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Radiant Energy - This is the type of energy found in photons and electromagnetic waves. It can be in the visible or invisible electromagnetic wavelengths. This involves aspects of radiant heating with convection and conduction. Measuring the earth's radiant energy flux helps to determine the parts of the atmosphere and earth's surface that absorb energy and heat up and that which radiate energy to space. [Bulletin American Meteorological Society; v77; 853-868; 1996: DOI:10.1175/1520-0477(1996)077<0853:CATERE>2.0.CO;2] [Atmospheric Environment; v45; 665-672: 2011; DOI:10.1016/j.atmosenv.2010.10.025]

Radiation Flux - This is defined as the way heat from the sun is absorbed, scattered, returned, or deposited around the earth and can be expressed as some radiation arrival rate per the unit of area of the surface. The radiation flux can be the total amount of radiation that the earth collects, emits, and disperses. [Oil and Gas Journal ; v91; 45-48; 1993.] [Nature; v343; 251-254; 1990; DOI:10.1038/343251a0]

Radiative Forcing - Reduction of radiation reflecting from earth per units of increase of a chemical component that causes this change in the atmosphere. The result is often used to compare the relative effect of greenhouse gases which can alter climate on earth. This means that components that are present in the atmosphere at low concentrations (e.g. methane at 1.7 ppmv) but with high radiative forcing have a greater effect on global warming than higher concentration components with a lower radiative forcing. [Atmospheric Environment; v41; 2820-2830; 2007; DOI:10.1016/j.atmosenv.2006.11.038] [Atmospheric Environment; v45; 2860-2868; 2011; DOI:10.1016/j.atmosenv.2011.02.071]

Radical (free radical) - A highly reactive molecule or atom with an unpaired electron. The species is often represented by a formula with a single dot as the unpaired electron (see immediately below). Important atmospheric examples include atomic chlorine, chlorine monoxide, hydroxyl radical, NO and NO2. [Organic Chemistry; Wade, L. G.; Prentice-Hall, New Jersey; page 125; 1991.] [Environmental Chemistry; Manahan, Stanley E.; Lewis Publishers; Boca Raton; page 218; 1994.][Atmospheric Environment; v45; 3141-3148; 2011; DOI:10.1016/j.atmosenv.2011.02.064]

Radical Chain Reaction - Free radicals are molecules that, through photolysis or chemical reaction, have an unpaired electron in their outer valance shell. These radicals are very reactive and thus have a short life. When a free radical reacts with a more stable molecule, the radical often pulls an atom from it and becomes a stable molecule itself. The original molecule then becomes a free radical and will react with other species of atoms and molecules in a long series (or chain) of reactions until the process reaches the termination phase (see below). In this phase two free radicals combine, sharing the pair of electrons and breaking the chain. [Journal of Organic Chemistry; v56; 5743-5; 1991; DOI:10.1021/jo00020a004] [Journal of Organic Chemistry; v58; 3953-3959; 1993: DOI:10.1021/jo00067a030]

Radical Propagation - Free radicals react with molecules, atoms, other radicals in long series of chain reactions. A radical is formed in the atmosphere through photolysis or chemical reaction. Radicals are very reactive and will pull atoms or electrons from other molecules to fill their outer valance shell. New radicals may formed from the original molecule which then react with another molecule, etc. This reaction propagates from one species to another until a termination reaction occurs. The termination reaction usually involves the reaction of two free radicals to form a stable product. [Journal of Organic Chemistry; v50; 5442-4; 1985; DOI:10.1021/jo00225a107] [Journal of Organic Chemistry; v56; 5743-5; 1991: DOI:10.1021/jo00020a004]

Radical Termination - Two or more radical species reacting to form non-radical products; the nighttime reaction of nitrogen dioxide radical with ozone to produce neutral nitrogen pentoxide is an example of radical termination:

NO2 + O3 ---> NO3 + O2 then

NO2 + NO3 ---> N2O5

[Atmospheric Environment; v16; 113-120; 1982; DOI:10.1016/0004-6981(82)90318-3] [Atmospheric Environment; v40.1189-1198; 2006; DOI:10.1016/j.atmosenv.2005.10.041]

