Air is a non-homogenous mixture of gases, solid particles, and liquids.

Air is characterized as an aerosol.
 

Nominal Composition

Major Components	(99%) in % by wt.
N2	75.51% by wt.	(0.7808)
O2	23.14%	(0.2095)
40Ar	1.28%	(0.0093)
Water Vapor		(0.0004)

Minor Components
CO2	325 ppm (parts per million)
Ne	18
He	5
CH4	2
N20	0.5
Xe	0.1

** 325 ppm means that 325 of each 1 million particles is CO₂

The Atmosphere is Divided into Four Major Regions

Recall atmosphere layers

Troposphere	0-17, Km (? 11 mi.)	> -56°C
Stratosphere	17-50	> -2°C
Mesosphere	50-90	> -85°C
Ionosphere (thermosphere)	90-100

95% of the air by weight is contained in the Troposphere.

• A Thermal barrier to prevent Troposphereic mixing with Stratosphere

• Good Ozone layer is in the stratosphere also acts as a thermocline to prevent mixing (notice chart of atmospheric temperature)
•  The Sun drives reactions and with the Atmosphere sets the Earth's temperature
• Earth maintains its temperature because of its atmosphere.
• Between 0 °C and < 100 °C

An equilibrium is established which accounts for the earth's surface temperature this is the Positive greenhouse effect.

It is an increase in the shift of this equilibrium that will increase the average temperature of the earth's surface and result in a
Negative  greenhouse effect.
 
The Sun - The Atmospheric Energy Driver

All energy driven reactions on Earth and are a result of large amounts of energy form the Sun.
 

The Atmosphere has different temperatures at different heights due to chemical reactions driven by the Sun's energy of ~1 Kw/m²/ day at surface and ~1,340 watts/m²/ day;  in the upper atmosphere perpendicular to the Sun.
 

What wavelengths are involved with the conversion of energy?
The sun is our only source of energy.  It transmits ~ 86 % of this energy at 400-700 nm (0.4 to 0.7 {µm} micrometers)
The Visible Light Spectrum.  [400 nm blue and 700 nm red]
~7 % is transmitted at < 400 nm or as ultraviolet (UV).
 (remember O₃)
~7 % is transmitted at > 700 nm or as Infrared (IR).
 

 Outdoor Air Pollutants

Sources and Types:

1. Carbon Oxides - Carbon Monoxide (CO), Carbon Dioxide (CO₂)

2. Sulfur Oxides -  Sulfur Dioxides (SO₂) & Sulfur Trioxide (SO₃)
{contributor to ground level Ozone, (O₃)}

3. Nitrogen Oxides - Nitric Oxide (NO), Nitrogen Dioxide (NO₂), &  Nitrous Oxide (N₂O)
{contributor to ground level Ozone, (O₃)}

4. Volatile Organic Compounds (VOCs) - Most organic compounds
 Example:  Methane (CH₄), Methanol (CH₃OH), Benzene    (C₆H₆), Chlorofluorocarbons (CFCs), formaldehyde (CH₂O), Propane (C₃H₈).
{contributor to ground level Ozone, (O₃)}

5. Suspended Particulate Matter (SPM) - Solid and Liquid Particles  both suspended in air.  Example: dust soot, pollen, asbestos,  ash, conglomerates, soil, salts, etc.
{contributor to ground level Ozone, (O₃)}

6. Photochemical Oxidants - Ozone (O₃), Peroxide (H₂O₂), Complex interactions with VOCs and NOx. Photochemical reactions

7. Radioactive Isotopes - Radon - 222, Iodine - 131, Strontium - 90,  Plutonium - 239, Potassium - 40,

8. Heat - Waste energy from fossil fuels; most energy sources and  uses produce waste energy as heat, Ex.: cars, power, plants,

9. Noise - A byproduct of energy: airplanes, cars, industry, lawn mowers, mechanical, radios, wind, electrical power line discharge, etc.
 
Outdoor Air Pollutants

 Figure 22-2 (Primary Pollutants)

Primary Pollutants Anthropogenic (Directly Emitted)
CO  Carbon Monoxide
CO₂  Carbon Dioxide
NO   Nitrogen Monoxide
NO₂  Nitrogen Dioxide
SO₂  Sulfur Dioxide
VOC  Volatile Organic Carbon
SPM,  Suspended Particulates  Matter
or PM-10

Secondary Pollutants     (Transformed in the environment)
SO₃  Sulfite
HNO₃ Nitric Acid
H₂SO₄ Sulfuric Acid
H₂O₂ Hydrogen Peroxide
O₃   Troposphere

Methane (CH₄) {a primary VOC}, Contributor to Photochemical Smog and Greenhouse Gas Emissions  from Anthropogenic Sources

World	270,000,000 metric tons
By World Region
Asia	130,000,000
North and Central America	45,000,000
Europe	26,000,000
USSR	34,000,000
Africa	19,000,000
South America	19,000,000
Oceannia (Australia, Fiji, etc.)	6,200,000

Outdoor Air Pollutants
Pollution Emissions
Anthropogenic compositional alterations to the atmosphere:
From 1989 Compilation (from United Nations Reference)
 

Emissions of Chlorofluorocarbons (CFCs)

World	580,000 metric tons
By World Region
Asia	140,000
North and Central America	150,000
Europe	180,000
USSR	67,000
Africa	16,000
South America	15,000
Oceannia (Australia, Fiji, etc.)	9,000

Ozone depletion is the main effect, remember on average each Cl atom destroys 100,000 (105) O₃ molecules (4 per molecule of CFC).

