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Chapter
Water in the atmosphere
Water’s role in the atmosphere is manifold: in the lower atmosphere its action as a
greenhouse gas helps keep the atmosphere warm by absorbing and re-emitting energy (reacting
at wavelengths <= 8 micrometers). Water is a significant source of the hydroxyl radical through
reaction with energetic (excited) oxygen atoms as comes from the photolysis
(photodissociation) of diatomic oxygen.
Evaporated water carries latent heat and releases it at condensation, so motion of water in
the atmosphere also represents a transport of latent energy. The presence of water in air raises
the heat capacity of the air.
Water is the primary component of cloud of liquid water or ice, which reflect visible
light. Liquid water or ice ‘droplets’ can grow and precipitate, taking along aerosols that have
dissolved in them. This suggests a mechanism for bringing together water soluble gases and
aerosols and keeping them in proximity for a while.
–==– –==– –==– –==– –==– –==– –==– –==– –==– –==– –==– –==– –==– –==– –==–
architecture of the atmosphere
I think something should go here…
This section sets up the architecture of the atmosphere
pressure
The earth’s gravity pulls on the atmosphere and the pressure helps hold it up. The pressure is
greatest at the surface due to the bulk of the atmosphere above it, and the pressure decreases
logarithmically with altitude. Since the atmosphere is fluid, the pressure can vary due to various
motions. The average pressure at sea level is taken to be the standard and is given the value of 1
and the units of atmosphere (atm), making it 1 atm (10132.5 Pa, 1013.25 mb).
sunlight
The sun heats the earth and the atmosphere. The earth’s surface absorbs energy from the sun and
releases it as heat, but not all of the sunlight reaches the surface. Some wavelengths of light are
absorbed by species of gas in the atmosphere, causing photochemical reactions and heating it.
Photolysis is a photochemical reaction which energizes a molecule of gas enough to break
molecular bonds and leave its fragments. When energy of certain wavelengths is absorbed from
a beam of sunlight, the energy remaining in the beam at those wavelengths is reduced. If there is
enough of the reactant gas in the atmosphere, it can completely absorb all of the energy at those
wavelengths. Past that point of final absorption, the beam of sunlight cannot drive that particular
photolytic reaction even if the reactant gas is present. In that case we say that the lower part of
the atmosphere (or the surface) is shielded from those wavelengths.
An example is the absorption in the ozone layer. The ozone layer lies a little above the
tropopause, centered about 30-35 km and about 20 km thick. It is produced by diatomic oxygen
that absorbs solar ultraviolet and dissociates into energetic atomic oxygen. Some of the atomic
oxygen reacts exothermically with water, methane, or nitrous oxide to release heat and form
highly reactive and very important radicals (OH, CH3, NO). [equations from Wayne p 12].
Virtually all ground-state atomic oxygen will combine with diatomic oxygen to form ozone.
[Wayne p214]. Ozone can absorb ultraviolet wavelengths  ≤ 310 nm (UV) dissociating into
diatomic and atomic oxygen, and these can recycle quickly back into ozone again.
Maybe move this into chemistry…
thermal layer structure
The atmosphere is often described layers that share certain characteristics. Here we introduce
the thermal description for vertical variation.
Sunshine keeps the temperature at the surface quite warm. The land and sea radiate heat into the
lower parts of the atmosphere. Temperature decreases with altitude until it reaches a minimum
and begins to increase again. The lowest layer of atmosphere, where we experience most of our
weather, is called the troposphere. It is a region of thermal instability: warmed air near the
surface becomes less dense and rises, with air from nearby or even the higher colder air replacing
it. The top of this layer, called the tropopause, is where the temperature stops decreasing. The
height of the tropopause varies, being highest at around 16 km or so near the equator, and lower
nearer the poles, closer to 10 km. The stratosphere lies over the tropopause. In this layer,
absorption of solar energy and chemical and photochemical reactions heats the air a little.
Heated air here must compete against warmer air higher, so there is little thermal convection.
