Uptake rate - measurements and values on bulk materials

J. Aemsol Sci. Vol. 29,

Suppl.1,pp. S6394640,

1998

Q 1998 Published by Elsevier Science Ltd. All rights reserved

Pergamon

Printed in Great Britain 0021~8502/98 $19.00 + 0.00

UPTAKE RATE - MEASUREMENTS AND VALUES ON BULK MATERIALS Ch. George and Ph. Mirabel Equipe de Physico-chimie de l’Atmosph&re CNRS - ULP 28, rue Goethe, F-67083 Strasbourg Cedex, France KEYWORDS Uptake and mass accommodation coefficients ; measurement techniques.

The atmospheric importance of gas/aerosol interactions is now highlighted by several processes e.g., the ozone hole, acid rain (Wayne, 1991) and the formation of radicals in the marine boundary layer (Behnke et al, 1997). The kinetics of these processes (i.e. physical and/or reactive uptake) are related to fundamental properties of the colliding gas and/or of the surface of the aerosol. The rate at which a trace gas molecule may be transferred, from a well mixed gas phase at a given concentration nmixd, into the condensed phase can be derived from the kinetic theory of gases which allows the calculation of the maximum flux (I+,,=crossing the interface: l =- is the trace gas average thermal velocity and CLis the mass accommodation coefficient that characterises the gas / liquid collision. It represents the probability that a molecule, impinging on the interface, will be transferred into the condensed phase. However, equation (1) does not describe the overall uptake kinetics since it does not account for limitations introduced by diffusion processes (in both phase i.e. gas and liquid), by saturation phenomena of the interface or by slow chemical transformation in the condensed phase. In order to take such limitations into account, one can extend equation (1) by introducing the uptake coefficient y which allows the determination of the effective flux 0,~ actually crossing the interface: @, =bnti,,y

(2)

In this equation which is very similar to equation (l), Q has been replaced by the uptake coefficient y. This parameter can be defined as the probability that a striking molecule will be taken up by the condensed phase (similarly to a) but considering now the overall uptake process. Therefore, the uptake coefficient y will be a function of the diffusion rates in both phases (described by their respective diffusion coefficients), of the accommodation process (described by the accommodation coefficient a), of the solubility (which depends upon the Henry’s law constant H) and of the reactivity in the liquid phase (controlled by the rate constant k). Therefore uptake rate measurements from the gas phase will provide useful information on such fundamental properties of the gas under study. Table I: Characteristics of the experimental set-up. Technique Droplet train Wetted-wall Working pressure (Torr) IO-100 760 Typical exposure time (s) 10-3- 1o-2 1 Gas phase diffusion limitations small limitations limited Surface area exposed (cm2) 10-l lo* Nature of the surface droplet film Detection limit y 2 1o-5 lo-‘lyI lOA S639

Liquid Jet 760 lOA - 1o-3 limited 10-l jet 10% y I 1o-2

SE40

Abstracts

of the 5th International

Aerosol Conference

1998

The overall principle of measuring y is very simple i.e., it consists in exposing a aqueous phase (with a given surface) to the gas under investigation and in following its decay with time or the appearance of products in both phases. During the last decade, several techniques have been developed allowing the experimental determination of y. These techniques principally differ only by the shape of the condensed phase and its exposure time of the gas phase. Their main characteristics are listed in Table I. In some cases, the measured uptake kinetics can be deconvoluted into its elementary steps (as cited above) according to (Kolb et al, 1997): -1 1 1 cc> 2 -_=-+ -+& (3) y a 4HRT,& (& 1 where Da is the aqueous phase diffusion coefficient and t is the interaction time. Table II gives some example of experimentally determined mass accommodations. In most cases, this coefficient exhibit a negative temperature dependence which can be explained by: AH’ AS’ ln a =-(4) RT + R 1 l--d 1 Nathanson et al (1996) have considered that mass accommodation can be viewed as a multistep process where the trace gas first thermally accommodates on the droplet surface, with unit probability, and remains adsorbed until it undergoes a further step into the liquid or until it is released back to the gas phase. Nathanson et al (1996) hypothesised that the rate-limiting step in the mass accommodation is part of the physical solvation process. Table II: Some of mass accommodation coefficients relevant to atmospheric chemistry (see Kolb et al. 1994. Schweitzer et al.. 1997) Gaseous AI-I* AS* a Species kcal/mol cal/mol/K cf, 260-285 -12.7 -53.7 0.07-0.0 1 CH3C(O)CH3 260-29 1 -8.1 -34.9 0.15-0.03 CH3C(O)OH 264-278 -2.7 -14.0 0.16-0.10 CH3S(02)0H 273 -5.5 -22.5 0.18 H202 260-29 1 -34.9 0.10-0.02 HC(O)OH -7.9 HCl 274 -8.8 -34.6 0.2 HN03 268 -6.6 -27.6 0.2 In regard to the number of studies made during the last year, the understanding of uptake kinetics but also of the underlying molecular processes has been significantly enhanced. Such knowledge in turn will lead to a better understanding and description of atmospheric chemistry. REFERENCES Behnke, W.; George,C.; Scheer,V.; Zetzsch, C. J. Geophys.Res.

1997,102,3795. Kolb, C.E.; Worsnop, D.R.; Zahniser MS.; Davidovits, P.; Hanson, D.R.; Ravishankara A.R.; Keyser, L.F.; Leu, M.T.; Williams, L.R.; Molina, M.J.; Tolbert, M.A.; Laboratory Studies of Atmospheric Heterogeneous Chemistry; Current Problems in Atmospheric Chemistry, J.R. Barker, ed.; Advances in Physical Chemistry Series: 1994. Nathanson, G.M.; Davidovits, P.; Worsnop, D.R.; Kolb, C.E.; J. Phys. Chem. 1996, 100, 13007. Schweitzer, Mirabel, Ph.; George, Ch. J. Phys. Chem. 1997, 102,593. Wayne, R.P. Chemistry of atmospheres; 2”d edition, Clarandon Press, Oxford, 1991