meto 637 lesson 15. polar meteorology in the winter months the poles are in perpetual darkness. this...
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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 (<1m) HNO3 rich particles. These have
a mass mixing ratio of about 10 ppbm.• Type II PSC are larger (from 10 m 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
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Chapman Layers
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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