level yields a reservoir for a further rise at the next day.
In this connection long-term trends of the ozone concentration will be discussed briefly. From Figure 8 it is obvious that in the relatively short measuring period in Berlin the average maximum value on summer days indicates no significant trend.
Indeed a 1 to 2 % increase in the ozone concentration per year since the mid- 70s is to be assumed for other stations, particularly far away from conurbations, e.g. on the Zugspitze (cf. German Bundestag 1990). This increase of the wide area background concentration is probably due particularly to the rise in traffic emissions in the 70s and 80s. Whether for instance the decline of the hydrocarbon output brought by the decommissioning of two-stroke vehicles in the new federal states will provide an improvement in the ozone burden in Berlin can first be answered in a few years.
Map 03.06.3: Infringement of Standard Values 1992
In this map the average spatial distribution of the ozone concentration in Berlin is shown. The days were selected on which individual stations of the Berlin Air Quality Monitoring Network recorded measurements in excess of threshold values. The choice of the threshold values was the “maximum pollution concentration” (MIK) according to VDI 2310 with 120 µg/m3 as half-hourly average and the threshold value of the EC Guideline on the notification of the population with 180 µg/m3 per hour average (cf. Tab. 1).
To make clear the dependence of the ozone burden on the local nitrogen oxide emission, the frequency with which the values were exceeded was collated with the distribution of nitrogen oxide emission from traffic. In the comparison of both statements it becomes clear that high nitrogen oxide emissions lead to clearly fewer infringements of the ozone threshold values in the vicinity of a measuring point. So the MIK is overstepped at the city expressway value four times more rarely than at the stations on the outskirts of town, where in the summer the value of 120 µg/m3 an was overstepped on more than 100 days. The values at the remaining measuring points, not lying directly on main thoroughfares in the inner city lie at 70 to 80 infringements between them. The station Mitte lies clearly under it with 57 days. However, due to the central situation western which run past busy streets has reacted and because of a part of the ozone with the nitrogen oxides. The photochemical formation mechanisms, for the most part, cannot compensate for this decomposition because the ozone forming are processes described above are complex and take time during which the air has already moved to a large extent from the city to the outside areas and the surrounding countryside. The Berlin forests lie however as a rule in these areas and therefore in the influence area. Thus for the year 1992 limits to the protection of the vegetation and several indicators listed in Table 1 were overstepped at the forest measuring point Grunewald. For instance the daily median value from the EC Guideline of 65 µg/m3 was overstepped on 141 days and the WHO value of 60 µg/m3 for the entire vegetation time was clearly exceeded, with 81 µg/m3 actually measured.
A similar picture emerges for the infringement of the EC threshold values, for which behavioral recommendations for sensitive persons are published by the Berlin Department of Urban Development and Environmental Protection. On the outskirts of town this occurred from 19 to 23 days in 1992. In the inner city residential areas this happened from 11 to 17 days, in Mitte only six times and at the city expressway not at all. The respective infringement numbers at the measuring points are in first approximation transferable to other urban areas with more comparable nitrogen oxide emissions.
The infringement frequencies at the tower measuring point in Frohnau are very much higher. It could already be seen from Map 03.06.2 that there ozone decomposing effects hardly play a role and therefore high concentrations are more frequently measured. Also at the measuring point Grunewald the infringement frequencies of the MIK value in the sampling at 10 m elevation above the trees are somewhat higher than in 4 m amount in the forest, where ozone can be decomposed at the plant parts. So that is joined however at the same time a plant damaging effect, which presents additionally an essential part for the impairment of the entire forest ecosystem (see results of Long-Term Ecological Monitoring Program for Forest Ecosystems, cf. SenStadtUm 1993).
Maps 03.06.4, 03.06.5 and 03.06.6: Case Study with High Ozone Concentrations in August 1992 in Berlin
To best be able to present the interaction of the different meteorological parameters and other factors on the ozone concentration in Berlin, a sample episode with high ozone load from 5 to 9 August 1992 is described (cf. Lutz 1994).
The then prevailing general weather situation was determined by a high-pressure area over Central Europe, which itself shifted south on 6 August. Thereby cool sea air from the North Sea could advance up to the Mittelgebirge and also temporarily displace the previously very warm air mass in the Berlin region. On 8 August, however, the boundary shifted of this subtropical air mass again over Berlin and northward, so that on this day the temperatures climbed from the previous 27 °C to almost 35 °C.
