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5.5 Characterisation of the ozone phenomenology from a meteorological perspective
Surface ozone concentrations depend on complex combinations of
chemical processes and meteorological influences. Characterising the ozone phenomenology
in the European Union is therefore essential to better assess the impact of man-made
emissions. The following sections give an overview of the main results of JRC-ERLAP's
study concerning the interpretation of widespread ozone episodes observed in the European
Union in 1994, 1995 and 1996 (Koffi et al., 1998). The first two sections focus on
the characterisation of the Europe scale meteorology associated with exceedances of the
population information threshold (i.e. concentrations above 180 mg.m-3). The
contribution to observed ozone levels of the photochemical formation, horizontal advection
and vertical transport processes are then discussed for eight selected episodes, on the
basis of additional chemical and meteorological measurements and backward trajectory
studies. The tables and figures mentioned in this section are given in Appendix 1.
5.5.1 Ozone exceedances and weather type over Middle Europe
The Grosswetterlagen (GWL) daily classification from Deutscher Wetterdienst in Offenbach (Hess and Brezowsky, 1969) was firstly used as a convenient description of the synoptic meteorology that prevailed during 1994, 1995 and 1996 growing seasons (i.e. April to August periods). It defines 29 different synoptic weather situations over Middle Europe, in terms of cyclonic/anticyclonic circulations and predominant wind direction (Table I.1):
5.5.2 Ozone conducive meteorological parameters
Eight characteristic wide-spread ozone episodes were selected during the 1994, 1995 and 1996 growing seasons, in order to characterise the meteorological situations which are conducive to high ozone levels (Table I.4). The selection was made on the basis of the previous analysis and the temporal evolution of the exceedances of the population information threshold reported in each of the 15 Member States. Daily European scale thematic maps were prepared for the 62 days of the selected episodes for the following parameters (see examples in Figure I.1).
The main conclusions are:
The beginning of the occurrence was always associated with the centre of an anticyclone, or with less well defined high pressure systems. The spatial and temporal evolution of the ozone exceedances area is shown to be usually associated to a corresponding displacement, in time and space, of the high pressure system. The end or decrease of the ozone exceedances were linked either to the arrival of low pressure systems over Europe or to a slackening of the anticyclone, or to a transition between two different but anticyclonic weather types over Middle Europe. Particular differing scenarios are also observed in the spatio-temporal distribution of the ozone exceedances that cannot be explained by the associated spatio-temporal evolution of the anticyclone. Moreover, horizontal advection can also be suspected during the eight episodes.
A good correlation between the highest daily maximum surface temperature and the location of the ozone exceedances is observed for five out of the eight episodes, whereas the main location of exceedances coincided with high surface temperatures also during the three other. On the other hand, no clear connection is observed between the pollution area and the average diurnal surface solar radiation (12-18 hours UTC). The relative sunshine duration, or the daily maximum surface radiation might be more appropriate predictors. Unfortunately, no data is available for these parameters at the required spatial and temporal scales.
The stations which reported ozone exceedances at the beginning of the episodes (i.e. the first and second days) were mainly located within areas with relatively low wind speeds in the lower tropospheric layer, whereas no obvious relationship was observed during the following days. The previous assumption about horizontal advection processes that would be involved during most of the selected episodes is supported in all cases by the wind speed and direction in the low and middle troposphere. The wind fields also provided useful information for the understanding of the above-mentioned special spatio-temporal evolution of the pollution area that were related to wind direction changes, from continental to oceanic, and vice versa. The persistence phenomenon observed during one of the selected episodes was shown to be associated with persistent low winds in the boundary layer, which induced stagnation/accumulation of the pollution.
Whilst the continental thematic maps provide a useful description of the European scale meteorological conditions associated with ozone episodes, they do not allow a detailed interpretation. However, local to synoptic meteorological effects are discernible from additional chemical and meteorological data and backward-trajectory studies, as illustrated in the three following sections.
5.5.3 Photochemical formation of ozone
As shown in the above section for Middle and North European countries, major episodes of high concentrations of ozone are associated with slow-moving, high-pressure systems. The often cloudless and warm conditions, associated with these systems are favourable for the photochemical production of ozone. These systems are also characterised by widespread sinking of air through most of the troposphere, which is warmed adiabatically and thus tends to produce a pronounced high elevation (1-3 km) inversion of the normal temperature profile serving as a strong lid to contain pollutants in a shallow layer in the troposphere. Moreover, because associated winds are generally light, there is greater chance for pollutants to accumulate in the atmospheric boundary layer (National Research Council, 1992). All these conditions lead to accumulation of ozone and other chemical pollutants from one day to the next. As an example, the wide-spread anticyclone observed during the episode of June 1996 ( during a period with a ridge of high pressure over Middle Europe) and the location of the ozone exceedances are shown in Figure I.2.
Previous studies focusing on South European countries showed that the diffusion, transport and chemical cycles of atmospheric pollutants in Mediterranean areas are significantly different from those observed in North Europe. The complex processes that cannot be highlighted from the sparse information collected in these countries in the framework of Ozone Directive, are summarised below:
Examples of the impact of such meso-scale meteorological processes on the surface ozone levels observed in Spain and Greece during the years 1994, 1995 and 1996 are given below:
5.5.4 Long-range transport
A back-trajectory model was used to study the advection processes involved during the eight selected ozone episodes (Koffi, 1997). The backward trajectories calculated for twenty selected reception sites in nine out of the 15 Member States clearly demonstrate:
5.5.5 Downward transport from the upper troposphere and lower stratosphere
Accumulated air pollution in the higher troposphere and even stratospheric ozone intrusion can intrude the lower atmospheric layers, where by vertical turbulence they are mixed down to ambient air masses. There has been a continuing debate since the 1960s/1970s about the role of vertical transport vs. local photochemical formation with regard to the annual tropospheric ozone budget. The current consensus view is that in situ chemical production is the major contributor to the observed ozone levels in the ambient tropospheric air. Nevertheless, the contribution of ozone transport down from the free troposphere may not be negligible (Davies and Schuepbach, 1994).
The ozone episode of May 1995 is in fact a characteristic example of
such vertical exchanges: The wind vertical velocity over Europe shows a subsidence of the
air masses during the three days of high ozone level concentrations, with its centre over
Benelux (Figure I.8). The concentrations of Beryllium 7 (a tracer of air coming from the
upper troposphere) measured in Luxembourg in 1995 show a significant peak during the
corresponding days (Figure I.9). Moreover, peaks of 7Be concentrations are also
observed during the 1995 summer ozone episodes, which suggest a contribution of free
tropospheric ozone in addition to photochemical formation, during summertime.
For references, please go to https://www.eea.europa.eu/publications/TOP08-98/page009.html or scan the QR code.
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