Biogeochemical reactions in the troposphere Major constituents
Nitrogen
OCN 401
Atmosphere
lecture II
Nitrogen, very inert, triple bond v strong 945 kJ/mole, hard to
break cf O2 498 kJ/mole
Reactive, N as NH 3, NO 3-, NO 2- is in short supply and at times
limits plant growth in the ocean and on land
Can form reactive N from N2 by "fixation" through lightning
→ NO ~ 3 x 10 12 g/yr
Bacterial reduction under anoxic conditions → most important
fixation route ~200 x 10 12 g/yr
N2 + 8H + + 8e - +16 ATP → 2NH3 +H 2 +16ADP, uses lots of energy
Oxygen
Synthetic fertiliser production ~ 100 x 10 12 g/yr
Mean Residence Time ~ 4,000 years
Balanced by denitrification, happens when no O2 available for
organic matter diagenesis
Oxygen accumulated from photosynthesis → organic carbon
5CH2O + 4H + + 4NO 3- → 2N2 + 5CO 2 + 7H 2O
Nitrification alone would deplete the atmospheric N 2 in 20 million yrs
N-fixation and denitrification not driven by atmospheric N
concentrations so biosphere not driving atmospheric N content
Carbon Dioxide
Contemporary organic C in plants = 0.03% of atmospheric O2
Long term burial of sedimentary C from oceanic photosynthesis
balances rest of atmospheric O 2
Balance between burial of org C and its oxidation controls
atmospheric O2 levels at ~ 21%
Is a link between burial rate and atmospheric O2 levels - through
deep water oxygen concentrations and atmospheric oxygen levels
Mean Residence Time 5 years
Affected by rock weathering, photosynthesis and dissolution in the
ocean
Rate of uptake by plants and ocean function of atmospheric CO 2
levels, therefore buffers atmospheric concs
Weathering uptake is slow, buffers on 100,000 yr timescale
--But, rate of photosynthesis is also important, not so simple!
Oxidation of reduced crustal minerals (Fe, S) is another sink for O 2
but does not change with atmospheric O 2 levels, i.e. not controlling
Current increases in atmosphere CO 2
are non-steady-state
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Trace biogenic gases
Most supplied by biogenic (microbial activity) and anthropogenic
(combustion, fossil fuel use) activities, most concentrations are
increasing
Have higher concs than expected with 21% O 2 atmosphere
Reactive, short residence times days → few years
Removal, oxidation in troposphere, wet deposition of products
Oxidation reactions in the Troposphere
O2 unreactive, O=O double bond is strong 498 kJ/mole
Ozone and OH radicals are main oxidising agents
NO2 produces ozone --has natural sources but anthropogenic
activity has raised its level and ozone levels
NO2 + hv ↔ NO + O
Overall
Equilibrium
O + O 2 ↔ O3
NO2 + O 2 ↔ NO + O 3
O3 (ppm) = 0.021 [NO2] / [NO]
increased concentrations of NO drive rx backwards (reduce O 3)
NO and NO 2 known as NO x
OH catalyses oxidation of CH 4 and other non-methane
hydrocarbons → CH 2O (formaldehyde)
OH + CH 4 →CH 3 + H 2O
CH 3 + O 2 → CH 3O2
CH 3O2 + HO 2 + CH 3O2H + O 2
CH 3O2H → CH 3O + OH
CH 3O + O 2 → CH 2O + HO 2
Net rx
CH 4 + O 2 → CH 2O + H 2O
OH is then a catalyst in this reaction for the oxidation of CH 4 by O2
Ozone produces hydroxyl radicals via its degradation
O3 + hv ↔ O 2 + O ( 1D) (excited oxygen)
<310 nM
O ( 1D) + H 2O ↔ 2ΟΗ radicals
2ΟΗ + 2Ο 3 ↔ 2HO 2 + 2O 2
2HO2 ↔ H2O2 + O 2
Hydroxy radicals are the main oxidising agent in the clean
troposphere
Average OH concentration is ~ 9.7 x 105 molecules/cm3 but highly
variable
OH has lifetime of only a few seconds
Can measure by laser or using CH 3CCl 3 / CH 2CCl 3 ratio
The formaldehyde is further oxidised
CH 2O + OH + O 2 → CO + H2O + HO 2
and the CO is oxidised to CO 2
CO + OH → CO 2 + H
H + O 2 → HO2
HO2 + O 3 →OH + 2O 2
net reaction CO + O 3 → CO 2 + O 2
Get consumption of OH and O3
OH scrubs atmosphere of reduced C
OH also oxidises NO 2 → HNO 3 v. fast
and SO2→ HSO 3 slow and eventually H2SO4
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Polluted troposphere NO> 3.8 ppt, HO 2 from CO oxidation
reacts with NO 2 not O 3
SO 2 also reacts with H 2O 2 in rain to form H2SO 4
HO2 + NO → OH + NO 2
NO2 + hv → NO + O
O + O 2 → O3
CO + 2O 2 → CO 2 + O 3
H2S and (CH 3)2S from ocean are oxidised by OH and other means
OH scrubs atm of reduced N and S gases
Net rx
Anthropogenic release of materials like CO competes for OH
radicals in atmosphere, allows build up of other reduced gases
e.g. CH 4
Other measurements suggest OH concentrations may have been
constant over recent decades
i.e. getting production of O3, not destruction
simlarly for CH 4
CH 4 + 4O 2 → CH 2O + H 2O +2O 3
Are getting a net increase in O 3 and OH in high NO (polluted)
environments
But in presence of cloud H2O may not get build up of OH because:
O3 + H 2O2 ↔ 2O2 + H 2O
“Polluted air”
from biomass
burning
Need to understand changes in OH and O3 as affects predictions of
future CH4 levels
Models predict build up of OH and O3 because of increase in NO
emissions
OH concentrations appear stable -- may be increase in "dirty"
conditions that produce OH are matching increases in reduced
gases that remove OH
H2O2 increases in Greenland ice indicates oxidising capacity of
N hemisphere may be increasing from human activity
Recent rapid
increases in
oxidising capacity
of atmosphere
or local effect?