Instead, displaced air tends to return to its previous level, and vertical movement of the air is
primarily caused by wave motions initiated by disturbances from below in the troposphere. The
stratopause is where the temperature is a local maximum about 50 km. The temperature again
decreases with height through the mesosphere up to the mesopause about 85 km. The next
layer up is the thermosphere, where the temperature again increases with height. The
atmosphere is well-mixed by various motions, vertical, zonal (east-west), and meridional (northsouth) motions of the air in the turbosphere (so named for turbulence, but also called the
homosphere for homogeneity), which encompasses layers from the surface up to about 100 km,
(the turbopause or homopause). Above that the gases tend to layer according to their mass,
momentum, and gravity, with only the lightest molecules and atoms farthest from the planet.
There mixing is dominated by diffusion; so, the composition differs according to altitude, and it
is called the heterosphere. The thermosphere extends from the mesopause to variable 500-1000
km. Beyond that lies the outermost named layer of atmosphere, called the exosphere, which
extends to about 10,000 km.
The way the temperature changes with altitude is called a thermal profile, and this description of
layers is based on the thermal profile. (The thermal profile differs somewhat from place to place
on the earth and over the seasons, so we often refer to an average profile as a standard.)
greenhouse gases (this section may have to be moved)
It might be better to combine this section with spectroscopy and detection (radiative transferrelated things).
The properties of a particular kind of gas, its location and concentration determine its affects on
the atmosphere. We’ve already seen that absorption of ultraviolet light can lead to a
photochemical change. A greenhouse gas absorbs and re-emits infrared wavelengths according
to its own particular spectrum. When the gas is lower in the atmosphere, as in the stratosphere, it
radiates heat back towards the earth and in the middle atmosphere; but, when the same gas is
higher in the atmosphere, it radiates heat down towards the middle atmosphere and upward
outward to space. The first effect is called greenhouse warming, but the second is cooling.
Because earth receives energy from the sun (1370 Wm–2), it must reflect or radiate the excess to
space as heat to maintain its energy budget (and avoid overheating or overcooling). Sufficient
amounts of energy filter through the atmosphere to reach the surface and support the biological
environment and its processes, collectively called the biosphere. The biosphere processes
chemicals from the air, sea and land through respiration and digestion and contributes gases like
the greenhouse gases carbon dioxide, water vapor, and methane to the atmosphere.
Processes involved in changes in the composition of a parcel of
atmosphere
The amount of a substance in a place is equal to the amount that was there already plus any
change: the change is simply the difference between what increases it and what decreases it.
The atmosphere hosts a variety of chemical and photochemical reactions that increase gas
species as reaction products, and decrease the gas species that are reactants. Also, since the
atmosphere is fluid, circulation into and out of a region is important in changing or sustaining the
amount of a gas species in a location and distributing it to other parts of the atmosphere.
Change = production – loss + transport in – transport out
Here is a list of processes that cause change of a quantity in the atmosphere in any particular
location
Transport processes (for material):
convection (wind, upwelling, waves, eddies),
differences due to profile: decreasing temperature with altitude
in troposphere and mesosphere or increasing in stratosphere
coagulation, sedimentation, precipitation (of aerosols, not gases)
Production and loss mechanisms:
phase change (evaporation, deposition/condensation—reservoir)
depends on temperature and pressure (saturation and vapor pressures of water)
chemical reaction with other gases in the atmosphere
(rates directly related to reactant concentrations and
inversely related to product concentrations)
photolysis and photochemical reactions
(rates on reaction rates and also depend on sunlight, wavelength and shielding).
heterogeneous chemistry of a gas with or on an aerosol particle (solid or liquid).
(rates depend on concentrations of gas, and particle density, surface area.
If liquid water, it may also depend on solubility and what else is in the water.
