13Feb

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Outline for Today’s Lecture: Ozone the good ozone (O3): definition, regulation, effects of the bad ozone (O3): definition, regulation, effects of

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Ozone what is it? how is it formed? What happens to it? what does it do and why is it important?

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Ozone (O3) is a gas that occurs in two layers of the atmosphere, the stratosphere and the troposphere. The stratospheric or "good" ozone layer, which extends upward from about 10 to 30 miles above the earth's surface, protects life on earth from the sun's harmful ultraviolet rays (UV-b). However, ozone found in the troposphere, the layer of the atmosphere that extends from the earth's surface to about 10 miles up, is deemed ground level or "bad" ozone. At ground level, ozone is an air pollutant that damages human health, vegetation, many common materials, and is a key ingredient of smog. What is Ozone?

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Thickness of the ozone layer The term "ozone layer" is often misunderstood. Its not really a single layer in the atmosphere where ozone is concentrated. Rather it means that a higher fraction of ozone molecules are found in the stratosphere (at altitudes between 18 and 40 km) compared to levels the troposphere below or the mesosphere above. In reality, only about 10 out of every million molecules in the atmosphere are actually ozone but 90% of all the ozone present in the atmosphere is found in the stratosphere

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Why Don't Stratospheric (good) and Tropospheric (bad) Ozone Mix? The troposphere contains 75% of all atmospheric gases and 99% of water vapor. The air in the troposphere is in constant motion, with both horizontal and vertical air currents. The combination of vigorous air movement and water vapor creates weather. The troposphere is capped by a thin layer known as the tropopause, which is a region of stable temperature that helps to confine most weather phenomena and "bad" ozone to the troposphere.

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Why Don't Stratospheric (good) and Tropospheric (bad) Ozone Mix? The lower part of the stratosphere contains the ozone layer. The ozone layer prevents UV radiation from reaching the earth's surface by absorbing the rays, causing the ozone layer and the air above it to warm. The warm air tends to remain in the upper stratosphere, and cool air remains lower. The layering of warm and cool air prevents vertical mixing, so the air moves only in a horizontal direction, making the stratosphere very stable, but also creating a kind of giant lid. [Helpful to commercial airlines that often fly in the lower stratosphere because the air is relatively warm and stable, but not helpful to be able to mix ozone between the stratosphere and the troposphere].

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The stratosphere is dynamically isolated from the lower atmosphere due to the temperature inversion that exists at the tropopause (the boundary between the troposphere and stratosphere). As a consequence, living organisms do not have direct contact with stratospheric ozone.

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The ozone layer prevents the sun's UV rays from reaching life on earth. This UV radiation is of a suitable energy to damage living cells. A 1% reduction of the O3 in the stratosphere could lead to: ・An increase in skin cancers in animals and humans ・A suppression of the human immune systems ・Some inhibition to plant life, and an increased susceptibility to pests ・Reduction in growth of phytoplankton, endangering the food chain ・A decrease in aquatic lifeforms Why do we care about the stratospheric ozone layer?

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How does ozone protect us from UV radiation?

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Absorption of ultra-violet radiation by ozone and other compounds in the stratosphere.

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Ozone absorbs UV radiation in its formation and breakdown O2 absorbs UV light to form 2 oxygen radicals: O2 + hv  O + O (1) (and release of heat)

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The oxygen radicals can then react in one of three ways: O+O2  O3 (2) O+O  O2 (3) O+O3  2O2 (4) Reaction 2 is the reaction in which ozone is produced.

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This ozone can absorb ultra-violet radiation and undergo photodissociation: O3 + hv  O2 + O (5) This reaction is vital in shielding the earth from the sun's UV radiation.

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less ultra-violet radiation reaches the lower parts of the atmosphere and, as a consequence, the surface of the Earth is protected from damaging radiation. since ultra-violet radiation is important for both ozone formation and destruction, the amount of ozone that can accumulate is limited - as ozone formation increases, ozone destruction also increases. ultra-violet radiation is highly energetic. This energy is transformed to heat during reaction leading to a warming of the stratosphere. This is the reason why the temperature trend in the stratosphere is opposite to that seen in the troposphere. Summary: Consequences of UV-ozone interactions for the atmosphere

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The Breakdown of Ozone: Chlorofluorocarbons in the stratosphere… The "good" ozone that occurs naturally in the stratosphere is gradually being destroyed by man-made chemicals (chlorofluorocarbons, etc.). Used as refrigerants in air conditioners and fridges, blowing agents in expanded plastics (e.g. polystyrene), aerosol propellants, and cleaning solvents to dissolve grease, particularly in dry cleaning.