Biogeochemical reactions in the Stratosphere
Ozone produced by light 180-240 nm,
main absorber of sunlight
O2 + hv → O + O
O + O 2 → O 3
More produced in stratosphere
than troposphere, as more uv
Lost by mixing into troposphere
Absorbs uvB 200-320 nm, warms
stratosphere
O3 + hv → O2 + O
O+O 3 → 2O2
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Also lost by reaction with OH
O3 + OH → ΗO 2 + O 2
ΗO2 + O 3 → OΗ + 2O 2
And lost by reaction with NO produced by photolysis of tropospheric
N2O (nitrous oxide) that gets into stratosphere
This is the only known sink for N2O
N2O →N 2 + O ( 1D)
N2O + O ( 1D) → 2ΝΟ
ΝΟ + Ο 3 → ΝΟ 2 + Ο 2
O3 → Ο + Ο 2
NO2 +Ο →ΝΟ + Ο 2
Net reaction 2O3 → 3Ο2
NO2 eventually removed from stratosphere by OH
Steady state concentration of O3 = 7 x 1018 molecules/m3
max at 30 km, max production at equator, greatest density at poles
Problem of CFC's and ozone first postulated in 1974, US regulatory
action taken in 1977
Ozone also removed from stratosphere by reaction with Cl from
CFCs and other sources
Cl + O 3 → ClO + O 2
O3 + hv → O + O 2
ClO + O → Cl + O 2
net rx is 2O 3 + hv → 3O 2
O3 lost BUT Cl is conserved
Cl eventually removed as HCl
http://wps.prenhall.com/wps/media/objects/394/403681/Destsruction
In dry air ClO reacts with NO 2 to form ClONO 2 and both are removed
In presence of ice-clouds (Springtime Antarctica)
ClONO 2 + HCl → Cl 2 + HNO 3
Cl 2 + hv → 2Cl, active Cl that destroys O 3
CFC's inert in troposphere, not v soluble in water so can reach
stratosphere
Montreal protocol signed in 1987
Residence time of CFCs wrt oxidation by OH ~ 80 yrs
Cl produced from uv interactions with CFCs
CCl 2F2 → Cl + CClF 2
Loss of stratospheric O 3 mirrors increase in Cl
Sources of Cl to stratosphere
Cl from seasalt short residence time removed in troposphere, no
reason to expect production rate change
Industrial and volcanic HCl removed by rain in troposphere
Violent volcanic activity can add to strat. e.g. Pinatubo eruption,
lead to <1% increase in Cl
Natural Cl to strat from CH3Cl (MRT 1.8 yrs) from marine algae,
higher plants and forest fires
Even though small production rate CFCs are main source of Cl to
strat. because stable in trop.
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Antarctic ozone hole first seen in 1985
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Reserved for Sept 1 2004
Montreal protocol signed in
1987
Atmospheric CFC concs
starting to decline by 1994
Current (August 26, 2004)
estimates of Southern Hemisphere
ozone concentrations and the
extent of the area of the hole
http://toms.gsfc.nasa.gov/ eptoms/dataqual/ozone_v8.html
http://www.cmdl.noaa.gov/publications/annrpt24/fig.5.9.jpg
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http://www.nasa.gov/vision/earth/lookingatearth/25TOMSAGU.html
Other materials
Br compounds may be more potent at destroying O3
CH 3Br agricultural fumigant (MRT ~ 0.8-2yr) increasing 3%/yr
in atmosphere
CHBr 3 from algae and biomass burning also adding Br to atmosphere
CH 3I and inorganic F cpds from algae short MRT do not get to strat.
Increase in stratosphere F from CFC destruction
Pinatubo aerosols
increased O3 destruction
see from satellite
images (TOMS)
Ozone depletion
seen over Arctic during
winter 1999/2000
Junge layer of SO4 aerosols at 20-25 km MRT 5yr
Ozone hole developing
5 days after eruption
From oxidation of SO 2 from volcanic eruptions and oxidation of COS
from wetlands
Stratospheric SO 4 is increasing, aircraft?
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