Chapter 18 Chemistry of the Environment

Transcription

1 Chapter 18 Chemistry of the Environment Earth s Atmosphere Outer Regions of the Atmosphere Ozone in the Upper Atmosphere Chemistry of the Troposphere You will need to read the Course Pack to compliment the Textbook it is available at the UVic Bookstore and includes, Ozone hole over Antarctica Acid Rain Global Warming and The Greenhouse Effect

2 18.1 Earth s Atmosphere The temperature of the atmosphere varies in a complex manner as altitude increases The atmosphere is divided into 4 regions At the boundary between regions (suffix - pause), temperature extremes are observed Gases mix slowly at these boundaries

3 The pressure of the atmosphere decreases in a regular manner as altitude increases The atmospheric pressure (100kPa) at Sealevel, decreases to 0.3Pa at 100km.

4 The sun bombards us with radiation and energetic particles The atmosphere is our first line of defense! The aurora borealis (Northern Lights) is caused by collisions of high speed electrons and protons from the Sun with air molecules The charged particles are channeled towards the polar regions by Earth s magnetic field

5 Composition of the Atmosphere Because of the great variation in atmospheric conditions, the composition of gases in the atmosphere is not uniform. Parts per million (ppm) are commonly used when referring to the trace constituents

6 Parts per million For substances in aqueous solution 1ppm is equivalent to 1 mg in 1 kg PV = n RT so Volume is proportional to n So 1 mole in 1 million moles of gas = 1 ppm Sample Exercise 18.1

7 18.2 Outer Regions of the Atmosphere The Sun emits radiant energy over a wide range of wavelengths As the radiation and high energy particles pass through the outer atmosphere it causes two types of chemical change:

8 Photodissociation Photons of sufficient energy are absorbed by a molecule causing Homolytic splitting 99% of the oxygen molecules in the upper atmosphere are dissociated Very few Nitrogen molecules are dissociated Nitrogen has a very high bond enthalpy and does not readily absorb photons

9 Photoionization Molecules absorb radiation (high energy photons) causing a valence electron to be lost, (ejected)

10 18.3 Ozone in the Upper Atmosphere Ozone absorbs much of the radiation in the nm wavelength range (UV) The rate at which ozone forms depends on the factors that vary in opposite directions as the altitude increases

11 Ozone cycle The cycle of ozone formation and dissociation is responsible for the temperature peak observed at the stratosphere boundary Depletion of the Ozone Layer It has been known for 25 years that the ozone layer can be depleted due to reaction with chlorine from Chlorofluorocarbons (CFCs) Used as coolant gases and propellants Un-reactive and insoluble so they are able to survive extended periods in the atmosphere

12 The dark area represents the area of lowest O 3 concentration - The Hole centered over Antarctica The C-Cl bond is easily broken as the molecule diffuses to higher altitudes and absorbs photons with wavelengths between 190 and 225nm

13 The Chlorine monoxide generated is photo-dissociated back to Cl(g) and O(g), this generates more Cl(g) that can react with more ozone! Overall these 3 equations can be combined (eliminate the like species) to give the following reaction: In this catalytic mechanism, 1Cl destroys 50 O 3 on a spring day over Antarctica * Course Pack!

14 18.4 Chemistry of the Troposphere 99% of the Troposphere is made up of N 2 and O 2, however minor constituent gases can have a profound effect on the Troposphere Many of the minor constituents are present naturally However Human activity, primarily combustion, can increase the concentration of these constituents causing adverse effects.

15 Acid Rain Oxides of Sulfur and Nitrogen cause acidification of rain Sulfur Compounds Sulfur Compounds occur naturally due to decay of organic matter and volcanic activity, however Human activity accounts for an approximate three fold increase in Sulfur compounds, primarily sulfur dioxide (SO 2 ) Nitrogen Compounds Nitrogen Compounds, primarily Nitrogen monoxide (NO), also occur naturally, being formed due to lightening. However NO(g) can also be formed during hydrocarbon combustion (transportation), and in Industrial processes involving fossil fuel combustion.

