Particle concentration profile atmosphere

Effective thermal conductivity values of porous materials Ap,efr range between 0.1 and 0.5Js IK 1 in gaseous atmospheres [6] and are only slightly larger than those for the gas phase. Straightforward combination of eqs 23 and 37 and integration leads to a simple general result that relates the temperature and concentration profile over a particle ... 

It is the fluctuating element of the velocity in a turbulent flow that drives the dispersion process. The foundation for determining the rate of dispersion was set out in papers by G. 1. Taylor, who first noted the ability of eddy motion in the atmosphere to diffuse matter in a manner analogous to molecular diffusion (though over much larger length scales) (Taylor 1915), and later identified the existence of a direct relation between the standard deviation in the displacement of a parcel of fluid (and thus any affinely transported particles) and the standard deviation of the velocity (which represents the root-mean-square value of the velocity fluctuations) (Taylor 1923). Roberts (1924) used the molecular diffusion analogy to derive concentration profiles... 

SCR protons and alpha-particles penetrate only a very few cm into solid matter. Therefore, SCR-produced cosmogenic nuclides are normally not expected to be observed in meteorites, because their outermost few cm usually were ablated in the Earth s atmosphere. SCR nuclide concentration profiles therefore mostly have been calculated for the Moon, i.e., for samples with lunar chemical composition irradiated at 1 AU from the Sun. Most of the recent work has been done by R.C. Reedy and coworkers (e.g., Rao et al. 1994 Reedy 1998a,b) and R. Michel and coworkers (Michel et al. 1996 Neumann... 

Let us assume that we introduce a particle of radius Rp consisting of a species A in an atmosphere with a uniform gas-phase concentration of A equal to c. Initially, the concentration profile of A around the particle will be flat and eventually after time xdg it will relax to its steady state. This timescale, xdg, corresponds to the time required by gas-phase diffusion to establish a steady-state profile around a particle. It should not be confused with the timescale of equilibration of the particle with the surrounding atmosphere. We assume that cx remains constant and that the concentration of A at the particle surface (equilibrium) concentration is cs and also remains constant. 

We conclude that the characteristic time for attaining a steady-state concentration profile around particles of atmospheric size is smaller than 1 ms. Since we are interested in changes that occur in atmospheric particles and droplets over timescales of several minutes, we can safely neglect this millisecond transition and assume that the concentration is always at steady state and the concentration profile is given in the continuum regime by (12.9) and the flux by (12.12). 

The collision rate is initially extremely fast (actually it starts at infinity) but for t 4Rp/nD, it approaches a steady-state value of /coi = 8nRp DNq. Physically, at t — 0, other particles in the vicinity of the absorbing one collide with it, immediately resulting in a mathematically infinite collision rate. However, these particles are soon absorbed by the stationary particle and the concentration profile around our particle relaxes to its steady-state profile with a steady-state collision rate. One can easily calculate, given the Brownian diffusivities in Table 9.5, that such a system reaches steady state in 10-4 s for particles of diameter 0.1 pm and in roughly 0.1 s for 1 pm particles. Therefore neglecting the transition to this steady state is a good assumption for atmospheric applications. 

Figure 6. At Niinatak lake, Greenland, the sediment Pb concentration profile shows a slight surface enrichment. The baseline Pb concentration (line with no symbols), estimated using the method of Norton Kahl (1987), does not account for this surface enrichment, which might be taken as evidence for an atmospheric pollution signal. However, the acid-extractable Mn and Al profiles resemble that of Pb. The similarity of the profiles for elements with such differing mobility can only be explained by a physical transport/sorting mechanism. A plausible explanation is that all three elements are associated with fine particles formed in catchment inceptisols. A small excess of Pb above both Mn and Al in the top c. 2 cm may indicate an atmospheric pollution component.

Some typical examples of vertical concentration profiles for O3 are shown in Figure 28.25 the data shown were measured after the volcanic eruption of Mount Pinatubo in June 1991. For comparison, aerosol back-scatter data are included as well. A link between the amount of aerosol particles and the concentration of O3 in the lower stratosphere seems to be evident, when comparing the concentration values at heights below 20 km with fewer aerosol particles in the atmosphere the concentration of O3 is lower once more (for details of a long-term study of this hnk over the period 1988 to 2002, see Park et al. (2006)). 

As mentioned, the type of concentration-depth profiles observed in oceans should also be observed in lakes. However, the vertical concentration differences in lakes are often not as pronounced as in the ocean. The reason for this is, that the water column in lakes is much shorter mixing and stagnation in lakes is much more dynamic than in the oceans. Due to the presence of high concentrations of different particles in lakes, the release of trace elements from biogenic particles may not be clearly observed, due to readsorption to other particles. This would mean that low concentrations are observed throughout the water column, but that concentration differences are small. Atmospheric inputs to the upper water layers may also make it more difficult to observe a depletion of certain elements in the epilimnion. 

