Commecs Perspective Patron Cdr (Retd) Abdul Razaq PN Convener Dr. Uzma Naveed Incharge Rohana Tariq February 2018 Topic: Troposphere-II Chemistry of Troposphere Quratulain Imran Lecturer – Chemistry Atmosphere has been separated into four regions, very different in structure, thermodynamics, photo- chemistry and dynamics. This partition is best reflected by the atmospheric vertical temperature profile, whose points of inflection are used to distinguish the four regions. Figure: The atmospheric temperature profile (US Standard Atmosphere, 1976). To the four regions correspond very different temperature gradients. Starting from the ground, they are called ‘troposphere’, ‘stratosphere’, ‘mesosphere’ and ‘thermosphere’, and the boundaries which separate them are termed as ‘tropopause’, ‘stratopause’ and the ‘mesopause’. Page | 1 The atmospheric thermal structure is ultimately defined by a combination of dynamic and radiative transfer processes. The troposphere is heated from the ground, which absorbs solar radiation and releases heat back up in the infrared. The temperature of the air in this region therefore decreases linearly with altitude. The tropopause, situated between 8 km (at high latitudes) and 15 km (at the equator), marks the end of this linear decrease and the beginning of the stratosphere, where lies the bulk of atmospheric ozone (the ‘ozone layer’). The presence of ozone is vital for life on Earth, as it absorbs the dangerous part of incoming ultra-violet radiation. As a result, the stratosphere heats up and has a positive temperature gradient. The temperature peaks at the stratopause at approximately 50 km in altitude, then falls linearly again in the mesosphere, as ozone heating diminishes. The region of the atmosphere above the mesopause is called the thermosphere and is radically different from the three lower regions. It cannot be treated as an electrically neutral medium because energetic solar radiation ionizes the molecules and atoms to form a plasma of free electrons and ions that interact with the Earth’s magnetic field. The word troposphere means ‘turning sphere’, which symbolizes the fact that, in this region, convective processes dominate over radiative processes. The troposphere is indeed marked by strong convective over-turnings, whereby large parcels of warm air travel upwards to the tropopause, carrying water vapor and forming clouds as they cool down (the stratosphere, on the other hand is a very stable a stratified environment where heat transfer is mainly radiative). The troposphere contains the bulk of atmospheric water vapor, the majority of clouds and most of the weather, both on a global and a local scale. Because pressure decreases exponentially with altitude, it also contains over 75% of the total mass of the atmosphere. Most importantly, however, it is in contact with the Earth’s surface and therefore interacts directly with other climate subsystems, such as the biosphere (the land and vegetation), the hydrosphere (the oceans), the cryosphere (the ice caps), the lithosphere (the topography), and most all, with the human world. Chemistry of Troposphere: Nowhere around the Earth is the air perfectly clean: besides nitrogen, oxygen, inert gases, carbon dioxide and water vapor, it contains many trace pollutants. Be they emitted by natural or anthropogenic sources, they heavily influence our climate system. Natural sources include volcanoes eruptions, swamps, wild animal emissions, forest fires and dust, while anthropogenic ones include industrial activities, fossil fuel burning, car usage, emissions from domestic animals and agriculture. Fortunately, the atmosphere has up to now avoided any substantial accumulation of pollutants, thanks to a remarkable natural ability to cleanse itself. There are three end removal processes. The first is chemical conversion to non-polluting constituents, such as H2O or O2. The second is dry deposition, whereby gases are absorbed by plants, water or soil. It is of limited significance because it often only applies to gases in the boundary layer on a local scale. The third is wet deposition, or removal by precipitation, and is only effective for species that have enough solubility in water, which is not the case in general. There are, however, a number of tropospheric species capable of oxidizing these pollutants so that they become soluble. Although these species are only present in minute amounts, they constitute the pivot of tropospheric chemistry. Ironically, the discovery of the oxidizing capacity of the troposphere came relatively late and through indirect reasoning. In 1970, Pressman and Warneck noted that, although the emission of CO had been steadily increasing over the 50's and 60's, there was no repercussion on its tropospheric concentration. The puzzle was solved a year later when Levy (1971) found a route for the formation of OH radicals in the troposphere and suggested that they could be a major sink for CO. The importance of OH radicals Page | 2 and other oxidants as the detergents of the atmosphere has been recognized ever since. They are, in descending order of importance: 1. The hydroxyl radical OH. Hydroxyl is a short-lived free radical and by far the most effective scavenger in the troposphere. It is the main oxidant for CO, CH4 and higher hydrocarbons, H2S (hydrogen sulfide) and SO2 (sulfur dioxide). 