Radioactive Decay - The spontaneous breakdown of certain kinds of atomic nuclei into one or more nuclei of different elements, involving the release of energy and subatomic particles such as neutrons. [Crutzen, Paul J. and Graedel, T.E. Atmospheric Change: An Earth System Perspective. Freeman Press; 1993; p. 436.] [Applied Radiation and Isotopes; v64; 93-100; 2006; DOI:10.1016/j.apradiso.2005.07.006] [Applied Radiation and Isotopes; v69; 1070-1078; 2011; DOI:10.1016/j.apradiso.2011.03.027]

Radiocarbon Dating - Establishing the relative age of various materials with the use of carbon-14. This involves measuring the amount of 14C and of 12C and comparing the measured ratio to the one established by the production of 14C in the upper atmosphere by cosmic rays. When an organism is alive, the 14C/ 12C ratio in its biomass is constant (because of constant atmospheric 14C production, diffusion to the lower atmosphere, absorption and incorporation by organisms, and because of constant 14C radioactive decay); however, when the organism dies, the ratio begins to change---at a very predictable rate---because the (dead) organism is no longer incorporating new carbon. Therefore knowing the carbon-14/carbon-12 ratio now in some artifact gives a very good measure of how long the carbon has been "dead." The object however must obviously have organic material either in it or on it for this dating method to work. Also the length of time one can "look back" in time is limited because the amount of carbon-14 must be detected with some certainty for the age to be known with confidence, and the longer the sample has been dead the less 14C is present because it is continually decaying. [New Scientist; v274; 18; 1996.] [Scientific American; v150; 24; 1996.] [Bowman, S. Radiocarbon dating--Interpreting the past. British Museum Press; London; 1995.]

Radiosonde - Instrument designed in the 1930's to be carried into upper atmosphere via balloon to measure temperature, humidity, and barometric pressure; radio transmission is the means of returning data to surface. Resolution can be a little as a few meters of altitude. [Atmospheric Environment; v40; 3816-3822; 2006: DOI:10.1016/j.atmosenv.2006.02.034] [Journal Atmospheric Solar-Terrestrial Physics; v73; 544-550; 2011; DOI:10.1016/j.jastp.2010.11.016]

Rainout - The process of the removal of atmospheric constituents by precipitation. One item necessary for a rain drop to form is a condensation nucleus. Pollutants in the air such as nitric acid and sulfuric acid form nucleation sites (particles) upon which water vapor often condenses and forms a droplet. As these droplets grow they collect more moisture and may ultimately fall out as precipitation. This accounts for some of the acid rain deposition on the planet's surface and a means by which atmospheric aerosols, gases, and particles are removed. [Environmental Science & Technology; v25; 2012-23; 1991: DOI:10.1021/es00024a005] [Environmental Science & Technology; v19; 1044-8; 1985; DOI:10.1021/es00141a003]

Rayleigh Scattering - The scattering of light by a body with a particle diameter (Dp) to wavelength ratio less than 0.03 is termed Rayleigh Scattering. The wavelength of light scattered is dependent on the Dp, and the amount of light scattered is dependent on the number of particles present per unit volume. Shorter visible wavelengths, such as blue, are scattered by smaller particles than are the longer wavelengths like red. This is the reason why, during daylight, the sky is blue when looking away from the sun. See also Visible Light. [American Journal of Physics; v56; 948-50; 1988: DOI:10.1119/1.15370] [Atmospheric Environment; v43; 6136-6144; 2009; DOI:10.1016/j.atmosenv.2009.09.010]

Reaction Rate - The speed of chemical reactions is expressed as a rate of change in the concentration of a participating species with respect to time. The reaction rates of chemical species in the atmosphere are extremely important parameters when considering sources and sinks of gas-phase components. For instance, the reaction rate for the reaction of hydroxyl radical with atmospheric propane in the troposphere is very fast, yielding an alkyl radical that goes through a series of reactions to yield ozone in urban environments. Conversely the reaction rate of propane with the nitrate radical ("NO3 dot") is ~10,000 times slower under the same conditions. Therefore, just considering these two chemical sinks, hydroxyl controls propane's lifetime in urban settings.[Evaluated Kinetic Data; International Union of Pure Applied Chemistry; 2008.] [Crutzen, Paul J. and Graedel, T.E. Atmospheric Change: An Earth System Perspective. Freeman Press; 1993; p. 434.] [Atmospheric Environment; v34; 1543-1551; 2000; DOI:10.1016/S1352-2310(99)00407-0]

Reactive Organic Gases (ROG's) - Volatile organic compounds, excluding methane, found in the atmosphere which are capable of producing radicals upon reaction with common atmospheric oxides and radicals; these compounds are often responsible for positive feedback cycles involving the production of ozone. [Atmospheric Environment; v33; 2471-2485; 1999.]