The approach described above is by no means complete or exclusive. For example, Lamb et al. (1975) have proposed an alternative route to assess the adequacy of the atmospheric diffusion equation. Their approach is based on the Lagrangian description of the statistical properties of nonreacting particles released in a turbulent atmosphere. By employing the boundary layer model of Deardorff (1970), the transition probability density p x, y, z, t x, y, z, t ) is determined from the statistics of particles released into the computed flow field. Once p has been obtained, Eq. (3.1) can then be used to derive an estimate of the mean concentration field. Finally, the validity of the atmospheric diffusion equation is assessed by determining the profile of vertical dififiisivity that produced the best fit of the predicted mean concentration field. 

There has not been a measured decrease in rain concentrations of PAHs over the period from 1991 to 1998 [ 148], unlike the observed decreases in concentration of these same compounds in the gas phase. This is likely due to the dominance of particle scavenging on the observed precipitation concentrations. PAHs are present in both the gas and particle phases, and Cortes et al. [33] showed that only gas-phase PAH concentrations showed a decrease with time. Particle bound atmospheric PAH concentrations exhibited no such decrease, and precipitation does not as well. Simcik et al. [148] compared the profiles of individual compounds and found the distribution in precipitation closely resembles that associated with particles in the air (r2 = 0.925) but not the gas phase (r2 = 0.085). They went on to to explain that precipitation is an effective scavenger of particle phase PAHs, and so the lack of decrease in particle-bound PAHs in the air has lead to the same lack of decrease in PAH concentration in rain. Earlier measurements revealed concentrations of 109 to 459ngL-1 across all Great Lakes on the US Canada border [149], and support the conclusion that concentrations of PAHs in precipitation are not decreasing appreciably. 

The mechanism of deposition in the size range 0.1-1 /tm, and the appropriate vg values, have been the subject of some dispute. Sulphate particles in the urban and suburban atmosphere have median diameters of about 0.5 /um (Heard Wiffin, 1969 Whitby, 1978). Using the results of Fig. 6.9, and weighting vg according to the mass of sulphate in various size ranges, Garland (1978) calculated a mean value of vg for sulphate aerosol of 1.0 mm s-1. Nicholson Davies (1987) measured the profile of SO4- concentrations, and also wind speed, above agricul-... 

Aerosol concentrations and size distributions can be investigated remotely using sun-photometry. Characterization of volcanic aerosol is important in smdies of plume chemistry, atmospheric radiation, and the environmental and health impacts of particle emissions. Watson and Oppenheimer (2000, 2001) used a portable sun-photometer to observe tropospheric aerosol emitted by Mt. Etna. They found distinct aerosol optical signatures for the several plumes emitted from Etna s different summit craters, and apparent coagulation of particles as the plume aged. More recently. Porter et al. (2002) have obtained sun-photometer and pulsed lidar data for the plume from Pu u O o vent on Kilauea, Hawaii, from a moving vehicle in order to build profiles of sulfate concentration. 

In Figure 10.21 the depth profile of the Pb concentrations of the Central Pacific is compared with that of Lake Constance. In each case, the Pb concentrations of the surface waters are higher than in the deep water in both cases atmospheric transport plays a significant role in supplying Pb to the surface water. The decrease in the concentration of Pb with depth occurs by particles that scavenge Pb(II) most efficiently. Patterson (e.g.. Settle and Patterson, 1980) has used data on the memory record of sediments to compare... 

Analogous surface maxima were observed studying the distribution of Cu along profiles of the Pacific Ocean and Indian Ocean (83, 135). In both cases the authors emphasized the presence of a minimum of concentration at a depth of about 500 m and they explained this as the effect of an important local surface source. In particular, Boyle et al. provided evidence that the surface maxima may be transient features resulting from the advecting of Cu-rich near-shore surface water into the more central regions of the oceans, while Saager et al. hypothesized the contribution of atmospheric particles to the surface concentration (19). 

Fox and Staley (1976), which ranged from 66 ng/m for benzo[a]pyrene to 120 ng/m for pyrene. Benner and Gordon (1989) postulated that the observed decrease in PAH concentrations over the 1975-85 decade resulted from the increasing use of catalytic converters in U.S. automobiles over that period. These authors also reported concentrations of PAHs in a typical vapor-phase sample from the Boston Harbor Tunnel for four PAHs included in this profile anthracene (32.3 ng/m ), fluoranthene (25.6 ng/m ), phenanthrene (184 ng/m ), and pyrene (28.3 ng/m. They emphasized that the vapor-phase samples included PAHs inherently present in the vapor phase as well as the more volatile 3- and 4-ring PAHs that may be desorbed from particles during sampling. These results underscore the need to evaluate both particle- and vapor-phase samples to obtain more reliable estimates of total atmospheric PAH concentrations. 

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