1. Accumulation of Pollutants ¯ Question 1: Do pollutants accumulate ? ¯ Question 2: Are there some natural cleaning processes which remove them ? ¯ Residence time of air contaminants gives us an idea on accumulation of pollutants ¯ Residence time can be used to determine the approximate duration in which an air contaminant can be removed from the atmosphere ¯ Residence times of CO and CO 2 are 65 years and 15 years respectively indicates that CO is quite reactive and CO 2 more stable

2. Cleansing of the Atmosphere ¯ Processes involved: ¬ Deposition of the pollutants, ¬ Conversion of pollutants to other forms (due to oxidation, absorption etc.) Notes: * Rain, snow and dew washes smoke, dust, pollen and gaseous waste products present in the atmosphere * Particles of diameter less than 2 microns settle down and are removed by impaction with surface obstacles like trees, rocks etc.

3. Cleaning Processes for SO 2 Cleansing of SO 2 : Ô Total world emission of SO 2 is about 100 million tons/year Ô SO 2 is removed from the atmosphere in about 40 days Ô Two ways to remove it are: 1. SO 2 + O 2 SO 3 H 2 SO 4 Ammonia Sulfate, Calcium Sulfate 2. SO 2 + Sulfites oxidation Sulfate (Precipitate) Ô Major known sinks: Scavenging, chemical reactions, soil and surface water absorption, dry deposition H20H20 + NH 3, Lime NH 3, directly calcite of lime

4. Cleaning Processes for H 2 S and NH 3 Cleansing of H 2 S: Ô Total world emission is about 300 million tons/year Ô No evidence of accumulation H 2 S Chemically React - PbO, FeO (metal oxides) Insoluble metal sulfides Ô Odor can be removed by the addition of a few tenths of nitrogen oxides which act as catalyst to form H 2 SO 4 ( in laboratory) Cleansing of Ammonia: Ô Ammonia is removed from the atmosphere by reaction with acids or acid forming oxides

5. Cleaning Processes Cleansing of Fluorides: ³ Hydrogen fluoride and Silica Fluoride readily attack materials like carbonates, silicates and are readily removed from the atmosphere Cleansing of CO: ³ Reaction with the hydroxyl radical. ³ Oxidized in the upper atmosphere by atomic oxygen to yield CO 2 Cleansing of CO 2 : ³ Consumption in photosynthesis ³ Removed slowly silicate rocks limestone and dolomite Cleansing of NO x : ³ Nitric Acid reacts with ammonia or lime Ammonia nitrate/Calcium nitrate

6. Cleaning Processes Cleansing of Smoke: ± Can be removed at source ± Settles as a result of gravitational effects, agglomeration, or rain Cleansing of Hydrocarbons: ± Reactive hydrocarbons are removed by a series of photochemical reactions