ATMOSPHERIC CHEMISTRYThe atmosphere is a thin layer of gases which surrounds the earth. 78% N2 21% O2 0.9% Ar

0.03% CO2 plus trace gases

thermosphere mesosphere 53 mi

stratosphere 31 mi

troposphere 6 mi earth

- 90% of earth’s atmosphere lies in the troposphere - The atmosphere is classified by altitude and temperature - The temperature generally decreases with increasing altitude except in the stratosphere where absorption of hv increases the temperature

- 90% of atmosphere lies 10 miles from surface – troposphere: weather area.- 9% of the atmosphere lies 10-50 miles above the surface- stratosphere and mesosphere- O3 located 12-21 miles above the earth’s surface

TROPOSPHERE CHEMISTRYPhotochemical SmogPhotochemical smog results mainly from the burning of hydrocarbon fuels. The major ingredients in photochemical smog are NOx, HC, and hv. Minor components include SOx and O3.

Under normal conditions, low lying ground air (lower troposphere) circulates faster and is warmer that the above air(upper troposphere). The net effect allows pollutants to be swept up and out of the low ground area.

Thermal InversionSunlight helps create an inversion pattern while mountainsprevents both the polluted air from escaping and fresh airfrom circulating. During thermal inversion, a layer of warm air resides above cooler ground air, thus trapping pollutants in the lower region. It is in this region that dangerous levels of NOx, SOx, and O3 build up.

PHOTOCHEMICAL SMOG

NOx

Initial Steps:N2 + O2 2•NO2•NO + O2 2•NO2 brown haze•NO2 + hv •NO + O*

Termination Steps:O* + O2 O3

O* + HC IRRITANTS Aldehydes, ketones andO3 + HC IRRITANTS peroxyacl nitratesO3 + •NO •NO2 + O2 restarts #3

SOx

S + O2 SO2 (or) Toxic Gas2H2S + 3O2 2SO2 + 2H2O2SO2 + O2 2SO3 Toxic Gas

SO3 + H2O H2SO4 “Acid Rain”

Solar RadiationUV-C 200 - 280 nmUV-B 280 - 315 nmUV-A 315 - 400 nm

Thermosphere O2 + hv ( 242nm) O + O- Oxygen atoms are produced by cleaving

O2

molecules (endothermic)- Oxygen atoms are highly reactive O + O O2 (exothermic)- This is an energy conversion process:

UV-C (high energy) vibrational energy(low energy) + (heat)

This can occur since (O2) molecules in thermosphere is very low and (UV-C) is great.

OZONE

- Allotropic form of oxygenO2 is odorless O3 is smellylight blue liquid dark blue liquidat - 183º C at -112ºC

- Commercially produced by passing UV radiation or an electrical discharge through O2.

- Commercially used as a water disinfectant and as a bleach

UV 3O2 2O3

ele

OZONE O3

- Detectable around electrical equipment, faulty fluorescent lamps, or thunderstorms in the troposphere

- Ozone is stable at lower elevations (low temperatures) but reactive enough to be a key component in smog.

- Ozone is unstable at higher elevation (high temperature or if a catalyst is used) and breaks down. 2 O3 3 O2

Ozone and Oxygen

• Oxygen molecule is O2.

• Ozone is O3.

• Short wavelength UV dissociates O2.

• O atoms are reactive.

• O3 absorbs longer wavelength UV recreating O2 and O atoms.

• Equilibrium.

2422

uv nmO O O

2 3O O O

3203 2

uv nmO O O

2 3uvO O O

PROTECTIVE UMBRELLAOne of the most important functions of the earth’s atmosphere is to shield the planet from harmful UV radiation from the sun.

High energy radiation from the sun, travels through space until it enters the stratosphere. The gases present in the stratosphere absorb the high energy photons

O3 + hv O2* + O*

N2 + hv 2N* (hv=UV-B)O2 + hv 2O*

Chapman mechanism (1930’s)O2 + hv (< 242 nm) O + OO* + O2

* M O3

O3 + hv (< 325 nm) O2* + O*

O3 + O* 2O2

Good Refrigerants Are Not Necessarily Good For the Air

• Water is not a good refrigerant because of its high boiling point.