2. The nitrate radical NO3. At night, this radical takes over from hydroxyl as the dominant oxidant in the atmosphere: hydroxyl is formed by photolysis and its concentration peaks during daytime while NO3 does not survive sunlight. 3. The oxygen atom. The exited state of the oxygen atom has the ability to oxidize unsaturated hydrocarbons and other gases containing a double bond such as CS2 and COS in the upper troposphere. 4. Peroxy and hydroperoxy radicals HO2 and RO2 (where R is an alkyl). HO2 and RO2 are very much intertwined with hydroxyl in the oxidation cycle. They are not as efficient as hydroxyl, but react with themselves to form H2O2, an important oxidant in cloud droplets. 5. Hydrogen peroxide H2O2. This strong acid reacts very efficiently in cloud droplets and oxidizes a number of trace gases, in particular sulfur dioxide. Highly soluble, it also accounts for a large part of the excess acidity in rain. Together, these oxidants determine the lifetime and the abundance of trace species, acting as a atmospheric regulators. The reverse is also true: the abundance of trace species regulate the oxidizing capacity of the atmosphere, since an increase in the emission of a given pollutant reduces the abundance of its principal oxidant. The resulting positive feedback may even eventually lead to an increase of other pollutants. Role of Different Chemicals in Tropospheric Chemistry: This table (below) describes some of the chemicals in the troposphere, and some of the chemical reactions that happen in the air Chemical Formula Role in Tropospheric Chemistry Carbon dioxide Carbon dioxide is a kind of greenhouse gas. When we breathe, CO2 we take in oxygen and breathe out carbon dioxide. Plants and some kinds of microbes use carbon dioxide during photosynthesis to make food. Burning fuels also puts carbon dioxide into the atmosphere. Carbon monoxide CO When things burn, they mostly make carbon dioxide. Sometimes they make carbon monoxide, too. Carbon monoxide is a poisonous gas. Volcanoes and engines make carbon monoxide. Hydrocarbons CxOy Hydrocarbons are chemicals made up of hydrogen and carbon atoms. When fuel burns, it puts some hydrocarbons into the air. Hydrocarbons help to make smog, a kind of air pollution. Methane CH4 Methane is a kind of greenhouse gas. Nitrogen N2 Most of the gas in Earth's atmosphere is nitrogen. About 4/5ths of the air is nitrogen. The nitrogen cycle explains how nitrogen moves around in the environment. When fuel burns hot, like it Page | 3 does in the engine of a car, nitrogen combines with oxygen to make nitrogen oxides. Nitrogen Oxides NO & NO2 Nitrogen oxides are a kind of pollution. Burning fuels like gasoline in air makes nitrogen oxides. Most nitrogen oxides come from cars and trucks. They help to make smog. They also mix with water droplets in the air to make nitric acid. Nitric acid is a part of acid rain. Nitric Acid HNO3 Nitric acid is part of acid rain. Nitric acid forms when nitrogen oxides mix with water droplets in the air. Nitrogen oxides are a kind of pollution that comes from the engines of cars and trucks. Oxygen & Ozone O2 & O3 About 1/5th of the gas in the atmosphere is oxygen. When you breathe, your body uses the oxygen to keep you alive. Ozone is a special kind of oxygen that has three atoms instead of two. PAN(Peroxyacytyl C2H3O5N PAN is a kind of air pollution. Smog has PAN in it. PAN forms nitrate) when nitrogen dioxide, oxygen, and Volatile Organic Compounds (VOCs) get together. Smog Smog is a mixture of smoke and fog. Photochemical smog is a kind of air pollution. It has nitrogen oxides, ozone, VOCs, and PAN in it. Photodissociation When a photon of sunlight breaks apart a molecule Sulfur Oxides SO2 & SO3 Sulfur dioxide and sulfur trioxide are types of pollution. People make them when we burn coal and oil. Volcanoes also give off sulfur oxides. Sulfur dioxide mixes with water droplets in the air to make sulfuric acid. Sulfuric acid is in acid rain. Sulfuric Acid H2SO4 Sulfuric acid is in acid rain. Sulfuric acid in the air is made when sulfur dioxide gasmixes with water droplets. The sulfur dioxide gas comes from volcanoes and from coal and oil that people burn for fuel. Photochemical Smog Ayesha Bibi Lecturer– Chemistry As scorching hot days continue this summer, heatstroke and heat exhaustion have sent record numbers of people to hospitals. Drink lots of water, stay indoors and use air-conditioning, doctors say. However, if you have irritated eyes or a sore throat, or feel dizzy or nauseated after being outside on a hot, windless day, you may be suffering from something very different.