stratospheric ozone

Stratospheric Ozone

• Stratosphere is about 10,000 m to 49,000 m

• Temperatures about -50oC until 30,000 m and then increase to about 23oC. Increase due to absorption of UV by ozone/oxygen.

• Ozone protects earth surface from UV rays.

Ozone Formation/Destruction in the Stratosphere

Ozone Depletion• World-wide monitoring

shows stratospheric ozone decreasing for more than two decades.

• Since the 1970s, an ozone hole has formed over Antarctica in which up to 60 % of the total ozone is depleted.

• Depletion of ozone primarily due to ChloroFluoroCarbons (CFCs)

What are CFCs

• CFCs are commonly used as refrigerants, solvents, and foam blowing agents.

• The most common CFCs are CFC-11, CFC-12, CFC-113, CFC-114, and CFC-115.

CFC Production

• Freon 11 is fluorodichloromethane and has a structure of

CCl3F

• Carbon tetrachloride, methyl bromide and methyl chloroform and other halons also destroy the ozone layer

CFC production in US

• HCFC (hydrogenated CFCs) are more reactive in troposphere and so less destructive to ozone layer

• Brominated compounds also more reactive in troposphere but Br is 40 times more efficient at destroying ozone than chlorine.

How do CFCs affect ozone?

• CFCs - unreactive in the troposphere and move unchanged to stratosphere

• Short-wave UV light in stratosphere breaks down CFCs and chlorine is released

• Much of the chlorine reacts to form HCl or other compounds that do not react with ozone

Cl + O3 ClO + O2

ClO + O Cl + O2

ClO + ClO Cl2O2 Cl2 + O2

Cl2 can photodissociate to form more chlorine atoms

“A single chlorine atom can destroy as many as 100,000 ozone molecules during residence in

stratosphere”

Ozone Depleting Potential All CFCs are not created equal

• The ozone depleting potential (ODP) is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11. Thus, the ODP of CFC-11 is defined to be 1.0. HFCs have zero ODP because they do not contain chlorine.

• Other CFCs and HCFCs have ODPs that range from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetrachloride has an ODP of 1.2, and methyl chloroform's ODP is 0.11.

CFC characteristics

Compound Lifetime (years)

ODP GWP-2 CAS No.

CFC-11 (CCl3F) Trichlorofluoromethane

45 1 4600 75-69-4

CFC-12 (CCl2F2) Dichlorodifluoromethane

100 1 10600 75-71-8

CFC-113 (C2Cl3F3) Trichlorotrifluoroethane

85 0.8 6000 76-13-1

CFC-114 (C2Cl2F4) Dichlorotetrafluoroethane

300 1 9800 76-14-2

CFC-115 (C2ClF5) Monochloropentafluor

ethane

1700 0.6 10300 76-15-3

Ozone Depletion Means

• Increase in UV radiation (UVB)

• UVB has been linked to skin cancer, cataracts, damage to crops, damage to marine organisms (like reefs), damage to plastics

• Picture shows squamous cell carcinoma in elderly patient

Skin cancer is definitely related to UV/sunlight exposure

• Basal cell carcinoma • Malignant melanoma

Montreal Protocol

• 1989 Global Treaty to protect ozone layer

• Currently 140 countries are parties

• Timetable to reduce and end production and consumption of 8 major halocarbons

• Many governments committed to early phase outs

Global Climate Change

• Earth’s temperature -balance between solar radiation input and reflection and reradiation of energy from earth back into space.

• Atmospheric gases trap some outgoing energy, retaining heat - The Greenhouse Effect

Solar energy blocked by atmospheric sulfates (radiating force in watts per square meter)

What are greenhouse gases?

• Carbon dioxide –burning of fossil fuels, wood products.

• Methane – production of coal, gas and oil. Decomposition of wastes at landfills. Raising livestock.

• Nitrous oxide – industrial and agricultural activites.

• CFCs, HFCs, sulfur hexafluoride – generated in industrial processes.

• Water vapor, ozone.

Global Warming Potential (GWP)

Global Warming Potential: a number that refers to the amount of global warming caused by a substance  The GWP is the ratio of the warming caused by a substance to the warming caused by a similar mass of carbon dioxide. Thus, the GWP of CO2 is defined to be 1.0 . Methane is 21. Nitrous Oxide (N2O) is 310. CFC-12 has a GWP of 8,500, while CFC-11 has a GWP of 5,000. Various HCFCs and HFCs have GWPs ranging from 93 to 12,100. SF6 is 23,900. Water, a substitute in numerous end-uses, has a GWP of 0.  

GHG Emissions from various sources in the US

Methane emissions in US

CFC Emissions in US

Nitrous oxide emission in US

Percentage of total energy consumed in US as fossil fuels, renewables and nuclear.

Effects of climate change

• Ecosystem decline such as Everglades, coral reefs

• Flooding/desertification

• Human health issues

• Agricultural impacts

Change in ecological systems

Increases in precipitation and extreme

weather events can lead to flooding

Health Issues• Warmer climates in

temperate zones can mean that tropical and subtropical diseases can enlarge their range.

• Infectious diseases like cholera more likely in warm climates.

• Hotter temperatures increase deaths as it aggravates cardiovascular and respiratory problems.

Human populations at risk

Severe shifts in agriculture due to extreme weather, extended droughts and flooding. For example, glaciers have disappeared in Northern India, severely affecting the region where farmers rely on melting of glaciers for irrigation.