Of the massive quantities of water evaporated from the surface and circulated in the troposphere,
only about 3 ppmv of water vapor passes the cold tropopause into the stratosphere. Joining with
that is about 1.6 ppmv of methane gas, which oxidizes and ultimately yields two molecules of
water and for each one of methane.
circulation and features (or phenomena)
Now I’ve talked about the thermal description and profile. Next should come more on
circulation in the Troposphere, Stratosphere, & Mesosphere, because transport is significant to
what reactants are where. Then chemistry & spectroscopy.
Maybe sources and sinks with circulation…
This section addresses circulation patterns and certain features in the atmosphere, with attention
to the variations according to altitude and latitude, but not so much the finer detail of
longitudinal variations. This is background that may be useful in understanding the altitude vs.
latitude contour plots in the HALOE climatology section.
The earth rotates from west to east at about 460 m/s (at the equator) and pulls the
atmosphere along with it, outermost layers lagging. The prevailing equatorial surface winds are
from the east, while the prevailing surface winds in the mid latitudes and all cells’ upper
troposphere regions from the west. The general tendency for polar tropospheric air is to sink and
wind to blow from the east. However, …
As air rises or descends it has to conserve angular momentum. Descending air moves
closer to the earth’s spin axis and so the part of its motion around the spin axis should increase;
likewise, rising air should slow. Coriolis effects due to the earth’s rotation deflect winds
leftward in the north hemisphere (rightward in the south).
equatorial troposphere
Circulating air moves whatever is in it, so now we consider how it moves around and then we’ll
look at sources and sinks of water vapor. The atmosphere operates on solar energy—literally.
heat drives the system. [see Houghton, p 143.] The sun shines most directly on the earth near
the equator, within 23.5° due the tilt of the earth’s axis. At higher latitudes, the surface curves
away, so the sunlight must travel farther through the atmosphere (at an angle) and it is spread out
more when it reaches the surface. The same happens as the sun moves through the sky as the
earth rotates: the sun’s rays reach the local surface most directly at noon and at angles at other
times of day. (The size of your shadow gives you an idea of how far the sun’s rays spread out
when the sun is closest to zenith (directly overhead) and how much when the sun is lower in the
sky.)
The sun’s heat on the surface (land and sea) melts ice and snow and evaporates water.
Water vapor is lighter than dry air, so moist air tends to rise. It cools as it expands from
decreasing pressure. Warmer air has a greater capacity for water vapor. As it cools, its capacity
decreases and its saturation vapor pressure decreases; thus the relative humidity (the ratio of the
partial pressure of water to the saturation pressure of water) increases. If or when it reaches
100%, vapor will start condensing, forming mist or clouds around condensation nuclei, which are
abundantly available. With more condensation, the water (or ice) droplets grow larger, removing
more vapor from the air. With less water vapor, the air becomes denser and less buoyant.
The tropopause is where the air stops rising (or almost stops). (The tropopause is
highest at the equator and is lower at the poles.) Cooled and drier, the air is no longer buoyed
up, so it does not pass the tropopause into the warmer increasing temperature region of the
stratosphere, instead it is mostly pushed aside by more upwelling air. North of the equator,
where the air is pushed northward, the coriolis effect from the earth’s rotation turns it rightward
towards the east. In the south, the southward moving air is in the same manner turned to the left,
again towards the east. The air descends at about 30° and mixes with descending air from higher
(mid-) latitudes. Air returns toward the equator along the surface, turned towards the west, and
gathers more water vapor along the way. The complete cycle is known as Hadley cell
circulation.
Mid latitudes
There is a general circulation of air upward from the surface in the troposphere about 60° (-60°)
latitude, where the mid-latitude Ferrel circulation (30°-60°) and the polar cell (60°-90°) meet.
The warmer mid-latitude air tends to rise slantwise northward over the colder polar air. Again,
air spreads out near the tropopause heading towards the pole in the polar cell and away into the
mid-latitudes of the Ferrel cell.
Polar troposphere
The temperature profile is a little different near the pole—the tropopause is at about 8-10 km.
Cold polar air forms a high pressure region in the lower troposphere and the coriolis effect turns
the surface winds that are going away from the pole towards the west.
