The Earth’s Atmosphere

Temperature Temperature is a measure of the average speed of the molecules, faster motion = higher temperature. Temperature is a fundamental quantity for understanding the weather, radiation, and chemistry of the atmosphere. Temperature scales: Fahrenheit (F): water freezes at 32°F and boils at 212°F Celsius (C): water freezes at 0°C and boils at 100°C, T(F) = (9/5) T(C) + 32 Kelvin (K): water freezes at K and boils at K, T(K) = T(C)

Composition of the Atmosphere The atmosphere is comprised of a variety of gases: Major Constituents (99%): Nitrogen (N 2 ): 78% Oxygen (O 2 ): 21% Trace Constituents: Argon (Ar), about 0.9% Water vapor (H 2 O), up to ppmv Carbon dioxide (CO 2 ), 350 ppmv Ozone (O 3 ), near zero at the surface, up to 10 ppmv in the stratosphere Methane (CH 4 ), 1.7 ppmv and others….. ppmv = “parts per million by volume”

Vertical Structure of the Atmosphere Layers in the atmosphere are defined by temperature Earth's atmosphere thins out to near nothingness several hundred kilometers above the surface 99% of the total mass of the atmosphere exists below 30 km altitude

Troposphere and Stratosphere Troposphere 0 to 15 km altitude The lowest region of the atmosphere, where life & weather exist. Temperature decreases with altitude. Long-wave radiation emitted from Earth is absorbed by the atmosphere, the atmosphere becomes less dense with increasing altitude, less air to absorb Top of the troposphere is known as the tropopause Stratosphere 15 to 50 km altitude Temperature increases with altitude. Heating occurs because ozone (O 3 ) absorbs ultraviolet radiation from the Sun. Top of the stratosphere is known as the stratopause

Mesosphere and Thermosphere Mesosphere 50 to 90 km altitude Temperature decreases with altitude The lowest temperatures in the entire atmosphere are found at the mesopause during summer at high latitudes, 130 K (-226°F) can occur Top of the mesosphere is known as the mesopause Thermosphere 90 to 1000 km altitude (> 800 Km esosphere) Temperature increases with altitude above 90 km, and is constant above 200 km. This heating is due to absorption of solar radiation (wavelengths less than 0.2 microns) by molecular oxygen (O 2 ). The highest temperatures in the atmosphere can be found in the thermosphere, 2000 K can occur

7 Ozone and UV radiation

8 What is ozone?  The ozone molecule contains three oxygen atoms, hence O 3 is ozone.  How is ozone produced? –It is formed when oxygen absorbs solar radiation O 2 +h ( <240nm)  O+O O+ O 2  O 3 -It is destroyed by solar radiation O 3 + O  2O 2 –Ozone reacts with various other chemicals, and thus is highly reactive.

9 Why is the ozone layer important Ozone acts as the Earth’s protective shield against the Sun’s harmful Ultraviolet radiation (UV radiation). Without the ozone layer, life would not exist on Earth!

10 UV radiation UV radiation is emitted from the sun with wavelength from nm (nanometers) UV radiation is divided into three ranges –UV-A, nm –UV-B, nm –UV-C, nm

UV light and life UV light is high energy light When it strikes molecules it can cause them to beak into ions or free radicals The free radicals in turn damage large molecules such as proteins and DNA Damage to DNA causes mutations It is a mutagen Mutations can lead to cancer

UV light & DNA UV-B has a direct effect on DNA DNA absorbs UV light of 260nm The action of UV forms thymine dimers This can cause a gene mutation when the DNA replicates UV- B Thymine dimer ACGCTGGCTTAGT TGCGACCGAATGA ACGCTGGCT=TAGT TGCGACCGAATGA DNA

13 UV light & DNA

Specific damage by UV light Sunburn Clouding of the cornea and formation of cataract Skin cancer (melanoma) Reduced rates of photosynthesis Immune system damage

15 Effects on Human Health  Over exposure may: –Increase risk of non- melanoma and malignant melanoma skin cancer  Higher risks of malignant melanoma from severe sunburns – especially in childhood  Risk of malignant melanoma has increased 10%  Risk of nonmalignant melanoma has increased 26% malignant Non-malignant

16 Good ozone/bad ozone

17 Ozone in the Atmosphere Ozone is mainly produced in the tropics because of the higher amounts of solar radiation. Ozone is mainly destroyed at middle and higher latitudes by chemical processes. Ozone is also largely affected by atmospheric winds. Thus, ozone naturally has large variations in space and time.

18 Summary Ozone is produced and destroyed naturally in the atmosphere (ozone bucket picture). Ozone is responsible for absorbing much of the sun’s harmful UV radiation. UV radiation varies due to time of day, season, cloud amount, and ozone amount.

19 History of Ozone Depletion  CFCs developed in 40’s and 50’s –Refrigerants, propellants, fire retardants  1970’s CFCs detected in atmosphere. –Many of these have long atmospheric lifetimes (10’s to 100’s of years)  1974 Rowland and Molina propose that CFC’s can destroy ozone in the stratosphere. –CFCs broken apart by UV radiation forming chlorine which can destroy ozone quickly:  O 3 +Cl  ClO+ O 2 (Catalytic Reaction)  ClO+O  Cl+O 2

20 Chemical Mechanism Chlorofluorocarbons (CFC’s) : –man-made, non-toxic and inert in the troposphere –In the stratosphere are photolysed, releasing reactive chlorine atoms that catalytically destroy ozone

21 A combination of low temperatures and elevated chlorine and bromine concentrations are responsible for the destruction of ozone in the upper stratosphere thus forming a “hole”. (Kerr, 1987)

22 Images of Antarctica

23 Where is the ozone hole?  Ozone hole largely restricted to polar areas, in particular over Antarctica  A “mini” ozone hole was observed also over Arctic area  Polar meteorology  Polar vortex – winter polar night  Polar stratospheric clouds (-80C)

24 What are the consequences of an ozone depleted world? With ozone depletion, there will be higher rates of UV radiation. More UV radiation rates mean –Higher rates of skin cancer –Higher amounts of cataracts –Possible danger to plant and animal life Recall: Without the ozone layer, life on the Earth’s surface would not exist!!!

25 What is being done about ozone depletion?  Montreal Protocol (1988): international agreement to phase out ozone depleting chemicals –Developed countries have switched to HCFC’s (more ozone friendly) instead of CFC’s. –Developing countries has until 2010 to phase out CFC’s.

26 Ozone module prepared by Eugene C. Cordero Is the Montreal Protocol working? Observations indicate that chlorine is beginning to decline in the atmosphere. (Good news!) Still large uncertainties about illegal trade/use of CFC’s (??’s) Future Atmospheric models suggest that: atmospheric chlorine will return to pre-80’s in next 50 to 100 years. –a slow ozone recovery will follow decreasing chlorine concentrations

27 Ozone depletion and global warming No direct connection between these environmental issues. However: Global warming may enhance ozone depletion –global warming produces Tropospheric warming & stratospheric cooling –Therefore, if the stratosphere cools, then ozone destroying chemistry (e.g. ozone hole), will increase.