Chemistry Of The Atmosphere Lesson 8 PDF

Lesson 8: CHEMISTRY OF THE ATMOSPHERE UNIT 1: THE COMPOSITION OF THE ATMOSPHERE Introduction: Human beings can survive for couple days in the absence of water but this is not the same in the case of air. The air that we breathe is part of the atmosphere. In this lesson, the layers and compo...

Lesson 8: CHEMISTRY OF THE ATMOSPHERE UNIT 1: THE COMPOSITION OF THE ATMOSPHERE Introduction: Human beings can survive for couple days in the absence of water but this is not the same in the case of air. The air that we breathe is part of the atmosphere. In this lesson, the layers and composition of the atmosphere is revisited. Air and the Atmosphere Air is the term used to describe the homogeneous mixture of gases that makes life on earth sustainable due to its dioxygen molecule component. While the atmosphere refers to the thin air that is made up of a low-density fluid that extends few hundred kilometers from the surface of the earth thus surrounding the planet. The layers of the atmosphere The atmosphere is subdivided into different layers in accordance to their temperature profile Troposphere  is the lowest part of the atmosphere. It is where the planet Earth’s weather and air pollution takes place.  It extends from the ground level to 1- to 15 km. The temperature change within the troposphere with respect to altitude (known as lapse rate) decreases at a rate of 5 to 6 K/km.  An unwanted phenomenon may occur in the troposphere, known as thermal inversion. This happens when the lapse rate becomes positive; hence, the reversal of the normal behavior in the troposphere wherein the cold air at the surface of the earth is overlaid by the warmer air. Thermal inversion changes the dynamics of air movement resulting in air pollutants to be trapped near the earth’s surface. Prolong occurrence can have several adverse effects on the population’s health such as lung inflammation. Tropopause  It is the layer of atmosphere of constant temperature found between the troposphere and the stratosphere. Stratosphere  It has a temperature profile that increases with the altitude up to a maximum of about 273K and it reaches at around 50 km from the surface of the earth.  The layer’s temperature profile is what suppresses air to undergo vertical motions. Stratopause  It is the layer of atmosphere of constant temperature found between the stratosphere and the mesosphere. Mesosphere  It is with a temperature profile that is similar to the troposphere, that is the temperature increases as the altitude increases until it reaches the mesopause at an altitude of 85 km from the surface of the earth.  In this layer, the gases are thick enough to slow down the meteors hurtling to the atmosphere, which when burn up leave fiery tails in the night sky. Mesopause  is the layer of atmosphere of constant temperature found between the mesosphere and the thermosphere. Thermosphere  with a lapse rate of 5K/km. Thermopause  is the layer of atmosphere of constant temperature found between the thermosphere and the exosphere. Exosphere  The outermost layer of the atmosphere.  It extends from the thermopause to about 10000 km from the surface of the earth.  It is in this layer where satellites orbit the earth. The components of the atmosphere The atmosphere is a homogeneous mixture of various gases. Its main composition has remarkably steady for ages with nitrogen and oxygen being the main components. It is the abundance number of oxygen molecules in the atmosphere that makes life possible on earth. UNIT 2: PROCESSES IN THE ATMOSPHERE Introduction: Each of the layers of the atmosphere has distinct characteristics and compositions that influence the processes occurring therein. In this lesson, chemical process that are occurring in the troposphere and stratosphere are discussed. Processes occurring at the troposphere The composition of the stratosphere is dependent on the properties of gases that are present at the troposphere and the processes they undergo. If the trace gases in the troposphere are inert and do not react with the available free radicals, they will be transported to the stratosphere. Examples of these gases are chlorofluorocarbons (CFCs) and methyl chloride. However, there are trace gases that readily react to free radicals and sometimes cause formation of unwanted compounds in the atmosphere.  The presence of these free radicals in the troposphere are crucial to the formation of compounds that are responsible to pollution. According to the work Hiram I.I. Levy entitled “Photochemistry of minor constituents in the troposphere”, the reaction of the OH radical with methane and nitrogen dioxide produces carbon monoxide and nitric acid, respectively. While dioxygen molecules are abundant in the atmosphere, they are not considered as the principal oxidative species in the troposphere. This is because reactions of dioxygen molecules require higher activation energy compared to that of the free radicals such as the hydroxyl radical, OH. Where are these free radicals come from? Major sources of OH is the ozone photolysis with ultraviolet (UV) radiation during daytime, producing excited oxygen atom which rapidly reacts with water vapor. During night time, the nitrite radical NO2 reacts with ozone (O3) to produce the nitrate radical NO3. Moreover, when the compound peroxyacetyl nitrate (PAN, CH3COOONO2) thermally