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Environmental Chemistry MODULE No. 18: Composition of Atmosphere

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Environmental Chemistry MODULE No. 18: Composition of Atmosphere ____________________________________________________________________________________________________ Subject Chemistry Paper No and Title 4, Environmental Chemistry Module No and Title 18, Composition of Atmosphere Module Tag CHE_P4_M18 CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Composition of earth’s atmosphere 3.1 Particles: formation of particulate matter, physical and chemical properties and its effects 3.2 Ions and radicals with reactions: hydroxyl and hydroperoxy in atmosphere and reaction with trace gases. 4. Summary CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere ____________________________________________________________________________________________________ 1. Learning Outcomes-: After studying this module, you will: 1. Be able to differentiate between air and atmosphere. 2. Know about chemical composition of earth’s atmosphere. 3. Know the physical and chemical properties of particles, ions and radicals. 4. Know the effect of particles, ions, and radicals. 5. Understand the reactions of ions and radicals. 6. Know the variation in earth’s atmosphere with altitude in various levels of atmospheric layers. 2. Introduction The atmosphere of the earth is the blanket of surrounding gases maintained in its place by earth’s gravity. Atmospheres of all the planets of our solar system are different but some planets have similarities between their atmospheres. This includes the four innermost planets Mercury, Venus, Earth and Mars (the ‘terrestrial’ planets) and the four outer planets Jupiter, Saturn, Uranus and Neptune (the ‘gas giant’ planets). 3. Composition of earth’s atmosphere The earth’s atmosphere is composed of air. Nitrogen, Oxygen, and Argon, constitute the major gases of the air. This is the mixture of gases, water vapour and particles. Fig1. Composition of atmosphere CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere ____________________________________________________________________________________________________ The chemical composition of the earth’s atmosphere varies with altitude, temperature and pressure profile of the different regions of atmosphere and also due to the penetration of incoming solar radiation. (Fig 2) Fig2. Different regions of the atmosphere 3.1 Particles Particles are very important constituent of the atmosphere. They are small sized solid and liquid matter in the atmosphere of earth. The number of particles per unit volume of air varies from several hundred per cubic centimetre in pure air to more than 105 per cubic centimetre in polluted air. Their size is in the range of 0.1-10µ.The particulate mass level ranges from10 µg/m3 in clean air to 60-2000 µg/m3 in the polluted urban areas. Colloidal sized solid and liquid matters present in the earth’s atmosphere are known as aerosols. Particles of natural origin with diameter less than 0.2 µ are called Aitken particles or Aitken nuclei, after a pioneer of aerosol science, John Aitken. The sources of aerosols define their composition. Wind-blown mineral dust comes from mineral oxides and other material blown from the earth’s crust; this particulate is light-absorbing. The second largest contributor of global aerosol is Sea salt and consists mainly of sodium chloride originating directly from the ocean (sea spray); other constituents of sea salt reflect the composition of sea water, and thus include magnesium, sulphate, calcium, potassium, etc. sea spray aerosols contain both inorganic as well as organic compounds, which affect their chemistry. The particles which occur naturally in the atmosphere are bacteria, fog, pollen and volcanic ash. CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere ____________________________________________________________________________________________________ The size and chemical nature of the particles are more important than the particulate material present in the earth’s atmosphere. 3.1.1 Formation of particulate matter 1). Inorganic particulate matter-: Metal oxides form a major group of inorganic particles in the atmosphere. They originate from fuel combustion. Iron oxide particles, for example, originate in the combustion of pyrite- containing coal. Calcium oxide particles also come from coal combustion. Part of the CaCO3 in the ash fraction of coal is converted to calcium oxide and discharged through the stack. Combustion of leaded gasoline in automobiles releases lead and lead halide particles in the exhaust. Aerosol mists are formed from sulphuric acid, which in turn is obtained by oxidation of sulphur dioxide which collects vapour to form small liquid droplets. In the presence of basic air pollutants such as ammonia or calcium oxide, salts are formed. Table -1. Some constituents of inorganic particulate matter and their origins CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere ____________________________________________________________________________________________________ 2). Organic particulate matter-: Organic particulate matter is organic in nature and is obtained from number of sources such as emission from vegetation, automobiles, combustion of fuels etc. It has been estimated that the average formula for the organic particulate matter is C32H48O3.8S0.083X0.06 (X = halogen) and a size ranges up to 1µ. Such particulate matter is a potential health hazard. One class of components of organic particulate matter, polycyclic aromatic hydrocarbons (PAH) have carcinogenic effect. They commonly occur in upper atmosphere at about 20 µg/m3 level. They originate from the pyrolysis of higher paraffin’s present in fuels and plant material. High molecular weight compounds are pyrolysed during combustion to yield C10H22, which undergoes further pyrolysis: The PAH compounds are usually adsorbed on soot particles. Soot is produced on combustion of fuel in automobiles and thermal power plants. In urban areas it accounts for half of the total particulate load. The hydrogen content of soot is 1-3% and oxygen content about 5-10% due to partial surface oxidation. Soot acts as a carrier for toxic organics due to its large surface area. The size range of these particles is in between 0.1 – 20µ. 3.1.2 Physical and Chemical Properties of Particulate Matter (PM) The size and shape of particles comes under physical properties. These particles include liquid droplets, regular or irregular shaped crystals or aggregates of odd shape. Particles are often classified by size, e.g. the particles having diameter less than and up to 10µm are named as PM10 and PM2.5 refers to those of diameter up to 2.5µm. • The chemical composition of these particles vary a lot they can be dilute water solution of acids or salts or can be organic liquids and even materials present in the earth's crust (dust) or soot (unburned carbon) and even toxic metals. • Optical properties such as light extinction, absorption and scattering help in determination of dust, smoke and haze visibility. Physical and chemical properties of ambient PM, in turn, decide their optical properties • Particles produced by any PM source have specific physical, chemical and optical properties. Hence PM should be viewed as containing several pollutants since each type of aerosol has its unique properties and sources and different control is required. 3.1.3 Effects of Particulate Matter • They affect the atmosphere by forming cloud and fog • They help in balancing the temperature of the atmosphere on earth through light reflection. • They serve as nuclei for the development of ice crystals and water droplets. • They provide a surface for many atmospheric reactions like neutralization of water droplets, oxidation reactions (e.g. catalytic effect of tiny particles of metal oxides), photochemical oxidation reaction etc. • Particulate matter is responsible for having adverse effects on human health, especially fine particles such as PM10 and PM2.5, which enter the human body through the respiratory tract (such particles are called Respirable Particulate CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere ____________________________________________________________________________________________________ Matter or RPM). PM10, when inhaled, penetrates deep into the lungs, bypassing the body’s respiratory filters and can give rise to chronic respiratory diseases such as bronchitis, bronchial asthma and pulmonary oedema. PM2.5 is more dangerous than PM10 as they can affect the exchange of gases inside the lungs by reaching the peripheral regions of bronchioles when inhaled. Air pollution due to particulate matter is recognized by WHO to be responsible for disease named chronic obstructive pulmonary disease and lung cancer, in addition to infections of the respiratory system. The removal of particulate matter from exhaust gases is an important strategy for air pollution control. 3.2 Ions and Radicals The upper layer of atmosphere known as ionosphere contain significant numbers of electrons and + + + positive ions (O2 , O , NO ). They persist for long durations without recombining to give neutral species due to rarefied conditions in the upper atmosphere. In the ionosphere the ions are primarily produced by ultraviolet light. At night, when there is no UV light, the positive ions present in
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