Trace gases coefficients

O. Dugstad, T. Bjomstad, and I. Hundere. Measurements and appliea-tion of partition coefficients of compounds suitable for tracing gas injected into oil reservoirs. Rev Inst Franc Petrol, 47(2) 205-215, March-April 1992. 

If the flow of the carrier gas (e.g., He) is given by Fg (cm3 s 1) and An is the change in the trace gas concentration due to uptake by the droplets, then the number of gas molecules taken up per second is just FgAn. The number of gas-droplet collisions per second per unit area is given (Eq. PP) as J = NgudV/4, where N is the number of gas molecules per unit volume and wav is the mean molecular (thermal) speed. If Ad is the surface area of one droplet and there are N droplets to which the gas is exposed, then the total available surface area is (N Ad), the total number of gas-droplet collisions is J = (N Ad)NgudV/4, and the measured mass accommodation coefficient becomes... 

Kirchner, W., F. Welter, A. Bongartz, J. Karnes, S. Schweighoefer, and U. Schurath, Trace Gas Exchange at the Air/Water Interface Measurements of Mass Accommodation Coefficients," J. Atmos. Chem., 10, 427-449 (1990). 

Kirchner, W Welter, F Bongartz, A., Karnes, J., Schwdghofer, S., and Schurath, U. (1990) Trace gas exchange at the air/water interface measurements of mass accommodation coefficients, J. Atm. Chem. 10,427-449. 

Heterogeneous reactions involving water droplets in clouds and fogs are important mechanisms for the chemical transformation of atmospheric trace gases. The principal factors affecting the uptake of trace gases by liquid droplets are the mass accommodation coefficient of the trace gas, the gas phase diffusion of the species to the droplet surface and Heniy s Law saturation of the liquid. The saturation process in turn involves liquid phase diffusion and chemical reactions within the liquid droplet. The individual processes are discussed quantitatively and are illustrated by the results of experiments which measure the uptake of SOj by water droplets. 

Due to the limitations of gas diffusion, the uptake of the trace gas is reduced by a factor of n /n. To take this into account, we define a diffusion limited observable mass accommodation coefficient 74 as... 

Interfacial mass transfer is an important consideration in many dynamic processes involving the transport of a gaseous species across a gas-liquid interface. In particular the rate of trace gas incorporation into aqueous drops in the atmosphere has recently received much attention because of its relevance to acid precipitation (1,2). In the present paper, mass accommodation coefficient measurements are reported for O3 and SO2 on water surfaces, using an UV absorption-stop flow technique. The results are incorporated into a simple model considering the coupled interfacial mass transfer and aqueous chemistry in aqueous drops. Some implications of the measured accommodation coefficients on the oxidation of SO2 by O3 in cloud water are discussed. 

The flux of trace gases and particles from the atmosphere to the surface is calculated by multiplying concentrations in the lowest model layer by the spatially and temporally varying deposition velocity, which is proportional to the sum of three characteristic resistances (aerodynamic resistance, sublayer resistance, and surface resistance). The surface resistance parametrization developed by Wesely (1989) is used. In this parametrization, the surface resistance is derived from the resistances of the surfaces of the soil and the plants. The properties of the plants are determined using land-use data and the season. The surface resistance also depends on the diffusion coefficient, the reactivity, and water solubility of the reactive trace gas. 

Trace gas destruction is limited by the relatively small total ion concentration, n. Taking an upper limit oi k 10 cm s as the rate coefficient for an ion-molecule collision, a lower limit of about one day is obtained for the lifetime of a molecule against destruction or removal by ion processes. For noncatalytic ion processes the lifetime can be much larger. 

Reactions taking place on the surface of solid or liquid particles and inside liquid droplets play an important role in the middle atmosphere, especially in the lower stratosphere where sulfate aerosol particles and polar stratospheric clouds (PSCs) are observed. The nature, properties and chemical composition of these particles are described in Chapters 5 and 6. Several parameters are commonly used to describe the uptake of gas-phase molecules into these particles (1) the sticking coefficient s which is the fraction of collisions of a gaseous molecule with a solid or liquid particle that results in the uptake of this molecule on the surface of the particle (2) the accommodation coefficient a which is the fraction of collisions that leads to incorporation into the bulk condensed phase, and (3) the reaction probability 7 (also called the reactive uptake coefficient) which is the fraction of collisions that results in reactive loss of the molecule (chemical reaction). Thus, the accommodation coefficient a represents the probability of reversible physical uptake of a gaseous species colliding with a surface, while the reaction probability 7 accounts for reactive (irreversible) uptake of trace gas species on condensed surfaces. This latter coefficient represents the transfer of a gas into the condensed phase and takes into account processes such as liquid phase solubility, interfacial transport or aqueous phase diffusion, chemical reaction on the surface or inside the condensed phase, etc. 

Fig. 1-10. One-dimensional vertical eddy diffusion coefficient Kz derived from trace gas observations in the stratosphere (1) from nitrous oxide, (2,3) from methane. (1) Schmeltekopf el al. (1977), (2) Wofsy and McElroy (1973), (3) Hunten (1975).

