Sinks

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Sinks and Photochemistry
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Sinks

• To maintain steady state concentrations in the
  atmosphere, the rates of removal of atmospheric
  species must balance the rates of their
  production
• Chemical reactions act not only as sources but
  also as sinks.
• But one could argue that chemical reactions are
  not true sinks, just transformations!
• At some point the element must be removed.
• H2, He light enough to achieve escape velocity
  (11.2 km/sec) at high temperatures reached in
  outer regions of the thermosphere (exosphere)
• estimates based molecular velocities and
  temperatures say
• t(H2) 10 3 .5 yr
• t(He) 10 8 1 yr
• but t(H2) observed to be 10 years
• removed through chemical reactions and in soil by
  micro-organisms.

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Sinks

• Some unreactive gases are too heavy to escape and
  have no identified sinks
• e.g. Argon is produced by radioactive decay of
  40K (electron capture)
• 40K e- ? 40Ar g (t1/2 1.25 x 109 yr)
• Argon has been accumulating since the Earth was
  formed.

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Sinks

• Ultimate removal of trace components occurs via
  two pathways
• (i) Dry Deposition
• direct transfer of gases and particles to the
  ground
• (can also be onto wet surfaces oceans)
• (ii) Wet Removal
• trace constituents dissolved in rainfall.
• Soluble gases, e.g. SO2 , tend to dry deposit
  relatively quickly.
• water is of great importance for deposition.
• SO2 deposited more effectively on dew covered
  crops.

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Sinks

• Flux of gas to the ground is expressed as
• F Vg c
• Flux deposition velocity
• concentration of gas
• Even for the deposition of very soluble gases
  onto wet surfaces,
• Vg lt 1 cm/sec
• (column of gas deposited onto the ground)
• Gas trapped in the first km of the amosphere is
  absorbed by Earths surface in 105 sec ( 28
  hr).
• If Vg 1 cm/sec, deposition can represent
  significant removal rates.
• Surface morphology is an important aspect of
  deposition.
• Surface area

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Sinks

• Oceans are a particularly important sink.
• On solid surfaces, soils (soil microbes) can use
  H2 and CO and act as a sink
• weathering of inorganic minerals is also
  important over long timescales (millions of
  years)
• oxygen removal via oxidation of sulfide and
  ferrous iron
• the reactions of the acid gases such as CO2 and
  SO2 can give carbonate and sulphate minerals

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Examples of sinks

• Ocean
• Confirmation of long-range Saharan dust
  transport.
• Soil dust sampled over the tropical Atlantic two
  decades ago appeared to come from the Saharan
  Desert subsequent work provided additional
  evidence that windborne dust from arid African
  and Asian regions is indeed the principal source
  of mineral aerosols found in global troposphere.
  In a striking and definitive confirmation of this
  hypothesis, ABLE-1 fights above Barbados observed
  a massive infusion of Saharan dust at these
  longitudes. Dense layers of the dust were mapped
  for the first time by an airborne lidar system
  and, at peak intensity, by aerial photography as
  well. Such dust is eventually deposited onto the
  sea surface, thousands of kilometers from the
  source. Successive episodes can add mineral
  nutrients to the ocean in amounts comparable to
  those injected by the Amazon River.

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Examples of sinks

• Forests
• Examination of air chemistry over a tropical
  forest.
• The world's tropical forest are important sources
  and sinks for many gas and aerosol species.
  ABLE-1 flights over Guyana provided the first
  comprehensive study of the wet tropical forest
  boundary layer from an airborne platform. They
  showed that this layer is a source of CO,
  isoprene (C5H8), and dimethyl sulfide (DMS), as
  well as a sink for O3. The Guyana forest was also
  revealed to be a major source of chemically
  important aerosols.

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Examples of sinks

• Marine life
• Assessment of marine DMS contributions to
  atmospheric sulfur.
• Marine phytoplankton are a major source of DMS.
  Measurements of tropospheric DMS concentrations
  confirmed earlier conclusions that marine DMS
  production accounts for fully half of natural
  sulfur dioxide (SO2) emissions, are important in
  clean-air areas of the troposphere.

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Examples of sinks

• Arctic
• Arctic removal of oxides of nitrogen.
• Arctic tundra was found to remove important trace
  nitrogen compounds from the atmosphere with high
  efficiency through biological processes. This
  reduces nitrogen oxide concentrations in the
  lower and middle regions of the Arctic
  troposphere to exceedingly low levels and so
  prevents significant ozone formation.

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Examples of sinks

• Amazon Rain Forest
• Natural Sink for tropospheric ozone.
• The undisturbed rain forest is the most efficient
  sink for O3 yet discovered. Amazonian O3
  decomposition rates were found to be 5 to 50
  times higher than those previously measured over
  pine forest and water surfaces. The disappearance
  of such a strong ozone sink through deforestation
  would have global implications for atmospheric
  chemistry.

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Aside Rain forest as a source

• Seasonal degradation of Amazonian air quality.
• Air above the Amazon jungle is extremely clean
  during the wet season but deteriorates
  dramatically during the dry season as the result
  of biomass burning. This degradation is caused
  mostly by burning at the edges of the forest.
  Under the worst conditions, trace-gas
  concentrations at aircraft altitudes approach
  those typically observed over industrialized
  regions. This is a spectacular example of
  long-range transport of pollution into a pristine
  environment.