decomposes in the presence of NO also produce OH radicals and convert NO2  Processes occurring at the stratosphere The stratosphere is where the ozone layer can be found. Stratospheric ozone protects life in the surface of the earth by screening the harmful ultraviolet rays from the sun through the photodissociation mechanism. The ozone in not consumed and the UV radiation is converted into heat. This is the reason behind the positive lapse rate profile of the stratosphere. Because the stratosphere is warmer compared to the troposphere, an inversion between the two layers occur. This traps the molecules found in the troposphere. On the other hand, O2 can be consumed if O collides with O3, the process known as the recombination reactions. This process is very slow and if it is the only means by which O3 is consumed, then the ozone layer will be thicker than it really is. Another way by which O3 is consumed is catalysis of the recombination reactions by available free radicals such as NO , NO2 , H , OH, HO2 , Cl , ClO and ClO2. The thickness of the ozone layer is therefore due to the competition among these three reactions. Aerosols It is not only the gaseous compounds that have impact to the atmospheric conditions. Solid and liquid particulates, known as aerosols, also have consequences in atmospheric temperature and climate because they have the ability to alter the radiative processes of the Earth. An example of which is the soot as a product from the burning of fossil fuels. UNIT 3: AIR POLLUTION, SOURCES AND EFFECTS Introduction: The atmosphere has its own ways of recreating the balance to make life sustainable for all its habitats. However, changes occur due the presence of pollutants in the atmosphere. In this lesson, we are to discuss the common compounds that pollute the air and how they do it. Air Pollutants Air pollutants are products and by-products of different process such as combustion, industrial activities, natural resource processing and commercial services. They can be classified as primary pollutants (those that are directly emitted to the atmosphere) and secondary pollutants (those that are products of various chemical reactions in the atmosphere). There are ten groups of air pollutants according to the US EPA (Environment Protection Agency) official Hazardous Air Pollutants (HAPs) list: 1. Carbon dioxide (CO2) and Carbon monoxide (CO) Carbon dioxide is produced from complete combustion of fuels and metabolic processes while carbon monoxide is derived from the incomplete combustions processes and from the photochemical transformation of volatile organic compounds (VOCs) by the free hydroxyl radical ( OH) generated in the atmosphere such as methane. Carbon dioxide is considered toxic because it can replace oxygen uptake by hemoglobin. 2. NOx: Nitrogen oxide (NO), nitrogen dioxide(NO2) and dinitrogen oxide (N2O) Nitrogen oxide is a product of combustion through the oxidation of air nitrogen at high temperature and can also be produced by lightning. It can also oxidize into nitrogen dioxide which is a precursor of acid rain and smog formation. Meanwhile, the dinitrogen oxide is released by soil bacteria and can be converted as nitrogen oxide into the atmosphere. 3. SOx: Sulfur dioxide (SO2) and sulfur trioxide (SO3) Sox are produced from the oxidation of sulfur-containing fuels and dihydrogen sulfide, H2S. H2S is a toxic gas that is produced from biological decay of organic matter and various human activities such as geothermal and industrial processes. 4. Photochemical oxidants Photochemical oxidants are considered as secondary pollutants as they are products of chemical reactions from other pollutants. The common photochemical oxidants are ozone (which can be produced through lightning or photochemical reactions), peroxybenzonyl nitrate (PBzN), peroxyacyl nitrate (PAN), hydroxyl radicals ( OH) and nitrogen oxide derivatives. In was seen from the previous lesson on how free radicals react with other compounds in the atmosphere. 5. Particulates and aerosols Particulates and aerosols are complex mixture of extremely small particles and liquid droplets. They are existing in different forms such as ocean salt crystals, soil particles, minerals and metallic compounds. These small particles may come from wild fires, volcanic eruptions, incomplete combustion, mineral processing, industrial processing among others. The two most common particulates are the PM2.5 and PM10. PM2.5 are those that has an average diameter od ≤2.5μ and is considered dangerous because they are often carcinogenic yet the body is not able to release them. Moreover, they are also active in smog formation and other photocatalytic activities. PM10, on the other hand, are those that have diameters within the range of 2.5μm ≤ d ≤10μm. 6. Metal and metalloid compounds and vapors These may consist of hazardous particulate matter or vapors of metals such as Pb, Hg, As, Ni, Be, and Cu that are generated by combustion and incineration processes, engine wear, lubricating oil components, lamp wastes, and mineralization of geologic formations. 7. Polycyclic aromatic hydrocarbons and derivatives (PAHs) These are aromatic compounds of at least three fused aromatic rings and are hazardous. They are products of incomplete combustion of fossil fuels and vegetable matters, petroleum products and petroleum refineries. 