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Development Team Paper No: 8 Atmospheric Processes Module: 12 Atmospheric Chemistry Development Team Principal Investigator Prof. R.K. Kohli & Prof. V.K. Garg &Prof. Ashok Dhawan Co- Principal Investigator Central University of Punjab, Bathinda Dr. Sunayan Saha Paper Coordinator Scientist, ICAR-National Institute of Abiotic Stress Management, Baramati, Pune Dr. Puneeta Pandey Content Writer Central University of Punjab, Bhatinda Dr. Amrita Daripa Content Reviewer ICAR-National Bureau of Soil Survey and Land Use Planning, Nagpur, Maharashtra 1 Anchor Institute Central University of Punjab Atmospheric Processes Environmental Sciences Atmospheric Chemistry Description of Module Subject Name Environmental Sciences Paper Name Atmospheric Processes Module Name/Title Atmospheric Chemistry Module Id EVS/AP-VIII/12 Pre-requisites Basic knowledge of elementary physics and chemistry Understand the structure of the atmosphere and its thermal stratification Know the chemical composition of the atmosphere Objectives Know the thermo-chemical and photochemical reactions occurring in the atmosphere Understand the mechanism of formation of smog, its reactions and effects Keywords Atmosphere, Electromagnetic spectrum, Thermochemical, Photochemical, Smog 2 Atmospheric Processes Environmental Sciences Atmospheric Chemistry TABLE OF CONTENTS 1. Aim of the Module 2. Introduction 3. Chemical composition of the earth atmosphere 3.1 Nitrogen (N2) 3.2 Oxygen (O2) 3.3 Argon (Ar) 3.4 Carbon Dioxide (CO2) 3.5 Trace Elements 4. Thermochemical reactions in atmosphere 4.1 Electromagnetic spectrum 4.1.1 Gamma rays 4.1.2 X-rays 4.1.3 Ultra-Violet light 4.1.4 Visible light 4.1.5 Infrared light 4.1.6 Microwaves 4.1.7 Radiowaves 4.2 Reactions taking place in earth’s atmosphere 4.2.1 Troposphere 4.2.2 Stratosphere 4.2.3 Mesosphere 4.2.4 Thermosphere 5. Photochemical reactions in atmosphere 5.1 Smog formation 5.1.1 Hydrocarbons and photochemical smog 5.1.2 Photochemical reactions of methane 5.1.3 Mechanism of smog formation 5.1.4 Nitrate radical 5.1.5 Photolyzable compounds in the atmosphere 5.1.6 Inorganic products from smog 5.2 Effects of smog 5.2.1 Human health and comfort 5.2.2 Damage to materials 5.2.3 Effects on the atmosphere 5.2.4 Toxicity to plants 6. Summary 3 Atmospheric Processes Environmental Sciences Atmospheric Chemistry 1. Aim of the Module After going through this module, you shall be able to: Understand the structure of the atmosphere and its thermal stratification Know the chemical composition of the atmosphere Know the thermo-chemical and photochemical reactions occurring in the atmosphere Understand the mechanism of formation of smog, its reactions and effects 2. Introduction The earth’s atmosphere is a mixture of gases and aerosols; some of which may be in rather fixed proportions throughout the atmosphere; while, some may vary in proportion depending on altitude and region. The layer of the atmosphere in which its gaseous composition is generally uniform (does not change with altitude) is called as ‘homosphere’ and extends up to an altitude of 80 km. At altitudes beyond 80 km, the chemical constituents of air change significantly with height and hence that layer is known as ‘heterosphere’. Nitrogen (N2) by far is the most abundant of all gases present in earth's atmosphere. About 3.5 times less than that of N2 is the quantum of oxygen (O2) gas in atmosphere and these two gases comprise about 99 % of dry air volume. Within the rest about 1 %, many different gases and non gaseous constituents are accommodated and some of which like CH4 and CO2 are known to produce huge influence on our planet through chain of events starting with the rise of temperature. The nature of chemical interaction among these atmospheric constituents under the ambience of solar light and or earth radiation in different layers of the atmosphere is what comprises the subject matter of this “atmospheric chemistry” module. 3. Chemical composition of the earth atmosphere Principle gases composing the earth’s atmosphere is given in Table 1 followed by their brief description. 4 Atmospheric Processes Environmental Sciences Atmospheric Chemistry Table 1: Gaseous composition of earth’s atmosphere (Battan, 1984) S. No. Constituents Percent by volume of Concentration dry air (ppm) of air 1 Nitrogen 78.08 780800 2 Oxygen 20.94 209400 3 Argon 0.934 9340 4 Neon 0.00182 18.2 5 Helium 0.000524 5.24 6 Methane 0.00015 1.5 7 Krypton 0.000114 1.14 8 Hydrogen 0.00005 0.5 9 Important Variable gases A Water vapour 0-5 B Carbon Dioxide (CO2) 0.034 340 C Carbon Monoxide --- <100 (CO) D Sulfur Dioxide (SO2) --- 0-1 E Nitrogen Dioxide --- 0-0.2 (NO2) F Ozone (O3) --- 0-10 3.1. Nitrogen (N2) Nitrogen gas constitutes 78% by volume of atmosphere. Nitrogen in its native form is inert; however, it can be converted into nitrites and nitrates during nitrogen cycling. Ammonification and denitrification during this cycle converts these nitrogenous compounds back to ammonia and then 5 Atmospheric Processes Environmental Sciences Atmospheric Chemistry nitrogen; which eventually reaches back to atmosphere. The residence time of nitrogen in atmosphere is 100 million years. 