Atmospheric advection

Some air pollutants, because of their prolonged lifetime in the atmosphere, advect and disperse from the emission sources and envelope the whole planet. Examples are the stratospheric 03-destroying CFCs and the global-warming-causing greenhouse gases. 

Long, P. E., and Pepper, D. W., A comparison of six numerical schemes for calculating the advection of atmospheric pollution, in "Proceedings of the Third Symposium on Atmospheric Turbulence, Diffusion and Air Quality." American Meteorological Societv, Boston, 1976, pp. 181-186. 

At times when the surface pressure gradient is weak, resulting in light winds in the atmosphere s lowest layers, and there is a closed high-preSsure system aloft, there is potential for the buildup of air pollutant concentrations. This is especially true if the system is slow-moving so that light winds remain in the same vicinity for several days. With light winds there will be little dilution of pollutants at the source and not much advection of the polluted air away from source areas. 

The Gaussian Plume Model is the most well-known and simplest scheme to estimate atmospheric dispersion. This is a mathematical model which has been formulated on the assumption that horizontal advection is balanced by vertical and transverse turbulent diffusion and terms arising from creation of depletion of species i by various internal sources or sinks. In the wind-oriented coordinate system, the conservation of species mass equation takes the following form ... 

The motions on the largest spatial scales amount to the aggregate of the world s synoptic weather systems, often called the general circulation. Both with respect to substances that have atmospheric lifetimes of a day or more and with regard to the advection of water, it is useful to depict the nature of this general circulation. The mean circulation is described to some extent in terms of the Hadley and Ferrell cells shown in Fig. 7-4. They describe a coupled circulation... 

Three regions of the atmosphere are seen to have significant zonal components of flow and thus of advection. The mid-latitude troposphere at the surface tends to exhibit westerly flow (i.e., flow from west to east) on the average. This region contains the familiar high- and low-pressure systems that cause periodicity in mid-latitude weather. Depending on the lifetime of the substances of concern, the motion in these weather systems may be important. 

In addition to dissipation of the substance from the model system through degradation, other dissipative mechanisms can be considered. Neely and Mackay(26) and Mackay(3) have also introduced advection (loss of the chemical from the troposphere via diffusion) and sedimentation (loss of the chemical from dynamic regions of the system by movement deep into sedimentation layers). Both of these mechanisms are then assumed to act in the unit world. This approach makes it possible to investigate the behavior of atmosphere emissions where advection can be a significant process. Therefore, from a regulatory standpoint if the emission rate exceeds the advection rate and degradation processes in a system, accumulation of material could be expected. Based on such an analysis reduction of emissions would be called for. 

Interphase Material Transfer. In some cases there is unidirectional bulk transfer of material and associated chemical between compartments (e.g. sediment deposition or atmospheric particle fallout) in which case the rate is given by an expression similar to that for advection in which Gg (m3/h) is the rate of transfer of the material namely... 

The Level II calculation (Fig. 4) has the same distribution as Fig. 3. The inflow of 1.30 m mol/h is largely removed by advection (1.284 m mol/h) with contributions by sediment burial (.0074 m mol/h), by sediment reaction (.0030 m mol/h) and by water reaction (.0006 m mol/h). This assumes that water to air transfer is rapid thus providing a resistance to this transfer, as in level III will alter the fate considerably. Atmospheric distribution of PCBs is likely to be important. The residence time of 400 h is largely controlled by air advection. 

The concentration of small ions in the atmosphere is determined by 1) the rate of ion-pair production by the cosmic rays and radioactive decay due to natural radioactive substances, 2) recombination with negative ions, 3) attachment to condensation nuclei, 4) precipitation scavenging, and 5) transport processes including convection, advection, eddy diffusion, sedimentation, and ion migration under the influence of electric fields. A detailed differential equation for the concentration of short-lived Rn-222 daughter ions including these terms as well as those pertaining to the rate of formation of the... 

Sea ice is represented in the model as a two-dimensional surface covered with a snowpack. Ice advection, rheology and snow cover are calculated from the sea-ice model embedded in MPIOM [Hibler (1979)]. The only source of pollutants for the ice compartment is deposition from the atmosphere. Once pollutants enter the ice compartment they can diffuse into the snow pore space air, dissolve in the interstitial liquid water or adsorb to the ice surface. Together with the sea ice the pollutants undergo advection. Sinks considered for the ice compartment are volatilisation to the atmosphere and release into the ocean with melt water. 

