Day Four: Forest Decay and Acid Rain (Jizera Mts)

Day Four: Forest decay and Acid Rain (Jizera Mts)

4.1 Acid rain

4.1.1 History

Acid rain is not a recent phenomenon. The term was first coined in 1852 by a chemist, Robert Angus Smith. The 35-year-old chemist, while studying the quality of air in and around his home town Manchester, England, found local rainfall to be unusually acidic. He suspected a connection between this occurrence and sulphur dioxide emissions when coal was burnt by local factories. He even noticed that the acid in the air was causing textiles to lose their colours and metals to corrode. The air over Manchester was partly cleaned by building tall chimneys at power stations and factories which release pollutants high into the air. These chimneys improved the air quality locally by dispersing the pollutants, but they were only blown across into neighbouring countries.

The UK's early awareness of pollution was related to its early industrialisation. But even in less industrialised parts of Europe, observers were beginning to worry about the effects of smoke from coal. In 1881, Norwegian scientists observed polluted snowfall and attributed it to a large town or industrial district in England. Just before World War I, mass deaths began to occur among fish in the rivers of southern Norway. Lakes, too, began losing their fish populations. But it was only in the 1950s that the link with acid rain was established.

In his book Acid Earth, John McCormick recounts that research in Austria and England from 1911-1919 showed that rain with dilute sulphuric acid inhibited plant growth. In 1924, a farmer from Manchester even sued the power station at Barton in England for compensation, arguing that sulphur emissions were damaging his crops. The House of Lords, however, rejected his claim.

4.1.2 Chemistry

Acid deposition

Acid deposition occurs through both wet (polluted rainfall, acid rain) and dry processes (interception of gases and particles at the surface).

Acid rain is a human-related phenomenon. Since industries are so keen on burning fossil fuels (coal and oil) they tend to release a lot of sulphur into the air.

Acid rain is produced by the conversion in the atmosphere, of sulphur dioxide (SO2) and nitrogen oxides (NOx) to sulphuric acid and nitric acid. The processes are complex and depend not only on the physical dispersion of the pollutants but also on the rates of chemical conversion (Porteus, 1996). Rain is naturally slightly acidic, from reaction with CO2 in the atmosphere to form dilute carbonic acid, but these processes can make it much more so. Unpolluted rain is normally slightly acidic, with a pH of 5.6. Carbon dioxide (CO2) from the atmosphere dissolves to form carbonic acid. When pollutants combine with the rain, the acidity increases greatly. Measurements of pH taken in different areas of the Czech Republic (Moldan et al. 19..) gave values ranging between 3.1 and 6.0.

Sulphur combines with the oxygen already present in the air to form sulphur dioxide (SO2). Also, since we like to drive big fancy cars rather than ride bikes or walk, we cause the formation of nitrogen oxides (NO or NO2 or NO3, etc) in air from burning gasoline.

The primary conversion of sulphur dioxide is through an aqueous phase reaction with hydrogen peroxide (H2O2) that exists in clouds. On the other hand, nitrogen dioxides in the air react with hydroxide (OH) radicals formed photochemically. The result of these chemical reactions is the formation of acids in the atmosphere. These acids are, most notably, sulphuric acid (H2SO4) and nitric acid (HNO3).

CHEMICAL REACTIONS:

Formation of SO2 and nitrogen oxides (NO & NOx):

S(in fuel) + O2 ------> SO2

N2 + O2 ------> 2NO

NO + 0.5O2 -----> NO2

Formation of hydrogen peroxide
VOC + sunlight + HO2 (in air) ------> H2O2
(VOC = Volatile Organic Compounds)

Formation of acids :
SO2 + H2O2 and O3 (in clouds) ------> H2SO4
SO2+ OH + O2 (in air) ------> H2SO4
SO2 + oxidants (from wet surfaces) --> H2SO4

Sulphuric acid has a lifetime in the atmosphere of only a few days. It is oxidised in water droplets by hydrogen peroxide or ozone or in the gas phase by the hydroxyl radical, to form sulphate (Adams and Smith, 1995).

