NATURAL CLEANING PROCESS OF THE ATMOSPHERE

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?

NATURAL CLEANSING PROCESS

Nature regularly transforms oxygen into ozone through forces such as lightning and ultra-violet light striking the earth.

Ozone breaks down the pollutants and reverts back to oxygen at ground level.

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 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

DEPOSITION OF TOXIC AIR POLLUTANTS

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.

DEPOSITION FLUX

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

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.

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.

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.

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.

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.

SINKS FOR AIR CONTAMINANTS

PLANTS Research has shown that these plants are the most effective all-

around in counteracting off-gassed chemicals and contributing to balanced internal humidity.

Areca Palm

Australian Sword Fern

Boston Fern

Dwarf Date Palm

English Ivy

Ficus Alii

Janet Craig Dracaena

Lady Palm (Rhapis excelsa)

Peace Lily (Spathiphyllum)*

Reed Palm

Rubber Plant (Ficus elastica)

Weeping Fig

Zamioculcas Zamifolia

POLLUTED SOIL

PhytoremediationUsing plants to clean up polluted soilFast growing, easy to maintain, and

capable of transforming the pollutants into a non-toxic form.