Atmosphere chemical residence times

Air and water are the two most important fluids on earth. The atmosphere and the hydrosphere are complementary in their role of transporting and transforming chemicals. The atmosphere is the fastest and most efficient global conveyor belt. Yet, certain chemicals prefer the aquatic milieu which is of global dimension, as well. In the hydrosphere typical transport velocities are significantly smaller than in the atmosphere. Therefore, residence times of chemicals in the water are usually much larger than in the atmosphere. 

A major difficulty of the physico-chemical studies is to link the observations in the atmosphere and the kinetic data determined in the laboratory (elementary reaction kinetics and atmospheric simulation in smog chambers). The most amenable method of comparing field information with kinetic data is by means of the chemical residence time r, defined by the relationship ... 

Transformation of parent contaminants into secondary products may occur during the processes of atmospheric diffusion and transport as a result of physical, chemicjd, and photochemical processes (22). Chemical conversion within the atmosphere may also change the physico-chemical characteristics of contaminants, dramatically altering their atmospheric residence times and fates from those of the parent contaminants. The complex reactions within the atmosphere that are driven by chemical processes such as hydroxyl scavenging... 

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. 

Thus, the chemical reactivity of the elements in seawater is reflected by the residence time. It is important to note, however, that while residence times tell us something about the relative reactivities, they also tell us nothing about the nature of the reactions. The best source of clues for imderstanding these reactions is to study the shape of dissolved profiles of the different elements. When we do this we find that there are six main characteristic types of profiles as described in Table 10-8. Notice that most of these reactions occur at the phase discontinuities between the atmosphere, biosphere, hydrosphere, and lithosphere. 

From Table 14-5 it is obvious that the residence time of P in the atmosphere is extremely short. This does not represent chemical reaction and removal of P from the atmosphere but rather the rapid removal of most phosphorus-containing particles that enter the atmosphere. 

Due to its chemical inertness, vaporizable nature (enthalpy of vaporization = 59.15 kJ/mol), and low water solubility (at 20°C, 2 x 10 6 g/g), elemental mercury vapor has over one year of residence time, long-range transport, and global distribution in the atmosphere [3-8]. 

Lyman, W. 1982. Atmospheric Residence Time. In Handbook of Chemical Property Estimation Methods, Environmental Behavior of Organic compounds. Lyman, W.J., Reehl, W. F., and Rosenblatt D.H., eds. McGraw Hill Book company, New York, NY. 10-2-10-33. 

The most important transformation process for di-w-octylphthalate present in the atmosphere as an aerosol is reaction with photochemically produced hydroxyl radicals. The half-life for this reaction has been estimated to be 4.5 14.8 hours (Howard et al. 1991). Actual atmospheric half-lives may be longer since phthalate esters sorbed to wind-entrained particulates may have long atmospheric residence times (Vista Chemical 1992). Direct photolysis in the atmosphere is not expected to be an important process (EPA 1993a HSDB 1995). 

The atmospheric and chemical processes controlling the spatial and temporal variability of psychoactive substances in urban atmospheres are largely uncertain, mostly due to the fact that the atmospheric residence time of these compounds is so far unclear. The transport, transformation and deposition/atmospheric removal... 

Once emitted, individual compounds may react chemically in three post-emission stages First, while suspended in the atmosphere before being sampled, the compound may react in the presence of solar radiation and various reactive species such as hydroxyl radicals and ozone. For example, the average residence time in the Los Angeles atmosphere of a parcel of air is of the order of ten hours. 

A number of studies have documented that concentrations of some of the directly emitted species found in outdoor atmospheres can be quite high indoors if there are emission sources present such as combustion heaters, gas stoves, or tobacco smoke. In addition, there is evidence for chemistry analogous to that occurring outdoors taking place in indoor air environments, with modifications for different light intensities and wavelength distributions, shorter residence times, and different relative concentrations of reactants. In Chapter 15, we briefly summarize what is known about the chemical composition and chemistry of indoor atmospheres. 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

Values of Wp for particle-associated chemicals are generally in the range 105 to 106 (Eisenriech et al., 1981 Bidleman, 1988). A value of W = 10s is calculated to result in a residence time for chemicals due to wet deposition in the well-mixed troposphere (with a scale height of 7 km) of 20 days for a constant precipitation rate of 1 m yr1 (the residence time = (height of atmosphere considered)/WJ, where J is the precipitation rate). 

With a typical value of Vdp = 0.2 cm s 1 for particles of this size range, the calculated residence time of particles and particle-associated chemicals due to dry deposition in the well-mixed troposphere is 30 days. Modeling of the transport of particle-associated 210Pb leads to an estimated residence time of particles in the atmosphere of 5-15 days due to combined wet and dry deposition (Balkanski et al., 1993), consistent with the above calculations. 

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