Water atmospheric residence time

Although thermodynamically it is relatively simple to determine the amount of water vapor that enters the atmosphere using the Clausius-Clapeyron equation (see, e.g.. Chapter 6, Equation (1)), its resultant atmospheric residence time and effect on clouds are both highly uncertain. Therefore this seemingly easily describable feedback is very difficult to quantify. 

In the atmosphere, the vapor pressure of the isomeric cresols, 0.11+0.30 mmHg at 25.5 °C (Chao et al. 1983 Daubert and Danner 1985), suggests that these compounds will exist predominantly in the vapor phase (Eisenreich et al. 1981). This is consistent with experimental studies that found all three isomers in the gas phase of urban air samples, but they were not present in the particulate samples collected at the same time (Cautreels and Vancauwenbergh 1978). The relatively high water solubility of the cresol isomers, 21,520- 25,950 ppm (Yalkowsky et al. 1987), indicates that wet deposition may remove them from the atmosphere. This is confirmed by the detection of cresols in rainwater (Section 5.4.2). The short atmospheric residence time expected for the cresols (Section 5.3.2.1) suggests that cresols will not be transported long distances from their initial point of release. 

Chemical degradation reactions, primarily reaction with hydroxyl radicals, limit the atmospheric residence time of benzene to only a few days, and possibly to only a few hours. Benzene released to soil or waterways is subject to volatilization, photooxidation, and biodegradation. Biodegradation, principally under aerobic conditions, is the most important environmental fate process for water- and soil-associated benzene. 

Several industrial chemicals have been created that are highly volatile, resistant to degradation, act conservatively in groundwater, are unusual or absent in the natural environment, and can be detected at very low concentrations. If these compounds show a relatively regular increase in atmospheric concentration, then their concentration in shallow groundwater systems can be correlated with the time of recharge of the water, and residence times can be determined. 

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... 

In Section 4.6.4, the role of CFCs in stratospheric ozone destruction was discussed. CFCs also are of concern because they are radiatively active in portions of the infrared spectrum not strongly attenuated by water vapor, C02, CH4, or N20. Currently, a CFC molecule added to the atmosphere absorbs about 10,000 times as much long-wave infrared radiation as does a C02 molecule. C02 has a radiative forcing of 1.8 X 10-5 W/(m2 ppb(v)), whereas CFCs range from 0.22 to 0.32 W/(m2 ppb(v)) (Prather et al., 1996). CFCs also have long atmospheric residence times, ranging from 50 to 1700 years. The locations of some CFC absorbance bands are shown in Fig. 4-42. Unlike the several radiatively active trace gases that have both natural and... 

The vapor pressure of 2,4-DNP is 1.49x1 O 5 mm Hg at 18 °C (Mabey et al. 1981). Organics with vapor pressures of 10" to 10" mm Hg at ambient temperature should exist partly in the vapor and partly in the particulate phase in the atmosphere (Eisenreich et al. 1981). Nitrophenols were detected experimentally in the particulate phase in air (Nojima et al. 1983), although the method used to collect atmospheric particulate matter was not suitable for collecting vapor-phase dinitrophenols. The distance of atmospheric transport of dinitrophenols will depend on atmospheric residence times. The residence time of dinitrophenols, based on the estimated rates of various reactions, is long enough to allow atmospheric transport (see Section 5.3.2.1). The removal and transport of atmospheric dinitrophenols to land and water by physical processes, such as wet and dry deposition, will depend on the physical states of these compounds in the atmosphere. Since dinitrophenols have been detected in rain, snow, and fog (Alber et al. 1989 Capel et al. 1991 ... 

As much of the potential riverwater input of dissolved iron is rapidly stripped out of water during estuarine mixing, the main external source of iron to the open oceans is from the (very limited) dissolution of wind-blown soil and dust. This is material derived primarily from the great Asian, African and Middle Eastern deserts (Plate 6.2, facing p. 138) of the northern hemisphere. Dust has an atmospheric residence time of only a few days, much shorter than hemispheric atmospheric mixing times. This means that the southern hemisphere oceans receive much lower atmospheric dust inputs than those in the northern hemisphere. For example, dust inputs to the North Pacific are 11 times greater than those to the South Pacific. 