• Chlorofluorocarbons (CFCs) are excellent refrigerants.

• AKA Freons:– Non-toxic, non-

flammable, chemically inert.

Ozone Depletion and CFCs

• The ozone layer in the stratosphere absorbs harmful UV radiation.

• Reduction in the ozone layer was linked to presence of CFCs.

A hole over Antarctica

Theorized in 1974, experimentally detected in 1984.

Upsetting the Applecart• Even the stable CFC molecule is

cleaved by UV.• Reactive Cl radicals catalyze

decomposition of O3. 3 2ClO O O

DEPLETION OF OZONE (O3) IN THE STRATOSPHERECatalyst

A substance which increases the rate of reaction but is not consumed in the reaction. A catalyst lowers the barrier thus allowing the reaction to proceed faster

Catalytic Cycle x = catalystX• + O3 XO + O2

XO + O X• + O2

O3 + O 2O2 Net Rx1. X is not dissipated2. O2 is the dominate reactive species3. XO is not reactive with O2

4. XO requires O atoms to complete the cycle5. Most of the O atoms are in the upper stratosphere6. Net result still has more O3 in lower stratosphere

X• free radical (unpaired electron) Cl•

CFC CATALYSIS

CFC CH3Cl, CCl4, CH3CCl3, CFCl3 (Freon 11) CF2Cl2 (Freon 12)

CF2Cl2 exampleCF2Cl2 + hv CF2Cl• + Cl•Cl• + O3 ClO• + O2 ClO• + O Cl• + O2

The single Cl• radical can undergo step 2 around 100,000 time before being terminated (Cl• + CH4 HCl + CH3)(ClO• + NO2 ClONO2) HNO3

Around 200 reactions have been identified as possible participants in ozone depletion.

All is Not Lost

• The Montreal Protocol mandated cessation of CFC use by 1997:– 57 nations signed the

document.

• Modified CFCs with some of the F or C atoms replaced by H atoms do not threaten the ozone layer but…

GREENHOUSE EFFECT

STEP 1Solar radiation reaches the atmosphere primarily as visible light. A smallerportion of the solar spectrum is infrared. 30% of this light is reflected back in to space while the remaining amount is absorbed by molecules on the surface of the planet. Visible light travels past the atmosphere and collides with surface molecules like sand and vegetation. The surface molecules absorb the visible radiation; some of this energy is lost as vibrational energy and the remaining energy is re-emitted as infrared (lower energy).

N2 and O2 are transparent to visible light, these components of the atmosphere absorb predominantly UV radiation and do not participate in the greehouse effect. The primary greenhouse gases are H2O, CO2, and O3. Secondary gases are CH4, CFC’s and NO. These gases are also transparent to visible light and absorb infrared.

GREENHOUSE EFFECT

STEP 2The infrared radiation produce by the interaction of visible light with the surface molecules and that emitted by the sun is absorbed by the greenhouse gases in the atmosphere. These gases have several vibrational modes so they are capable of absorbing lr light. The energy absorbed by these gases is radiated out in all directions as infrared radiation. Some of the re-emitted light is released into space and the rest is directed back down to earth and the processes repeats itself.

The overall effect is the accumulation of large quantities of lr radiation in the

atmosphere, resulting in an increase in the global temperature.

CO2

Things Are Heating Up

• Since 1880 the average temperature on earth has increased.

• Greenhouse gases:– Carbon dioxide– CFCs– Sulfur hexafluoride– Nitrous oxide– Perfluorocarbons– Methane

The Kyoto Protocol: A Step in the Right Direction

• Evidence for global warming has prompted world-wide action.

• The Kyoto Protocol 1997 calls for reduction of greenhouse gases to 8 % below 1990 levels.

• Ratified by most industrial nations – except U.S.A.