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These gases stable (live as much as 75 years) and can remain intact for years while moving through the troposphere until they reach the stratosphere. There they are broken down by the intensity of the sun's ultraviolet rays and release chlorine and bromine molecules, which destroy "good" ozone. One chlorine or bromine molecule can destroy 100,000 ozone molecules, causing ozone to disappear much faster than nature (or man) could ever replace it. Here chlorine will be used as the example: When CFC's are destroyed they form Cl radicals. These chlorine radicals react with ozone: Cl+O3  O2+ClO (6)

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This scheme shows ozone transport. The simulation below shows how measured ozone concentrations vary between the poles and the equator (low values = blue, high values = red).

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Even though ozone is produced in the tropics, concentrations aren't particularly high since intensive solar radiation means that ozone loss in this area is also high. Ozone is transported from the tropics towards the poles where it accumulates. Ozone can accumulate in these regions as solar radiation isn't strong enough here to cause much ozone destruction. Unless disturbed by the formation of the Antarctic ozone hole in spring, ozone levels are generally highest in the cold subpolar regions. [Concentrations are slightly lower at the poles themselves, particularly in winter, as no additional ozone can be formed here because there isn't any sunlight!]

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This scheme shows ozone transport. The simulation below shows how measured ozone concentrations vary between the poles and the equator (low values = blue, high values = red).

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Until >Montreal Protocol agreement, ozone depletion rapidly grew worse {1989; CFC production declined and stopped by 1996}

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1956-2003

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Ground-level or "bad" ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. ”Bad" ozone: where, how, what and why

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Photochemical smog is a complex pollutant “soup” generated when hydrocarbons and oxides of nitrogen react in strong sunlight, forming a series of secondary pollutants, which react with themselves and with primary pollutants in complex ways (which we will largely ignore). The major components of photochemical smog are: Ozone Peroxyacyl nitrates Aldehydes Alkyl Nitrates Air borne particles The formation of a photochemical smog is typically initiated by photolysis of nitrogen dioxide, which leads to the production of ozone and in turn the reformation of NO2. This is one of the most important photochemical reactions in the lower atmosphere

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Simplified Story: How Does Ground Level Ozone Form? Ozone is formed when certain compounds react in the presence of direct sunlight. VOCs + NOx + Sunlight = Ozone VOCs, (volatile organic compounds) are widely used as ingredients in household products including; paints, varnishes, wax, fuels, cleaning, disinfecting, cosmetic, degreasing, and hobby products. In addition to all of the man made sources of VOCs, natural sources of VOCs exist. For example, trees naturally release small amounts of VOCs. NOx, (nitrogen oxide gases) group of highly reactive gases. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. When high levels of VOCs and NOx are present in the air, they can react. When they react in the presence of sunlight and hot weather, ground level ozone forms.

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(1) NO2 + hv  NO + O When nitrogen dioxide is excited by light nitric oxide and atomic oxygen are formed. The rate of this equation is k1[NO2]. The rate of the reaction (k1) is proportional to the concentration of nitrogen dioxide and to the intensity of light. At night k1 approaches 0 and reaches its maximum during hours of high sunlight intensity.

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The first reaction of the atomic oxygen tends to be with the more abundant diatomic oxygen (O2) (2) O2 + O + M  O3 + M M is an unreactive 3rd molecule which is needed to absorb excess energy from the reaction. The rate coefficient of this reaction is k2.

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There is a third and final reaction which completes this reaction: O3 + NO  NO2 + O2 (3) This reaction has the rate coefficient k3.

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Each of the 3 reactions is very fast, and a photostationary state of equilibrium is established, this governs the ratio of NO2/NO in air, which in turn can be shown to be proportional to the concentration of ozone. [NO2]/[NO] = k3[O3]/k1 (4) The presence of excess ozone increases and the lowest [NO2]/[NO] ratios are during hours of extreme day light.

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Ozone pollution is a concern during summer months because strong sunlight and hot weather result in high ozone concentrations Many urban and suburban areas in the US have high levels of "bad" ozone. But many rural areas of the country are also subject to high ozone levels as winds carry emissions hundreds of miles away from their original sources. Even though largely a summer problem, that is also when crops and trees grow, and when people tend to be outside!

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Why do we care about ground level (or tropospheric) ozone? Greenhouse warming Human health Effects on terrestrial ecosystems

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…some good news….

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….some bad news….. (average concentrations increasing over large geographic areas)

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Such as the world

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Concentration vs. Dose Responses to Ozone

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Reich & Amundson 1985

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Ollinger et al. 1997 Ecological Applications Implications for crops and forests?

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Ozone reduces forest production by 7-9% in New England

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Feltzer et al 2004

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