6.4 Acid Deposition

This is a transboundary issue

affects countries that don’t necessarily cause the pollution

Acidity

a measure of H+ concentration in a solution

pH < 7 (H+ > OH-)

• pure water = pH 7 (H+ and OH- ←→ H2O)

• bases = pH > 7 (cleaning product), alkaline

rain’s normal pH is between 5-6

• CO2 + H2O → H2CO3 (carbonic acid) affects ocean animals

• rain pH < 5 when NOx + SO2 creates HNO3 (nitric acid) + H2SO4 (sulfuric acid)

human blood → pH 7.4

bleach → pH 13

Buffers

bring solutions toward neutral

ex: add a weak base to a strong acid to raise pH closer to 7

SO2 (sulphur dioxide)→ Sulphuric acid RXN

sulphur dioxide (primary pollutant) + O2 → sulphur trioxide →

sulphur trioxide + H2O → sulphuric acid (secondary pollutant, acid in rain)

NOx → Nitric/Nitrous Acid RXN

nitrogen monoxide (primary) + O2 → nitrogen dioxide →

nitrogen dioxide (could be primary too if directly emitted) + H2O → nitric acid (HNO3) + nitrous acid (HNO2, secondary/acid in rain)

Types of Deposition

Dry deposition

• gravity forces pollutants down toward surface

• plants + buildings can directly absorb the pollutants

Wet deposition

• precipitation (rain, snow, sleet) brings pollutants down to surface from clouds

Dispersion + Distribution

• winds disperse SO2 + NOx

• when there’s little/no wind, pollutants accumulate near source

• smokestacks reduce local pollution by sending emission higher into troposphere where there’s less wind

• topography downwind of source can affect distribution

  • mountains can force moist air up — precipitation that forms can be more acidic

Impacts of acid deposition

• increase of acidity has a negative effect on aquatic + terrestrial ecosystems + structural damage

• increasing acidity can cause increase in aluminum ions which has a negative effect on life

• use of buffer can neutralize the effects of acid rain in water + soil

  • BAND-AID solution

  • limestone (calcium carbonate) is used

  • calcium + magnesium can also be used

Aquatic ecosystems

• acid rain and acidic RUNOFF can both lower pH (acid → 0-6.9)

• aquatic species are very sensitive to pH changes

  • fish reduce O2 uptake when exposed to Al+ (aluminum ion) due to excess mucus that forms over the gills

  • changes in pH can interfere with shell + exoskeleton formation

Ocean Acidification (by CO2)

• increase in CO2 in atm → increase T of ocean surface

• as T of ocean increases → CO2 absorption decreases → more CO2 in atm

  • cold water absorbs more CO2

    • polar waters r absorbing CO2 faster than tropical waters (which releases CO2 faster)

• currents move warm water around the planet (warm tropical → poles)

• wind also plays a role: mixes upper + lower depths of water to mix CO2 saturated water down

  • surface T increases → wind doesn’t churn layers as it should

Increase in H+ decreases carbonate ions

more H+ means more acidity

H+ grabs carbonate ions to make bicarbonate

carbonate is needed by shell fish + corals to make shells (less carbonate for the animals that need it)

Process of Acidification

CO2 (atm) + H2O → H2CO3

H2CO3 → H+ (increase acidity) + bicarbonate (makes CO3 2- unavailable)

H+ + CO32- → HCO3- (H+ bonds with carbonate ions → make more bicarbonate) EXCESS H+ stealing CO3 2-

Necessary life function: CO3 2- (carbonate ions that are taken away by H+) + Ca 2+ → CaCo3

• carbonate needed to build shells or reefs (calcification by marine organisms)

How are Terrestrial Ecosystems Affected?

• Increase in soil acidity increases Al+ → make plant nutrient uptake more difficult → damage to root cells

  • essential nutrients plants need are leached

• increase the amount of toxic metals in the soil (which can leach into water)

• acidity can also damage plant cuticles (protective layer on leaves) + reduce tolerance to disease + lower temps

• thus crop yield, BD, forests are impacted negatively

pH could also affect the enzymes that help perform PS

Stone erosion

• chemical weathering

• acid rain can erode limestone + marble

• pores can allow acid to penetrate stone + further damage the structure

calcium carbonate (limestone) + sulfuric acid ←→ gypsum + CO2

• gypsum can flake, occurs from acid penetration + allows for deeper acid penetration which can destroy stones

Metal corrosion (iron, steel)

reaction between acid rain + metal

weakens its structure

Management

Manage polluters + emissions of NOx + SO2 from stationary + mobile sources

same methods used to manage photochemical smog apply here

• education

• legislation

• mitigation

International Cooperation

Countries must be mindful that their actions affect other countries

Convention of Long-Range Transboundary Air Pollution (Geneva Convention)

• focused on reducing NOx + SO2 (LRTAPs)

• 51 parties (N. America, Europe, Russia/nations of ex-USSR)

• 2012: most recent amendments to set emission standards from the Gothenburg Protocol

  • further reduce emissions of soot, other particulate matter

  • more flexibility in how obligations are implemented

Canada-US Air Quality Agreement (1991)

Recognized problem b/t the 2 countries (emissions can travel long distances)

Began to work tgt to reduce emissions

• led to reductions in acid rain

• 2000: reduce transboundary smog emissions

Emission levels have fallen in response to the standards put in place

Clean Air Interstate Rule (CAIR)

EPA’s national law

address soot (particulate matter), smog (ozone) that leads to lots of death + illness each year

requires 28 eastern states to reduce sulfur dioxide (SO2) + nitrogen oxides (NOx) that lead to pollution in downwind states

US EPA Acid Rain Program (ARP)

requires major emission reductions of SO2 + NOx from the power sector

Cap and Trade System in the US

allocation of emission permits distributed to energy producers

companies can buy/sell their permits

• companies that are under emissions limit may profit off the sale of their permit (sell off the surplus)

On site clean up + restoration

How do we restore the natural balance of an ecosystem after pollution + increase in acidity?

Help nature + CO2 absorption

• Reforestation

• Regenerate grasslands

Can add lime, but this is only temporary fix

Need to deal with the source

• Individuals/states/nations relying lens on FF

• retool society especially those directly reliant on FFs (workers who drill the oil)

Mitigation = PRICEY