Recombination - In chemical terms, a recombination is one of the ways in which free radicals are lost (the other being disproportionation). In this case, two free radicals join to create one product. This could be from a molecule that broke apart (ionized) and reconstituted, or two free radicals from different sources that form a new molecule. [Planetary and Space Science. 26, 559-569; 1978] [Journal of Electroanalytical Chemistry. 161, 235-245; 1984.]

Reflected Infrared Radiation - This is the type of electromagnetic energy that is made up of near-infrared (0.75-1.4 micrometers in wavelength) and short-wavelength infrared (1.4-3 micrometers in wavelength).This is a thermal type of radiation. [Journal of Geophysical Research; v99(D9); 18,669-18,684; 1994.]

Reflectivity - A proportionality describing the light energy reflected by a surface; in atmospheric content the reflector includes things like the surface of the earth or particles in the atmosphere. Also see albedo. [Journal of Photochemistry and Photobiology B: Biology; v87; 1-8; 2007.]

Refraction - the change in direction of light as it passes between two objects of differing densities; the angle of refraction and the angle of incidence are related by Snell's Law: n1*sinθ1 = n2*sinθ2 [International Journal of Solids and Structures; v 42, issue 13; Pages 3621-3643; June 2005] [Journal of Applied Geophysics; V 57, I 3; Pages 193-211.]

Relative Humidity - This is the ratio between the actual water vapor content of the atmosphere and the maximum water vapor content possible at that given temperature. If the temperature of a given parcel of air rises, the amount of moisture it can hold increases and as such its relative humidity decreases. If there is no change in temperature but the moisture content decreases then the relative humidity will again decrease because the ratio of actual water vapor present is less than the maximum amount the air could hold. Conversely, if the water content does not change but the temperature falls, the relative humidity increases until saturation and possible precipitation occurs. [Ocean; v17; p34-6; 1984.] [Science News; v132; p236; 1987.]

Reservoir Species - In reference to ozone destruction, compounds formed in the stratosphere that sequester reactive chlorine and thereby remove it from a role in the catalytic destruction of ozone. Classic examples are HCl and ClONO2. [Surface Science; v502-503; 285-289; 2002.] [Atmospheric Environment; v34; 3951-3962; 2000.]

Residence Time - The amount of time a substance can remain in a reservoir. The reservoir can be aquatic, atmospheric, or terrestrial. In atmospheric chemistry an example of reservoir time would be the amount of time NOx can remain in the atmosphere before being removed by deposition or chemical reactions. [Journal of Environmental Management; v84; 20-26; 2007.]

Return Stroke - A short-lived, high amplitude discharge of electricity that accompanies a cloud-to-ground lightning event. The bright lightning flash that is visible to the human eye is actually composed of extremely rapid electric discharges called strokes. The return stoke follows a downward extending leader, or conductive path of ionized air. The electrons in the cloud flow down this conductive pathway toward the surface of the earth. As electrons continually migrate down the path, electrons remaining higher up on the path in the cloud begin to consecutively move down the channel to the surface. Since the path of electron flow is progressively lengthened upward, the discharge of the elevated electrons high in the cloud to a lower place in the cloud and then down the pathway to the ground is called the return stroke. [Journal of Atmospheric and Solar-Terrestrial Physics, v. 62, Issue 3, p. 169-187]

