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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. . |