16 The high temperatures associated with these processes results in NO(g) Rainwater is naturally slightly acidic, due to the formation of carbonic acid (H 2 CO 3 ) from carbon dioxide and water

17 The ph of freshwater sites around the US Don t think the West coast is OK: Rain fall and fog near LA has been measured to have a ph less than 2! Effects of Acid Rain Acid Rain causes the acidification of freshwater

18 Acid Rains attacks metals and stone (calcium carbonate) This stone statue shows evidence of acid rain damage Acid rain also damages forests acid rain causes forest soils to lose valuable nutrients such as calcium and magnesium. It also increases the concentration of aluminum in the soil, which interferes with the uptake of nutrients by the trees

19 Sulfur Powdered limestone (CaCO 3 ) can be used to remove SO 2 from the gases formed by combustion of coal and oil

20 Carbon Monoxide Carbon monoxide (CO) is formed by the incomplete combustion carbon compounds and is present in unpolluted air at levels of 0.5ppm. CO is relatively unreactive and has little effect on vegetation and materials.

21 Photochemical Smog What we recognize as smog, the brownish haze that hangs above major urban areas is largely NO 2, nitrogen dioxide NO 2 (g) is formed by oxidation of the NO(g) produced by the reaction of nitrogen and oxygen at the very high temperatures in combustion engines.

22 Nitrogen oxides and ozone are just some of the components of photochemical smog Catalytic Convertors use precious metals Rh and Pt to catalyse the conversion of NO and CO to CO 2 and N 2

23 Global warming and the Greenhouse effect The greenhouse effect describes the trapping of infra-red (IR) radiation (thermal energy) emitted from the earth, by gases, primarily carbon dioxide and water vapor in the atmosphere. This trapped heat is re-emitted in all directions, some towards the earth. The effect gives the earth thermal consistency.

24 IR is only absorbed by molecules with polar bonds All molecules vibrate with a specific energy, the vibrations of molecules possessing polar bonds causes the dipole moment to change during the course of vibration The excited molecules lose the excess energy through collisions with other molecules Although the level of water vapor in our atmosphere has not changed appreciably the levels of other greenhouse gases, especially CO 2 have increased since the 1900 s. Greenhouse gases include all molecules with polar bonds, CH 4, NO x, CFCs etc

25 Carbon Dioxide Carbon dioxide is naturally present in the atmosphere, the carbon cycle, describes the mechanism of how carbon is transferred to and from the atmosphere. Details in the course pack US sources of CO 2 electricity production industry transport residential heating Increase in Greenhouse gas emissions disrupt the earth s delicate thermal balance and causes Global Warming

26 Global Warming Increases in greenhouse gases have caused the earths surface temperature to increase abnormally over the past 150 years This graph (from Wiki) plots the temperature change during this time period The small increase may seem insignificant but has had very noticeable effect on global weather patterns with related adverse consequences. 9 out of the 10 hottest years recorded occurred during 1994 to 2004 Signs of global warming *More extreme weather Total annual rainfall increased in the last century, however, typically dry areas north and south of the equator became even drier, while cooler climates became wetter, hurricanes and storms have become more frequent

27 Shorter Winters Over the last 3 decades winters have become 11 days shorter (on average) and warmer, there are fewer frost days Warmer BC winters are failing to control the population of the mountain pine beetle Mosquito borne diseases such as malaria, dengue fever and West Nile virus are becoming prevalent in areas where they were previously not viable Melting Ice cover Antarctica OK? Contains 90% of the worlds ice. Glaciers, Arctic and Greenland all in danger, changing fragile eco-systems

28 Warmer oceans and rising sea levels A warmer ocean is the dominant factor in rising sea levels. Water is most dense at 4 o C. Sea levels are predicted to rise between 15 and 95cm over the next 100 years. The Pacific island of Tuvalu is already falling victim to rising sea levels Warmer oceans are also killing coral and threatening sea-life A quarter of the worlds coral has disappeared and another 30% is expected to be gone in 30 years Warmer water contains less dissolved oxygen stressing fish populations and adversely effecting marine life