NOx + sunlight + OH (air) ------> HNO3

NOx arises from oxidation of nitrogen during combustion; with fuels such as coal, much of the nitrogen comes from the fuel itself; in addition, combustion air introduces atmospheric nitrogen, which is oxidised in high temperature flames; a third contributor of NOx is a reaction between components of the hydrocarbon fuel and molecular nitrogen. Careful control of combustion temperature can limit the oxidation of atmospheric nitrogen and control of combustion conditions can help too; post-combustion removal of NOx is also used in large plants. In vehicles, catalytic converters can remove NO as well as unburnt fuel.

These compounds then fall to the earth in either wet form (such as rain, snow, and fog or dry form (such as gas and particles). About half of the acidity in the atmosphere falls back to earth through dry deposition as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. In some instances, these gases and particles can eat away the things on which they settle. Dry deposited gases and particles are sometimes washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone. The combination of acid rain plus dry deposited acid is called acid deposition. Prevailing winds transport the compounds, sometimes hundreds of miles, across state and national borders.

The main sources of sulphur emissions to the atmosphere are shown in the table below. Coal-fired power plants are the largest single source of SO2 but increasing use of flue gas desulphurisation (FGD) is reducing this. FGD is installed in 18 countries, in plant with a total capacity of 168GWe, and more are planned (Soud and Takeshita, 1994). One of the largest FGD systems is at the 4,000MWe Drax power plant in the UK. Eventually this FGD system will remove 90% of that station's sulphur emissions. Other combustion technologies, such as fluidised bed combustion, can contain emissions of SO2 and also reduce the amount of NOx produced.

Table 4.1: Main sources of sulphur

Source / Amount Mt(S)/y
Human activities / Fossil fuels (mainly SO2) / 70-80
Smelting / 6.8
Biomass burning (SO2) / 0.8-2.5
Natural sources / Oceans / 19-51
Volcanic emissions (mainly SO2) / 7-10
Soil erosion / 1-5
Soils and plants / 0.2-4

Source: Adams and Smith, 1995

4.1.3 Acid Rain: A Transboundary Problem

Once in the air, pollutant gases are carried by the wind and hence deposition can take place a long distance from the source. If large quantities of acid are deposited in one place then it may have drastic consequence for:

• humans;
• wildlife;
• vegetation;
• soils;
• crops;
• freshwater;
• buildings.

Acid deposition is clearly a transboundary problem as about 5% of sulphur deposition in Germany and Sweden is of UK origin and in Norway the figures are as high as 9-12%. Similarly, a considerably high portion of acid deposition over the Czech Republic comes from distant sources in Germany.

Problems occurring from acid deposition have been recognised in Scandinavia for a long time, but the problem has only been given attention in the UK during the last few decades. In the UK the acidity of rain is greatest In the South East and least in the North West. However the North West receives much higher rainfall and hence more acid deposition takes place here.

Day Three:Acid rain and forest decline (Jizera Mts)

4.2 Effects of acid rain

Sometimes, the environment can naturally adapt to acid rain. For example, in locations where there is a large amount of lime occurring naturally in the soil, the soil will have no problem with acid rain. The lime, which is a base, will neutralize the acid in the rain, thus minimizing their effects.

However, in locations where there is not a way to naturally compensate for the acid rain, the acids can cause a lot of harm to things that we care about and enjoy. For example, some animals, like frogs and fish, have difficult time adapting to and reproducing in an acidic environment. Also, the leaves of many plants and trees can be severely damaged by acidic precipitation. Acid rain leaches calcium and magnesium from the soil. This causes a decrease in the ratio of calcium to aluminium in the soil, which stimulates the uptake of aluminium by roots. The uptake of aluminium by trees and plants can be destructive. Finally, in cities and towns all over the world, stone structures, such as buildings, ancient monuments etc. are being deteriorated by the corrosive effects of acidic rainfall.

Precipitation containing these acids will affect the ecology of the land and water that it reaches. For example, if the pH of surface water falls below 5.5, algal growth is enhanced which reduces the amount of oxygen available to other species (Alloway and Ayres, 1993). Many fish cannot tolerate acid waters, with an effect on number and variety of species at pH~6. Acid deposition may also affect the growth of trees in forests.

