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Vol. 10 No. 2 - April 2004

Tropospheric Ozone in the Mediterranean Basin: Evidence of its Effects

By: Maria J. Sanz


Atmospheric Pollution : Historic Evolution

 Atmospheric pollution is a relatively modern concept; the first book on this topic was written by John Evelyn in the 17th century (Fumifugium). Although it was known from several centuries as an undesirable product of civilization, it is only from the 19th century onwards, when real concern about it was raised. It became a real problem in certain European regions with the Industrial Revolution. The first problems derived from smelters, furnaces and primitive chemical industries: their effects were restricted to the urban areas surrounding the industries and had an episodic character. Thus, it is not strange that the first reports of injury to forests were from parks in large cities such as London. In fact, the first important public reactions were after several episodes from the 30’s to the 50’s, which even caused the deaths of a number of people. But it is not until after the 50’s when air pollution was regarded as a problem of public interest in several industrial countries and when measures were taken (e.g.. The Clean Air Act in 1964 in England.

 The first transnational report of air pollution affects dates from the 30’s, when the injuries to vegetation detected in the Columbia river valley, in the Rocky Mountains, was identified as caused by the emissions of a smelter from Trail, Canada. In Europe, acidification of lakes and soil in the Northern Countries was attributed to acid rain at a regional scale in the 60’s. Perception of air pollution has changed in the last two decades: urban air pollution was the major concern in the 70’s, but currently air pollution encompasses several other problems which have appeared in the last 20 years. At present, urban pollution is  a problem in many of the largest cities, and photochemical compounds are a problem at regional and even global scales. The most important pollutants are: tropospheric ozone and photooxidants in general (e.g. PAN, nitrogen oxides...), pollution in large cities (including several organic pollutants),  “acid rain” long range transport of the pollutants, new toxic gases, and “global warming” due to the increase of greenhouse gases

Ozone in Southern Europe and the Mediterranean Basin

The occidental Mediterranean Basin is surrounded by mountain ranges with altitudes over 1500 m.  In coastal mountains the warm up of the eastern oriented slopes start with the sunrise.  In summer, this situation favours the early development of the slope winds that "pull or reinforce” seabreezes.  In this process, slopes act as orographic chimneys connecting directly surface winds with fluxes in altitude, the latter being subject to compensatory subsidence along its return in altitude towards the sea.

 The above processes decrease during the afternoon and stop at nightfall.  On the next day, the lowest strata are transported inwards with the new sea breeze and the described circulations replenish the ozone rich layers in altitude. As a result of this process, a clear vertical stratification of the air mass occurs over the sea, along the coasts, the most recently formed strata being accumulated over the oldest ones, which are close to the sea surface. The strata act as reservoirs since the pollutants emitted in previous days or their products may be the “background” for subsequent days. Moreover, strata can move along the coasts contributing to inter-regional and long range transport of atmospheric pollutants.

 These processes have been documented experimentally in several projects of the European Commission, three of which under the leadership of Spanish teams. MECAPIP and RECAPMA projects were initially co-ordinated from CIEMAT and later from Fundación Centro de Estudios Ambientales del Mediterráneo (CEAM), together with SECAP project. The CEAM, was founded by Spanish and Valencian governments, with the support of the European Commission, to specifically deal with these subjects in the Mediterránean Basin.  The results achieved indicate that the strata system is extended along the coast, reaching up to 2 a 3 km of altitude over the sea. Over the land it has a variable width (up to 100 km), and it extends to more than 300 km over the sea. The continuity of these processes over the whole Mediterranean Basin has also been documented. Experiments with tracers conducted in the eastern Spain have documented that the time needed for the first return is two days, for the given meteorological and site conditions of the experiment. Recent numerical simulations with mesoscale models and re-analysis of data of the RECAPMA project show that air pollutant masses in the occidental Mediterranean Basin in summer may require over 5 days to renovate 50%, and 7-10 days for a 80% renovation. Under strong summer sun radiation, coastal re-circulations acts as "large natural photochemical reactors" in which NOx emissions and other precursors (VOCs) are transformed into acidic compounds, aerosols and photooxidants, including ozone, which frequently reach relatively high concentrations.

 The threshold for ozone injury to vegetation (65 ug/m3, 24 h), is exceeded systematically during more than six months of the year, the threshold for heath protection (110 ug/m3, 8 h) is exceeded intermittently during more than four months in the highest of the monitoring stations considered, and the threshold for information to population (180 ug/m3, 1 h) is also sometime exceeded, from April to August/September. These situations are the rule more than the exception along all the coasts of the occidental Mediterranean Basin, illustrating the episodes with chronic levels of O3, caused by the re-circulation of the air masses. On the other hand, peak episodes with high ozone concentrations during a few days may also occur; they are typical from Central Europe, and are originated under stable anticyclonic conditions.

 It is remarkable that the described processes were not documented before 1986-91, in spite of the protocols and of the large international programs (e.g. EMEP and other models of atmospheric pollution) and is used as a base for the European Directives. Probably, the large grid (150 x 150 km) used for the calculations in those models did not allow to reproduce the above mentioned processes. To model such re-circulation processes in the Iberian Peninsula, Salvador et al. (1999) used a smaller grid of 10 x 10 km complemented with a local nested grid of 2 x 2 km for the Valencian Community (eastern Spain); this allowed to reproduce 95% of the variability of the atmospheric fluxes. Part of the above mentioned results and their possible consequences have been taken into account for the definition of priorities within the 6th Framework Research Programme of the European Union (European Commission, 2001).

