N ATURAL C LEANING P ROCESS OF THE A TMOSPHERE

O BJECTIVES 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?

N ATURAL C LEANSING P ROCESS 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.

R ESIDENCE T IME FOR A IR C ONTAMINANTS The time for which an air contaminant stays in atmosphere is known as the residence time. TypeContaminantsApproximate Residence Time Major species N2N yr O2O2 10 yr Tracer species CO 2 15 yr CH 4 10 yr H2H2 N2ON2O150 yr CO65 days NH 3 20 days NO/NO 2 1 day O 3 (troposphere)< 1 yr HNO 3 1 day CFC 1165 yr Sulfur Compounds SO 2 40 days COS> 0.3 * 10 5 hrs CS 2 > 1.8 * 10 5 hrs CH 3 CH hrs (CH 3 )S31 hrs H2SH2S53 hrs

D EPOSITION OF T OXIC A IR P OLLUTANTS

C LEANSING OF THE A TMOSPHERE * 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.

D EPOSITION F LUX

D EPOSITION V ELOCITY 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

C LEANSING P ROCESSES OF SO 2 The total world emissions are about 100 million tons/year. SO 2 is removed from the atmosphere in about 40 days. The two theories for removal are mentioned below: 1. SO 2 + O 2 > SO 3 + H2O > H 2 SO 4 + NH3, Lime > Ammonia Sulfate, Calcium Sulfate. 2. SO 2 + NH3, directly calcite of lime > Sulfites oxidation > Sulfate (Precipitate) Ammonia sulfate is a good fertilizer.

C LEANSING P ROCESSES OF H 2 S AND NH3 Cleansing of H 2 S The total world emissions is about 300 million tons/year. Major sources are volcanoes and biological decay. There is no evidence of accumulation. H 2 S 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 H 2 SO 4. Cleansing of Ammonia Ammonia is removed from the atmosphere by reaction with acids or acid forming oxides.

C LEANSING P ROCESSES OF F LUORIDES AND C ARBON M ONOXIDE 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 CO 2. No evidence of accumulation.

C LEANSING P ROCESSES OF CO 2 AND NO X Cleansing of CO 2 Consumption in photosynthesis. Removed slowly > silicate rocks > limestone and dolomite. The concentration of CO 2 began increasing in the 18 th century. Data reported in literature indicate that the increase has accelerated. Cleansing of NO x Nitric Acid formed reacts with ammonia or lime > Ammonia nitrate or Calcium nitrate.

C LEANSING P ROCESSES OF S MOKE AND H YDROCARBONS 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.

S INKS FOR A IR C ONTAMINANTS

P LANTS 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

P OLLUTED S OIL Phytoremediation Using plants to clean up polluted soil Fast growing, easy to maintain, and capable of transforming the pollutants into a non-toxic form.