Sources of trace components in the atmosphere

1
Sources of trace components in the atmosphere
2
The Natural Components of Air

• Bulk composition of the atmosphere has been
  relatively well understood from early on
• Focus on variable trace components
• Despite low concentrations, they have a profound
  influence on chemistry of the atmosphere
• The Earths atmosphere is unique among planets
• presence of oxygen ? living organisms

3
Air Biological Sources

• The effect of living organisms is large
• respiration releases CO2
• photosynthesis produces O2
• The 1018 kg oxygen present in the atmosphere is
  virtually all due to photosynthesis
• The amount of carbon taken up by photosynthesis
  and released back to the atmosphere by
  respiration each year is 1,000 times greater than
  the amount of carbon that moves through the
  geological cycle on an annual basis.

4
Air Biological Sources

• CH2O O2 ? CO2 H2O
• ? respiration
• ? photosynthesis
• reservoir of organic carbon in reduced form
• The reservoir of organic carbon is small compared
  to the amounts of oxygen available

5
Air Biological Sources

• In the long term, organic carbon will not control
  the amount of oxygen in the atmosphere.
• oxidation of oxidizable inorganic material in
  Earths minerals e.g. Fe
• By far the largest reservoir of Earth's oxygen is
  within the silicate and oxide minerals of the
  crust and mantle (99.5). Only a small portion
  has been released as free oxygen to the biosphere
  (0.01) and atmosphere (0.36).
• 4FeO 3O2 ? 2Fe2O3
• At present, the availability and reactivity of
  reduced organic carbon material controls the
  oxygen carbon dioxide balance in air.

6
Biological Sources

• Large numbers of photosynthesizing organisms can
  alter the O2 and CO2 levels.
• relatively small effect on O2 concentrations
• but at night (no light) respiration from these
  organisms can produce large amounts of CO2
  (relatively large effect since atmospheric CO2
  concentrations are small)
• The amount of CO2 in atmosphere is 1.4 x 1016 mol
• sources respiration, combustion, decay
• sinks photosynthesis, exchange with oceans
• t 2-4 years (moderately well mixed)

7
Carbon Dioxide in the atmosphere has been
steadily rising since regular measurements began
in 1958. The graph above shows both the long-term
trend and the seasonal variation. (Graph by
Robert Simmon, based on data from the NOAA
Climate Monitoring Diagnostics Laboratory)
http//earthobservatory.nasa.gov/Library/CarbonCyc
le/carbon_cycle3.html
8
Air Trace Compounds

• Plants, animals and associated micro-organisms
  can produce a variety of exotic atmospheric trace
  components

9
Air Compounds of Carbon

• Methane (CH4) is produced in marshes, paddy
  fields and in animal guts through microbial
  degradation of organic matter (cow emissions)
• CO2 4H2 ? CH4 2H2O
• from the degradation of alcohols or other
  compounds

10
Air Compounds of Carbon

• Larger hydrocarbons can also be produced through
  biological activities but are present in much
  smaller quantities
• soils, microbes
• forests (terpenes, essential oils)
• animals (pheromones)

11
(No Transcript)
12
Air Compounds of Nitrogen

• Largely the result of microbial activity
• Ammonia production is correlated with animal
  urine.
• NH2CONH2 H2O ? 2NH3 CO2
• urea
• NH3 if soil is alkaline
• NH4 if soil is acidic
• Nitrogen used in the biosphere is used to make
  amino acids. However, amino acids can also be
  broken down.
• CH2NH2COOH 3/2 O2 ? 2 CO2 H2O NH3
• (glycine)

13
Air Compounds of Nitrogen

• Some organisms use NH3 as an energy source
  (similar to oxidation of reduced carbon
  compounds)
• nitrification NH3, NH4 ? NO3-
• NH3 3/2 O2 ? H NO2- H2O
• nitrite
• nitrobacteria oxidize nitrite to nitrate
• NO2- 1/2 O2 ? NO3-

14
Air Compounds of Nitrogen

• Some organisms perform the reverse process ?
  denitrification
• NO3- ? N2 , N2O
• nitrate
• C6H12O6 6 NO3- ? 6 CO2 3 H2O 6 OH- 3 N2O
• 5 C6H12O6 24 NO3- ?30 CO2 18 H2O 24 OH-
  21 N2

15
Air Compounds of Sulphur

• Biological sources are most important
• Dimethylsulphide (DMS), (CH3)2S is produced in
  marine and soil emissions
• dominant source is marine phytoplankton in upper
  layers of the ocean
• Also in the ocean,
• methylmercaptan, dimethyldisulphide, carbonyl
  sulphide, and carbon disulphide
• carbonyl sulphide can be made by oxidation of
  carbon disulphide in the atmosphere or hydrolysis
  in water
• CS2 H2O ? COS H2S
• Oceans and sulphur-rich soils are the most
  effective regions for production of sulphur
  compounds

