Mass accommodation coefficient

Heterogeneous uptake on surfaces has also been documented for various free radicals (DeMore et al., 1994). Table 3 shows values of the gas/surface reaction probabilities (y) of the species assumed to undergo loss to aerosol surface in the model. Only the species where a reaction probability has been measured at a reasonable boundary layer temperature (i.e. >273 K) and on a suitable surface for the marine boundary layer (NaCl(s) or liquid water) have been included. Unless stated otherwise, values for uptake onto NaCl(s), the most likely aerosol surface in the MBL (Gras and Ayers, 1983), have been used. Where reaction probabilities are unavailable mass accommodation coefficients (a) have been used instead. The experimental values of the reaction probability are expected to be smaller than or equal to the mass accommodation coefficients because a is just the probability that a molecule is taken up on the particle surface, while y takes into account the uptake, the gas phase diffusion and the reaction with other species in the particle (Ravishankara, 1997). 

Leyssens, G., Louis, F., and Sawerysyn, J.-P. Temperature dependence of the mass accommodation coefficients of 2-nitrophenol, 2-methylphenol, and 4-methylphenol on aqueous surfaces, J. Phys. Chem. A, 109(9) 1864-1872, 2003. 

Bongartz, A., J. Karnes, U. Schurath, Ch. George, Ph. Mirabel, and J. L. Ponche, Experimental Determination of HONO Mass Accommodation Coefficients Using Two Different Techniques, J. Atmos. Chem., 18, 149(1994). 

Mass accommodation coefficient (a) is the fraction of gas-condensed phase collisions that result in uptake of the gas by the condensed phase ... 

We therefore first briefly discuss the analysis of systems that involve diffusion in the gas and liquid phases, uptake, and reaction in the bulk liquid or at the interface. Following that, we give a brief description of some of the most common methods used to measure mass accommodation coefficients and reaction kinetics for heterogeneous atmospheric reactions. Included are some new approaches that appear to be especially promising. For a review of this area, see Kolb et al. (1995, 1997). 

Uptake at the interface. If the gas molecule is taken up at the surface, it enters the interface region and then the bulk. The efficiency of uptake involving crossing the interface is described by the mass accommodation coefficient (a) defined earlier. Molecular-level mechanisms by which gas molecules are taken up into liquids are discussed elsewhere (e.g., see Davidovits et al., 1995 Taylor et al., 1996, 1997 and Nathanson et al., 1996). 

FIGURE 5.16 Schematic of resistance model for diffusion, uptake, and reaction of gases with liquids. Tg represents the transport of gases to the surface of the particle, a the mass accommodation coefficient for transfer across the interface, rso, the solubilization and diffusion in the liquid phase, riM the bulk liquid-phase reaction, and rinlcrl.ll c the reaction of the gas at the interface. 

Uptake across the interface into solution (a). By definition, this is described by the mass accommodation coefficient, a, and l/a is the interfacial resistance. ... 

Fast gas transport, high solubility, and/or fast reaction. In this case, 1 /ync( approaches 1/a i.e., the maximum value for the measured uptake approaches the mass accommodation coefficient. 

That is, the net measured uptake measures the mass accommodation coefficient, corrected for the rate of transport of the gas to the surface. 

Adapted from Schwartz (1984a), and Shi and Seinfeld (1991). ka = particle radius, Dg = gas-phase diffusion coefficient, D, = liquid-phase diffusion coefficient, H = Henry s law constant, a = mass accommodation coefficient, u.w = mean thermal speed, and k = first-order aqueous-phase rate constant. 

Another important factor to recognize is that the net uptake coefficient determined using Knudsen cells may not represent the true uptake or trapping of the gas by the surface if reevaporation into the gas phase occurs, which must be taken into account in such cases. In principle, the mass accommodation coefficient is the... 

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... 

While the experiments are thus conceptually straightforward, this is not always the case with respect to the interpretation and extraction of the true mass accommodation coefficient because of the simultaneous occurrence of all of the processes depicted in Fig. 5.12. The approach to extracting a from the measurements of the net gas uptake was treated above in Section E.l. 