Waves, stratosphere
The boundary of the polar air cell develops waves so that it dips farther towards the equator in
some places and not as far in others. These are the dips and crests of a planetary wave called a
Rossby wave, and they circulate around the polar region. They change in number and shape and
sometimes pinch off as independent cyclones (or anticyclones) and move down into lower
latitudes. These and other phenomena, like the up and downwelling of the main circulation cells
and airflow against and up over large mountain ranges (like the Rockies and the Himalayas), can
start disturbances in the troposphere that propagate into the stratosphere. The disturbances
propagate as gravity waves (the restoring force is gravity, water surface waves are also gravity
waves.) that move up into and through the stratosphere. The waves can carry momentum and
energy without the particles in the wave moving much (in water waves, the molecules move
mainly in a cyclic motion). They can also propel properly placed objects (air in this case) in the
direction of the wave, as water waves can propel a surfer. Waves change when thethe properties
of their medium changes, like water waves approaching a shallower region near a beach or
passing into warmer parts of the stratosphere. The waves release the energy when they break,
like water wave crests that overshoot the wave body and flow down to make a tube or crash
down into the water in front of the wave body. In the atmosphere, the wavelengths of the waves
determine how far they will go into the stratosphere (and mesosphere) before breaking and
releasing the last of their energy. Unlike the thermal convection in the troposphere caused by
solar heating of the surface, it is wave motions that affect the atmosphere above the stratosphere
and generate circulation in the stratosphere.
“The extratropical stratosphere and mesosphere, up to altitudes of about 80 km, act on the
tropical stratosphere like a gigantic, seasonally and interannually variable suction pump, whose
action depends on the Earth's rotation together with certain wavelike and turbulent eddy motions. …
The effect is to pull air gently but persistently upward and poleward out of the tropical troposphere
and lower stratosphere, then push it back down toward the extratropical troposphere, most of it
through the winter stratosphere via complicated, chaotic pathways.” [McIntyre, 1995 p 228 ]
The resultant pattern of motion in the stratosphere is called Brewer-Dobson circulation. It tends
to be upward slowly from the tropopause near the sun’s declination, increasing upward and
flowing towards the poles. Lower stratospheric air near the summer pole, up to about 30 km,
remains slow-moving and higher stratospheric air joins the summer upwelling. On the winter
side of the solar declination air moves towards the winter pole (slowly lower in the stratosphere
and faster higher). Some air circulates back down towards the winter mid-latitudes and mixes
into the troposphere.
“Typical large-scale upwelling velocities in the tropical lower stratosphere (altitudes 15-20
km) are seasonally variable roughly from 0.2 mm s-1 in northern summer to 0.4 mm s-l in northern
winter, or roughly 6-13 kilometres per year with the largest values confined mainly to the northern
winter.” [McIntyre, 1995 p 228]
Although for several weeks during summer, the north pole receives more sunshine in its 24-hour
day than sunny Mexico receives in its shorter daylight hours, McIntyre emphasizes that the wave
effects far outweigh the solar input local to that part of the atmosphere:
“There is a widespread misconception that the rising branch of the global-scale stratospheric
circulation is locally 'caused' by solar heating, as in a certain sense is true of the tropospheric Hadley
circulation. But this is one of the ways in which the stratosphere differs from the troposphere. What is
important for this purpose, in the stratosphere, is the far larger scale of the circulation and the far
larger scale of the radiative heating and cooling, together with the relaxational character of the
infrared contribution already mentioned. What this turns out to mean is that, under the conditions
characteristic of the stratosphere, the main effect of solar heating is to raise temperatures rather than to
cause any persistent global-scale circulation.” [McIntyre, 1995 p 237]
The wave motions extend above the mesosphere too.