Let us now assume that the residence time of a system is equivalent to the period of time that the system behaves as a closed system thermodynamically. With this assumption it is useful to qualitatively compare the residence times of different aqueous systems in the hydrosphere to the halftimes of some example reactions and reaction types. This has been done schematically in Fig. 2.2. In essence, as we examine the diagram, we can assume reactions are at equilibrium in waters whose residence times significantly exceed the half-times of reactions of interest. Note that the half-times of some solute-solute and solute-water reactions (these include some complexation and acid-base reactions [see Chaps. 3 and 5]) are shorter than the residence times of raindrops and so can be assumed to be at equilibrium in rain. These are homogeneous reactions. However, the other types of reactions shown, including atmospheric gas exchange, which is heterogeneous, are too slow to have... 

The accumulation mode results largely from gas-to-particle conversion by chemical reaction, the condensation of water and other vapors, and the attachment of particles from the uitrafine range by coagulation. A smaller part of the accumulation mode is directly emitted as primary particles. This mode is. stable with respect to deposition, interacts little with the coarse mode, and has a relatively long atmospheric residence time. It ts for these reasons that it is called the accumulation mode. 

Studies of the atmospheric input of chemicals to the open ocean have also been increasing lately. For many substances a relatively small fraction of the material delivered to estuaries and the coastal zone by rivers and streams makes its way through the near shore environment to open ocean regions. Most of this material is lost via flocculation and sedimentation to the sediments as it passes from the freshwater environment to open sea water. Since aerosol particles in the size range of a few micrometers or less have atmospheric residence times of one to several days, depending upon their size distribution and local precipitation patterns, and most substances of interest in the gas phase have similar or even longer atmospheric residence times, there is ample opportunity... 

Vapor-phase solvents can dissolve into water vapor, and be subject to hydrolysis reactions and ultimately, precipitation (wet deposition), depending on the solubility of the given solvent. The solvents may also be adsorbed by particulate matter, and be subject to dry deposition. Lyman asserted that atmospheric residence time cannot be directly measured that it must be estimated using simple models of the atmosphere. Howard et al. calculated ranges in half-lives for various organic compounds in the troposphere, and considered reaction rates with hydroxyl radicals, ozone, and by direct photolysis. 

Halogenated organic substances are a potential risk to the stratospheric ozone, provided their residence times in the atmosphere are long enough for them to reach the stratosphere. The impact on the ozone chemistry increases with atomic number, i.e., bromine is more aggressive than chlorine. The atmospheric residence times of the most stable compounds are of the order of a hundred years, while others break down within a few days. Residence times are longer in seawater, except in anoxic waters Ballister and Lee, 1995 Tanhua et al., 1996). 

Atmospheric residence times for significant gases in the atmosphere are given in Table 5.1. N2 is very large because it is an unreactive gas and is relatively insoluble in water. You might expect CO2 and O2 to have similar values since these two gases are locked into the carbon cycle. However a major difference is that CO2 dissolves and reacts readily in water to form carbonic acid. O2 is relatively insoluble and therefore the main reservoir of O2 is the atmosphere, whilst that for CO2 is in the ocean. CO2 therefore has a minor presence in the atmosphere, but the rates of formation and removal are similar to that of O2, hence the residence time of O2 is about a thousand times longer. 

Both vapor-phase and Hquid-phase processes are employed to nitrate paraffins, using either HNO or NO2. The nitrations occur by means of free-radical steps, and sufftciendy high temperatures are required to produce free radicals to initiate the reaction steps. For Hquid-phase nitrations, temperatures of about 150—200°C are usually required, whereas gas-phase nitrations fall in the 200—440°C range. Sufficient pressures are needed for the Hquid-phase processes to maintain the reactants and products as Hquids. Residence times of several minutes are commonly required to obtain acceptable conversions. Gas-phase nitrations occur at atmospheric pressure, but pressures of 0.8—1.2 MPa (8—12 atm) are frequentiy employed in industrial units. The higher pressures expedite the condensation and recovery of the nitroparaffin products when cooling water is employed to cool the product gas stream leaving the reactor (see Nitroparaffins). 

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