Natural Cleaning Process of Atmosphere

 

Residence Time for Different Contaminants in Air

Type	Contaminants Approximate Residence Time
Major species	N2 106 yr O2 10 yr
Tracer species	CO2 15 yr CH4 10 yr H2 10 yr 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 3 - 13 hrs (CH3)S 31 hrs H2S 53 hrs

  The residence time gives us an idea of how long it takes for an air contaminant to be removed from the atmosphere. For example, the residence times of carbon dioxide and carbon mono-oxide are 15 years and 65 days respectively. Carbon monoxide is quite reactive, while CO₂  is much more stable.
There are two methods involved in the cleansing of the atmosphere:

	Deposition of the pollutants, and
	Conversion of pollutants to other forms ( due to oxidation, absorption etc.)

In the atmosphere, rain, snow, and dew tend to wash smoke, dust, pollen, and gaseous waste products. Particles less than 2 microns in diameter are not washed by rain and tend to remain suspended. These particles slowly settle down and are then removed by impaction with trees, rocks, or other surface obstacles. The particles may travel for many miles without regard to national boundaries. Pollutants are deposited on the earth surface due to dry deposition and wet deposition. The dry deposition rate can be computed using the following formula:
Dry deposition rate = Deposition velocity * Pollution concentration
The deposition velocity is generally determined by performing experiments. 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 are given below:

Species	Deposition velocity(cm/s)
O3	0.2 to 0.7
NO	0.01 to 0.1
NO2	0.1 to 0.8
MNO3	0.5 to 5.0
NH3	0.2 to 0.6
PAN	0.1 to 0.6
SO2	0.2 to 3.0
H2S	0.2 to 0.4

In the following sections a discussion is given on cleaning processes involved with various contaminants.
 
Cleansing 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.

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.
 
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.

 

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. 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.   A summary of major known sinks for different air contaminants is given in the following table.     Major Sinks For Common Air Contaminants Contaminants Major known sinks SO2 Scavenging, chemical reactions, soil and surface water absorption, dry deposition H2S Oxidation to SO2 N2O Photodissocation in stratosphere, surface water and soil absorption NO Oxidation to NO2 NO2 Photochemical reactions, oxidation to nitrate, scavenging NH3 Reaction with SO2 , oxidation to nitrate, scavenging CO Soil absorption, chemical oxidation O3 Photochemical reactions, absorption by surfaces (soil and vegetation) and surface water Non reactive Hydrocarbons Biological action Reactive Hydrocarbons Photochemical oxidation CO2 Biogenic HCl , Cl2  Precipitation