4.2.1 Impacts of Acid Rain on Soils

The critical load for acid deposition will depend on the buffering capacity of the soil – on how quickly the minerals in the soil can be freed by weathering, and so enabled to neutralize the acid.

One effect of soil acidification is to impoverish the soil's nutrient status, according as base cations, such as potassium, calcium, and magnesium, are leached out of it. This may in turn result in nutrient deficiencies and imbalances, which are thought to be among the causes of forest decline in Europe.

Another effect is to increase concentrations of potentially toxic metals, such as aluminium, in the soil water. This is especially the case when the pH of the mineral soil has fallen below 4.4. Increased soil-water concentrations of aluminium can produce damage to trees' roots, and aluminium ions, when leached out into lakes and streams, can become transformed and so toxic to organisms such as fish.

4.2.1.1 Introduction

Soil is the basis of wealth upon which all land-based life depends. The damage that occurs to ecosystems from acidic deposition is dependent on the buffering ability of that ecosystem. This buffering ability is dependent on a number of factors, the two major ones being soil chemistry and the inherent ecosystem sensitivity to acidification. Indirect damage to ecosystems is largely caused by changes in the soil chemistry. Increasing soil acidity can affect micro-organisms which break down organic matter into nutrient form for plants to take up. Increasing soil acidity also allows aluminium (a common constituent of soil minerals) to come into solution. In its free organic form, aluminium is toxic to plant roots and can lock up phosphate, thereby reducing the concentrations of this important plant nutrient.

4.2.1.2 Effects of the soil and underlying bedrock on acid rain

Soils containing calcium and limestone are more able to neutralise sulphuric and nitric acid depositions than a thin layer of sand or gravel with a granite base. If the soil is rich in limestone or if the underlying bedrock is either composed of limestone or marble, then the acid rain may be neutralised. This is because limestone and marble are more alkaline and produce a higher pH when dissolved in water. The higher pH of these materials dissolved in water offsets or buffers the acidity of the rainwater producing a more neutral pH.

4.2.1.3 Acid Sensitive Areas

In regions where the soil is not rich in limestone or if the bedrock is not composed of limestone or marble, then no neutralising effect takes place, and the acid rainwater accumulates in the bodies of water in the area. This applies to much of the mountain ranges encircling the Czech Republic, where the bedrock is typically composed of granite, gneiss or quartzite. These rocks have no neutralising effect on acid rainwater. Therefore over time more and more acid precipitation accumulates in lakes and ponds.

The water bodies most susceptible to change due to acid precipitation are those whose catchments have shallow soil cover and poorly weathering bedrock, e.g. granite and quartzite. These soil types are characterised by the absence of carbonates that could neutralise acidity. The run-off water from such areas is less buffered than from areas such as limestone catchments, with an adequate level of carbonate. Such catchments and waters are termed acid sensitive (poorly buffered), and can suffer serious ecological damage due to artificially acidified precipitation from air masses downwind of major emissions.

Notable high risk areas in Bohemia are Krkonoše Mountains, Jizera Mountains, Ore Mountains. These areas are vulnerable because of their relatively high elevations, small watersheds, and naturally acidic soils. Different types of bedrock contain variable amounts of contain variable amounts of alkaline chemicals. Regions with bedrock containing less alkali have a lower capacity for reducing acidity, and thus are more sensitive to acid deposition.

4.2.2 Soil-calcium depletion linked to acid rain and forest growth

By Gregory B. Lawrence and T. G. Huntington, USGS

Ca

Since the discovery of acid rain in the 1970’s, scientists have been concerned that deposition of acids could cause depletion of calcium in forest soils. Research in the 1980’s showed that the amount of calcium in forest soils is controlled by several factors that are difficult to measure. Further research in the 1990’s, including several studies by the U.S. Geological Survey, has shown that (1) calcium in forest soils has decreased at locations in the northeastern and southeastern U.S., and (2) acid rain and forest growth (uptake of calcium from the soil by roots) are both factors contributing to calcium depletion.