16
Trace Geochemical Sources

• The most important geochemical source of trace
  gases at the Earths surface are volcanoes.
• massive source, but highly localized and very
  sporadic
• e.g. Mount Etna can produce SO2 at a greater rate
  than all the industries of Europe combined, but
  at other times emits only small amounts.
• When eruptions are violent, volcanic emissions
  can be injected directly into the stratosphere.
  Most emissions are to the troposphere via a more
  gentle flux of gases.
• The major components are
• sulphur gases SO2, H2S, COS, CS2
• Halogens (hydrohalic acids) also in large
  quantities
• Organohalides in smaller quantities
• not expected from components in volcano vents
  (reactions of volcanic gases with vegetation)

17
(No Transcript)
18
Trace Gases Oceans

• Oceans are also a significant source of gases
  (although much is biological)
• Gases can be present at very high concentrations
• Diffusion across the air / sea interface into the
  atmosphere
• CO, (CH3)2S, H2S, CH4, N2O, and metals also

19
CO2 Oceans

• carbon dioxide exchange is largely controlled by
• sea surface temperatures
• circulating currents
• biological processes of photosynthesis and
  respiration
• carbon dioxide can dissolve easily into the ocean
  
• the amount of carbon dioxide that the ocean can
  hold depends on
• ocean temperature
• amount of carbon dioxide already present.

20
CO2 Oceans

• Cold ocean temperatures favor the uptake of
  carbon dioxide from the atmosphere
• Warm temperatures can cause the ocean surface to
  release carbon dioxide.
• Cold, downward moving currents such as those that
  occur over the North Atlantic absorb carbon
  dioxide and transfer it to the deep ocean.
• Upward moving currents such as those in the
  tropics bring carbon dioxide up from depth and
  release it to the atmosphere.

21
(No Transcript)
22
Trace Gases Forest/Savannah Fires

• hard to distinguish between natural fires and
  those either directly or indirectly the result of
  human activity
• can produce large quantities of unburnt and
  pyrolized organic compounds
• CH3Cl, CO, Nitrogen oxides
• smaller amounts of sulphur compounds (SO2, COS),
  metals
• difficult to estimate global emissions because
  the number and sizes of fires, emission factors
  for individual compounds can vary widely -
  laboratory measurements required.

23
CO2 Fire

• plays an important role in the transfer of carbon
  dioxide from the land to the atmosphere.
• consume biomass and organic matter to produce
  carbon dioxide (along with methane, carbon
  monoxide, smoke),
• vegetation that is killed but not consumed by the
  fire decomposes over time adding further carbon
  dioxide to the atmosphere.

24
(No Transcript)
25
Trace Gases Atmospheric Sources

• Chemical reactions in the atmosphere act as
  sources and sinks of new trace gases
• e.g. Lightning strikes
• N2 O2 ? 2 NO (shock wave, 4000 K)
• it is estimated that each lightning strike
  produces 4 x 1026 molecules of NO ? 8 Tg / yr
  (N)
• At high concentrations this is oxidized to
  nitrogen dioxide
• 2 NO O2 ? 2 NO2
• and can dissolve in water
• 2 NO2 H2O ? HNO2 HNO3
• nitrous acid nitric acid

26
Trace Gases Atmospheric Sources

• Most reactions occur at ambient temperatures or
  (if they have a high activation energy)
  photochemically.
• Major reaction pathways in the atmosphere involve
  the oxidation of reduced gases (the atmosphere is
  highly oxidizing)
• reduced N compounds (NH3, N2O) ultimately produce
  nitric acid or nitrate ion
• NH3, N2O ? NO ? NO2 ? HNO3, NO3-
• reduced sulphur compounds ((CH3)2S, COS, CS2,
  H2S) will be oxidized to sulphuric acid, sulphate
  ion, or related compound
• (CH3)2S, COS, CS2, H2S ? SO2 H2SO4, SO42- (a
  range of oxidized products)
• e.g. (CH3)2S ? (CH3)2SO, (CH3)2SO2, (CH3)2SO3H

27
Trace Gases Atmospheric Sources

• oxidized compounds are either fairly involatile
  (sulphate, nitrate ions) or have a high affinity
  for water (sulphuric acid, nitric acid).
• material is removed from the atmosphere through
  rainfall or sedimentation and deposition onto the
  Earths surface.
• Oxidation increases the rate of removal of sulfur
  and nitrogen compounds from the atmosphere
• Note that not all gases need to be oxidized to
  become soluble in water (rainfall removal)
• e.g. NH3, SO2 (moderate)

28
Carbon storage

• Over periods of years to decades, significant
  amounts of carbon can be stored or released on
  land.
• when forests are cleared for agriculture the
  carbon contained in the living material and soil
  is released, causing atmospheric carbon dioxide
  concentrations to increase.
• When agricultural land is abandoned and forests
  are allowed to re-grow, carbon is stored in the
  accumulating living biomass and soils causing
  atmospheric carbon dioxide concentrations to
  decrease.