Rule of 90 - The systematic numbering system originally created by the Du Pont Chemical Company in the early 1930s, which is now the standard method in the naming of chlorofluorocarbons (CFCs). Chlorofluorocarbons, such as dichlorodifluoromethane (CCl2F2), and trichlorofluorocarbon (CCl3F), were given the nicknames CFC-12 and CFC-11, respectively, by Du Pont so that they could be remembered more easily. In order to derive the chemical structure of a chlorofluorocarbon from its common name, the rule of 90 states that you would add 90 to the number at the end of the name. In the case of CFC-12, add 90+12 to get 102. The hundreds place, the one in this example, represents the number of carbon atoms in the molecule. The tens place, the 0, represents the number of hydrogen atoms in the molecule, and the ones place represents the number of fluorine atoms in the molecule. According to the rule of 90, CFC-12 has 1 carbon atom, 0 hydrogen atoms, and 2 fluorine atoms. The final aspect of the rule of 90 pertains to the necessity of carbon to form four bonds. If, after assigning hydrogen and fluorine atoms to it, the carbon atom still requires more bonds to reach the required four, those bonds will be to chlorine atoms. So far, the one carbon atom of CFC-12 has only formed two bonds, both with fluorine atoms. Thus the remaining two bonds must be chlorine atoms. Analogously, CFC-11's chemical formula is CCl3F. [Science, v242; 666-668; 1988.] [Science, v249; 31-35; 1990.]

Runaway Greenhouse Effect- This describes and explains the difference in the surface temperatures between the Earth (average surface temperature 15 C) and Venus (450 C). Note that Venus is not this temperature because of its proximity to the sun. This phenomenon on Venus is caused by a high atmospheric concentration of greenhouse gases, which in turn cause overheating of the planet. The runaway effect causes a permanence in the warming of the planet at the surface. On Venus, the loss of liquid water from evaporation and subsequent photolysis of H2O and loss of atomic H to space in the planet's early life caused the buildup of carbon dioxide in Venus' atmosphere. Since CO2 is a greenhouse gas, it absorbs the infrared radiation reflected from the planet's surface, which heats the atmosphere and prevents the removal of CO2 by rain out and depositional processes that capture CO2 as carbonates. [Journal of the Atmospheric Sciences; v59; 2801-2810; 2002.][Earth and Planetary Science Letters; v286; 503-513; 2009.DOI:10.1016/j.epsl.2009.07.016]

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Satellite - Satellites are electronic devices placed into orbit to monitor and examine regions which we may not be able observe from earth. These are used routinely to monitor atmospheric ozone, ultraviolet radiation, and many other constituents. Satellite altimetry and radar interferometry are used to measure magnetic field intensity and track Antarctic ice flow. Geosynchronous satellites--whose orbit is 24 hours--are used for communication. Arthur C. Clarke proposed the concept of geosynchronous satellites in 1945. [Science; v256; 1526-30; 1993.] [Atmospheric Environment; v45; 6267-6275; 2011; DOI:10.1016/j.atmosenv.2011.08.066] [Space Policy; v26; 78-80; 2010; DOI:10.1016/j.spacepol.2010.02.009 ]

Saturated Adiabatic Lapse Rate - This is the lapse rate (the rate of change of the temperature in the gases in the atmosphere along different altitudes) of a mass of moist air as it rises in the atmosphere. This rate begins to slow as the dew point is reached and condensation forms. This rate of cooling goes from 4 degrees C/km all the way to 9 degrees C/km depending on humidity. Contrast with the dry adiabatic lapse rate which is 9.8 degrees C per km of rise. The saturated lapse rate is always less than the dry rate because the lapse rate (cooling) is less for air that contains water vapor which releases its latent heat as it cools, suppressing the cooling. [Journal of Applied Meteorology; v13; 318-324; 1974.[Science of the Total Environment; v390; 97-10; 2008; DOI:10.1016/j.scitotenv.2007.08.035]

Sea Level - This is the average level of the ocean over the entire earth. Tidal fluctuation is taken into account when determining sea level. Mean Sea Level (MSL) is used to reference the height of structures above ground level and depth below the ocean's surface. Atmospheric pressure is also referenced from sea level. At sea level, 760mm (29.92 inches) of mercury is normally displaced by the pressure of the air column above that point on the earth. [Nature; v367; p54-7; 1994.] [Environment; v36; p23-4; 1994.]

