Environmental problem targeted

The photo-oxidizing pollution

Ozone (O3) is a secondary pollutant that is produced during the atmospheric photo-oxidation of Volatile Organic Compounds (VOCs) under the presence of nitrogen oxides (NOx). The ozone concentration in any given area results from a combination of formation, transport, destruction and deposition. There are two formation ways in the troposphere. The first source is an exchange of air between stratosphere and troposphere, which constitutes a downward flux of ozone into the troposphere. There is also a chemical route: the photo-dissociation of NO2 in NO and an oxygen atom, which then can form ozone by reaction with O2. In addition, the O3 formation is becoming more pronounced by the presence of VOCs. Unlike, in the troposphere, ozone can be removed by a large number of chemical reactions and by dry deposition.

Furthermore, ozone is an important air quality issue and causes serious health problems and damage to materials and ecosystems. Atmospheric lifetimes of ozone precursors are long enough (several days) to allow them to be transported on long distances but the range of the impact depends on meteorological and geographical conditions. Rural areas are influenced by the large-scale dispersal of precursors produced at urban and regional scales. These areas are the most representative of background pollution at global scale.

Yes, the ozone is a problem, because:

-  Background levels increasing to phytotoxic levels

A large number of studies have been carried out on near-surface ozone trends in Europe in the latter period. Many authors report an increase of the annual mean or median ozone values during the 1990s at European sites such as Mace Head, Ireland (Simmonds et al., 2004).

In rural station across France, between 1995 and 2003, a slight increasing trend of the O3 levels (+ 0.6 %.year-1) is observed (Sicard et al., 2009). For the “Provence-Alpes-Côte-d’Azur” region, between 2000 and 2008, an increase of + 2.9%.year-1, + 4.3 %.year-1 and + 0.7%.year-1 of the mean values in urban, suburban and urban stations, were obtained respectively, from median values (Sicard et al., 2009, 2011). The mediterranean basin will be one of the most strongly effected areas by 03 increase.

By 2030, global surface ozone is calculated to increase globally from 1.5±1.2 ppb to 4.3±2.2 ppb, using the ensemble mean of model results (Dentener et al., 2006). At the global scale, the ozone background levels could triple by the year 2100, up to 80ppb following the considered scenario (IPCC, 2008).

-  Evidence of ozone impacts on vegetation

Various sanitary studies (Contran et al., 2007; Dalstein et al., 2002, 2005a,b,c and 2008; Paoletti et al., 2007; Ulrich et al., 2006) showed an important impact of ozone concentrations increase on ecosystems and forests.

A review of 50 years of research on the impacts of ozone on forests reveals that the gas can induce visible leaf injuries, decrease chlorophyll content in leaves, accelerate leaf deterioration, decrease photosynthesis, alter the allocation of carbon, decrease fitness, and produce a variety of other physiological effects in plants (Karnosky et al., 2007). This reduced growth lead to reductions in economic profit for forest owners and a reduced CO2 uptake.

Ozone enters plant tissues through the stomata, the small pores in leaves and stems where gas exchange takes place. Ozone also reduces the modulation ability of the stomata, that is, their ability to open and close. Once inside the plant, ozone reacts with cellular components found in plants and produces various organic compounds, oxidizers, and free radicals. These products can destroy cell proteins and membranes and lead to impaired cell functioning and death (Fuhrer & Booker, 2003).

Necrosed parts appear on leaves after an exposure of some hours to ambient air concentrations (> 80µg.m-3, information threshold fixed in 180µg.m-3). After an exposure of some hours, ambient ozone concentrations affect the cultures productivity.

Some of the most common types of injury in broadleaf plants are:

  • Stippling - Upper leaf surface interveinal stipple, discoloration that occurs between the leaf veins. It usually appears as red or brown spots.
  • Chlorosis - Loss of chlorophyll that manifests itself as non-green pigmentation in discrete patches on the leaf. Discoloration of light-exposed leaf surfaces.
  • Bronzing / Flecking – Tan, brown or black areas on the upper surface of the leaf that result from the death of certain cells in the plant.
  • Bifacial necrosis - Dead areas on both sides of the leaf.
  • Accelerated senescence – premature loss of leaves, flowers or fruit.

In conifers, one can observe:
  • Tipburn – dead tissue (usually red or brown) spreading from the tip of needles downwards.
  • Photobleaching - Specific discoloration of the needles, on light-exposed surfaces.
  • Chlorotic mottle - Small yellow or light green spot or marbling with diffuse outline, in particular on the upper side and the tip of the needles.
  • Accelerated senescence.

At the moment, the European standards use the AOT40 index (Accumulated dose over a threshold of 40ppb) to protect vegetation. AOT40 is a numerical index that describes the cumulative effects of agricultural crops, forest and other ecosystems exposure in terms of hourly accumulated exposure over a threshold of 40 ppb expressed in units of ppb.h.

See the technical report “Evidence of widespread ozone damage to vegetation in Europe (1990-2006)” published by the ICP Vegetation consortium in 2007.  

- Greenhouse gases

Ozone is an important Greenhouse gases, the third most important greenhouse gas, and as such is a significant contributor to climate change, with a short residence time, thus, an possible immediate climate benefits from reductions. The warming effects of tropospheric ozone are about 25% of that contributed by carbon dioxide CO2. At the moment, the greenhouse gas tropospheric ozone is the most worrying atmospheric pollutant for ecosystems and forests.

- Impacts on agricultural: a threat to food security

Many crops are ozone-sensitive including wheat, maize, legumes, tomato and lettuce. For the year 2000, loss in economic value for crops in Europe due to ozone was 6.7 billion (based on production and sensitivity for 23 crops). The ozone may reduce crop yields by up to 40% (ICP vegetation report, 2006). Ozone pollution in EU27 was predicted to be causing an average of 13.7% yield loss for wheat (an ozone sensitive crop), with an economic loss of €3.2 billion. For tomato (a moderately ozone sensitive crop) economic losses of €1.02 billion representing 9.4% of production value were estimated. For both wheat and tomato, economic impacts were predicted to decrease by 38% to €1.96 billion and €0.63 billion respectively by 2020 (Mills et al., 2011).

-   Human health risks

Extensive research (World Health Organisation) has demonstrated the associations between exposure to ozone and ill-health endpoints such as respiratory, cardiovascular disease and congestive heart failure. The effects of air pollutants on lung function depend largely on the type of pollutant and its environmental concentration, the duration of pollutant exposure and the total ventilation of exposed persons. Ozone can enter the body through inhalation and can reach the respiratory system. Acute exposure to high ozone levels can induce changes in lung function, airway inflammation and increased airway responsiveness to broncho-constrictors. Ozone exposure has also been associated with increased numbers of hospital admissions from respiratory diseases, including asthma.

The average Relative Risk (RR) is the probability for the occurrence of the specified health outcomes per 10 µg.m-3 increase of a given pollutants. For an 8h averaged O3 concentration of 120µg.m-3 the daily mortality risk increase for cardiovascular diseases is of 4.8% in the European regions (RRO3-8h = 1.004).