13.1 Ozone

Ozone is an air pollutant that has adverse health effects on humans (see Chapter 12). It also has adverse impacts on vegetation (EPA, 2007). These effects may be significant on crop yields and, therefore, may translate into economic impacts due to reduced agricultural crop production. Ozone may also show impacts on forests, as is the case, for example, in California.

The first symptoms of ozone deposition on vegetation appear on the outer layer of leaves. They are more important on leaves that are exposed to sunlight. The effects vary depending on the type of vegetation (e.g., deciduous trees, conifers, agricultural crops). The main symptoms are a discoloration of the leaves exposed to sunlight (photobleaching), small spots on the leaf surface (stippling and molting), and/or a brownish coloration on the upper parts of the leaves (bronzing). The mechanism leading to those symptoms may be summarized as follows. Ozone is absorbed by the plant stomata and reacts with organic molecules, such as isoprene and ethylene, which are present in the extracellular plant fluid. Oxidizing organic compounds are formed, which react with the proteins of the cellular membrane of the plant. This deterioration of the membrane leads to the visible effects corresponding to the exposure of the vegetation to high atmospheric ozone concentrations. Some other secondary effects also occur, such as a reduction in CO₂ fixation (either via a perturbation of the enzymatic function or via stomata damage). The perturbation of the photosynthesis of the plant leads to accelerated aging of the leaves and a decrease in the root/shoot ratio and grain/biomass ratio. Therefore, there is a decrease in the grain yield and number of healthy leaves.

Exposure of vegetation to ozone must be quantified with a function that represents exposure above harmful levels. To that end, the AOT40 function (Accumulated ozone exposure over a threshold of 40 ppb) has been widely applied. The calculation of AOT40 is generally limited to daylight hours (solar radiation >50 W m⁻²). It corresponds to the cumulative exposure of vegetation to ozone concentrations greater than 40 ppb during daylight hours over a three-month period (from May to July) for agricultural crops and over a six-month period (April to September) for trees:

AOT40=∑i=1NdhMax([O3]i−40; 0)(13.1)

where N_dh represents the total number of daylight hours during the period of interest and [O₃] is expressed in ppb. For example, the European Union has a target value of 3,000 ppb h for crops based on an AOT40 calculated using a daylight-hour period of 8 am to 8 pm Central European Time. Other exposure threshold values may be used. For example, a threshold value of 60 ppb is used in the SUM60 function. It is calculated in the same way as AOT40, but with an ozone concentration threshold of 60 ppb. The U.S. EPA currently uses a sigmoidal cumulative function, W126, which does not use an adverse exposure concentration threshold, but instead gives more weight to the higher concentrations. It is calculated for daylight hours between 8 am and 8 pm, i.e., for a twelve-hour period, as follows:

W126j=∑i=8am7pm[O3]i(11+(4403 exp(−126 [O3]i)))(13.2)

Next, the function is calculated over a three-month period:

W126=∑j=1NwW126j(13.3)

where N_w is the total number of days during the three-month period (i.e., N_w = 91 or 92). (Adjustments are made for periods with missing O₃ data.) W126 is calculated as a moving three-month average between April and October and the maximum value of the three-month averaged W126 is selected. Next, the average of these maximum values is calculated over a three-year period. The U.S. EPA currently uses W126 to estimate vegetation exposure to ozone and a range of 13,000 to 17,000 ppb h is considered acceptable to protect vegetation from ozone damage. Although there is no direct relationship between the U.S. health-based air quality standard (a concentration of 0.070 ppm averaged over 8 hours, referred to as the primary ambient air quality standard) and W126, the U.S. EPA considers that the primary standard should be protective of vegetation and has not proposed a secondary standard that would be specific to vegetation exposure.

The adverse effects of ozone on vegetation depend on the types of crops and trees, as some species are more resistant than others. Nevertheless, the impact of ozone on vegetation translates into significant economic impacts for agriculture. For example, ozone levels for the year 2000 have been estimated to lead to decreases in global crop yields ranging from 2 to 15 % depending on crop type (soybean, wheat or maize) and the methodology used (Avnery et al., 2011).

13.2 Acid Rain

The acidity of water is represented on a logarithmic scale by the pH. It is the negative value of the base 10 logarithm of the activity of the proton H⁺ (for a dilute solution, the activity is equivalent to the concentration; see Chapter 10):

Pure water has a neutral pH of 7 at 25 °C, since H⁺ and OH⁻ are present in identical concentrations. However, liquid water in the atmosphere does not have a neutral pH, even in a pristine atmosphere. Carbon dioxide (CO₂), which is naturally present in the atmosphere, is a weak acid, which is soluble in water. For an atmospheric CO₂ concentration of 400 ppm (parts per million), the pH of a cloud droplet or raindrop is calculated to be 5.6 (see Chapter 10). Therefore, acid rain has a pH that is less than 5.6.

Chemical species that lead to acid rain are strong acids, i.e., they dissociate quasi totally in water into H⁺ and anions. In the atmosphere, the main strong acids are sulfuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl). The discovery of acid rain goes back to the beginning of the industrial era. Although the original references given in the English literature list mostly the work of R.A. Smith (1852, 1872), the first scientific article on that topic was published by a French pharmacist, M. Ducros (1845), who proposed that the acidity of the rain was due to nitric acid, resulting, for example, from the formation of nitrogen oxides during thunderstorms. A reanalysis of the data obtained by Smith on precipitation in the region of Manchester suggested that acidity was mostly due to HCl. It is possible that coal combustion in that region led to high atmospheric concentrations of HCl (chlorine is one of the compounds present in coal and it is emitted mostly as HCl during coal combustion).