3.2. Oxygen (O2) It occupies 21% by volume of atmosphere. It plays a major role in respiration and combustion. The turnover time of CO2 in the atmosphere is 3000 years. 3.3. Argon (Ar) It is an inert gas that occupies about 1% by volume of atmosphere. 3.4. Carbon Dioxide (CO2) This gas occupies 0.038% (376 ppm) of the atmosphere. The concentration of this gas has been rising since the last century due to enhanced anthropogenic activities. It is also a potent greenhouse gas since it traps long-wave solar radiations. The turnover time of O2 in the atmosphere is 4 years. 3.5. Trace Elements This includes various elements and compounds in varying proportions such as Water Vapour (H2O), Sulfur Dioxide (SO2), Nitrogen Dioxide (NO2), Carbon Monoxide (CO), Ozone (O3), Chlorofluorocarbons (CFCs), Methane (CH4), dust and other particulate matter. Water vapour is also a potent greenhouse gas, the constitution of which varies in the atmosphere with a residence time of 11 days. SO2 and NO2 are by-products of fossil fuel combustion and automobile exhausts. They can lead to formation of nitric acid and sulfuric acid in the atmosphere, thus contributing to ‘acid rain’. Ozone gas lies in highest concentration in stratosphere and plays the vital role of absorbing ultraviolet radiations; in troposphere, it acts as a secondary pollutant causing respiratory ailments and eye irritations. CFCs cause depletion of the ozone layer in the stratosphere; hence, it is of prime environmental concern. CH4 is another significant greenhouse gas which is even more potent than CO2. Dust and other particulate matter are airborne solids suspended in air; also known as ‘Aerosols’. Aerosols play an important role in scattering and reflecting solar radiations, thus affecting the albedo. Major sources include sea salt from evaporated sea spray, wind-blown dust, debris from volcanoes and 6 Atmospheric Processes Environmental Sciences Atmospheric Chemistry fires, and from anthropogenic sources. Particulates can also act as cloud condensation nuclei (CCN) onto which water vapor condenses, thus affecting the rainfall pattern. 4. Thermochemical reactions in atmosphere 4.1. Electromagnetic spectrum The earth’s radiation budget is regulated by regular input of solar energy. At a temperature of about 6000K, the sun radiates an enormous amount of energy, but the earth intercepts only about 5x10- 10 of it since the earth subtends a small angle when viewed from the sun. The average flux of the solar radiation at outer limit of earth’s atmosphere, falling on a surface perpendicular to the incoming rays, is called solar constant. All electromagnetic waves travel at the speed of light i.e. 300,000,000 metres per second. Various spectra of electromagnetic radiations are described below: 4.1.1. Gamma rays Gamma rays are high frequency waves, carrying a large amount of energy given off by stars, and some radioactive substances. Gamma rays are used in ‘Radiotherapy’ to kill cancer cells. Figure 1: Electromagnetic Spectrum (www.earthobservatory.nasa.gov) 4.1.2. X-rays X-rays are very high frequency waves, and carry a lot of energy. They are useful in medicine and industry to see inside things since they can pass through most substances. 7 Atmospheric Processes Environmental Sciences Atmospheric Chemistry 4.1.3. Ultra-Violet light It is emitted by sun and also made by special lamps. UV light is used in sterilizing microbial contamination in surgical equipments, operation theatres and laboratories; detecting forged bank notes in shops, and hardening some types of dental filling. 4.1.4. Visible light Visible light lies in the range of 0.4-0.7µm and includes radiations that can be perceived by human eye. White light is made up of various wavelengths ranging from violet to red (the colours of rainbow). 4.1.5. Infrared light Infra-red waves are released as heat, because they're given off by hot objects, and can be felt as warmth. Infra-Red waves are also given off by stars, lamps, flames and anything else that's warm, i.e., which has temperature above absolute zero (273K). Infra-red waves are used for remote controls for TVs and video recorders, to help heal sports injuries by physiotherapists, in burglar alarm systems, and for weather forecasting by infrared satellite data. 4.1.6. Microwaves They are basically extremely high frequency radio waves with wavelength ranging from 1mm to 1m. They find application in cooking, mobile phones, traffic speed cameras, and for radar, which is used by aircraft, ships and in weather forecasting. 4.1.7. Radiowaves These are the lowest frequencies in the electromagnetic spectrum, and are used mainly for communications.
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