Map 03.06.4 Influence of the long-distance transport from 7 August 1992 (low) and 8 August 1992 (high)
The varying origin of the air arriving in Berlin on 7 and 8 August can be seen using the calculated path (trajectory) of an air package, which arrives in the afternoon in Berlin on 7 and/or 8 August, respectively. The path traveled was calculated using a model for the three-dimensional distribution of measured air pressure, wind and temperature values in Europe, developed at the Meteorological Institute of the Free University of Berlin (cf. Reimer et al. 1991).
On 7 August, the air moved in only 36 hours from the mouth of the English Channel to the North German lowland all the way to Berlin and thus assuming a relatively maritime character. In the air mass division of the Berlin weather map, it is classified as warmed subpolar sea air. The air mass on the next day originated on the contrary from the south of France and moved relatively slowly from southwest to the northeast from Germany onward. The distribution of nitrogen oxide emission in Germany should give an impression of the amount of the encumbrance which the air mass has absorbed on their way after Berlin at ozone forming materials. This depends once on the contact to the emission concentrations in the trajectory and on the other hand by the speed with which the corresponding areas are covered. The path of the air from 7 August crossed the northern part of the Dutch-Belgian industrial region quite quickly. Since also in the further course the trajectory over north Germany no emission concentrations were crossed, the encumbrance of the air with nitrogen oxides and hydrocarbons on 7 August can be seen as relatively slight. The comparatively long traveling time and the crossing of the emission areas in Baden Württemberg, Saxony Anhalt and Saxony might have led to a by far higher encumbrance of the air mass on 8 August.
Map 03.06.5 Correlations of different parameters from 7 August 1992 and 8 August 1992
The graphic shows the course of different measurements from 7 to 8 August. With the meteorological quantities which were registered altogether at the measuring point in Schöneberg, is to be read the different air mass character. The temperature climbed in the afternoon the 7 August despite intense solar radiation only on 27 °C, which along with the northern wind direction on the subpolar air mass source points. On this day, the ozone at both stations Mitte and radio tower Frohnau still remains under 150 µg/m3. In Mitte the ozone concentration is clearly reduced in the early hours and after 18.00 o’clock in comparison to the curve in 324 m elevation. The curve in the diagram below shows at this time especially high NO2 concentrations, which depict the final product of the ozone decomposition through NO. On account of poorer air exchange in the early and evening hours, the ozone is decomposed by the traffic exhaust fumes emitted in the inner city. The NO2 maximum at the station Mitte in the afternoon of 7 August is also caused by increased traffic emissions. The rise of the ozone concentration is however hardly affected by it, because the intense solar radiation intensifies the photochemical formation of ozone and the air exchange thus ozone-rich air is transported again and again to the measuring point. Therefore differences in the afternoon between the concentration at the tower and the station Mitte are hardly distinguishable.
At the Funkturm (radio tower), the NO2 concentration was very low on 7 August, so that the ozone level reached there during the day continued particularly into the night of 8 August. In the second half of the night, it climbed even further to more than 180 µg/m3, although the NO2 concentration also increased at the same time. At the same time the Berlin region was subjected to a subtropical warm air mass from the south. This warm front covered all of northern Germany on 8 August, and subsequent days leading to temperatures of 35 °C and more. Also the ozone burden reached a clearly higher level than that of the day before, although the global radiation curve no longer reached such high values. This can be explained by the increasing turbidity of the air, which can be clearly seen in the significantly higher dust concentration on 8 August. This points, along with the higher NO2 values at the tower, to an increased pre-pollution of the air mass. The appraisal of the air trajectory depicted in Map 03.06.4, which reached Berlin on 8 August, came to a similar result. If one considers the nationwide distribution for the ozone value measured two days before the afternoon of 6 August, (cf. Fig. 7) one can see that the air which reached Berlin on 8 August in the afternoon, was found 48 hours previously still over Baden Württemberg. There at this time already ozone concentrations far over 200 µg/m3 had been registered. This was quite different in the case of the air which arrived in the afternoon of 7 August: it originated from the Northwest German region in the area of cool sea air with low ozone concentrations. The significant rise in the ozone values in Berlin from 7 to 8 August is thus predominantly a consequence of the changed characteristics the transported air mass, which displayed a higher level of precursor materials and ozone on 8 August. Also the nighttime rise in the ozone values at the tower on 8 August around 3.00 o’clock is consistent with this conclusion. At this time local ozone generation is to be excluded because of the lack of radiation. The strong intrusion of the ozone in the morning is, as presented in the discussion of Map 03.06.2, the result of initial vertical transport of polluted and with it low-ozone air from the ground to the tower measuring point.