One should note that acid rain is generally used as a term that covers more generally both wet and dry deposition. Wet deposition includes scavenging by rain, but also by snow and hail, as well as occult wet deposition (from mountain clouds and fogs). Dry deposition affects both particles (which may contain sulfate and nitrate) and gases (such as nitric acid); see Chapter 11. Therefore, the correct scientific term is “acid deposition,” rather than “acid rain.”

In the 1970s, regions such as Scandinavia in Europe and Canada in North America showed significant modifications in their lakes with significant decreases in their fish population. In addition, some forests such as the Black Forest in Germany showed important signs of decline. An analysis of the causes of these environmental changes showed that the pH of lakes had decreased significantly (i.e., lake acidification). In the forests, the rain acidity led to a change in the geochemistry with a modification of the chemical equilibria of the various inorganic chemical species (e.g., Reuss and Johnson, 1986). In particular, species such as magnesium (Mg) and calcium (Ca), which are nutrients for vegetation, became soluble ions. Thus, they were washed out by the rain and were no longer available for absorption by the roots of the trees. In addition, some elements that are toxic to vegetation, such as aluminum (Al), became available for absorption by roots. As a result, these different factors led to the death of a large number of trees in some regions. The main acids responsible for the acidification of lakes and soils were identified to be sulfuric acid and nitric acid.

These two acids are not emitted in the atmosphere in significant amounts, but are formed in the atmosphere by the chemical oxidation of primary pollutants emitted in the atmosphere (see Chapters 8 and 10):

Therefore, sulfuric acid and nitric acid are secondary pollutants.

Sulfur is present in coal, oil, and various minerals. Therefore, it is emitted during the combustion of coal (power plants, residential heating …), oil-derived fuels (diesel, gasoline …), and some industrial activities (smelters …), mostly in the form of sulfur dioxide (SO₂). Nitrogen oxides (nitric oxide, NO, and nitrogen dioxide, NO₂) are emitted during all combustion processes because nitrogen (N₂) and oxygen (O₂) present in the air react at the high temperatures of the combustion process to form these nitrogen oxides (mostly NO; NO_x is used to represent the sum of NO and NO₂). Since it takes some time for those primary pollutants to be oxidized into acids, acid rain pollution tends to cover long distances (several hundreds of kilometers). In addition, there are some natural sources of SO₂ and NO_x. Volcanic eruptions lead to significant emissions of SO₂. Oceans emit dimethyl sulfide (DMS), which gets oxidized slowly into SO₂. Soils emit nitrogen oxides (NO_x, as well as nitrous oxide, N₂O, a greenhouse gas). Lightning produces NO_x in the high atmospheric layers (as suggested by Ducros, 1845). Table 2.1 summarizes the main categories of SO₂ and NO_x sources (see Chapter 2).

Another effect of acid rain is the degradation of buildings and statues (Brimblecombe, 2003). Some stones are calcareous (i.e., calcite, some marbles, freestone …). This calcareous compound is calcium carbonate, which reacts, for example, with sulfuric acid to lead to calcium sulfate (gypsum). This reaction leads to a change in the stone cohesiveness; the stone is no longer homogeneous in its chemical composition and some erosion of the stone occurs.

Acid deposition simulations are performed with three-dimensional (3D) atmospheric chemical-transport models. Such models were initially developed during the 1980s. The chemistry of the oxidation of SO₂ into sulfuric acid and of NO_x into nitric acid is rather well known (see Chapters 8 and 10). Figure 13.1 shows a comparison between wet deposition fluxes (precipitation) of simulated and measured sulfate at different stations of the National Acid Deposition Program (NADP) monitoring network in the United States. The spatial correlation between the simulation and measurements is satisfactory (determination coefficient of 0.77) and the model bias is low (8 %).

Figure 13.1. Comparison of simulated and measured wet deposition fluxes of sulfate in the United States for the year 1996.

Source: Reproduced with authorization from Vijayaraghavan et al. (2007), © American Meteorological Society.

SO₂ and NO_x emissions have been regulated in the 1980s in order to reduce acid deposition. For example, in the United States, SO₂ and NO_x emission regulations have been promulgated using a cap-and-trade approach (see Chapter 15). At the international level, the Göteborg protocol was introduced in 1999. It is officially called the “Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone”; therefore, it is also known as the “Multi-effect Protocol,” because it addresses several forms of air pollution (see Chapter 15). As of August 2017, 25 countries (including the U.S.) and the European Union had ratified this protocol, which defines ceiling targets for national emissions that must be attained by a given date (currently 2020). SO₂ regulations have targeted mostly coal-fired power plants, smelters, and the sulfur content of gasoline and diesel fuels. NO_x regulations have been mostly driven by the contribution of NO_x species to ozone formation. Regulations have targeted emissions from on-road traffic, refineries, and fossil-fuel fired power plants. Long-term monitoring of sulfate and nitrate, as well as rain pH, has shown that these regulations were efficient and led to a decrease in acid deposition downwind of those sources. Most of the lakes and soils have seen their pH increase. However, some ecosystems recover faster than others, depending on their physico-chemical characteristics, and some lakes and forests are still under recovery. Nevertheless, it appears that overall the public policies introduced to reduce SO₂ and NO_x emissions have been effective in North America and in Europe, so that ecosystems that were adversely impacted in the 1970s are recovering and, in some cases, have recovered to their pre-industrial status. However, the acid deposition problem may now be present in some regions of Asia due to a fast industrial growth over the past several years.