4.2.2.1 Calcium cycling in forests

Calcium is an essential nutrient for tree growth that is used in the formation of wood and in the maintenance of cell walls, the primary structure of plant tissue. Trees obtain calcium from the soil (fig. 1), but to be taken up by roots, the calcium (a positively charged ion) must be dissolved in soilwater. Calcium adsorbed to negatively charged surfaces of soil particles (termed exchangeable calcium) is also available to roots because both adsorption and desorption of calcium occurs readily through the process of chemical equilibrium. Some calcium is deposited onto forests in an available form in dust and precipitation or is returned to the soil through decomposing leaves and branches that form the forest floor. Most of the calcium in soil, however, is bound within the mineral structure of rocks within the underlying mineral soil, which prevents it from dissolving in water or becoming adsorbed to particle surfaces. Weathering, the physical and chemical breakdown of rocks, gradually releases calcium to soil water where it can (1) be taken up by roots, (2) adsorb to particle surfaces, or (3) be leached out of the soil by negatively charged ions in percolating water. As calcium is released by weathering, acidity in the form of hydrogen ions is neutralized, which increases the pH of soil water and the surface waters into which it flows. Calcium leached out of the soil into surface waters also serves as an essential nutrient for aquatic plants and animals.

Figure 3.3.1. Calcium cycle in forest ecosystems. Inputs to the pool of available calcium in the soil result from weathering of rocks, atmospheric deposition, and litterfall. Outputs from this pool result from root uptake and leaching out of the soil.

4.2.2.2 Calcium research in the 1970’s and 80’s

The discovery of acid rain (acids deposited from the atmosphere in rain, snow, particles and gases, more accurately referred to as acid deposition) in the 1970’s prompted concern that deposition of acids could increase leaching of calcium, and other neutralizing cations, such as magnesium, sodium, and potassium (referred to as base cations) from forest soils. Acid deposition provides (1) hydrogen ions, which displace cations adsorbed to soil surfaces, and (2) sulfate and nitrate ions, which tend to keep these cations dissolved in soil water that eventually drains into streams and lakes. Results of preliminary research in the 1970’s suggested that elevated leaching might deplete soil calcium, decrease forest growth, and acidify surface-waters (Cowling and Dochinger, 1980). These concerns were somewhat alleviated in the Northeast by the discovery that the soil contained large amounts of calcium. Although most of the soil calcium was known to be in unavailable forms in rocks and minerals, the rate at which calcium was released by weathering was considered to be sufficient to offset increased leaching caused by acid rain (Johnson and others, 1982). In the Southeast, the amount of calcium in soils was found to be generally less than in the Northeast, but was thought to be somewhat protected from cation leaching by the high capacity of these soils to adsorb sulfate, a negatively charged ion. The fine texture of soils in the Southeast enhances sulfate adsorption, which removes the negative charge from soil water that is necessary to leach the positively charged cations. Considerable uncertainty as to the status of soil calcium remained, through the 1980’s, however, because the processes involved in weathering and leaching were found to be extremely complex. At the close of the research phase of the National Acid Precipitation Assessment Program (NAPAP) in 1990, decreases in the availability of calcium in forest soils from either acid rain or forest growth had not been confirmed, although the possibility of future depletion from acid deposition was acknowledged (NAPAP, 1993).

4.2.2.3 Calcium research in the 1990’s

Continued research in the 1990’s has documented distinct decreases in soil calcium over the past 4 to 5 decades, in both the Northeast (Johnson and others, 1994a) and the Southeast (Richter and others, 1994). These decreases were attributed primarily to the uptake of calcium by trees in excess of inputs from weathering. Follow-up studies have suggested, however, that both acid deposition (Markewitz and others, 1998) and a decline in deposition of calcium from the atmosphere, a trend that has been underway since the 1970’s, may also have contributed to the decrease in the availability of soil calcium in the East (Johnson and others, 1994b). Investigations of forest health have identified relations between low calcium availability and a reduction in stress tolerance of red spruce (Shortle and Smith, 1988; DeHayes, 1992; Schlegel and others, 1992), and sugar maple (Wilmot and others, 1995; Long and others, 1997). Related studies have also suggested that forest harvesting could reduce calcium availability through the removal of calcium stored in trees, which could lower the growth rates of the regenerating stand (Federer and others, 1989; Hornbeck and others, 1990). Relationships among acid deposition, calcium availability, and forest productivity remain uncertain, however.