Mesospheric circulation “is driven and controlled by gyroscopic pumping, due mainly to the
violently breaking gravity waves in the lower thermosphere, above about 80 km. So strong is this
pumping that the resulting refrigeration of the summer polar mesopause near 80-90 km makes it the
coldest place on earth, as well as the sunniest place on earth, with temperature minima sometimes as
low as 110 K.” [McIntyre, 1995, p 238]
The general motion of air in the mesosphere is a cell connecting the polar regions:
[The wave effects force changes in zonal winds and generate] “a mean equatorward wind of
the order of 10 m s-1 in summer and a similar poleward flow in winter. This in combination with
radiative forcing, induces vigorous upward motion (several centimeters per second) as a result of mass
continuity. The rising air cools adiabatically, resulting in very low temperatures in summer. The
opposite circulation occurs in the other hemisphere and downwelling warms the wintertime
mesopause.” [Thomas 1991]
The seasonal nature suggests that increase in solar heating of the surface ultimately strengthens
the atmospheric disturbances that produce the waves.
Figure 1 This is a composite, so the scale of arrows below 55 km may not match the scale of arrows above 55
km. [http://www.ccpo.odu.edu/SEES/ozone/class/Chap_6/index.htm, accessed April 2008]
This results in a much colder mesopause region at the summer pole. At the winter pole there is a
corresponding downwelling, and the air is heated as it descends into higher pressure levels. This
results in a warmer than average mesopause over the winter pole. Air traverses the mesosphere
from the summer pole to the winter pole at a rate of about 10 m/s.
(This would be 3.6 km/hour. Since one degree of latitude is about 111 km (at 100 km altitude), it
would take about 31 hours to cross one degree for direct meridional flow. Going farther, in 90
days the flow would traverse 70 degrees, (but less if slowing to a stop) or in 45 days, half of
summer, 35 degrees.)
The sun heats the air and surface, though much is reflected by snow and ice (as much as
there is), and drives a ‘vigorous upwelling of several cm/s [Thomas 1991]. Air descends at the
winter pole during the 24-hour night, and there is a net flow in the mesosphere about 10 m/s that
spans the globe. [Thomas 1991, I think] The mesospheric flow must find some way to reverse.
As the summer pole approaches equinox, the upwelling becomes less vigorous, and then as it
becomes the winter pole, downwelling starts. Of course, this circulation pattern reverses every
six months.
Polar regions
polar vortex
These cover the main meridional motions of the winds in the lower and middle atmosphere
troposphere. There are also significant zonal (east-west) flows of air. The coriolis effect is
independent of altitude and acts on zonal acceleration or deceleration, but its influence is zero on
constant zonal flow. So, certain zonal circulation patterns are relatively stable and persist until
they are disturbed. The polar vortex is such a pattern that forms in the stratosphere as cold air
descends over the winter pole, circulating west to east, in the same direction of the earth’s
rotation, but faster, with speeds “perhaps 100 meters per second or more by spring”. The
southern vortex is more stable than the northern because the land and ocean distribution is
somewhat symmetric, whereas the land and ocean near the north pole alternate and generate
more airflow disturbances.
The southern polar vortex tends to isolate the air inside the vortex from air outside. In the
darkness of the polar winter, there is no sunlight to drive photochemistry of the gases inside; so,
other reactions dominate. The winter vortex is also home to polar stratospheric clouds in the
range of 15-25 km. This altitude region over a pole remains in darkness from early October
through to the end of February in the north and early April to the end of August in the south.
(Depending upon how wide the vortex is (and where it is centered), its edges will be in darkness
for a slightly shorter period of time.)
vortex chemistry, polar stratospheric clouds
During the south polar winter, the stratosphere and troposphere are significantly colder (and the
mesosphere warmer) and clouds can form in the stratosphere. The triple point pressure of water
is approximately 611 Pa (0.006 atm), corresponding to the air pressure about 34 km. The
temperature at the stratopause falls below water’s freezing point, so water vapor can condense to
form polar stratospheric clouds in supercool liquid and ice phases.
[http://en.wikipedia.org/wiki/Polar_stratospheric_clouds] “Some of the clouds are water ice, but
more prevalent are clouds of nitric acid and water.”