Sea-Level Pressure - This is the pressure at sea-level (the average level of the ocean over the entire earth). The average sea-level air pressure is 1,013 millibars, 760 mm Hg, 101 kilopascals, 29.92 inch of mercury, 14.7 lb/ sq in, or 1.00 atmosphere. The normal range of sea level air pressure worldwide is 970-1040 millibars. [Journal of Physical Oceanography; v8; 233-246; 1978.] [Journal of Marine Systems; v87; 177-193; 2011; DOI:10.1016/j.jmarsys.2011.03.012]

sec-Butylbenzene - Chemical given off by vehicle emissions and coal burning plants due to incomplete combustion of hydrocarbons. It is a colorless liquid, plasticizer, used in organic synthesis, and a solvent for coating compositions. It has a molecular weight of 134.22 g/mol, and a melting point of -82.7 degrees Celsius. [Journal of Molecular Catalysis A: Chemical; v192; 171-187; 2003/]

Sediment - Particles carried by water or air that are deposited in other areas or elevation are called sediment. Usually associated with water flow, sediment will accumulate at the mouth of a river or stream as it empties into a larger, slower moving body of water. Sediment deposition is also effected by particle size.[Geological Magazine; v131; p568-70; 1994.] [Ecology; v74; p919-34; 1993.]

Sensible Heat - This is the process in which energy from heat is released into the atmosphere through conduction and convection, as contrasted by latent heat, released by the condensation of water vapor. Atmospheric circulation can then move the heat (energy) horizontally. [Journal of the Atmospheric Sciences; v54; 498-509; 1997.] [Building and Environment; v46; 2542-2551; 2011; DOI:10.1016/j.buildenv.2011.06.012]

Sink - This is a removal process for chemicals in the atmosphere such as dissolution in bodies of water, removal via rain, photolysis, or by reactions with other atmospheric components. For instance, because of high chemical stability, the major atmospheric sink for the chlorofluorcarbon CCl3F is photolysis in the upper atmosphere. In the troposphere this compound is extremely long-lived because it isn't very water soluble, doesn't react with other tropospheric components, and isn't microbially attacked in soils or by plants. [Total Quality Environmental Management; v2; 459-464; 1993.] [Environmental Science and Technology; v32; 3077-3086; 1998; DOI:10.1021/es9802807] [Marine Chemistry; v94; 27-41; 2005; DOI:10.1016/j.marchem.2004.07.009]

Sintering - A change in crystal structure, specifically ice, which leads to stronger and more compact packing with time. This can change the space available for various atmospheric components trapped by the initial precipitation/snow fall. Because of this process, scientists are able to determine historical air temperatures by studying the air pockets within ice structures deep below surface. [Icarus; v144; 367-381; 2000.] [Science of the Total Environment; v342; 5-86; 2005; DOI:10.1016/j.scitotenv.2004.12.059][Polar Science; v5; 319-326; 2011; DOI:10.1016/j.polar.2011.02.001]

Smog - This is a term used to describe a mixture of smoke and fog. Smog occurs when high concentrations of moisture is combined with smoke (often containing oxides of sulfur and nitrogen) in the presence of high temperatures or thermal inversions and the absence of wind. These conditions cause polluted air to stagnate over industrial or urban areas and can create a respiratory health hazard. Large coastal industrial centers with surrounding high ground are more prone to smog. There is often a diurnal (over a day) variation in the process of smog formation because one of the necessary components for its formation is sunlight. This is usually referred to as photochemical smog. [Journal of Environmental Health; v56; 38; 1994.] [Science; v313; 137; 2006.] [Atmospheric Environment; v41; 1521-1538; 2007.] [Atmospheric Environment; v45; 5716-5724; 2011; DOI:10.1016/j.atmosenv.2011.07.024]

Smoke - It is the product of incomplete combustion and consisting mostly of carbon and small liquid and solid particles. Smoke is primarily caused by the burning of wood, coal, gasoline, or other biomass. Smoke particles are usually less than 1.0 micrometers in size. Wind intensity, particle size, air temperature, and atmospheric pressure all effect how smoke behaves in the environment. [Nature; v325; p701-3; 1987.] [Environmental Science & Technology; v28; p1375-88; 1994.]

