Week 8, Lesson 1 Chapter 18 – The Atmosphere
The Atmosphere Supports Life • The small fraction of the Earth’s environment that supports life is known as the biosphere. • The atmosphere consists of a mixture of gases extending about 100km above the Earth’s surface. • The troposphere is the layer closest to the Earth. • The interactions that occur in the upper layers of the atmosphere are mostly influenced by the high-energy radiation from the Sun. • The chemistry in the troposphere, however, is significantly affected by human activity on Earth such as deforestation and the release of millions of tonnes of waste gases from industrial processes. • The various layers of the atmosphere have very specific roles in maintaining life in our environment.
The Atmosphere & Radiation • High-energy UV radiation from the Sun is damaging to living organisms. • Ozone gas can absorb some radiation and so acts as a protective filter. • In the stratosphere, within a band 10-30km above the Earth’s surface, ozone occurs at concentrations significantly higher than in the lower regions of the atmosphere. • This region is known as the ozone layer. • Ultraviolet radiation is classified as UV-A, UV-B and UV-C. • Ozone absorbs all UV-C, most UV-B but little UV-A.
The Atmosphere & Radiation cont… • In the ozone layer, high energy UV radiation from the sun causes the double bond of O2 to break to form highly reactive oxygen atoms. • Some of the reactive oxygen atoms can then combine with other oxygen molecules to form ozone molecules. • Overall, with the effect of human activity, the rate of formation of ozone is about the same as its rate of depletion, so the concentration of ozone in the stratosphere remains fairly constant.
The Atmosphere & Climate • The Earth has maintained its average temperature over time because of the balance between the radiation received for the Sun and that reflected back into space. • Radiation of shorter wavelength – visible and UV radiation – can pass through the atmosphere from the Sun to the Earth’s surface. • The Earth then radiates back lower energy, longer wavelength infrared radiation.
The Atmosphere & Climate cont… • Gases in the troposphere, such as CO2, H2O, and CH4, absorb some of the reflected infrared radiation before it can reach space and reradiate some of it back towards Earth as heat. • This process is known as the greenhouse effect because it warms the Earth’s surface in the same way the Sun’s energy is used to heat a greenhouse.
The Atmosphere & the Water Cycle • The Earth’s weather patterns are mainly determined by the amount of water vapour in the air. • If it were not for the evaporation of water and its constant cycling as rain and snow back over the land masses of the world, we would have no fresh water supply to sustain life of Earth.
The Atmosphere & Essential Gases • Nitrogen and oxygen are the two most abundant gases in the atmosphere, making up 99% of clean dry air at sea level. • Both of these as well as much less abundant CO2, are essential in maintaining life on Earth.
Oxygen and Carbon Dioxide • Oxygen which makes up almost 21% of the atmosphere is required for respiration in plants and animals. • In respiration, glucose reacts with oxygen to produce carbon dioxide and water. • Respiration is the major energy producing process of most living things.
Oxygen and Carbon Dioxide cont… • Carbon dioxide makes up only 0.035%, by volume, of the atmosphere. • However, this small amount provides, either directly or indirectly, all food used by plants and animals. • In photosynthesis, plants combine carbon dioxide and water, in the presence of green pigment chlorophyll and sunlight to produce glucose and oxygen.
Oxygen and Carbon Dioxide cont… • Through the process of photosynthesis and respiration, both oxygen and carbon dioxide are cycled in nature. • The process, called carbon-oxygen cycle. • Photosynthesis in plants is the most significant process that consumes carbon dioxide, which is then released into the atmosphere during respiration in plants and animals.
Oxygen and Carbon Dioxide cont… • The cycling of both oxygen and carbon dioxide in the processes of photosynthesis and respiration is quite rapid. • Carbon dioxide is also returned to the atmosphere when an organism’s wastes are decomposed or when it dies. • This is a much slower recycling process.
Oxygen and Carbon Dioxide cont… • Other processes that contribute to the carbon-oxygen cycle include: • Volcanic action and and the combustion of fossil fuels, which add carbon dioxide to the atmosphere. • The slow production of coal, oil and natural gas over a period of several million years from the remains of dead organisms, and the formation of calcium carbonate from dead coral and the skeletons of marine animals in the sea. This process removes carbon from the cycle.
Oxygen and Carbon Dioxide cont… • The amount of carbon present at any particular stage of this cycle depends on the rate at which it enters and leaves that stage. • At present, atmospheric carbon dioxide levels are increasing dur to various factors including the rate at which we are burning fossil fuels.
Nitrogen • All living creatures need a source of nitrogen in order to produce essential large biomolecules such as proteins and DNA. • Most living things cannot use it in nitrogen gas form. • Some microorganisms break down the triple covalent bond within the nitrogen molecule to form simple nitrogen containing ions such as ammonium or nitrate. • These can be used by plants to make more complex nitrogen containing molecules. • Animals rely on these complex molecules as their source of nitrogen.
Nitrogen cont… • A small amount of nitrogen fixation occurs when lightening discharges convert nitrogen gas to nitrogen (II) oxide gas (NO). • This is released by volcanic action and is present in industrial and automobile emissions. • It reacts with oxygen to form nitrogen(IV) oxide gas, or nitrogen dioxide • Nitrogen dioxide reacts with rain water to form a mixture of nitrous acid and nitric acid. • The rain becomes as mildly acidic fertiliser. • Nitrogen is returned to the atmosphere when bacteria and other organisms in the soil cause organic matter to decay. • Some stages of the nitrogen cycle have been significantly affected by industrial and agricultural activities.
Human Activities and the Atmosphere • The rate of change in composition of the atmosphere has increased dramatically in the past 200 years as a result of human activity. • Contributing factors include: • Clearing of forests to make way for agriculture and so reducing the amount of carbon dioxide taken up by photosynthesis. • Burning of coal to produce electricity, which converts sulfur on coal to sulfur dioxide, which is released into the atmosphere. • Using fossil fuels in transport, which results in significant emissions of carbon and nitrogen oxides and unburnt hydrocarbons. • Using propellants in aerosols and foaming agents, which interfere with the balance of atmospheric gases.