[http://www.epa.gov/ozone/science/hole/whyant.html#psc]
In the darkness of the polar winter, certain chemical reactions build up compounds of chlorine
and nitrates. When the sun rises again, photolysis breaks them down and many of the products
are highly reactive radicals that oxidize ozone leaving diatomic oxygen. Subsequent reactions
produce intermediates that also consume ozone, and further reactions reproduce the original
compounds. The net effect is a rapid reduction of ozone (and atomic oxygen) in the vortex,
faster than photolysis of diatomic oxygen produced can replenish it.
“In the absence of polar stratospheric clouds, most stratospheric chlorine is locked up in
relatively inert compounds. However, the surfaces of the ice particles in the clouds allow these
compounds to react, converting the chlorine into ozone-destroying forms. The reactions are
different from those occurring when gases mix over midlatitudes, and can only take place on
cloud particles.” [http://www.epa.gov/ozone/science/hole/whyant.html#psc]
(Reactions that occur between materials of different phases, e.g. gas and aerosol (solid or liquid)
are called heterogeneous chemistry, whereas homogeneous chemistry are the reactions that occur
between materials of the same phase, e.g. gas and gas.)
These three photolysis reactions are parts of cycles that reproduce the radicals, Cl, OH, and NO,
which react directly with ozone to reduce it to diatomic oxygen.
ClONO2 + h → ClO + NO3
NO3 + h → NO + O2
HOCl + h → OH + Cl
[equations: Wayne, p163]
“Nitrogen dioxide (NO2) … turns into nitric acid when it mixes with water vapor.”
[http://www.windows.ucar.edu/tour/link=/physical_science/chemistry/nitric_acid.html&edu=hig
h]
“The nitric acid in the clouds comes from nitrogen oxides (NOx) (the process of removing NOx
from the atmosphere is called denoxification). Nitric acid normally slows the ozone depletion
reactions, so its removal allows ozone destruction to continue unabated.”
[http://www.epa.gov/ozone/science/hole/whyant.html#psc]
Since all nitrates are highly soluble in water, the nitric acid remains in the water…
… unless what? If the water evaporates, or if there is some chemical or photochemical reaction
to change the nitrate into something else…
“Clouds and raindrops have a major effect on gas-phase species through the scavenging
mechanism [of reactive condensation of the gas molecule onto pre-existing aerosol particles].
Rainout (removal of gases by cloud droplets) is believed to be more important than washout
(removal of gases by raindrops) because of the longer lifetime and greater surface area of cloud
droplets compared with raindrops. Water-soluble species, such as the acids, acid anhydrides, and
peroxides are obviously particularly susceptible to removal by these mechanisms.” Aqueousphase reactions in which water acts as a reactant are proceed very quickly. [Wayne, p 232]
Subsequent evaporation of the cloud particles leaves the contents, providing an atmospheric
source for them.
polar mesospheric clouds
Abundance of condensation nuclei…
In the polar regions during summer, a region in the mesosphere is much colder than at any other
time of year—cold enough for ice to form on the abundant nucleators into polar mesospheric
clouds. These PMCs act as a reservoir of water. The summer mesopause flow can push the
clouds into warmer regions where it vaporizes, contributing to the water levels equatorward of
the summer polar regions (at the mesosphere level). Such a flow would move not only the water
ice, but also anything else with the atmosphere, including dust and aerosols.
During the north polar summer, upwelling air expands adiabatically resulting in an extremely
cold mesopause. Water vapor condenses into ice forming polar mesospheric clouds, beginning
about five weeks before solstice. They form a very thin and sparse cloud cover about 83 km
altitude, so high that when they are thick enough, observers in the night-time latitudes below the
24-hour day terminator can see them illuminated by sunshine passing over the pole and call them
noctilucent clouds. The clouds act as a reservoir of water, but as they are driven equatorward by
the mesospheric airflow, they absorb heat in warmer latitudes and sublime again into vapor.