Solar Radiation - This is electromagnetic radiation (light energy) emitted by the sun. This energy is transmitted through space in the units of electromagnetic energy called photons. The strength of the solar energy that reaches our outer atmosphere is called the solar constant and has a value of approximately 2.0 calories per minute per square centimeter. Roughly one-half (1.0 cal min-1 cm-2) of all the solar radiation that enters the upper atmosphere is available at the earth's surface. Of the fifty percent loss, approximately 32 percent is reflected back into space by either clouds, dust, or aerosols; the remaining 18 percent is absorbed by clouds, dust, water, and atmospheric gases. The short high-energy wavelengths that are absorbed by the Earth are re-radiated at longer, lower energy wavelengths mostly in the IR wavelength region. These lower-energy wavelengths are, in part, prevented from escaping into space by the greenhouse gases in the atmosphere. Solar radiation is the fuel source that provides a mechanism for all biological processes on earth. Without solar radiation, life on earth would not exist. (This statement, of course, neglects the contribution to the earth's surface temperature from radioactive decay in the earth.) [Journal of the Atmospheric Sciences; v51; 1978-88; 1994.] [Journal of the Atmospheric Sciences; v49; 762-72; 1992.]

Solar Wind - The outflow of charged particles from the solar corona into space. Because of the high temperature of the particles of the corona (mostly protons and electrons), they are moving at speeds higher than the solar escape velocity. At the orbit of the Earth, these particles are moving at about 500 km/sec. Some of these particles are captured by the magnetic fields of the planets, forming their magnetospheres. [Advances in Space Research; v37; 456-460; 2006.] [Planetary and Space Science; v54; 1482-1495; 2006.]

Sorption - A class of processes by which one material is taken up by another. Absorption is refers to the process of the penetration of one material into another; adsorption to the action of one material being collected on another's surface. [Crutzen, Paul J. and Graedel, T.E. Atmospheric Change: An Earth System Perspective. Freeman Press; 1993; p. 435.]

Spectrophotometry - Laboratory technique used to measure the amount of light that is absorbed. [Harris, D. Quantitative Chemical Analysis, 7th ed; Freeman: New York.]

Spring Equinox - In the Northern hemisphere, March 21 or 22; on these dates, the rays of the sun are striking vertically at noon at the equator (0 degrees latitude), for the earth is in such a position in its orbit that the axis is tilted neither toward nor away from the sun. For latitudes above the equator, the dates are earlier then these dates. [Lutgens, Frederick K. et al. The Atmosphere-An Introduction to Meteorology 6th ed. pg. 29; 1995; Prentice-Hall: New York.]

Stratopause - The boundary between the stratosphere and the mesosphere. It occurs at a height in the atmosphere of approximately 50 kilometers; however this depends on latitude. The atmosphere is characterized by a decrease in pressure with respect to increased altitude. More importantly, regions within the atmosphere like the troposphere, stratosphere, and mesosphere are distinguishable because of distinct temperature gradients with relatively well-defined starting and ending points. The stratopause is the highest portion of the stratosphere, with a temperature of approximately 0 degrees Celsius; the stratopause can also be described as the warmest region between the mesosphere and the stratosphere. [Journal of the Atmospheric Sciences; v50; 3608-24; 1993.] [Journal of the Atmospheric Sciences; v50; 3928-38; 1993.]

Stratosphere - The thermal atmospheric region of the atmosphere between the troposphere and the mesosphere. The lower boundary of the stratospheric region is marked by the tropopause and begins at approximately 13 kilometers; however, this altitude of the troposphere depends on latitude. The upper limit of the stratosphere is marked by the stratopause at approximately 50 kilometers. The stratosphere is characterized by relatively stable temperatures (between -80 and -50 degrees Celsius) in the lower regions, and begins warming near 20 kilometers, reaching its maximum temperature of approximately 0 degree Celsius at the stratopause. Stratospheric chemistry is of particular interest to scientists because ozone, the principal substance that shields the earth from incoming solar ultraviolet radiation, is found in the stratosphere. It should also be noted that wind currents in the stratosphere are primarily horizontal in nature. [Journal of the Atmospheric Sciences; v50; 3928-38; 1993.] [Nature; v370; 429-34; 1994.]