The SIFT can be used for accurate determinations of the concentrations of trace gases, M, in an air sample that has been introduced into the carrier gas, if the rate coefficients k are known for the reactions of a chosen injected precursor ion species with the JVL Measurements of the count rates at the downstream mass spectrometer system of the precursor ion and each product ion, and respectively, provide values of for each trace gas if I. Then following Equation [1] ... 

In terms of characterizing, the ultimate performance of PTR-MS as a measurement technique, it is useful to quantify the accuracy and precision of any quantitative determination. The reader is reminded of the definitions of these two terms accuracy reveals how close a series of measurements are to the true value of the desired quantity, while precision is a measure of how reproducible each consecutive measurement is. Thus if multiple measurements of a trace gas concentration are made under identical conditions, the best estimate of the concentration will be the mean of these values. However, there is no guarantee that the mean value will be close to the true value, since systematic errors may be incorporated in this determination. As an example, if the rate coefficient used in the application of Equation 4.1 differs from the true value by a factor of two, then this relatively large error will be incorporated into the determination of the gas concentration. All of the potential sources of error for concentration determinations discussed in Section 4.4 are sources of systematic error. 

Where (rg - rn) is the fin height and yt, is the fin thickness. For the TRACE gas cooler design, the quantity x" is about 1.5, resulting in a fin efficiency of about 0.6. The actual fin efficiency would be determined with a more detailed equipment design and subsequent testing, The TRACE model uses cylindrical geometry hydraulics and heat structures. The heat structure is nodalized at 40 axial and 5 radial nodes. Axial conduction is enabled. The material properties applied are for Alloy 600. The TRACE heat structure has the same dimensions as an individual tube. A multiplier is used to produce the correct total heat transfer area. The fin surfaces are not explicitly modeled in TRACE. Instead, a gas (outside tube) heat transfer coefficient multiplier is used to account for the additional effective area. 

Foley SF, Jackson SE, Fryer BJ, Greenough JD, Jenner GA (1996) Trace element partition coefficients for clinopyroxene and phlogopite in an alkaline lamprophyre from Newfoundland by LAM-ICP-MS. Geochim Cosmochim Acta 60 629-638... 

Foley SF, Barth MG, Jenner GA (2000) Rutile/melt partition coefficients for trace elements and an assessment of the influence of ratile on the trace element characteristics of subduction zone magma. Geochim Cosmochim Acta 64 933-938... 

Jenner GA, Foley SF, Jackson SE, Green TH, Fryer BJ, Longerich HP (1994) Determination of partition coefficients for trace elements in high pressnre-temperature experimental ran products by laser ablation microprobe-inductively conpled plasma-mass spectrometry (LAM-ICP-MS). Geochim Cosmochim Acta 58 5099-5103... 

Figure 2 The absorption coefficient a, normalized by the helium and hydrogen gas densities, pi and P2, respectively, as function of frequency in the H2 roto-translational band, at the temperature of 296 K (upper trace) and 196 (lower trace, shifted downward one step for clarity). Solid and dashed curves represent calculations with and without accounting for the anisotropy of the intermolecu-lar interactions, respectively. Also shown are measurements (as in Fig. 3.12, p. 85) from Ref. [17]...

Assume that the system is being used to deposit cadmium onto the wafers. The inlet gas is primarily He, which carries a 1% trace of monatomic Cd vapor. The inlet mixture temperature is at the nominal wall temperature of T — 800°C. The cadmium vapor reacts to form a film on the lower-temperature deposition zone with a sticking coefficient of Y =0.8. All other sections of the reactor walls are presumed to be chemically inert. The process is intended to run at a nominal reduced pressure of p = 0.05 bar. 

As equilibrium is approached, the net uptake of gas decreases and with it the observed mass accommodation coefficient 705 . Once equilibrium is reached, the rate of absorption of the gas is balanced by the rate of desoiption from the liquid and the net uptake of gas is zero. Linder dynamic conditions such as found in our experiment, the surface may saturate long before the trace species penetrates into the whole droplet. At that point, the rate of gas uptake is controlled by the rate of liquid phase diffusion of the dissolved molecule away from the surface. The surface density and characteristic time rp for the surface to reach saturation can be derived as follows. The net flux Jof trace species entering the liquid is... 

Partition (Distribution) Coefficients In describing the partitioning of a trace element among coexisting phases, we frequently use a partition (distribution) coefficient for a given element, defined as a concentration ratio C2/Cj. Here C is concentration, and the subscripts identify the phases often the normalizing phase is some convenient reservoir, such as a silicate melt, with which several other phases may equilibrate. For noble gases, it is often most convenient to normalize to a gas phase. If the concentrations are expressed in the same units, the distribution coefficient is dimensionless. It is conventional to cite noble gas concentrations in condensed phases in cm3 STP/g, however, and to describe the gas phase by partial... 

The crystal-melt partition coefficient KD = CJCh where Cs is concentration in a solid and Q is concentration in coexisting liquid, is a key parameter in trace element studies of igneous systems. A noble gas crystal-melt partition coefficient is the ratio of the gas solubilities considered here. As seen in Table 2.3, solubilities have now been reported for a variety of melt compositions, but solubility data are still very scarce for solids in general. 

---