Recognizing Ozone Effects to Vegetation in a Simple Way

 Photo-oxidants, and especially ozone, have been widely regarded as harmful to vegetation since the 80’s, although in the 60’s its effects were already detected in California. However, it is during the last decade when ozone become an issue of concern in Europe. Ozone pollution, unlike fluoride or sulphur dioxide, does not leave elemental residue that can be detected by means of analytical techniques in vegetative tissues. Thus, ozone injury in leaves, are the only evidence easily detected in the field. So far, experimental studies have focused mainly on explaining the mechanisms that produce damage, rather than to identify and characterise symptoms observed in the field at a regional scale. Recent researches have increased our knowledge on the subjacent mechanisms that explain the effects of ozone on crops, and to a lesser extent, on trees and other wild plants. A long term effect of this pollutant on forests may affect some of their functions, e.g. their role in water and energy balances, protection against soil erosion, cover of vegetation, and aesthetics of the landscape. One possible effect on plant communities might be the change in species composition and loss of biodiversity, an important potential threat when regions with many endemic plants are considered. Furthermore, before these problems are approached, more basic and detailed studies on the sensitivity of the species under different environmental conditions, including e.g. nutritional aspects, have to be undertaken. 

 Still, there are discrepancies between models and observed concentrations specially in the southern Europe. However, some recent modelling works are able to detect the so called “Mediterranean anomaly”. Thus, on the basis of modelling and experimental evidence, we strongly suggest that ozone occurs at concentrations causing visible foliar injury to sensitive plants in several parts of Europe.

 Visible symptoms are the result of oxidative stress, which also causes a cascade of physiological changes on the plant. Thus, although they are only a part of all the effects of ozone to plants (e.g. physiological changes, or growth and yield reduction, they may also occur before specific symptoms are developed). Their observation in the field is being a useful, cheap tool to detect areas in which ozone concentrations produce phytotoxic adverse effects since the 60s. After mid 90s, surveys have recorded ozone-like symptoms on numerous native tree, shrub, and forbs species in southern Switzerland, Greece, Italy, France and Spain. However, still little information is available on the effects of ozone on the multitude of native plant species throughout Europe. And not only in native species visible injury is being recognised in Europe, also crops also show such injuries quite frequently, specially in the Mediterranean, in countries like Spain in crops like potato, beans, artichoke, watermelon, etc. 

International Co-Opearation  Program for Forest Under UN/ECE CLRTP Ozone Initiative.

 The lack of ozone data was a serious limitation for the EU Intensive Monitoring (Level II) database. Besides the obvious connections with the potential effects on forests, ozone data are also relevant in relation to other themes which were subjected to important political agreement, like the tropospheric chemistry changes and the regional ozone formation (e.g. the CLRTAP multi-pollutant, multi-effect directive; the UN Biodiversity Convention; the EU Habitat Directive; the EU acidification strategy, the UN/ECE CLRTAP, the EU Air Quality directive.  The WG on Air Quality within the Expert Panel started a monitoring activity by using the tools passive sampling and visible injury assessment on Main Tree Species and Ground Vegetation on Intensive Monitoring Plots within ICP-Forests. Since much of the ozone data at European level comes from monitoring devices located in urban/sub-urban areas, a comprehensive dataset about forest sites can be considered an important input for a better understanding of ozone levels in remote areas across the European continent, including the Mediterranean region. Details about the methodological details of the program as well as information on injury identification can be found on the Coordination Center Webpage  (htp://www.gva.es/ceam/ICP-forests).

 First results on the passive samplers indicate that higher concentrations occur in southern Europe, 58 % of the Spanish Sites and 63 % of the Italian sites show a 6-month-time weighted-average concentration in the range of 45-60 ppb. In respect of ozone visible injury, nine countries reported results from 72 plots in the first ozone injury assessment in 2001 (DeVries et al. 2003). The adult trees that showed injury when the level II plots were prospected were: Fagus sylvatica, Carpinus betulus, Pinus nigra and Pinus pinaster, Pinus halepensis and Pinus strobus. Overall 24 % of the plots where main tree species were prospected showed injury. Within the specially light exposed plots stabilised to better detect ozone injury apart from the Level II plots but nearby, 27 species of trees, 19 of shrubs and 15 of herbs showed injury. From those, 17 species of shrubs and herbs were not known as sensitive in literature surveys (see the list of sensitive European species, www.gva.es/ceam/ICP-forests). Although the results have to be considered carefully,preliminary studies show the potential of visible injury as a tool to detect potential areas under ozone stress in Europe. In fact, it appears that there is a considerable agreement between the observed concentrations and the appearance of visible injury, specially in southern Europe.

Dr. Maria José Sanz is a Scientist at the Fundación Centro de Estudios Ambientales del Mediterráneo, Parque Tecnológico, c/ Charles R. Darwin, 14. Paterna, 46980  Valenci. Spain. .

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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