meto 637 lesson 15. polar meteorology in the winter months the poles are in perpetual darkness. this...

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.

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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