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METO 637

METO 637. Lesson 15. Polar meteorology. In the winter months the poles are in perpetual darkness. This causes extremely cold temperatures in the stratosphere (-80 o C). These cold temperatures favor the formation of ice clouds known as polar stratospheric clouds (PSC).

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METO 637

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  1. METO 637 Lesson 15

  2. 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

  3. Ozone Hole

  4. ClO and O3 in mid-September

  5. Partitioning of Chlorine

  6. Changes in species concentration near the vortex boundary

  7. 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.

  8. 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.

  9. Chapman Layers

  10. Chapman Layers

  11. Chapman Layers

  12. 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

  13. Schematic of biogenic emissions

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