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?

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 3 - 13 hrs (CH3)S 31 hrs H2S 53 hrs

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 of Toxic Air Pollutants

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