Transcript 637Lesson15

METO 637
Lesson 15
Polar meteorology
• In the winter months the poles are in perpetual
darkness. This causes extremely cold
temperatures in the stratosphere (-80oC).
• These cold temperatures favor the formation of
ice clouds known as polar stratospheric clouds
(PSC).
• It is significant that the years when the
temperature was lowest corresponded to the
years when the ozone depletion was largest.
• In addition a vortex forms around the pole as the
cold air descends. Wind speeds of 100 meters
per second or more have been observed
Ozone Hole
ClO and O3 in mid-September
Partitioning of Chlorine
Changes in species concentration near
the vortex boundary
Polar Stratospheric Clouds
• There are two main classes of PSC.
• Type 1 PSC are small (<1mm) HNO3 rich particles. These have
a mass mixing ratio of about 10 ppbm.
• Type II PSC are larger (from 10 mm to about 1 mm) composed
primarily of H2O-ice with minor amounts of HNO3 as
hydrates.
• They can constitute up to 1000 ppbm of the stratosphere.
• As noted before, the primary reaction that can be induced on
the surface of the PSC is
ClONO2 + HCl → Cl2 + HNO3
• The HNO3 is then retained in the PSC.
Perturbed Chemistry
• Most of the chlorine in the stratosphere is bound up in
two species, hydrogen chloride and chlorine nitrate:
Cl + CH4 → CH3 + HCl
ClO + NO2 + M → ClONO2 + M
• Normally homogeneous reactions only slowly convert
these reservoir species back to chlorine.
• However these two species can react on the surfaces of
PSC’s:
ClONO2 + HCl → Cl2 + HNO3
• The molecular chlorine is released as a gas, and the
nitric acid is retained within the PSC (as nitrates – NAT).
• The chlorine molecule can then be dissociated easily by
visible radiation.
Chapman Layers
We can write the number density at an altitude z as
n  n0 exp(  z / H )
for an increased in altitude dz the path that the suns rays
have to traverse is decreased by dz sec  where
 is the solar zenith angle. Hence
dI  In s (dz sec  )
combining the two equations we get
dI
 d (ln I )  n0 s sec  exp(  z / H )dz
I
Integratin g this equation
I  I  exp( n0 s sec  exp(  z / H ))
Chapman Layers
The rate, P, at which energy is removed from the
incident beam (or photons removed) is the decrease
in intensity per unit path trave rsed. That is
dI
 dI 
P
   cos 
dz sec   dz 
z
z
 I  n0 a cos  exp(   n0 a H sec  exp(  ))
H
H
Chapman Layers
Biogenic volatile organic compounds
• Troposphere contains a amazing mixture of VOC’s.
• Biogenic (naural) and anthropogenic (man-made)
• Natural VOC’s are often hydrocarbons, but also contain
partially oxidized VOC’s such as alcohols, aldehydes,
ketones and acids.
• Methane is by far the most abundant, but the sum of the
others are larger
• Non-methane hydrocarbons
• .Are much more reactive than methane.
• Play significant role in tropospheric chemistry
Schematic of biogenic emissions