Appendix 3 Ozone in southern Europe

Appendix 3: Ozone in southern Europe

Based on a contribution by Millán M. Millán, CEAM, Spain

In 1973 the European Commission began supporting research into the physico-chemical processes which govern the dynamics of air pollutants in various regions of Europe. In particular, the six European Remote Sensing Campaigns (Guillot et al. 1979; Guillot 1985; Sandroni and De Groot 1980; Le Bras 1988) were instrumental in documenting that polluted airmasses were mostly advected in Central Europe, i.e., Belgium (Ghent), France (Cordemais) and in the UK (Drax), while they showed marked diurnal oscillation cycles in the airsheds of Southern Europe, i.e., France (Lacq, Fos-Berre) and Italy (Turbigo). The hypothesis that pollutants could be re-circulated in some regions of Southern Europe formed the basis of other EC projects dealing with the dynamics of photo-oxidants in the Mediterranean Basin (Millán et al. 1991, 1992, 1996, 1997). In these projects, air pollutants were regarded as tracers of opportunity of the atmospheric flows in mid-summer (July).

Therefore, more knowledge is now available about specific meteorological processes in the Mediterranean and their links from the local to the sub-continental scales as shown in Figure 1. Other EC-supported projects (Le Bras 1993; Angeletti and Restelli 1994; Le Bras and Angeletti 1995; Cox 1997; Seufert 1997) have further documented that the formation of ozone in summer varies greatly across Europe, and even within the Mediterranean Basin. For example, the Western Mediterranean Basin is surrounded by high mountains and, in summer, is under weak levels of anticyclonic subsidence and strong insolation. These conditions favour the development of meso-scale processes and the re-circulation of airmasses. During the same period the Eastern Mediterranean Basin is under conditions of weak ascent and strong advection, i.e., the Etesian winds, and the development of re-circulations is largely inhibited (Millán et al. 1997; Kallos et al. 1997, 1998; Ziomas et al. 1998.

The Western Mediterranean Basin is surrounded by mountains 1500 m or higher. On summer days their east- and south-facing slopes are strongly heated and act like orographic chimneys, which favour the early formation of up-slope winds that reinforce the sea breezes and link the surface winds directly with their return flows aloft, and further, with their compensatory subsidence over the sea. The result is the formation of stacked layers along the coasts with the most recently formed layers at the top and the older ones near the sea. These reach 2 to 3 km in depth, have variable width over land (up to 100 km), and extend more than 300 km over the sea (Millán et al., 1992, 1996).

The layers act as reservoirs of aged pollutants, and the lower ones can be brought inland by the sea breeze of the next day(s), creating re-circulations, as illustrated in Figure 2. Tracer experiments on the Spanish East coast have shown that turnover times range from 2 to 3 days. During the night the land-based processes die out, and the reservoir layers can drift along the coasts and contribute to regional, inter-regional and long range transport of aged pollutants. Similar processes involving either re-circulations and/or oscillations of the aged airmasses have also been documented in the Central Mediterranean (Ciccioli et al. 1987; Fortezza et al. 1993; Georgiadis et al. 1994; Orciari et al. 1998).

At the larger scales, deep convection in some regions (e.g., Spanish and Turkish central plateaus), or strong up-slope winds in others (e.g., Alps and Atlas mountains), can inject aged air masses directly into the Mid-troposphere (3 to 6 km) and into the upper troposphere, i.e., 10 (+) km, where they can participate in long-range transport processes within Southern and Central Europe, and at the continental-global scales, respectively.

Under strong summer insolation, the coastal re-circulations become "large natural photo-chemical reactors" where most of the NO_x emissions and other precursors are transformed into oxidants, acidic compounds, aerosols and O₃, leading to the exceedance of EC thresholds (Figure 3). Relevant aspects of this problem are: (1) that the concept of upwind (background) and downwind of conurbations (polluted) is inappropriate in regions of complex coastal terrain where re-circulation processes are important, (2) that ozone is generated at the regional scale from emissions in urban centres and other NO_x source areas, and (3) that as much as 60%, or more, of the observed O₃ at any one site may result from advection within the recirculating air masses.

These situations are the norm, rather than the exception, for the coastal regions surrounding the Western Mediterranean Basin, and illustrate the existence of chronic-type O₃ episodes, created by atmospheric re-circulations, as compared with the peak-type episodes in Central and Northern Europe, which are created by combinations of long-range transport and atmospheric stagnation. They also reveal a problem of data interpretation for those responsible for monitoring networks from the local to the EC level.

Thus, in Southern Europe and the Mediterranean Basin the observed O₃ cycles depend strongly on the topographic location of the observing station and its relationship to the reservoir layers, the atmospheric circulations involved, and the chemical processes along each path (Derwent and Davies, 1994), as illustrated in Figures 2 and 3. As a result, each O₃ monitoring station shows a part of the whole, and could even be considered to represent a specific area, providing the relevant processes are understood for each site and the site itself has been adequately selected; however, no single station can be considered representative of regional processes, and much less of the whole situation.

Figure 1: A schematic summary of the observed and postulated circulations for the whole Mediterranean on a summer day.

Figure 2: Sketch of the diurnal circulations in the coastal regions of the Mediterranean Basin in summer. The entrance of the sea breeze during the day and the formation of stratified (reservoir) layers aloft is illustrated, and letters (a) to (d) indicate successive stages in this process. The upper part (DAY) shows a schematic of the ozone decay, and subsequent production, as it interacts with the emissions of nitrogen monoxide (NO) in a coastal city (Derwent and Davies, 1994). During the night drainage flows are formed and a stable airmass accumulates at the bottom of the valleys and over the coastal plains. The draining airmass can become blocked at some distance from the coast whenever the sea surface temperature is higher than that of the air. The lower part (NIGHT) shows a schematic of the ozone evolution along the path of the draining air. In these processes, stations located high above the coastal plains (#5) can remain within the reservoir layers during the night. The number codes correspond to the stations in Figure 3.

Figure 3: Daily ozone cycles in July for three stations located at various heights and distances inland from the Spanish East Coast. At the coast (Grao, type #1) the O₃ drops to low values during the night and rises sharply between 0700 and 1000, just before the onset of the sea breeze. This is associated with the fumigation of O₃ from the reservoir layers. After this time it remains nearly constant and can be regarded as the background O₃ entering with the sea breeze. In Villafranca (type #3), some O₃ is available from the reservoir layers, and the concentrations do not drop below 60 mg/m³ during the night. After the morning rise the O₃ remains nearly constant until the arrival of the sea-breeze front at 1200 and 1400, which produces a second rise in O₃ and the diurnal maximum by 16:00. Finally, the mountain site (Morella, type #4) remains within the reservoir layers during the night. The O₃ remains high, its daily cycle is dampened, and it shows a minimum by 08:00 which, not surprisingly, coincides with the time of maximum rise at the sites on the coast and valley floor.