Scavenging rates

Using expressions obtained for the collision efficiency for E(Dp,dp) in (20.50) and (20.51), one can estimate the scavenging coefficient, and the scavenging rate for a rain 

The scavenging coefficient A(dp) given by (20.50) describes the rate of removal of particles of diameter dp by rain with a raindrop size distribution N(Dp ). If one assumes that all raindrops have the same diameter Dp, and a number concentration Afo, then (20.50) simplifies to 

The only term depending on the aerosol size dp is the collection efficiency E. The number concentration of drops can be estimated using (20.52) as a function of the rainfall intensity by 

FIGURE 20.7 Scavenging coefficient for monodisperse particles as a function of their diameter collected by monodisperse raindrops with diameters 0.2 and 2 mm assuming a rainfall intensity of 1 mmh-1. 

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Catalysis plays an important part in the hydrazine/oxygen reaction. Copper salts were formerly added for this purpose, but in recent years certain organic substances, e.g. quinhydrone, have been employed and a number of proprietary activated hydrazines have been available. These are useful at low temperatures above 150°C scavenging rates with normal hydrazine are such that no great benefit is achieved by their use. 

Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cohen AS, O Nions RK (1991) Precise determination of femtogram quantities of radium by thermal ionization mass spectrometry. Anal Chem 63 2705-2708 Cohen AS, Belshaw NS, O Nions RK (1992) High precision uranium, thorium, and radium isotope ratio measurements by high dynamic range thermal ionization mass spectrometry. Inti J Mass Spectrom Ion Processes 116 71-81... 

Cochran JK, Barnes C, Achman D, Hirschberg DJ (1995) Thorium-234/Uranium-238 disequilibrium as an indicator of scavenging rates and particulate organic carbon fluxes in the Northeast Water Polynya, Greenland. J Geophys Res 100(C3) 4399-4410... 

Such OGCM modeling also suggests the importance of ice-cover in controlling the amount of Thxs advection (Henderson et al. 1999a). Low particle fluxes beneath sea-ice may lead to low scavenging rates in these areas, particularly where ice cover is permanent. In these areas, °Thxs may be advected to the edge of the ice sheet where it is... 

Edmonds HN, Moran SB, Hoff JA, Smith JN, Edwards RL (1998) Protactinium-231 and Thorium-230 abundances and high scavenging rates in the Western Arctic ocean. Science 280 405-407 Edwards RL, Gallup CD, Cheng H (2003) Uranium-series dating of marine and lacustrine carbonates. Rev Mineral Geochem 52 363-405... 

Spin trapping methods were also used to show that when carotenoid-P-cyclodextrin 1 1 inclusion complex is formed (Polyakov et al. 2004), cyclodextrin does not prevent the reaction of carotenoids with Fe3+ ions but does reduce their scavenging rate toward OOH radicals. This implies that different sites of the carotenoid interact with free radicals and the Fe3+ ions. Presumably, the OOH radical attacks only the cyclohexene ring of the carotenoid. This indicates that the torus-shaped cyclodextrins, Scheme 9.6, protects the incorporated carotenoids from reactive oxygen species. Since cyclodextrins are widely used as carriers and stabilizers of dietary carotenoids, this demonstrates a mechanism for their safe delivery to the cell membrane before reaction with oxygen species occurs. 

K and tu to be depth independent. Analogous to that in box-models, it has also been common to assume that the removal rate, J(z), is a first order process i.e., J(z) = i >C where t is a first order scavenging rate constant. The removal is speculated to occur through an irreversible j n situ scavenging or absorption process on to settling particulate matter. In this case, the scavenging residence time of the nuclide would be ... 

Some of these particles eventually sink to the seafloor, thus removing metals from the ocean. This process of surface adsorption followed by settling is referred to as particle scavenging. The rate and degree to which a dissolved metal is scavenged from the ocean depends on (1) its elemental nature, (2) the abimdance of particulate matter, (3) the concentrations of other solutes that can compete fc>r adsorption sites, and (4) the depth of the water column. Metal scavenging rates have been inferred from the concentrations of naturally occurring radionuclides, such as " Th, Th, and Th. 

Furthermore, the increase in the scavenging rate of photoproduced electrons resulting from the presence of ozone should decrease the recombination rate of electrons and holes, and thereby augment the formation rate of hydroxyl radicals from basic OH surface groups and adsorbed water molecules (Fig. 1). 

The removal rate of particle-reactive radionuclides on particles with a pore size of 0.4 xm or greater can be made using an empirically determined scavenging rate constant (Xs), which equals 1/residence time—where the residence time (t) of an element is defined as the ratio of the element s standing stock to the removal rate or supply. 

Bruland, K.W., and Coale, K.H. (1986) Surface water 234Th/238U disequilibria spatial and temporal variations of scavenging rates within the Pacific Ocean. In Dynamic Processes in the Chemistry of the Upper Ocean (Burton, J.D., ed), pp. 159-172, Plenum Publications, New York. 

In case of electron scavenging (and no Ps lifetime quenching, as is true for both Cl" and Tl+), no other positron states are present than free e+ and Ps then, the intensities from PALS and from DB are the same. The p-Ps and o-Ps intensities are expected to decrease so that the fwhm of the DB spectra should increase with solute concentration (the narrow components are suppressed). The variations of fwhm with C can be completely calculated, knowing the intensities Ij from PALS and the Tj previously established for a given solvent. This is illustrated by the solid line in Figure 4 for Tl+ this ion, as expected from its high solvated electron scavenging rate constant, is thus shown to suppress Ps formation by electron capture. 

Figure 1. Plot of the scavenging rate constants derived from advection-diffusion--scavenging models against the stability constant for simple hydroxo complexes, Ki...

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