Transcript Accumulation of Pollutants

Accumulation of Pollutants
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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
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Residence time can be used to determine the approximate duration in
which an air contaminant can be removed from the atmosphere
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Residence times of CO and CO2 are 65 years and 15 years
respectively indicates that CO is quite reactive and CO2 more stable
Cleansing of the Atmosphere
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Processes involved:
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Deposition of the pollutants,
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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.
Cleaning Processes for SO2
Cleansing of SO2:
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Total world emission of SO2 is about 100 million tons/year
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SO2 is removed from the atmosphere in about 40 days
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Two ways to remove it are:
1. SO2 + O2
Sulfate
SO3
H20
H2 SO4
NH , directly calcite of lime
3
2. SO2 +
(Precipitate)
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+ NH , Lime
3
Sulfites
Ammonia Sulfate, Calcium
oxidation
Sulfate
Major known sinks: Scavenging, chemical reactions, soil and surface
water absorption, dry deposition
Cleaning Processes for H2S and NH3
Cleansing of H2S:
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Total world emission is about 300 million tons/year
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No evidence of accumulation
H2S Chemically React - PbO, FeO (metal oxides) Insoluble metal sulfides
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Odor can be removed by the addition of a few tenths of nitrogen oxides
which act as catalyst to form H2SO4 ( in laboratory)
Cleansing of Ammonia:
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Ammonia is removed from the atmosphere by reaction with acids or
acid forming oxides
Cleaning Processes
Cleansing of Fluorides:
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Hydrogen fluoride and Silica Fluoride readily attack materials like
carbonates, silicates and are readily removed from the atmosphere
Cleansing of CO:
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Reaction with the hydroxyl radical.
Oxidized in the upper atmosphere by atomic oxygen to yield CO2
Cleansing of CO2 :
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Consumption in photosynthesis
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Removed slowly
silicate rocks
Cleansing of NOx :
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Nitric Acid reacts with ammonia or lime
limestone and dolomite
Ammonia nitrate/Calcium nitrate
Cleaning Processes
Cleansing of Smoke:
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Can be removed at source
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Settles as a result of gravitational effects, agglomeration, or rain
Cleansing of Hydrocarbons:
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Reactive hydrocarbons are removed by a series of photochemical
reactions