《环境化学》课程PPT教学课件讲稿（英文版）Unit 10 The ozone hole

The ozone hole

The Ozone Hole

The discovery of the ozone hole The British Antarctic Survey has been monitoring for many years, the total column ozone levels at its base at Halley Bay in the antarctica Monitoring data indicate that column ozone levels have been decreasing since 1977 This observation was later confirmed by satellite data (TOMS-Total Ozone Mapping spectrometer) Initially satellite data were assumed to be wrong with values lower than 190DU

The discovery of the ozone hole • The British Antarctic Survey has been monitoring, for many years, the total column ozone levels at its base at Halley Bay in the Antarctica. • Monitoring data indicate that column ozone levels have been decreasing since 1977. • This observation was later confirmed by satellite data (TOMS-Total Ozone Mapping Spectrometer) – Initially satellite data were assumed to be wrong with values lower than 190DU

220 205205 Antarctic ozone minimum (60.9o s) 97 101510 1979-1992 Nimbus 7 TOMS 1996-1994 Meteor 3 TOMS 180 1995 no TOMS in orbit 1996-1998 Earth Probe TOMS g160 154 1003146 1024 140 120 417/100 105 100 930 1980 1985 1990 1995 2000

October ozone hole over antarctic October 13 ⑦ 1979 1991 10020030040000

October ozone hole over Antarctic

Features of the ozone hole Ozone depletion occurs at altitudes between 10 and 20 km If 03 depletion resulted from the ClOx cycle, the depletion would occur at middle and lower latitude and altitudes between 35 and 45 km The ClOx cycle requires atom, but in the polar stratosphere the low sun elevation results in essentially no photodissociation of 02 The above observation could not be explained by the clox destruction mechanism alone Depletion occurs in the Antarctic spring

Features of the ozone hole • Ozone depletion occurs at altitudes between 10 and 20 km – If O3 depletion resulted from the ClOx cycle, the depletion would occur at middle and lower latitude and altitudes between 35 and 45 km. – The ClOx cycle requires O atom, but in the polar stratosphere, the low sun elevation results in essentially no photodissociation of O2. – The above observation could not be explained by the ClOx destruction mechanism alone. • Depletion occurs in the Antarctic spring

Special Features of Polar Meteorology During the winter polar night, sunlight does not reach the south pole a strong circumpolar wind develops in the middle to lower stratosphere, These strong winds are known as the polar vortex In the winter and early spring, the polar vortex is extremely stable, sealing off air in the vortex from that outside The exceptional stability of the vortex in Antarctic is the result of the almost symmetric distribution of ocean around antarctica The air within the polar vortex can get very cold Once the air temperature gets to below about -80C (193K) Polar Stratospheric Clouds (or pscs for short)are formed

Special Features of Polar Meteorology • During the winter polar night, sunlight does not reach the south pole. • A strong circumpolar wind develops in the middle to lower stratosphere; These strong winds are known as the 'polar vortex'. • In the winter and early spring, the polar vortex is extremely stable, sealing off air in the vortex from that outside. • The exceptional stability of the vortex in Antarctic is the result of the almost symmetric distribution of ocean around Antarctica. • The air within the polar vortex can get very cold. • Once the air temperature gets to below about -80C (193K), Polar Stratospheric Clouds (or PSCs for short) are formed

Polar vortex The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere The polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere(> 5 km)

Polar vortex • The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere. • The polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (> 5 km)

Polar Stratospheric Clouds (Pscs) PSCs first form as nitric acid trihydrate(Hno3: 3H20) once temperature drops to 195K As the temperature gets colder, larger droplets of Water-ice with nitric acid dissolved in them can form PSCs occur at heights of 15-20km

Polar Stratospheric Clouds (PSCs) • PSCs first form as nitric acid trihydrate (HNO3.3H2O) once temperature drops to 195K. • As the temperature gets colder, larger droplets of water-ice with nitric acid dissolved in them can form. • PSCs occur at heights of 15-20km

Why do pscs occur at heights of 15-20 km? The long polar night produces temperature as low as 183 k(-90oC)at heights of 15 to 20 km The stratosphere contains a natural aerosol layer at altitudes of 12 to 30 km

Why do PSCs occur at heights of 15-20 km? • The long polar night produces temperature as low as 183 k (-90oC) at heights of 15 to 20 km. • The stratosphere contains a natural aerosol layer at altitudes of 12 to 30 km

PSCs promote the conversion of inorganic Cl and Cl reservoir species to active c Pathway 1: HCI(g>Cl2(g Absorption of gaseous HCl by PSCs occurs very efficientl y HC(g)→HCI(s) Heterogeneous reaction of gaseous ciono2 with Hcl on the pSc particles HCIS)+ CloNo2> HNO3(s)+ cl2 Where s denotes the psc surface Note: The gas phase reaction between HCl and CIONO2 is extremely slow

PSCs promote the conversion of inorganic Cl and Cl reservoir species to active Cl Pathway 1 : HCl(g) → Cl2 (g) • Absorption of gaseous HCl by PSCs occurs very efficiently HCl(g) → HCl(s) • Heterogeneous reaction of gaseous ClONO2 with HCl on the PSC particles HCl(s) + ClONO2 → HNO3 (s) + Cl2 where s denotes the PSC surface Note: The gas phase reaction between HCl and ClONO2 is extremely slow