Pollutant Trends
From 1900 to 1970 dramatic increases in the U.S. in some pollutants were observed.

 NOx 690%
 VOC  260%
 SO₂ 210%

From 1986-1995 U.S. Concentrations of key pollutants decreased by:
NOx 14%
Ozone   6%
PM-10 22%  (Particulate Matter, <10 µm)
SO₂ 37%
lead 78%
CO 37%

From 1986-1995 U.S. Emissions of key pollutants decreased by:
NOx   3%
VOC   9%
PM-10 17%  (Particulate Matter, <10 µm)
SO₂ 18%
lead 32%
CO 16%

While you can measure Toposhpereic Ozone it is not (in general) a direct emission but is the result of reactions catalyzed or caused by other pollutants.

 Smog and Acid Deposition Conc.

Photochemical Smog is the interaction of electromagnetic radiation from the Sun and Primary Pollutants to form photochemical smog.
 

Table of Atmospheric Trace Gases in Dry Air Near Ground Level

NOx as Outdoor Air Pollutants

NO₂ & N₂O Emissions  1989   1980
North & Central Am.   21,600,000  22,300,000
Asia        1,400,000     ?
Europe             ~15,000,000   ?
USSR       4,190,000   ?

U.S. evaluation of sources
 Fuel combustion 46% of emissions
 Transportation 47% of emissions (specific fuel combustion)
 Industrial and other sources make up the remaining 7%

Principle anthopogenic source of NOx compounds is from fuel in high temperature combustion processes:
 
  N₂ + O₂   +  heat (>1,000 °C)<---> 2NO

 Then NO combines with O₂ in air or other oxidents to convert  NO to NO₂ within a few hours under normal conditions .

   2NO + O₂ ( or other oxidents, RO₂ )--->  2NO₂
 

All Oxides of nitrogen with water form nitric acid (HNO3 )
 N₂O, NO, N₂O₃, NO₂, N₂O₄, N₂ O₅ ~ =  NOx

   for reference:
   2NO₂    + H₂O  --->  HNO₂   + HNO₃

Anthropogenic Prevention

Primary method of prevention is
1. reduced temperature of oxidation of fuel (or do not burn fuel) and
2. scrubbing with various bases can be used if the process permits

Note: Natural source - Lightning and Nitrogen Cycle
 
Good Ozone

Hypotheses of CFC effect on Ozone presented

The possible depletion of Ozone by Anthropogenic gases was first proposed by H. S. Johnson of the U. of CA, Berkeley in the 1960s.

Effect on ozone of CFCs was theorized in 1974 by two chemists, Sherwood Rowland and Mario Molina from the University of California, Irvine.
(Noble Prize for this contribution)

Uses CFCs

Coolants in air conditioning and refrigerating
Cleaning of electrical parts
Fumigants for granaries and cargo holds
Bubbles in polystyrene plastic foam packaging &  insulation (DuPont - Styrofoam)
Propellants in aerosol spray cans (not in US since 1978s)

Demographics

MDCs (industrialized countries) use 84% of CFCs
US is 25% of global consumption of CFCs

Vehicle air conditioners account for about 3/4 of US CFC emissions

By year 2000, 75 countries will phase out all CFC use.
 
Why is Cl from CFCs so devastating to Stratospheric Ozone?

How long does a CFC last in the atmosphere
over 50-400 years?

Freons without any hydrogen atoms have an average life time of 100 years in the atmosphere.

Time is needed to transcend the Troposphere thermal barrier and get into the Stratosphere reach the Ozone layer and enter it.

One Cl atom can convert 10,000 to 100,000 O₃
to O₂ and O

Hydrogen containing CFCs only last a relatively short time and probably never make it to the Stratosphere.  CH₃CCl₃ and CHClF₂ only last 6-7 years.  This is the change in CFC formulation.

What is the difference between hydrogen containing (H)CFCs and halogen saturated CFCs?

Method of destruction in Troposphere.

CHClF₂   +  OH•  ---->   H₂O  +  CClF₂•

The hydroxyl radical is the natural air cleanser of the Troposphere and can work on (H)CFCs (not CFCs).
 