Also, If the cloud particles become large enough to precipitate, they will fall out of the cold area
of the mesosphere into lower warmer regions and evaporate.
Earlier I hadn’t understood properly and had the naïve idea that the air was upwelling from
near the surface driven more directly by the sun’s input.
add: I think part of that was because I saw that the SOFIE thermal profile for the (north)
summer polar region (actually 65-80°) has a warmer (e.g. weaker) ‘cold trap’ at the tropopause
than the south winter pole.
It would take about 10 days at 10 cm/s to travel from the surface to the mesosphere. The time
between solstice and the peak of PMC season is three weeks, about twice that long, so the
several cm/s would be about 5 cm/s. However, the vapor condensing into PMCs comes from a
little lower, and the vapor there is replenished by inflow from a little lower (and local sources),
and the air there is replenished by inflow from layers lower still, and so forth.
There are about three weeks from summer solstice to the peak of the PMC season.
The peak itself suggests a change in the PMC growth/shrinkage process.
Since PMC’s grow by deposition of water vapor onto extant ice particles (or nucleators), or
already
the water layers
Put something about them here so people know what to look for in the contour plots.
sources and sinks of water vapor in the atmosphere
Now that the idea of general circulation has been introduced, e.g. the transport processes, focus
on the production and loss of water vapor in the atmosphere.
The primary source of atmospheric water comes from solar evaporation of water from the
oceans and land surface. The temperature decreases away from the surface, and reaches a
minimum value at the tropopause (~15 km at the equator, ~10 km near the poles).
Air is very well mixed in the troposphere and we were talking about water vapor… in
the equatorial and mid latitudes , ~3 ppmv water vapor passes up through the tropopause. Note
that ~1.6 ppmv methane also passes into the stratosphere too, where it oxidizes to form water.
(Methane is produced naturally by decomposer organisms in the soil and in digestive processes
such as in cattle, and additional methane comes from industrial sources [double check this… ] .)
The cell is completed with the upper troposphere air in the 30-60° region moving
equatorward towards 30°.
Chemistry
All the chemical and photochemical processes continue in all regions, with rates affected by
reactant-product relative concentrations and the supply of them by convection and the supply of
energy through solar input and through release of latent heat by water condensation (?) and
perhaps electrical discharge during transient luminous events or maybe other as yet unobserved
phenomena. The concentrations present at any location are the net results of
Production – loss + transport in – transport out.
Ozone
The excess energy in O* makes it highly reactive and (many, most, all?) reactions in which it
participates are exothermic. [Wayne, p 12]
O* + H2O → OH + OH
O* + CH4 → OH + CH3
O* + N2O → NO + NO
[Wayne, p12]
(I’m missing the part where the OH reacts with something else… )
Methane
IF The net oxidation of methane produces two parts water for each part methane, then the
methane rising through the troposphere could provide about 3.2 ppmv more water vapor in the
stratosphere. Methane undergoes oxidation most of the methane is converted by ________ km.
[I’ll have to find where someone said that because I don’t see the other half!]
CH4 + 2O2 → CO2 + 2H2O, [?]
(I’d like to find it written somewhere.)
CH4 + OH → CH3 + H2O
CH4 + O2 → H2O + CO + H2
CO + OH → CO2 + H
[Wayne p 399]
(with OH, light) [Wayne p 22]
[Wayne p 22]
“Methane is photochemically stable near the surface ( because short –wavelength ultraviolet
radiation is filtered out by H2O vapor). [Wayne p 399]
Photolysis
(mentioned above in ‘sunlight’ section) .Higher in the mesosphere, water is less shielded from
solar radiation, acting itself more as a shield to lower atmospheric layers in particular
wavelengths. It becomes more subject to photolysis.
Heterogenous chemistry--Dust
Dust (and maybe other aerosols too?) provides a surface. Chemicals that are on such a surface
can interact not only with each other, but with the surface (and the chemistry of it) as well.