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Aside Rain forest as a source

• Combustive production of greenhouse gases.
• Biomass burning is also a copious source of such
  greenhouse gases as CO2, and CH4, as well as
  other pollutants (e.g. CO and oxides of
  nitrogen). Satellite observations during the dry
  season have detected some 6,000 fires at the peak
  of the burning, with associated haze covering
  millions of square kilometers

http//www-gte.larc.nasa.gov/able/able_fig3.htm
Effect of Biomass Burning is shown by variation
of tropospheric carbon monoxide with height above
Amazon rainforest and with longitude along Amazon
River, as measured by ABLE-2A flights at the end
of wet season (July) and beginning of dry season
(August). Note enormous CO increase in August as
a result of biomass burning.
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Aside Rain forest as a source

• Methane emissions from Amazonian wetlands.
• The Amazon River floodplain is a globally
  significant source of methane, supplying about
  12 of the estimated worldwide total from all
  wetlands sources.
• Enhancement of tropospheric carbon monoxide.
• Over Amazonia, CO is enhanced by factors ranging
  from 1.2 to 2.7 by comparison with adjacent ocean
  regions. The major Amazonian sources of CO are
  isoprene oxidation and biomass burning the
  latter probably accounts for the most of the CO
  enhancement observed by Space Shuttle instruments
  prior to ABLE-2.
• Importance of atmospheric circulation over the
  rainforest.
• ABLE-2 studies of spectacular atmospheric
  circulations over Amazonia have shed new light on
  links between the Amazon regions and global
  circulation. Individual convective storms
  transport 200 megatons of air per hour, of which
  3 megatons is water vapor that releases 100.000
  megawatts of energy into the atmosphere through
  condensation into rain. Replacement of forest
  with wetlands or pasture is likely to have a
  large impact on this enormous furnace, with
  attendant effects on atmospheric circulation
  patterns, and hence climate.

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Global Cycles and Residence Times

• Carbon Cycle
• CO2 exchange via photosynthesis and respiration
  involves enormous quantities
• otherwise, the pathways of (trace) reduced carbon
  dominate the cycle and are determined by gas
  phase reactivity
• Formaldehyde (HCHO) is produced from methane and
  is most reactive (short residence time of 1
  day)
• small amounts of long-lived gases such as CH4 and
  CO are transferred to the stratosphere and then
  oxidized in photochemically initiated reactions
• source of H2O in dry levels of atmosphere.
• CH4 leads to the generation of HCHO and CO
• Oxidation of large hydrocarbons leads to CO as
  well.
• Natural sources of CO are larger than that due to
  human activity.

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Global Cycles and Residence Times

• Sulfur Cycle
• Dominant biological sources of reduced sulphur
  ((CH3)2S, H2S) are from soils (microbes) and
  oceans (phytoplankton)
• Volcanic emissions SO2 and H2S
• most reduced sulfur is oxidized to sulfate (but
  some dry deposition)
• Oceans are a giant source of oxidized sulfur
  (sulfate within sea salt)
• Recycled back to oceans.
• Transferred to land.
• Neutral in terms of acidity
• SO42- produced via oxidation of SO2 generates H
  ions (acid rain)

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Global Cycles and Residence Times

• Nitrogen Cycle
• Large amounts of N2, N2O produced by
  denitrification
• Some N2O crosses the tropopause and is oxidized
  in the stratosphere (N2O is unreactive)
• N2 is removed (fixed) into terrestrial and marine
  biology and in lightning strikes (as NOx)
• N2 is fixed by the use of fertilizers!
• Industrial production of fertilizers is via the
  Haber process
• N2 3H2 ? 2NH3
• NH3 can be injected into soils or added as a
  nitrate salt
• NH3 2O2 ? HNO3 H2O
• air oxidation of NH3
• HNO3 NH3 ? NH4NO3

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Photochemistry!!!

• Photochemical and Radiation-Chemical Reactions
• Two kinds of radiation
• Electromagnetic Radiation Particle Radiation
• e.g. e.g.
• Infrared a particles (He nuclei)
• Visible ß particles (electrons)
• UV protons, neutrons etc.
• X-rays (particle accelerators)
• EM Radiation acts like a beam of particles
  (photons)

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Photochemistry!!!

• Photo- vs. Radiation-chemical distinction
  sometimes made on the basis of
• (i) whether or not ions are produced
• IR, vis, soft UV dont produce ions ?
  photochemical processes out to mesosphere
• Hard UV, X-Rays, particles produce ions ?
  radiation-chemical processes in ionosphere
• (ii) specificity
• low energy sources give reactions with simple
  stoichiometries ? photochemical
• high energy sources can be messy many
  fragments, products, recombination processes
  ?radiation-chemical

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Photochemical Reactions

• For radiation to bring about chemical change it
  must first be absorbed 1819 von Grotthuss
• Einsteins Law of Photochemical Equivalence
  (1912) one photon of radiation is absorbed by
  one molecule
• In all cases it is the light absorbed and not the
  incident intensity that directly controls the
  rate of a photochemical reaction.