Stratospheric Clouds - These are clouds found in the stratospheric region of the atmosphere. Unlike the moisture laden troposphere, the stratosphere is almost moisture free. A lack of moisture means that cloud formation in the stratosphere is a relatively uncommon occurrence. Because of the very low temperatures, nacreous clouds, a type of stratospheric cloud, occur during winter conditions at the higher latitudes. A second type of stratospheric cloud, called a polar stratospheric cloud (PSC), occurs in the polar regions during winter and early spring. Polar stratospheric clouds are of particular interest to scientists studying the Antarctic Ozone Hole. Polar stratospheric clouds form under extremely cold conditions, whereby nitric acid, water vapor and other trace chemicals freeze to form ice crystals. PSCs provide a solid surface on which chlorine-containing reservoir compounds can gather, and on this surface the chemical reactions involved in the depletion of ozone are greatly increased. [Science; v264; 527-8; 1994.] [Nature; v355; 534-7; 1992.] Science; v292; 61-63; 2001.] [Science; v290; 1756-1758; 2000.]

Stratospheric Ozone - This is also called the ozone layer, where ozone concentrations are as high as 10 parts per million, and is a vitally important region of the atmosphere. This layer of ozone is located approximately 20-50 kilometers above the earth's surface. Stratospheric ozone is important because it prevents most of the high-energy ultraviolet solar radiation from reaching the earth's surface. Photodissociation, a photochemical process, is responsible for the formation of the protective ozone layer in the stratosphere. In the upper atmosphere, diatomic oxygen absorbs high-energy ultraviolet radiation. The absorption of radiation causes the diatomic oxygen molecule to break forming two oxygen radicals. The oxygen radical can then recombine with other diatomic oxygen molecules to form triatomic oxygen, or ozone. In the middle regions of the stratosphere, ozone is found in concentrations as high as 10 parts per million. Ozone can also form in the lower portions of the troposphere, due to anthropogenic activity and by a completely different mechanism. Without the protective stratospheric ozone layer in the upper atmosphere, life (as we know it) on earth would not be possible. [Science; v263; 71-5; 1994.] [Lancet (North American edition); v342; 1156-8, 1993.]

Stratocumulus Clouds - Low altitude gray colored clouds composed of water droplets that have a patchy appearance. Each cloud patch consists of a rounded mass. This cloud has a somewhat uniform base and normally covers the entire sky. Between the patches blue sky can be seen. [Atmospheric Environment; v40; 1845-1855; 2006.]

Stratosphere/Troposphere Exchange - The exchange between the stratosphere and the troposphere is extremely important. It is caused by upwelling in the troposphere and when troposphere species are pushed into the stratosphere. It starts in the tropics and moves toward the poles. If certain ozone destroying species are brought into the stratosphere from the troposphere when they mix damage to the ozone layer and global warming can result. [Hall, Timothy M.and Mark Holzer. Adjective-diffusive mass flux and implications for stratosphere-troposphere exchange. [Geophysical Research Letters; v30; 1222; 2003. DOI:10.1029/2002GL016419][Journal of Atmospheric Sciences; v57; 1; 2000.]

Stratus Cloud - In general, clouds are classified and named according to their appearance and altitude at which they occur, being either high, medium, or low. Stratus clouds are low clouds, and as the root word suggests they are layered in appearance. They occur in broad sheets across most or all of the sky. Generally their formation is due to condensation within layered air that is not subjected to strong vertical movement. [Journal of the Atmospheric Sciences; v50; 3078-90, 1993.] [Journal of the Atmospheric Sciences; v43; 90-106; 1986.]

Styrene - chemical formula C8H8, also commonly called vinyl benzene. Styrene is an aromatic hydrocarbon originating from petroleum and natural gas by-products. It is used to manufacture resilient, lightweight packaging materials and fiberglass. Styrene is extremely volatile in air and evaporates quickly; as a result, it does not remain in the atmosphere for long periods of time. [Building and Environment; v42; 1949-1954; 2007.]