Use the data of Hu et al. (1995) in Fig. 5.19 to derive the second-order rate constant for the O, + I" reaction in the liquid phase assuming that solubility and gas-phase diffusion are not limiting factors. Also derive a value for the mass accommodation coefficient for O-, based on these data. The Henry s law constant for O-, can be taken to be 0.02 M atm-1, the temperature is 277 K, and the diffusion coefficient in the liquid phase 1.3 X 10-5 cm2 s-1. 

When a liquid becomes saturated with a gas, i.e., has reached Henry s law equilibrium, the rates of uptake and reevaporation are equal. Use Eq. (PP), the mass accommodation coefficient a, and the relationship between the gas-phase concentration N and the liquid-phase concentration Nt developed in Problem 5 to show that... 

McMurry, P. H., and M. R. Stolzenburg, Mass Accommodation Coefficients from Penetration Measurements in Laminar Tube Flow, Atmos. Environ., 21, 1231-1234 (1987). 

HONO also undergoes deposition at surfaces in competition with its formation by the N02 heterogeneous reaction with water. For example, the mass accommodation coefficient for HONO on water has been reported to be in the range of 4 X 10 3 to 0.15 over temperatures from 278 to 297 K (e.g., Kirchner et al., 1990 Bongartz et al., 1994 Mertes and Wahner, 1995). Thus aqueous particles and surfaces having adsorbed water can also act as a sink for gaseous HONO. This is consistent with the observations of Harrison et al. (1996) on the direction of HONO fluxes from the surface at various concentrations of N02 at N02 concentrations below 10 ppb in rural areas, surfaces were observed to be a net sink of HONO (e.g., see Harrison et al., 1996 and Harrison and Peak, 1997). 

Evidence for the uptake of NO, by aqueous solutions has been sought in both laboratory and field studies. A lower limit for the mass accommodation coefficient for NO, on liquid water of > 2.5 X 10 3 was reported by Thomas et al. (1989) and Mihelcic et al. (1993). Li et al. (1993) followed the formation of particulate nitrate in a rural area and, by comparing their measurements to model predictions, suggested that the mass accommodation coefficient for NO, on aqueous (NH4)2S04-NH4HS04-H2S04 aerosols is approximately unity, i.e. NO, is taken up into the particle on every collision. 

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). 

Thomas, K., D. Kley, D. Mihelcic, and A. Volz-Thomas, Mass Accommodation Coefficient for NO, Radicals on Water Implication for Atmospheric Oxidation Processes, International Conference on the Generation of Oxidants on Regional and Global Scales, Norwich, July 3-7, 1989. 

Table 8.5 shows the mass accommodation coefficients for S02, as well as for some other gases of tropospheric interest, on liquid water. It is seen that the uptake of most gases into liquid water is quite efficient. Interactions of gas molecules at the air-liquid interface may have additional implications other than the rate at which it is transferred into the aqueous... 

TABLE 8.5 Some Mass Accommodation Coefficients (a) for Gases of Tropospheric Interest on a Liquid Water Surface"... 

George, Ch., J. L. Ponche, and Ph. Mirabel, Experimental Determination of Mass Accommodation Coefficient, in Nucleation and Atmospheric Aerosols (N. Fukuta and P. E. Wagner, Eds.), A. Deepak Publishing, Hampton, VA, 1992. 

The condensation of low-volatility vapors on preexisting particles depends on a number of factors, including the rate of collisions of the gas with the surface, the probability of uptake per collision with the surface, i.e., the mass accommodation coefficient (see Chapter 5.E.1), the size of existing particles, and the difference in partial pressure of the condensing species between the air mass and the particle surface. While some of these parameters are reasonably well known, others are not. For example, mass accommodation coefficients for the complex surfaces found in the atmosphere are not well known. Indeed, the exact nature of the surfaces themselves, which determines the uptake and the partial pressures of gases at the surface, remains a research challenge. 

Because Type I PSCs may consist of NAT under some conditions, uptake of HC1 onto crystalline NAT as well as ice surfaces is of interest. As reviewed by DeMore et al. (1997), the mass accommodation coefficient for HC1 on both ice and NAT at stratospheric temperatures is very large, approaching unity. 

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