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Photochemical Primary Process

• Radiation is absorbed by a molecule which leads
  to the formation of species that undergo further
  reaction - primary process
• e.g. excited state ? atoms and free radicals
  (primary process)
• Photolysis photo light lysis splitting
• Atoms and radicals may go on to react
  subsequently (secondary processes)
• e.g. Acetone (280 nm light)
• CH3COCH3 h? ? CH3CO CH3 ? 2CH3 CO
• primary process

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Photochemical Primary Process

• Light must be absorbed for chemical change, but
  not all light absorbed leads to chemical change
  ....
• To understand the primary process one must
  understand excited molecules
• ? Concept of Potential Energy Curves

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For a diatomic molecule, the electronic states
can be represented by plots of potential energy
as a function of internuclear distance.
Electronic transitions are vertical or almost
vertical lines on such a plot since the
electronic transition occurs so rapidly that the
internuclear distance can't change much in the
process. Vibrational transitions occur between
different vibrational levels of the same
electronic state. Rotational transitions occur
mostly between rotational levels of the same
vibrational state, although there are many
examples of combination vibration-rotation
transitions for light molecules.
hyperphysics.phy-astr.gsu.edu/hbase/molecule/molec
.html
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Franck-Condon Principle

• Probabilities of transitions when light is
  absorbed are governed by the Franck-Condon
  Principle
• Relative speeds of electronic transitions are
  much faster then vibrational motions.
• Absorption of radiation t 10-15 10-18 sec
• Vibrational motion t 10-13 sec
• Intermolecular separations of molecules are the
  same before and after an electronic absorption
  transition. This leads to
• (i) vertical transitions
• (ii) transitions allowed only if h? equals the
  energy difference between two levels
• (iii) probability of a transiton is small if the
  two states correspond to a very different
  interatomic distance
• (iv) vibrational wavefunctions

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Frank-Condon excitation is often described as the
absence of the change of the nuclear coordinates
upon excitation (because the nuclei are so much
more massive than the electrons, an electronic
transition takes place very much faster than the
nuclei can respond), but in the quantum chemical
description of the FranckCondon principle, the
molecule undergoes a transition to the upper
vibrational state that most closely resembles the
vibrational wavefunction of the vibrational
ground state of the lower electronic state.
In this figure, the system is excited with white
light and the corresponding spectrum is shown
vertically. The wavefunction drawn in black has
highest overlap with that of the ground state
level. In this case the Frank-Condon factors are
related to the Einstein coefficients (and the
extinction coefficients). The Franck-Condon
factor is simply the overlap matrix element
between vibrational nuclear states. Incase only
one wavelength is used, the wavefunction of the
lowest vibrational level of the ground state
shown here would be projected onto one
vibrational wavefunction of the higher electronic
state.
http//home.uva.nl/r.m.williams/Introduction20to
20ET-30.htm
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In the electron transfer process that occurs
after Frank-Condon excitation of a donor-acceptor
system, the direct overlap of the vibrational
wavefunctions of the initial and the final state
plays a role. The two curves represent the local
excited state and the charge transfer state.
Electron transfer that is described by nuclear
tunneling is related to this overlap.
http//home.uva.nl/r.m.williams/Introduction20to
20ET-30.htm
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This figure gives a representation of several
aspects that occur during electron transfer. On
the left, the electron probability density (also
referred to as electronic position) at the donor
site and at the acceptor site and the evolution
of the electron density during the process is
displayed. On the right, the two parabola
represent the initial reactant state and the
final product state.
As the process proceeds the position on the
potential energy surface changes, and thereby the
energy gap between the two states becomes
smaller, until the barrier is reached, to
increase again in the final state of the process
(adapted from Marcus and Sutin) 
http//home.uva.nl/r.m.williams/Introduction20to
20ET-30.htm
Marcus, R.A. Sutin, N. Biochim. Biophys. Acta
1985, 811, 265.
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http//jegog.phys.nagoya-u.ac.jp/ayamada/rhod_pho
toiso_pot.html
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Potential Energy Surfaces

• Diatomic P.E. curves are straightforward to
  understand. For polyatomics things are more
  complicated
• ? Multidimensional P.E. Surface
• Pseudo-diatomic curves are sections or slices
  through the PES
• If a diatomic has sufficient energy to
  dissociate, it does so within the period of the
  first vibration ( 10-13 sec)
• For polyatomics, dissociation can take longer
  (10-8 sec) because energy is distributed among
  many modes of vibration.

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http//www.chem.hope.edu/polik/poster/HFCO97.htm
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Potential energy surface describes energy of a
molecule in terms of its structure. Molecules
move on the potential energy surface.

• Structure
• determined from the potential energy surface
• minimum corresponds to an equilibrium structure
• first order saddle point corresponds to a
  transition state for a reaction
• a reaction path is the steepest descent path
  connecting a transition state to minima

http//www.chem.wayne.edu/hbs/chm6440/PES.html
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http//www.che.hw.ac.uk/people/mjp.html
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Photochemical reactions to be continued