Sulfate Anion, SO42-- The sulfate anion in the atmosphere is the result of the oxidation of sulfur dioxide (SO2). The sulfate anion is water soluble and is removed from the atmosphere through precipitation.[Analytica Chimica Acta, v.217, 1989, p.135-147][Biochimica et Biophysica Acta (BBA) - Biomembranes, v.646, 1981, p. 88-98]

Sulfur Dioxide, SO2-This is a colorless gas consisting of a single sulfur atom and two oxygen atoms. Sulfur dioxide is a major primary pollutant in the atmosphere originating mostly from coal fired power plants and other fossil fuels combustion. In the atmosphere, sulfur dioxide is usually oxidized by ozone and hydrogen peroxide to form sulfur trioxide, a secondary pollutant. Sulfur trioxide, similar to sulfur dioxide, is extremely soluble in water. If these sulfur oxides are present in the atmosphere when condensation occurs, droplets of sulfuric acid (acid rain) are formed. Volcanic eruptions provide a natural source of sulfur dioxide in the atmosphere. However the real problem associated with the production of sulfuric acid in the atmosphere is not with the sulfur dioxide expelled by volcanoes. Anthropogenic production of sulfur dioxide, caused by the burning of fossil fuels, is largely responsible for damage caused by acid rain. [Science; v265; 497-9; 1994.] [Nature; v366; 327-9; 1993.]

Sulfur Hexafluoride, SF6- an inert gas categorized as hydrofluorocarbons. It is used as an insulator in circuit breakers and is used to produce other elements such aluminum and magnesium. Sulfur hexafluoride absorbs thermal infrared radiation and could increase global warming as its concentration in the atmosphere increases; therefore it is a greenhouse gas. [Agriculture, Ecosystems and Environment; v121; 30-46; 2007.]

Sulfuric Acid - In the atmosphere sulfur oxides (see sulfur dioxide) are converted to sulfuric acid. Oxides of sulfur and nitrogen combine with atmospheric moisture to produce acid rain. Although some sulfur oxides are introduced into the atmosphere by natural means, such as volcanic eruptions, the majority of the sulfur oxides responsible for the damaging effects of acid rain come from anthropogenic sources, mainly the burning of fossil fuels. Areas in the northeastern United States , eastern Canada , and northern Europe have suffered damage due to the effects of acid rain. In many areas damage to forests, crops, lakes, and streams are so severe that they are completely devoid of any life forms. Steps are now being taken in many parts of the world to reduce the amount of sulfur dioxide introduced into the atmosphere. In 1990, amendments where made to the Clean Air Act that places restrictions on the release of sulfur dioxides by power plants. The amendment calls for the reduction of sulfur emissions from a 1990 level of nearly 20 million tons per year to approximately 10 million tons per year by January 1, 2000. [Science; v265; 1136-40; 1993.] [Journal of the American Chemical Society; v116, 4947-52; 1994.]

Summer Solstice - In the Northern Hemisphere occurs on about June 21 of each year. The summer solstice is the longest day of the year and marks the beginning of the summer season. If the earth's axis were not tilted in relation to the plane of earth's orbit around the sun (the ecliptic) there would be no seasonal changes on earth. But because the earth's axis is tilted in relationship to its plane of orbit around the sun (~23.5 degrees) and assumes a constant direction--with the North Pole pointing towards the North Star, seasons occur, and daylight hours can fluctuate in length. Each day prior to the summer solstice the sun appears farther north in the sky; but at the summer solstice, the sun reaches its most northerly position in the sky, directly above the Tropic of Cancer, 23.5 degrees north latitude. Following the summer solstice the sun moves farther south in the sky, and on about September 23 it is directly overhead at the equator (the fall equinox). [ Isis; v77; 103-4; 1986.] [The Physics Teacher; v31; 508-9; 1993.]

Sunspot Cycle - This cycle, lasting an average period of 11 years, involves recurring increases and decreases in the number of sunspots that occur on the sun's surface. [New Science; v141; 14; 1994.] [Energy; v18; 1273-84; 1994.]

Sunspots - A dark area, which is strongly magnetic, that occurs in the area of the photosphere on the disk of the sun. [Science; v262; 1372-3; 1993.]

Surface Area (of a particle) - A function of the diameter and number of particles; used to study particulate matter in the atmosphere and to determine atmospheric concentrations. SA = (pi)D2, where D is the diameter of a particle. [Atmospheric Environment; v41; 2183-2201; 2007.]

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