8. Volatile organic compounds (VOCs) These are light hydrocarbons and unsaturated hydrocarbons and are precursors to smog formation and organic aerosols. They usually come from incomplete combustion processes, fugitive sources, industrial processes, petroleum processing and solvent intensive processes such as painting. 9. Halogenated hydrocarbons and polychlorinated organic compounds Hazardous compounds volatile chlorinated hydrocarbons and polychlorinated dibenzo compounds dioxins and furans belong to this category. They usually come from incineration processes and car emission. The ozone layer destructing chlorofluorocarbons (CFCs) are also included in this category. 10. Radionuclides Their presence is due to natural gas or particulate emissions from natural deposits of uranium where radon is liberated, as well as emissions from uranium processing, nuclear reactors, and fuel-reprocessing wastes. Internal Engine Combustion Process Combustion of fossil fuel leads to the production of main primary pollutant CO, NOx, VOCs and SO2. The combustion equation of an internal engine that is ideally operating is given by the equation below: If the hydrocarbon (CnHm) to oxygen ratio is not adequate, unreacted hydrocarbons and partially oxidized carbon will be emitted to the atmosphere. Moreover, with an excess O2, NO production is favored as it reacts with a sufficient amount of nitrogen in the atmosphere: In addition, the sulfur content of the fuels generated sulfur dioxide upon burning: The Greenhouse Effect The greenhouse effect (Figure 1) is a natural process by which earth maintains the heat on its surface to make life sustainable. Solar radiation passes through the clear atmosphere wherein most of the radiation are absorbed by the earth’s surface which keeps its warmth. Some of the heat from the earth radiates towards space but some of it is trapped by the greenhouse gases in the atmosphere. This keeps the planet warm enough to sustain life. However, the concentration of greenhouse gases in the atmosphere is gradually increased. This is trapping more heat from earth resulting for the earth’s temperature to rise. The phenomenon known as the global warming. Climate change, on the other hand, refers to the effect of global warming to the winds and ocean currents in ways that can cool some areas and warm others. The greenhouse effect is primarily due to the augmented emissions of CO2, NOx, water vapor and other greenhouse gases into the atmosphere. Water vapor is the largest contributor to greenhouse effect because it absorbs infrared radiation. It helps in the maintenance of atmospheric temperature during night time when the earth’s surface is emitting radiation into space. The second major contributor is the carbon dioxide due to the increase in energy production. Although carbon dioxide is being absorbed by the oceans and is being used by plants during photosynthesis, its generation is much faster than being absorbed or used. Another main contributor is the methane production due to the increasing demand on rice production (methane is produced in rice pads) and the intensive livestock breeding. Other greenhouse gases include chlorinated alkyl compounds (such as CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). Photochemical Smog Formation Photochemical smog is a consequence of photochemical reactions based on primary pollutants discharged into the atmosphere. Smog has many deleterious effects on human health as well as on plants and materials. Nitrogen oxides and hydrocarbons emitted from vehicles with sunlight and oxygen produce pungent nitrogen dioxide and toxic ozone. NO and NO2 can also react with other free radicals that come from hydrocarbon emission after reacting with strong oxidizers. The Stratospheric Ozone Destruction Stratospheric ozone protects life on the surface of the Earth by screening harmful UV radiation coming from the sun through a photodissociation mechanism. However, anthropogenic generated chlorine and bromine compounds in the atmosphere are responsible in the decrease in the ozone concentration in the stratosphere. Chlorine and bromine compounds are converted into it more active forms – free radicals. The Acid Rain Under normal conditions, the pH of rain is slightly acidic at 5.7 due to the dissociation of dissolved carbon dioxide in water: However, in polluted environments, oxides of sulfur and nitrogen are also present which when dissolved in rain droplets, its pH goes below 5.7. This is how the formation of acid rain occurs. Sulfur dioxide (SO2) come from the combustion of sulfur-containing fossil fuels. It is oxidized in the atmosphere to form sulfur trioxide (SO3) which reacts with water to form sulfuric acid: On the other hand, nitrogen oxides form due to the presence of nitrogen and oxygen gases in air. These two goes on a series of chemical reactions to produce nitric acid, another component of acid rain. Acid rain oxidizes materials such as copper and iron. They also cause deterioration of marble and other carbonate materials due to the decomposition reaction: Volcanic eruptions are natural sources of SOx, while lightnings are natural sources of NOx and ozone. Combustion processes remain as the top anthropogenic sources of SOx and NOx. ACTIVITY NO.5 1-3) Give 3 effects of air pollution. 4&5) Give 2 main gases of the atmosphere. 6-8) Give 3 air pollutants 9-10) Give 3 layers of the atmosphere