If the primary loss mechanism of atmospheric reaction is accepted as having a 17h half-life, the D value is 1.6 x 109 mol/Pah. For any other process to compete with this would require a value of at least 108 mol/Pah. This is achieved by advection (4 x 10s), but the other processes range in D value from 19 (advection in bottom sediment) to 1.5 x 10s (reaction in water) and are thus a factor of over 100 or less. The implication is that the water reaction rate constant would have to be increased 100-fold to become significant. The soil rate constant would require an increase by 104 and the sediment by 10s. These are inconceivably large numbers corresponding to very short half-lives, thus the actual values of the rate constants in these media are relatively unimportant in this context. They need not be known accurately. The most sensitive quantity is clearly the atmospheric reaction rate. 

Over recent years, increased computational power and improved efficiency have allowed significant developments and improvements to be applied to climate models [19], including the improved representation of dynamical processes such as advection [20] and an increase in the horizontal and vertical resolution of models. It has also enabled additional processes to be incorporated in models, particularly the coupling of the atmospheric and ocean components of models, the modelling of aerosols and of land surface and sea ice processes. The parame-terisations of physical processes have also been improved. 

The transport of heavy metals in the atmosphere is described by means of a monotone version of Bott s advection scheme. Pressure-based s-coordinate in the vertical makes possible to take into account an effect of the underlying surface elevation. Vertical eddy... 

The advection scheme of the regional model is improved to take into account the surface orography. Terrain following vertical structure of the model domain with higher resolution was incorporated. Wet removal of heavy metals from the atmosphere was enhanced by developing newparameterizations of precipitation scavenging. Both in-cloud and sub-cloud wet removal were modified on the basis of the up-to-date scientific literature data. 

To assess the relative importance of the volatilisation removal process of APs from estuarine water, Van Ry et al. constructed a box model to estimate the input and removal fluxes for the Hudson estuary. Inputs of NPs to the bay are advection by the Hudson river and air-water exchange (atmospheric deposition, absorption). Removal processes are advection out, volatilisation, sedimentation and biodegradation. Most of these processes could be estimated only the biodegradation rate was obtained indirectly by closing the mass balance. The calculations reveal that volatilisation is the most important removal process from the estuary, accounting for 37% of the removal. Degradation and advection out of the estuary account for 24 and 29% of the total removal. However, the actual importance of degradation is quite uncertain, as no real environmental data were used to quantify this process. The residence time of NP in the Hudson estuary, as calculated from the box model, is 9 days, while the residence time of the water in the estuary is 35 days [16]. 

We will now show that Eq. (4.31) may be obtained by solving the atmospheric diffusion equation in which diffusion in the direction of the mean flow is neglected relative to advection ... 

Above the atmospheric film lies the bulk atmosphere, which is well mixed by turbulence and advection and, hence, is homogeneous in gas composition. Below the sea surfece film lies the bulk seawater, which is also well mixed by turbulence and advection and is consequently homogeneous in gas composition. The thin films are regions in which turbulence and advection play minor roles, such that molecular diffusion controls the movement of gases. Because of the limited degree of air and water motion in... 

Oxygen Diffusion - advection Atmospheric concentration of oxygen... 

Particles in the accumulation range tend to represent only a small portion of the total particle number (e.g., 5%) but a significant portion (e.g., 50%) of the aerosol mass. Because they are too small to settle out rapidly (see later), they are removed by incorporation into cloud droplets followed by rainout, or by washout during precipitation. Alternatively, they may be carried to surfaces by eddy diffusion and advection and undergo dry deposition. As a result, they have much longer lifetimes than coarse particles. This long lifetime, combined with their effects on visibility, cloud formation, and health, makes them of great importance in atmospheric chemistry. 

The mathematics of the wall boundary model slightly changes if the media on either side of the interface are different. As an example, consider the volatilization of a dissolved chemical into the well-mixed atmosphere from a shallow puddle of water in which advective and turbulent motion is completely suppressed. Another example is the transport between a solid phase and a turbulent water body. 

In Illustrative Example 22.4 we reanalyze the case of the flux of trichloroethene (TCE) from a contaminated aquifer through the unsaturated zone into the atmosphere. As pointed out in Illustrative Example 19.2, measurements of TCE in the soil indicate that vertical advection of air may influence the TCE profile and the flux. 

In Illustrative Example 19.2 we discussed the flux of trichloroethene (TCE) from a contaminated aquifer through the unsaturated zone into the atmosphere. The example was based on a real case of a polluted aquifer in New Jersey (Smith et al., 1996). These authors compared the diffusive fluxes, calculated from measured TCE vapor concentration gradients, with total fluxes measured with a vertical flux chamber. They found that the measured fluxes were often several orders of magnitude larger than the fluxes calculated from Fick s first law. In these situations the vapor profiles across the unsaturated zone were not always linear. The authors attributed this to the influence of advective transport through the unsaturated zone. In order to test this hypothesis you are asked to make the following checks ... 

Explain the difference between the dispersion coefficient, dis, in a river and in the atmosphere. How is dis related to the mean advection velocity and to lateral turbulent diffusivity in each case ... 

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