1. Natural Cleaning Process of the Atmosphere

2. Objectives
What are the methods involved in the cleansing of the atmosphere?
Learn about deposition velocity and factors affecting it.
What are the cleansing processes involved with various contaminants?
What are the major sinks for common air contaminants?

3. Residence Time for Air Contaminants
The time for which an air contaminant stays in atmosphere is known as the residence time.
Type
Contaminants
Approximate Residence Time
Major species  N2
106 yr O2
10 yr
Tracer species 
CO2
15 yr
CH4 H2
N2O
150 yr CO
65 days
NH3
20 days
NO/NO2
 1 day
O3(troposphere)
< 1 yr
HNO3
1 day
CFC 11
65 yr
Sulfur Compounds 
SO2
40 days
COS
> 0.3 * 105 hrs
CS2
> 1.8 * 105 hrs
CH3CH
hrs
(CH3)S
31 hrs
H2S
53 hrs

4. Cleansing of the Atmosphere
* Methods involved in the cleansing of the atmosphere:
Deposition of the pollutants, and
Conversion of pollutants to other forms (oxidation, absorption etc.)
Deposition of contaminants on the earth surface:
Dry deposition: Toxic air contaminants are deposited to the land and water bodies by settling directly out of the air.
Dry deposition rate = Deposition velocity * Pollution concentration
Wet deposition: Toxic air contaminants are deposited to the land and water bodies through precipitation.

5. Deposition of Toxic Air Pollutants

6. Deposition Velocity The deposition velocity generally increases with:
(a)    solubility of pollutant
(b)    particle diameter and density
(c)    wetness and roughness of surface
(d)    wind speed and turbulence
Typical values of deposition velocity under experimental conditions

7. Cleansing Processes of SO2
The total world emissions are about 100 million tons/year. SO2 is removed from the atmosphere in about 40 days. The two theories for removal are mentioned below:
1.   SO2  +  O2         >  SO3     + H2O > H2 SO4    + NH3, Lime    > Ammonia Sulfate, Calcium Sulfate.
2.   SO2    + NH3, directly calcite of lime  > Sulfites    oxidation >  Sulfate (Precipitate)
Ammonia sulfate is a good fertilizer.

8. Cleansing Processes of H2S and NH3
Cleansing of H2S
The total world emissions is about 300 million tons/year. Major sources are volcanoes and biological decay. There is no evidence of accumulation.
H2S      Chemically React - PbO, FeO (metal oxides)   > Insoluble metal sulfides.
In the laboratory odor can be removed by the addition of a few tenths of nitrogen oxides, which act as a catalyst to form H2SO4.
Cleansing of Ammonia
Ammonia is removed from the atmosphere by reaction with acids or acid forming oxides.

9. Cleansing Processes of Fluorides and Carbon Monoxide
Cleansing of Fluorides
Hydrogen fluoride and SiF4 (Silica Fluoride) readily attack a variety of materials (carbonates, silicates, and organic compounds), and are readily removed from the atmosphere.
Cleansing of CO   
Reaction with the hydroxyl radical.
Oxidized in the upper atmosphere by atomic oxygen to yield CO2.
No evidence of accumulation.

10. Cleansing Processes of CO2 and NOx
Cleansing of CO2 Consumption in photosynthesis. Removed slowly > silicate rocks > limestone and dolomite. The concentration of CO2 began increasing in the 18th century. Data reported in literature indicate that the increase has accelerated. Cleansing of NOx Nitric Acid formed reacts with ammonia or lime > Ammonia nitrate or Calcium nitrate.

11. Cleansing Processes of Smoke and Hydrocarbons
Cleansing of Smoke
Can be removed at source Smoke particles slowly settle as a result of gravitational effects, agglomeration, or rain.  
Cleansing of Hydrocarbons
Reactive hydrocarbons are removed by a series of photochemical reactions.

12. Sinks for Air Contaminants