Chemical reactions must conserve quantities like charge, orbital angular momentum, and so forth
maybe be able to confer some of these properties to the surface, relaxing the constraints on
reactions and providing a reaction pathway which would have been forbidden otherwise.
Dust condensed from the vapors from meteors that ablate and burn in the atmosphere can
provide a surface. The dust settles due to gravity and coagulates into larger particles that
precipitates, but smaller particles can be kept aloft by upward eddy currents.
Additionally, dust particles from the earth’s surface made by erosion and evaporates of sea spray
can be lofted upwards. The time for passage for small particles of size ____ between surface and
mesosphere (either way) is on the order of decades.
It has been suggested that _____ _____ recombine on meteor dust may provide a local source of
water vapor in the ______ km altitude region. To help explain why there is so much observed
there when standard atmospheric chemistry says that there should be a loss there (Brasseur &
Solomon} [[[I haven’t seen that reference—didn’t find it in articles.]]]
Did that other article say that meteor dust is no so abundant over the summer polar regions?
Wouldn’t that suggest that the vigorous upwelling clears the air of meteor dust?
“The atmospheric circulation efficiently transports smoke particles to the winter pole, resulting
in as few as 10 particles cm-3 larger than 1 nm radius left at the summer polar mesopause. If
smoke particles in this size range are the only nucleation kernel, this would imply that there
could be no more than 10 ice particles per cubic centimeter at the summer mesopause. ”
Megner, L. Gumbel, J. Rapp, M. Siskind, D. E. Advances in Space Research
Reduced meteoric smoke particle density at the summer pole – Implications for mesospheric ice
particle nucleation.
But then how does the surface dust reach the mesosphere—through tropical upwelling
only/mostly?
Mentioned solar input, summer → cold polar mesopause
Next summarize data collection techniques: in situ probes (falling sphere, aircraft, rocket),
Collectionof what? Temp, dust, spectrograph of gases… airglow
Remote sensing (also by lidar, airplane-based sensors) Satellite remote sensing covers large
sections of atmosphere—more sampling & bigger set of samples than in situ measurements.
Miscellany
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Somewhere I should answer concisely why it is important to understand water vapor in the
atmosphere.
More topics:
spectroscopy & satellite detection,
overall composition, … I read something that reminded me that reaction rates depend upon the
relative concentrations of the reactants multiplied (and inversely on those of the products) and it
referred to the abundance of oxygen in the atmosphere—an oxidizing atmosphere.
At 60° latitude at equinox, the sunshine is about half as intense as at the equator just due to the
angle of the sun.
“Sunlit snow has been shown to be one of the most photochemically active, and strongly
oxidizing, regions of the entire troposphere, rather than simply a passive sink for the products of
tropospheric chemical processing. Photolysis of nitrate initiates very active chemistry that leads
to the release of a number of important trace gases. Initial measurements suggest that just above
sunlit snow the production of HOx from photolysis of HCHO, HOOH, CH3CHO and HONO are
all significant, and collectively dominate over photolysis of O3. The net result is a large
enhancement of OH and HO2 in air just above the snow. Oxidation by OH is the main sink for a
number of gases important for climate change and stratospheric O3 depletion, so this
enhancement may perturb chemistry in much of the free troposphere, and also modify the
chemical records of atmospheric composition ultimately preserved in glacial ice. While recent
work has shown that photochemical and physical processes in the snowpack can impact the
chemistry and composition of both the atmosphere and snowpack, these processes are, in
general, poorly understood. This is especially true for the processes that produce and consume
OH and HO2.” [http://www.acd.ucar.edu/ASR/2003/ASR2003ACDnarrativev11.htm, April
2008]
“Between 20 July and 10 August, the solar radiation records shows several extremely high shortlived peaks of up to 500 W / sq.m Such high numbers are possible when the sun shines through a
large opening in the sky and a great deal of additional radiation is reflected from the clouds
surrounding the opening.” Other items in context indicates the year 2004.
[http://www.arctic.noaa.gov/essay_untersteiner3.html]