Berlin Environmental Atlas

03.06 Near Ground Ozone (Edition 1993)

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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.

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