29
The Carbon Cycle

• The main reservoirs for carbon are
• sedimentary rocks,
• fossilized organic carbon including the fossil
  fuels,
• the oceans
• the biosphere.
• Carbon goes primarily through three cycles with
  different time constraints
• A long-term cycle involving sediments and the
  depths of the lithosphere.
• A cycle between the atmosphere and the land.
• A cycle between the atmosphere and the oceans.
• The last two cycles are faster and subject to
  human intervention.

30
Carbon cycle 1
Long-term Cycle This cycling between atmosphere,
oceans, and sediments involve a slow dissolution
of atmospheric carbon and carbon from rocks via
weathering into the oceans. In turn, the oceans
deposit sediments, and then some of the sediments
are thrown back into the atmosphere through
volcanic action.
This cycle occurs over hundreds of millions of
years. A larger portion of sediments is calcium
carbonate (CaCO3) because the ocean contains
large amounts of calcium.
http//telstar.ote.cmu.edu/environ/m3/s4/cycleOxyg
en.shtml
31
Carbon Cycle 2
Air and Land Cycle The second cycle between the
atmosphere and biosphere occurs over different
time scales ranging from days to decades. Carbon
dioxide is the basic "food" of the biosphere and
thus the biosphere is the agent for this cycling.
Photosynthesis (synthesizing starches and sugars
using light) is a main mechanism for cycling
carbon by the biosphere. The chemical reaction of
photosynthesis may be represented as
                                                  
            CH2O represents a unit of organic
matter six of the CH2O unit would be C6H1206
which makes the simple sugar (glucose or
fructose) and 11 of these units make C11H22O11, a
more complex sugar, sucrose, formed by the
combination of one glucose and one fructose.
Thousands of glucose molecules combine to form a
molecule of starch, or of cellulose. Thus
photosynthesis takes the atmospheric carbon in
CO2 and "fixes" it into the biosphere. The
subsequent cycling of the carbon in the biomass
is created.
32
Thus 750 Gt-C in the atmosphere cycling at the
rate of 80 Gt C/yr means that the lifetime of the
carbon in the atmosphere reservoir is about 9
years. When the organic matter is oxidized
through respiration, the reverse of
photosynthesis takes place.                     
                                           
http//telstar.ote.cmu.edu/environ/m3/s4/cycleOxyg
en.shtml
33
Respiration releases CO2 into the atmosphere.
Respiration and photosynthesis occur at nearly
equal rates over one year. Buried
biomass--eventually becoming fossils, including
coal--have historically had an effect of keeping
the carbon in the land. The accelerated burning
of fossil fuels is, however, releasing these
large stores into the atmosphere as combustion
products. Burning of biomass-based fuels such
as methanol and ethanol has been suggested an
alternate to fossil fuel combustion. Biomass
fuels have no net release of carbon dioxide.
34
Carbon Cycle 3 Air and Sea Cycle The oceans
contain much more carbon than the atmosphere.
Carbonates washed down from the rocks, over
thousands of years, dissolved CO2, and carbon in
the oceanic biomass constitutes this reservoir.
The carbon from the top layers of the ocean
cycles faster whereas the carbon in deep waters
may take thousands of years. The summary of the
three cycles is shown in the following figure.
http//telstar.ote.cmu.edu/environ/m3/s4/cycleOxyg
en.shtml
35
The Oxygen Cycle
http//telstar.ote.cmu.edu/environ/m3/s4/cycleOxyg
en.shtml
36
Oxygen reservoir capacities and fluxes Table 1
Major reservoirs involved in the oxygen cycle
(Conor Goodwilie) The following tables offer
estimates of oxygen cycle reservoir capacities
and fluxes. These numbers are based primarily on
estimates from (Walker, J.C.G.)
37
Table 2 Annual gain and loss of atmospheric
oxygen (Units of 1010 kg O2 per year)
38
Combustion and lightning fix nitrogen in the
atmosphere. When plant matter (biomass) is
burned, the organic fixed nitrogen is converted
into nitrogen oxides and released. The clearing
of forests by fire and burning of leftover debris
from farmland creates large emissions of nitrogen
oxide. The oceans and sediments also contain
large amounts of nitrogen as nitrates. Ammonia
(NH3) is another form of fixed nitrogen. Ammonia
is produced by bacteria after they consume
organic matter. This accounts for the ammonia
smell from the cat's litter-box resulting from
the bacterial emissions.
Before chlorofluorocarbons were invented, ammonia
was the most common refrigerant. While the figure
shows the main global routes of cycling nitrogen,
in some locations (for example the Los Angeles
basin, Mexico City, and in other industrial
cities), nitrogen oxides (NOx) and nitric acid
(HNO3) form a significant fraction of the local
tropospheric environment.
http//telstar.ote.cmu.edu/environ/m3/s4/cycleOxyg
en.shtml
39
The Sulfur Cycle
http//telstar.ote.cmu.edu/environ/m3/s4/cycleOxyg
en.shtml
40
http//www.biologie.uni-hamburg.de/b-online/e54/13
.htm
41
More cow emissions