Unit 6.4 Acid Rain* Flashcards
Outline the major sources of sulfur to the atmosphere and compare the magnitude of natural sources of sulfur to anthropogenic sources.
Almost all sulfur in the atmosphere occurs as SO2.
Natural sources:
- sulfides of metals in rocks
- sea spray
- biogenic gases
- volcanic activity
Anthropogenic sources:
- burning fossil fuels (coal and oil)
Natural and anthropogenic emissions are fairly evenly balanced.
State the major chemical components of anthropogenic acid rain.
Sulfur dioxide (SO2) is the main pollutant.
This can develop into sulfuric acid (H2SO4) in the atmosphere.
However, as deposition of sulfur decreases through legislation, nitrogen oxides (NOx) and ammonia (NH4) contribute an increasing proportion to acid deposition. However, these chemicals can be taken up harmlessly by plant roots, and so their impact in many areas has not been great.
Perform calculations on biogeochemical cycles, including those that determine the reservoir size, input/output flux or residence time of a component, given these two variables.
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Explain the effect of atmospheric carbon dioxide on the pH of ‘natural’ precipitation, and describe other natural factors that can influence pH of rain.
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Dissolved CO2 reduces the pH of unpolluted rainwater to pH 5.85
Dissolved CO2 dissociates into hydrogen carbonate, producing an H+ ion as a byproduct:
CO2 (aq) = HCO3/- (aq) + H+ (aq)
The hydrogen carbonate can in turn dissociate into carbonate, producing another H+ ion:
HCO3/2- (aq) = CO3/- (aq) + H+ (aq)
Summarise, in general terms, how SO2 is transformed to H2SO4 in wet- and dry-phase reactions.
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Recognise the environmental conditions that are likely to enhance the amount or concentration of acid deposition on vegetation and soils.
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Outline the major ways in which acid rain is buffered by soils through interactions with organic matter, ions derived from weathering, cation exchange reactions, and sulfate adsorption reactions.
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Sulfate absorption reactions:
- SO4/2- is attracted to protonated hydroxyl groups.
- SO4/2- reacts with iron or aluminium hydroxides to form iron or aluminium sulfate complexes, which consumes H+, releases OH-, and binds the sulfate anion to the soil.
- immobilising an anion reduces the mobility of cations, which can only leach in the company of an anion.
Define the acid neutralising capacity of a surface water and describe the major chemical components that make up ANC.
Acid neutralising capacity.
The sum of the molar concentrations of all proton acceptors (which can potentially neutralise H+ ions), minus the sum of the molar concentration of all OH- acceptors (which can potentially neutralise OH- ions), minus the sum of the molar concentrations of all proton donors (which can potentially release protons).
Incoming H+ ions are neutralised by proton acceptors (i.e. negative ions) until these are all used up, from which point acidity will increase.
In natural waters with normal pH, the principal proton acceptor is hydrogen carbonate (HCO3-). Other acceptors include carbonate, hydroxyl, and dissolved organic matter.
Describe the importance of dissolved inorganic aluminium in acidification of soil and surface water.
Most aluminium in soils occurs in organic acid complexes within humus, but in acidic conditions these complexes are reduced and inorganic aluminium ions are released.
In water, inorganic aluminium ions can react with hydroxyl ions to form one of several aluminium-hydroxyl complexes, and the relative proportions of these are linked to pH. All are toxic, although one (Al3+) is the most toxic.
Aluminium toxicity is thought to be the cause of fish deaths in highly acidified lakes in Scandinavia.
Explain the role of mobile anions in catchment and surface water acidification.
Cations do not leach without the accompaniment of an anion.
Therefore if anions are absorbed by the soil, base and acidic cations can not be leached. Retaining base cations allows the soil to maintain base saturation, while retaining acid cations reduces the acidity of soil water.
Explain the importance of catchment geology in determining the sensitivity of a stream or river to acidification by acid precipitation.
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Outline the three hypotheses that were first proposed to explain the observed acidification of surface waters.
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Explain how paleolimnology was used to settle some of the debates about the causes of surface water acidification.
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Understand the major mitigation strategies to halt or reverse the effects of acidification, and the European protocols for limiting the emission of the acid precursors SO2, NOx and NH3.
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Describe the role of models in understanding and predicting the effects of acid deposition.
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