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Berlin Environmental Atlas

03.06 Near Ground Ozone (Edition 1996)

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Ozone production and destruction in the troposphere

The up to 100 times higher ozone concentrations in the stratosphere (c.f. 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 (c.f. 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.

Fig. 5: Year Course of the Tropospheric Ozone Concentration of both Hemispheres (35 - 45 °C) (source: German Bundestag 1992)

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 (c.f. Schurath 1984), with aerial ozone sensing and test series from unindustrialized areas, this share lies (c.f. 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. This effect can be seen in the difference between the ozone values taken at the measuring station Grunewald at 4.5 meters and again at 10 meters above the forest stock (c.f. Fig. 7).

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 (c.f. UBA 1996).

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 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 (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 (c.f. 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

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 (c.f. Fig. 6). The necessary spur is provided by the UV radiation up to 400 nm.

Fig. 6: Schematic Display of Photochemical Ozone Formation in the Troposphere (Volz-Thomas et al. 1990)

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 (c.f. Map 03.06.8). 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 (c.f. Fig. 7) 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.

Fig. 7: Average Ozone Daily Course at BLUME-Stations on Summer Days (days with temperatures above 25 °C) in 1994 and 1995

[Statistical base of this Figure is also available as Excel-File (MS-Excel is required).]

Using the typical day courses for high ozone days at various BLUME measuring stations (c.f. Fig. 7) enables a clear description of these events.

The form of the day course curves at the three ground stations Grunewald, 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 takes effect on the city's edges because that is where the pollutants are transported- for example from the AVUS to the measuring station in Grunewald 1.5 km away. This results in a heavier ozone decomposition on the ground and therefore lower values than obtained above the tree crowns. Nonetheless this difference is also a result of the constant decay reactions of ozone with the materials on the ground and with reactive hydrocarbon compounds which the trees discharge especially in the afternoons. The latter could be a reason why this vertical ozone differential can be seen even in the afternoon after 4:00 p.m., a time of good vertical exchange.

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 the value of about 120 µg/m³, 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 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 (Parochialstrasse), 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. The ozone concentration above the near ground cold air layer has remained unaffected. The negligibly decreasing ozone level yields a reservoir at height of the tower for a further rise at the next day.

Since even under fair-weather situations considerable winds can be found at elevations over 300 m 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 (c.f. Map 03.06.4).

In this connection long-term trends of the ozone concentration will be discussed briefly. From Figure 8 it is obvious that in Berlin the average maximum value on summer days indicates no significant trend.

Fig. 8: Median Value of the Daily Ozone Maximums on Summer Days (days with maximum temperatures above 25 °C)

[Statistical base of this Figure is also available as Excel-File (MS-Excel is required).]

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 (c.f. 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. An increase can also be found at the rural stations in Baden-Württemberg although this would appear not to have continued over the last three years (c.f. UBA 1996).

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