Download 637Lesson13

METO 637
Lesson 13
Air Pollution
Air Pollution
Air Pollution
The Troposphere
• In the Stratosphere we had high energy photons so that oxygen
atoms and ozone dominated the chemistry.
• In the troposphere we have lower energy photons, and the
chemistry is dominated by the OH and NO3 radicals.
• OH is generated photochemically (i.e. only during the day),
NO3 is rapidly photolyzed during the day, so it can only
survive at night.
O(1D) + H2O → OH + OH : NO2 + O3 → NO3 + O2
• NO3 is generally less reactive then OH, its peak concentration
is higher.
• OH provides an efficient scavenging mechanism for both
natural and anthropogenic trace constituents
Dry and Wet Deposition
• Dry deposition – removal of gases and particles by a direct
transfer from the atmosphere to the surface.
• Wet deposition – removal of gases and particles carried to the
surface in water – rain, snow, fog etc.
• Dry deposition is known for SO2, O3, CO2, and SO3.
• Wet deposition of gaseous species requires that they be water
soluble. Terms used are rainout, or washout.
• Acid rain is an example of the rainout of sulfurous and nitric
acids, produced in polluted atmospheres.
Dry Deposition
• Three separate steps
(1) Species must be transported close to
the surface
(2) Species must cross to the surface
(3) Species must be taken up on the
surface.
• Dampness of the surface can be critical for
some species, e.g. SO2, O3, HNO3
Wet deposition
• Wet deposition – incorporation into falling
precipitation (washout), or into cloud
droplets (rainout).
• Chemical conversions can occur within the
droplets.
Dry and Wet Deposition
Boundary Layer
• Lower limit of the troposphere is seen as a layer and not
the surface – known as the boundary layer
• As the winds flow across the surface they undergo
friction – which generates turbulent eddies in the
atmosphere.
• This region of high turbulent mixing is generally confined
to altitudes up to 0.5 – 2km in the day, and lower at night
• The region above the boundary layer is known as the
free troposphere
• In the free troposphere the kinetic energy is associated
with large scale disturbances – periods of a day or more.
• In the boundary layer the periods are of the order of
minutes
Hadley Cell
GLOBAL CIRCULATION
• George Hadley first suggested in 1735 the general
concept of atmospheric circulation.
• He used a single cell, to explain the existence of
the easterly winds at the surface
• Cold air at the pole - high pressure at the surface.
Warm air at the equator - low pressure at the
surface. Pressure gradient force at surface will
move air from pole to equator at surface. Return
path at high altitudes.
• The action of the Coriolis force on this southerly
flow at the surface will produce easterly winds.
• But we find westerly winds at mid-latitudes –
why?
GLOBAL CIRCULATION
• In reality the atmosphere cannot maintain a single
cell, instead there are three cells, with boundaries at
about 30 and 60 degrees latitude.
• This results in sinking air at 30 N and 30 S. But
sinking air suppresses cloud development and
precipitation. Hence most of the worlds deserts occur
along these latitudes.
• At mid-latitudes the prevailing wind in the free
troposphere is westerly (mid-latitude westerlies)
• Between the equator and 30 degrees we get
easterlies – trade winds.
• The free troposphere also has lower temperatures
than at the surface – reaction rates are slower, and
pollutants can travel large distances before they are
removed chemically.
Fig. 7.6
Column tropospheric ozone