The dramatic increase of the dust concentration in the late afternoon of 8 August can be traced to a thunder storm which had exploded over the city. Dust deposited on the ground had been raised through the strong wind gusts preceding the rain. The point in time at which the storm began had been documented using the abrupt decline of the temperature. The ozone concentration dropped noticeably only at the station Mitte, which points however to simultaneously climbing NO2 concentration caused by the decomposition of NO. At the tower, there was no change despite the storm. Since ozone is hardly water-soluble, it is not diluted through the rain. Since the storm emerged only locally and not in connection with a clear weather change, the high ozone level remained in the air mass also through the night and into the next day.
Map 03.06.6 Highest half-hour median values on 7 August 1992 and 8 August 1992
The map shows the maximum half-hour values at the stations supplied with ozone instruments of the Berlin Air Quality Monitoring Network on 7 and 8 August 1992.
Independent of the location of the measuring point, the significant rise in the ozone values from 7 to 8 August becomes once more clearly noticeable. This can be traced predominantly to the change in the national burden for ozone and/or the precursor materials. That local effects contribute very little to the ozone production within the city boundaries can be seen already in the slight range of variance among the maximum values at the individual stations, if one discounts the station dominated by the destructive process at the city expressway. If the urban emissions would have provided a significant contribution to the ozone production within the city boundaries, the values would have to be higher at the stations lying, relative to wind direction, behind the center of town. The Map 03.06.5 shows that on 7 August, in the afternoon, the wind came from the northeast. Unfortunately, the measuring point in Berlin Buch on the northeast outskirts which has since been placed in service, did not exist then. Thus a Luv – Lee comparison is not possible. The potentially helpful comparison between the 170 µg/m3 over the Grunewald with the values measured at the Funkturm (160 µg/m3 ) yields only differences within the measuring precision of the instruments.
It is more interesting to examine the maximum value recorded on 8 August. The course of the wind direction from Map 03.06.5 points to a significant wind turn from the east (in the period until 14.00 o’clock) to the south from 15.00 to 16.30 o’clock. In this period the up to now highest half-hourly value of 293 µg/m3 was measured at the leeward station Heiligensee located in the northwest. It totaled well over 30 µg/m3 more than the maximum at the station near Luv in Marienfelde. What cannot be seen from the map, namely the relatively slight wind speed of under 1.5 m/s and the simultaneous increase in ozone and nitrogen oxides after the wind turn in Heiligensee, points to the additional formation of ozone from the urban emissions of the ozone precursor materials at the leeward northern outskirts. This local contribution to the ozone burden within the city boundaries is even at these low wind speeds, with somewhat more than 10 %, relatively slight. At the commonly higher wind speeds, the measurable increase might be only more in the surrounding countryside. An essentially higher ozone forming influence from precursor emissions has already been shown using airplane measurements in the leeward neighborhood of conurbations also (cf. Fricke 1983). Still it can be seen that local summer smog regulations, with short-term emission reducing measures imposed, do nothing to change the existing wide-area ozone burden, as exemplified in Figure 7. Only a relatively slight share of the ozone burden (here approx. 10 %) at the leeward outskirts is susceptible to local influence. The ozone concentration in inner cities is, because of its strong dependence on primary ozone-destroying pollutants only reducible through a clear wide-area and (!) local decrease the nitrogen oxide and hydrocarbon emissions.
Measures for the Protection of the Stratospheric Ozone Layer and for the Decrease of Near Ground Ozone Loads
As a measure against high ozone concentration in ground proximity, small area and short-term steps taken to decrease the emission present of ozone forming materials hardly constitute an effective means. If altogether ad hoc limits have a significant consequence on the ozone level, then only, if they are taken simultaneously for a large area. More meaningful would be nationally and internationally agreed concepts for (at least) halving the emission of precursor materials and that as soon as possible and not first in four years, as anticipated in the EC Guideline.
To prevent the thinning out of the ozone layer in the stratosphere globally or to limit it, reduction of the CFC emissions and the decrease in air traffic in the stratosphere must be striven for as fast as possible. A first attempt, as part of the Montreal conference of 1987, proved unsuitable because of its excessively long time limits for ending emissions and its abundant exceptions. At the follow-up conference in 1990 in London, a production stop could be agreed for most CFC compounds by the turn of the millennium. In the European context, this should happen by 1996. In Germany, the manufacture and application of most CFC has been forbidden since 1995. Exceptions have been granted until the year 2000 for partially halogenated materials which have a decreased ozone destroying potential. Decisive for a worldwide reduction of CFC emissions will be the assistance granted to developing and threshold countries to win them for the speedy decommissioning of their not insignificant CFC production capacities and grants of financial aid and support in the manufacture of substitute materials.