Mechanism of Ozone decomposition

CFC travels just above the O₃ layer in the Stratosphere
 

            Ultraviolet - UV
 CF₂Cl₂    ---->   CF₂Cl   +   Cl  Eq-1

It then drifts back into the lower Stratosphere where O₃ is in high concentrations
 

 Cl  +  O₃   ---->  ClO  +  O₂   Eq-2

Direct destruction of Ozone
 

 ClO  +  O   ---->    Cl  +  O₂   Eq-3

Short circuit of Step a and c  of Ozone UV protection cycle.

Life of Cl atom in atmosphere is 1-2 years.

In 1-2 years, one Cl atom will repeat Eq-2
100,000 times.

One Cl atom destroys ~ 105 the # of O₃ atoms.
 (A very efficient reaction mechanism)
 

Why does Ozone absorb UV energy?
 

Ozone is a Resonance Structure
 
 

 
 

This resonance gives it the unique UV energy absorbing property

 Photochemical Formation of Ozone in the Stratosphere and its interaction with UV light
 

Photodissociation Reaction

Step a
           Ultraviolet
 O₂(g)   ---->    2O(g)    Free Radical
 120 Kcal/Mol

Step b    240 - 300 nm
          Ultraviolet
O(g)  +  O₂(g)  + M(g)  ---->  O₃(g)  + M(g)
 
where M(g) is another molecule such as O₂ or N₂ or particle to carry away thermal energy
(Unchanged chemically)

How Ozone absorbs ultraviolet light

Step c  < 308 nm
    Ultraviolet
 O₃(g)   ---->   O₂(g) + O(g)*  +  636, 630, 558 nm
 26 Kcal/mol     (yellow to red light)

Back to Step b again to be regenerated if no pollutants are present

 Photochemical Formation of Ozone in the Troposphere and
its interaction with UV light VOC and NOx

Primarily a Summer time phenomena in U.S. due to heat needed in the reactions.

Tropical climates have the potential in all seasons.

 NOx + VOC + Heat + Sunlight ---->  Ozone

Specific important reactions

Step a: NO₂ decomposes to produce oxygen atoms
 NO₂ + Sunlight (<430 nm) ---->O + NO
       (NO₂ must be regenerated)

Step b: oxygen atom combines with oxygen molecule to form ozone with heat carried away
 O + O₂ (+N₂, O₂, M)  ---->   O₃ (+N₂, O₂, M)

Other Ozone building step and intermediates:
 O₃ + light (<320 nm)  ----> O*(O* Excited) + O₂

 O* + H₂O ----> 2 OH• (hydroxy radical)

 OHï + RH (VOC) (+ O₂) ----> H₂O + RO₂

 RO₂ + NO ----> RO + NO₂ (Back to Step a:)
 Thus NO₂ is the main ingredient however, NO₂ must be regenerated by something other  than Ozone itself as in to increase the Ozone level and this is the VOC through the OH radical.
  NO +  O₃  ---->   NO₂ + O₂

Thus formation of hydroxy radicals generates oxidized VOCs that regenerate NO₂ and prevent NO destruction of Ozone establish a high Ozone concentration.  So Ozone concentration depends on NO₂, H₂O, VOC, Heat and Sunlight to reach a high Tropospheric level.

Why is Tropospheric chemistry radically different

•  The mean free path for a reactive molecule is 10⁴ to 10⁹ different in these two regions of the atmosphere.  Mean free path in the Troposphere is only 1 x 10⁶cm while at 500 km it can be as long as 1 x 10⁶ cm a 1 x 10¹² difference.
•  In addition < 95% of the UV light is absent in the Troposphere due to the Ozone in the Stratosphere.
•  Water vapor exists in liquid form in the Troposphere to promote chemical reactions which is in the solid phase (ice) in the Stratosphere.
•  NOx and VOCs are not significantly present in the Stratosphere

Natural Troposphere atmospheric cleansers.

The single most important intermediate reactant in photochemical processes
 

    sunlight
 H₂O(g)    ---->  H(g)  + OH(g)

or

 O* + H₂O ---->  2 OH• (hydroxy radical)
 

Carbon monoxide is converted to carbon dioxide
CO(g) + HO(g)•      ---->    CO₂(g)  + H(g)
 
 

VOCs are removed by hydroxyl radical

CH₄(g) + HO(g)•      ---->  H₃C•(g)  + H₂O
 

 Summary

Increases in NOx and VOC concentration on hot (due to Sunlight) days when water vapor is highest and increases the O₃ level due to a complex series of reactions.

These conditions occur

Since NOx and VOC can be transported they can contribute to O₃ concentrations in other locations where water and Sunlight find them on a warm Summer day.
 

What can we reduce or eliminate to reduce Ozone?

Sunlight        No
Water vapor        No
Heat        No
Inversions         No
VOCs        Some
NOx        Some
Air Movement        No

Notes Table of Contents

Introduction to Environmental Science Home Page

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Revised 5/25/99