9. Acid Rain London Smog PDF

Preview

Summary

Professor Lew...

Description

9a. Introduction Acid Rain ● Important topic in 1970s ● Forms when a fog drops absorb anthropogenic pollutants such as SO2,, NO2, sulfuric and nitric acid ○ Leads to sick or dead trees ● Walsterben: dying forest ● Acidification leads to dying fish London Smog ● 1952 killer smog episode ● Physical symptoms: ○ Chest constriction ○ Difficulties breathing ○ Headache ○ Nausea ○ Vomiting ● Smoke + Fog = Smog ● Ingredients of London Smog ○ Low inversion height ■ 50-100m ○ Coal fires from house heating and industry release ■ SO2 (sulfur dioxide) and soot ○ Fog droplets take up SO2 and sulfuric acid and become very acidic ○ Acidic fog droplets + smoke particles are responsible for health effects 9b: Chemistry of Acids and Bases Acids: molecules dissociate into negative ions and H+ ● Acid tastes sour ● Sour liquids like lemon juice, vinegar are all acids Bases: molecules that dissociate into positive ions and OH- ions in the water ● Bases taste bitter (baking soda) ● Both acids and bases are corrosive Bases + Acids ● Bases can react with acids and form slats ● A salt solution can be neutral ○ Neither basic or acidic ● Base can act as buffer for an acid Acids/Bases ● Strong acids and bases dissociate completely in water

● ●

Weak acids and bases dissociate partially in water How can we quantify the acidity/basicity of acids and bases? ○ Use concentrations of H+ and OH-

→ Dissociation of Pure water ● A very small number of water molecules dissociates in pure (neutral water) ○ H2O (aqueous) ↔ H+ + OH● Equilibrium H+ and OH- concentration in pure water ○ [H+]eq = [OH-]eq = 10-7 mol / liter ○ 1 mol = 6.023 x 1023 molecules The pH sale ● pH: potenz (power) of hydrogen (H) ○ pH = -log10([H+)] ● Pure water = 7 Acid Buffer ● A buffer solution will remove any hydrogen ions or hydroxide ions that are added to it, thus resisting a change in pH ● Ex. Equilibrium between a weak acid, HA, and its base, A○ HA + H2O ↔ H3O+ + A● Add H+ → equilibrium moves to the left, as there are hydrogen ions (H + or H3O+) on the right ● Add OH- → equilibrium moves to the right, as hydrogen ions removed in the reaction (H+ + OH- → H2O) ● The added reagent is consumed in shifting the equilibrium ● pH changes by less than without the buffer 9c. Sulfuric Acid (H2SO4) ● Formation in the gas-phase ○ SO2 + OH + O2 + H2O -> H2SO4 + HO2 ● OH: hydroxyl radical ● HO2: hydroperoxy radical ○ Radicals are molecules with free electron and are chemically very reactive ● Formation in water ○ SO2 (Gas) -> SO2 (aqueous) : dissolution ○ SO2 (aqueous) -> H2SO3 : hydration ○ H2O2 + H2SO3 -> 2H + SO4 (sulfate ion) = +H2O (sulfuric acid) : mechanism Key Components of the Natural Sulfur Cycle ● Source ○ SO2 from volcanoes

○

●

● ●

● ● ●

Emission of organic sulfur compounds from the land and the ocean ■ Get chemically converted to SO2 and can further be converted through droplets

Sink ○ Rainout into the soil and the ocean ○ Can be buried in the ocean floor ○ Levels were rather low unless you are right next to a volcano Humans increased the amount of sulfuric dioxide into the atmosphere ○ Through burning coal and oil US SO2 Emission by source sector 1. Electricity generation 2. Fossil fuel combustion 3. Industrial processes Coal contains 1-6% of S by weight Oil 1-2 % S Gas ~0.5% S

NItric Acid HNO3 ● When NO2 is oxidized by OH -> HNO3 ● HNO3 (gas) -> HNO3 (aqueous) -> H+ + NO3● NO (emitted from combustion sources) + O3 -> NO2 + O2 ● HNO3 is very soluble and is rapidly taken up by water drops Nitrogen cycle ● Two natural sources ○ Lightening ○ Bacterial ins soil ● Very little nitric acids ● Combustion reacts with O3 turns into NO2 and then into HNO3 ● Highest level of NO2 are found in LA or highly industrialized areas NOx (=NO + NO2) emission ● Asia considerably increased ● N. America had stayed constant Sources of NOx in the US 1. On road vehicles 2. Natural road equipment 3. Electricity generation 4. Fossil fuel combustion 5. Industrial processes Bases in the Atmosphere NH3: ammonia is emitted by agriculture (cattle farms)

Ammonium Hydroxide: NH3 (gas) + H2O (liquid) -> NH4OH(aqueous) NH4OH (aqueous) -> NH4 + OH Lime: Ca(OH)2 (aqueous) -> Ca2+ + 2OH Acid rain is also impacted by the presence of bases in the atmosphere 9e. Aqueous Uptake and Deposition of Acids Uptake of gases in water ● Acid hit surface of water drop and enter into the water drop ○ H2SO4 enter water dropet -> 2H+ + SO4- (sulfate) ● Solubility: how much of a gas can be dissolved in water ● H2SO4 is taken up easily and has a high solubility ● SO2 has a low solubility Natrual pH of rain ● pH( natural rain) ~6.5 ● Uptake of CO2 into water droplets leads to Carbonic Acid H2CO3 ○ CO2 in water -> H2CO3 -> 2H2+ + CO3○ At ~390 ppmv CO2 natural rain is acidic ● This is not yet acid rain ● Acid (+ H2SO4) rain pH ~5-4 ● Acid fog minimum reported pH ~1.7 (as acidic as vinegar)

How do these acids get into the eco-system? Wet Deposition ● Acidic clouds rainout, introduce acid into the soil ● Acidic clouds can hit mountainside and be in contact with plants ● Acidic fog can be in contact with plants at the surface

● Acidic water ends up in the soil, severe effects Dry Deposition ● Gas phase acid molecules hit the surface of buildings, plants and stick to it

Acidity of acid rain ● A large east and west different ● West is around 5 ● pH on east coast is much lower 9f: Acidification Impacts and Remediation Influence on Ecosystems ● Main impact is acidification of soil ○ Release toxins that plants can take up ○ Weakened health of trees ○ Much more susceptible to insects -> death of trees ○ Effects depend on the “buffering capacity” of soil ● Acidification of lakes ○ Swedish fisherman found that number of fish was decreasing in 1960s ○ Realized that london smog was responsible ○ On the Northeast of US there are a lot of acidic lakes ○ Kales : typical pH ~6-8 ○ Lakes and streams collect rain water -> acidification ○ Some lakes are more sensitive than other because of their lower “buffering capacity) ■ Acidic lakes: pH of less than 5 ■ Negative influence on animals Destruction of Art How to Combat acid rain? ● Reduce SO2 and NO/NO2 emissions ● Scrubbers: take more than 95% of SO2 out of the exhaust of power plants ○ Costs money ● Low sulfur fuels: natural gas instead of coal ○ Alternative energy source How to implement most efficiently? ● Cap and trade system ○ Quite successful for reducing SO2 emission 1. Total emission is determined 2. Polluters have to obtain allowances in order to be able to emit 3. Polluters can opt to pay money to obtain allowances, or invest the money to reduce emission. If they reduce the emissions beyond the level for which they have allowances for, they can sell them. ● Economic model that makes it financially attractive ○ Cap has to be constantly decreased...