Residence time in the atmosphere

Gas Principal biological source Residence time in the atmosphere... 

The deposition velocities depend on the size distribution of the particulate matter, on the frequency of occurrence and intensity of precipitation, the chemical composition of the particles, the wind speed, nature of the surface, etc. Typical values of and dj for particles below about 1 average residence time in the atmosphere for such particles is a few days. 

Their residence time in the atmosphere is relatively short (months to a few years). Changes in their production rates are therefore relatively unattenuated and reflected in precipitation with good time resolution. 

Hydrochlorofluorocarbon-141b, or 1,1-dichloro-l-fluoroethane (HCFC141b), has been developed as a replacement for fully halogenated chlorofluorocarbons because its residence time in the atmosphere is shorter, and its ozone depleting potential is lower than that of presently used chlorofluoro... 

Atmospheric transport of chlordecone particles was reported as a result of emissions from a production facility in Virginia. Chlordecone concentrations at up to 15.6 miles away ranged from 1.4 to 20.7 ng/m (Epstein 1978). The long-range transport properties of chlordecone indicate that at least a portion of the emissions were of a fine particle size having a relatively long residence time in the atmosphere (Lewis and Lee 1976). 

The dry deposition velocity of lead-212, a thoron (thoron or radon-220 itself originating from thorium-232) decay product has been reported to be in the range 0.03-0.6 cm/sec (Bigu 1985 Rangarajan et al. 1986). These low deposition velocities indicate that the thoron daughter, stable lead, may have a long residence time in the atmosphere with respect to dry deposition. 

Weinstock, B Carbon Monoxide Residence Time in the Atmosphere, Science, 166, 224-225 (1969). 

Intrinsic to understanding the problem of air pollution is the necessity of understanding the chemical basis of the air pollution problem. If we are to have any grasp of the real effects of air pollution, much less predict future effects with any confidence, we must know the specific compounds involved, have some idea of their toxic nature and residence time in the atmosphere, and make some reasonable guess as to what the future composition of the atmosphere might be with respect to these phy to toxicants. For this reason Edward Schuck discusses the chemical basis of the air pollution problem. 

It is estimated that about 500 million tons of methane are being added to the air each year (Craig and Chou, 1982), largely by anaerobic production in rice paddies and wetlands as well as from the metabolism of ruminant domestic animals and, possibly, African termites (Rasmussen and Khalil, 1981 Zimmerman et d., 1982). This gas is slowly oxidized by reactions with Hydroxyl free radical. Its atmospheric content is around 5 gigatons, indicating that the residence time in the atmosphere is about 10 years. As Figure 12 shows, since 1965 the atmospheric concentration of methane has increased by about 3096. If this rate continues, the methane concentration will have doubled early in the 21st century. 

Nitrogen Oxide. The photochemical smog reaction involves nitrogen oxides, hydrocarbons, and sunlight. The global importance of this pollutant system depends upon the amounts of materials emitted to the atmosphere, their residence time in the atmosphere, and their reaction products. 

CO2, and has an average residence time in the atmosphere of 5-10 years. Carbon monoxide has an atmospheric residence time of only a few months. Its low concentration, —0.1 ppm, and its short residence time result from its chemical reactivity with OH radicals. Carbon monoxide is not a greenhouse gas, but its chemical reactivity affects the abundances of ozone and methane which are greenhouse gases. Non-methane hydrocarbons, another unstable form of carbon in the atmosphere, are present in even smaller concentrations. The oxidation of these biogenic trace gases is believed to be a major source of atmospheric CO, and, hence, these non-methane hydrocarbons also affect indirectly the Earth s radiative balance. 

Carbon dioxide is chemically stable and has an average residence time in the atmosphere of about four years before it enters either the oceans or terrestrial ecosystems. 

The atmosphere receives Nr mainly as air emissions of NOj NH3, and N2O from aquatic and terrestrial ecosystems and of NOj from the combustion of biomass or fossil fuels. NOj and NH3 (and their reaction products), can accumulate in the troposphere on a regional scale. However, because of their short residence times in the atmosphere and lack of potential for formation of N2 by denitrification, almost aU Nr emitted as NOj and NH3 is transferred back to the Earth s surface within hours to days. There is also an internal cascade of effects. (NO increases the potential first for ozone and then for aerosol formation.) Except for N2O, there is very limited potential for the long-term storage of Nr (and thus limited lag time), but there are significant effects from Nr while it remains in the atmosphere. There is no potential for denitrification back to N2 within the troposphere, and a large potential for Nr transfer to the next receptors—terrestrial and aquatic ecosystems. 

An Equilibrium Model for the Sea Abstracting from the complexity of nature, an idealized counterpart of the oxic ocean (atmosphere, water, sediment) may be visualized. Oxygen obviously is the atmospheric oxidant that is most influential in regulating (with its redox partner, water) the redox level of oxic water. It is more abundant—within the time span of its atmospheric residence time—in the atmosphere than in the other accessible exchange reservoirs. It is chemically and biologically reactive its redox processes (photosynthesis... 

During their residence time in the atmosphere, mineral dusts become coated by sulfates, nitrates, and other species (Dentener et al. 1996 Buseck and P6 sfai 1999 Zhang and Carmichael 1999 Song and Carmichael 1999 Buseck et al. 2000). These coatings are formed through chemical reactions such as the oxidation of SO2 and NO2 at the gas-solid interface, as well as by condensation of sulfuric and nitric acids. Once coated, the hygroscopic dusts act as cloud condensation nuclei and further oxidation reactions can take place in the aqueous medium (Wurzler et al. 2000). Subsequent evaporation of the cloud droplet yields a coated particle. 

If 10 Ci of cesium-137, having a half-life of 1.1 X 104 days, is blown into the upper atmosphere by a nuclear test, approximately how much ce-sium-137 may eventually return to the earth, if its typical residence time in the atmosphere is 2 years ... 

Gases with short residence times in the atmosphere are clearly those that can be removed easily. Some of these gases are removed by being absorbed by plants or solids or into water. However, chemical reactions are the usual reason for a gas having a short residence time. 

By contrast, kinetic measurements in the laboratory (which aim at determining the speed of reaction) show that gases that have slow rates of reaction with the OH radical have a long residence time in the atmosphere. Table 3.3 shows that... 

It is sobering to remember that despite the recent success in limiting production of CFCs, these stable substances have a long residence time in the atmosphere, between 40 and 150 years. This means their effects on the stratospheric 03 will continue for some time after the bans on their production. Current estimates suggest that the policies in place should see a decline in the stratospheric bromine and chlorine concentrations over the next 50 years, paralleled by a rise in ozone concentrations. 

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