Acid Precipitation

Definitions

Acid: substance that releases H+ in water

Base: substance that produces OH- in water

Molarity (M): the number of moles of solute present in 1 liter of solution

Some Acids and Bases

HCl

NaOH

NH3

HCl + NaOH

H+ + Cl-

Na+ + OH-

+ H2O NH4+ + OH-

Neutralization:

H2O + Na+ + Cl-

pH = - log10 [H+]

?

?

MH+ > MOH- acidic

MH+ < MOH- basic

MH+ = MOH- neutral

acidic basic

neutral

Dissociation of Water• Pure water naturally undergoes a slight

dissociation reaction:

H2O → H+ + OH-

• The pH of pure water, which is neutral, is 7, sopH = 7 = -log([H+])

-7 = log([H+])10-7 = [H+]

• Ditto for the OH- in water:pOH = 7 = -log([OH-])

-7 = log([OH-])10-7 = [OH-]

pH of Common Substances

CO2 as an Acid?

• exists in air at 370 ppm

• from natural processes

• an acid anhydride

CO2 + H2O H2CO3

carbonic acid

H+ + HCO3-

pH of unpolluted air

• So when rain water falls through the air a certain amount of CO2 is dissolved

• A certain about of this becomes carbonic acid• As a result, unpolluted rain has a pH of about 5.6• But human activities release acid precursors so

that in many places the pH of rain is much lower

Acid Precursors

• When gas pollutants e.g. sulphur dioxide, nitrogen dioxide dissolve in rain water, various acids are formed.

• CO2 + H2O H2CO3 (carbonic acid)

• SO2 + H2O H2SO4 (sulphuric acid)

• NO2 + H2O HNO2 (nitrous acid) &HNO3 (nitric acid)

Detailed Reactions

• Sulphuric Acid:

2SO2(g) + O2(g) 2SO3(g)

SO3(g) + H2O(l) H2SO4(aq)

• Nitric Acid:

2NO(g) + O2(g) 2NO2(g)

4NO2(g) + 2H2O(l) + O2(g) 4HNO3(aq)

Causes of Acid Precipitation

SO2: 70% NOx: 30%

Acid Rain

• Reactions to convert to acid take place in ~2 days - travel 1000 miles

• Down wind - Acid rain

• Dry Dep. vs Wet Dep.• Dry Deposition 50 %

of total

A Typical NADP/NTN* Site

• Precipitation sampler

• Recording rain gage

• Consistent sampling protocols

• Central laboratory

• Internal and external QA/QC

*National Atmospheric Deposition Program/ National Trends Network

NADP/ NTN Site Locations

Check out http://nadp.sws.uiuc.edu/nadpdata/state.asp?state=VA

Sources of SOx and NOx

C135H96O9NS

coal

SO2 Emissions across the United States

NO2 Emissions across the United States

Overview: Acid Precipitation

Affected Areas

• Canada– Acid rain is a problem in Canada– Water and soil systems lack natural alkalinity

such as lime base• Cannot neutralize acid

– Canada consists largely of hard rock such as granite

• Do not have the capacity to effectively neutralize acid rain

Affected Areas

• Industrial acid rain is a substantial problem in China, Eastern Europe and Russia and areas down-wind from them.

• Acid rain from power plants in the Midwest United States has also harmed the forests of upstate New York and New England.

• This shows that the effects of acid rain can spread over a large area, far from the source of the pollution

• The net acid deposition flux

=

the total acid deposition flux

–

deposition flux of base cations.

Effects of Acid Rain

• Harmful to aquatic life – Increased acidity in water bodies– Causes eggs of certain organisms (e.g. fish) to

stop hatching• Changes population ratios • Affects the ecosystem

• Fish usually die only when the acid level of a lake is high

• When the acid level is lower, they can become sick, suffer stunted growth, or lose their ability to reproduce.

• Aluminum liberated from soil can interfere with gills• Birds can die from eating "toxic" fish and insects.

Effects of Acid Rain

• Harmful to vegetation– Increased acidity in soil– Leeches nutrients from soil, slowing plant

growth– Leeches toxins from soil, poisoning plants– Creates brown spots in leaves of trees,

impeding photosynthesis– Allows organisms to infect through broken

leaves

• The needles and leaves of trees turn brown and fall off.

• Trees can also suffer from stunted growth

• Damaged bark and leaves can make them vulnerable to weather, disease, and insects

Effects of Acid Rain

Effects of Acid Rain

• Accelerates weathering in metal and stone structures– Eg. Parthenon in Athens,

Greece; Taj Mahal in Agra, India

Effects of Acid Rain• Impacts human health

– Respiratory problems, asthma, dry coughs, headaches and throat irritations

– Leeching of toxins from the soil by acid rain can be absorbed by plants and animals. When consumed, these toxins affect humans severely.

– Brain damage, kidney problems, and Alzheimer's disease has been linked to people eating "toxic" animals/plants.

There are natural sources…

• Terrestial, tidal, and nutrient-rich oceanic areas

• Volcanic eruptions

• Forest fires

• Outgassing from anaerobic wetlands

• Breakdown of amino acids in organic wastes

Preventative Measures• Reduce amount of sulphur dioxide and oxides of

nitrogen released into the atomosphere– Use less energy (hence less fuel burnt)– Use cleaner fuels– Remove oxides of sulphur and oxides of

nitrogen before releasing• Flue gas desulphurization• Catalytic Converters

Preventative Measures

• Use cleaner fuels– Coal that contains less sulphur– "Washing" the coal to reduce sulphur content– Natural Gas

Preventative Measures• Flue Gas Desulphurization (FGD)

– Removes sulphur dioxide from flue gas (waste gases)

– Consists of a wet scrubber and a reaction tower equipped with a fan that extracts hot smoky stack gases from a power plant into the tower

– Lime or limestone (calcium carbonate) in slurry form is injected into the tower to mix with the stack gases and reacts with the sulphur dioxide present

Preventive Measures

• (continued)– Produces pH-neutral

calcium sulphate that is physically removed from the scrubber

– Sulphates can be used for industrial purposes

Preventive Measures

• Use other sources of electricity (i.e. nuclear power, hydro-electricity, wind energy, geothermal energy, and solar energy)– Issue of cost

Reducing the effects of Acid Rain

• Liming– Powdered limestone/limewater

added to water and soil to neutralize acid

– Used extensively in Norway and Sweden

– Expensive, short-term remedy

Drawbacks of liming bodies of acidified water

• Very expensive

• Treatment needs to be repeated every 3-6 years

• Remote lakes are inaccessible

• Large pieces of lime dissolve slowly

• Doesn’t solve the problem for soil acidification

Policy

• 1979 - Geneva Convention on Long-Range Transboundary Air Pollution took place with the idea to limit sulfur and nitrogen emissions was successfully achieved

• 1995-1997 Title IV of the Clean Air Act was established to reduce the adverse effects of acid deposition through reductions in annual emissions of sulfur dioxide of ten million tons from 1980 emission levels.

Many policies and programs were established in the past couple of decades once governments started to become concerned with the harmful

effects of air pollution.