J. Phys. Chem. 1992, 96, 4973-4979 (18) Bareman, J. P.; Cardini, G.; Klein, M. L. Phys. Reu. Lerr. 1988,60, 2152. Bareman, J. P.; Klein, M. L. J . Phys. Chem. 1990, 94, 5202. (19) Harris, J.; Rice, S . A. J. Chem. Phys. 1988,89, 5898. (20) Smit, B.; Schlijper, A. G.; Rupert, L. A. M.; van Os,N. M. J. Phys. Chem. 1990, 94, 6933. Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; van Os,N. M.; Schlijper, A. G. Nature 1990, 348, 624. Smit, B.; Hilben, P. A. J.; Esselink, K.; Rupert, L. A. M.; van Os,N. M.; Schlijper, A. G. J. Phys. Chem. 1991, 95,6361. (21) Bishop, M.; Clarke, J. H.R. J . Chem. Phys. 1991, 95, 540. (22) Vacatello, M.; Busico, V.; Corradini, P. C a n . Chim. Ztal. 1984,114, 117. Vacatello, M.; Busico, V. Mol. Cryst. Liq. Crysr. 1984, 107, 341. (23) Harris, J.; Rice, S. A. J . Chem. Phys. 1988,88, 1298. (24) Millik, M.; Kolinski, A.; Skolnick, J. J . Chem. Phys. 1990, 93,4440. (25) Makovsky, N. N. Mol. Phys. 1991, 72, 235. (26) Mouritsen, 0. G. Chem. Phys. Lipids 1991, 57, 179. (27) Karaborni, S.; Toxvaerd, S. J . Chem. Phys. 1992, 96,5505. (28) van der Ploeg, P.; Berendsen, H. J. C. J. Chem. Phys. 1982,76,3271. (29) van der Ploeg, P.; Berendsen, H. J. C. Mol. Phys. 1983, 49, 233. (30) Edberg, R.; Evans, D. J.; Morriss, G. P. J . Chem. Phys. 1986, 84, 6933. (31) Toxvaerd, S. J . Chem. Phys. 1990,93,4290. Padilla, P.; Toxvaerd, S.;J. Chem. Phys. 1991, 94,5650; J. Chem. Phys. 1991, 95,509. (32) Braslau, A,; Penhan, P. S.; Swislow, G.; Ocko, B. M.; Als-Nielsen, J. Phys. Rev.A 1988, 38, 2457.
4973
(33) Toxvaerd, S. Mol. Phys. 1991, 72, 159. (34) Buff, F. P. Z . Elekrrochem. 1952, 56, 3 11. (35) Toxvaerd, S. J . Chem. Phys. 1981, 74, 1998. (36) Iwahashi, M.; Maehara, N.; Kaneko, Y.; Seimiya, T.; Middleton, S.; Pallas, N. R.; Pethica, B. A. J. Chem. Soc., Faraday Tram. 1 1985,81,913. (37) Bib, A. M.; Peterson, I. R. Adu. Marer. 1990, 2, 309. (38) Middleton, S. R.; Iwahashi, M.; Pallas, N. R.; Pethica, B. A. Proc. R. Soc. London A 1984, 396, 143. Pallas, N. R.; Pethica, B. A. Lungmuir 1985, I , 509. (39) Benattar, J. J.; Daillant, J.; Belorgey, 0.; Bosio, L. Physica A 1991, 172, 225. (40) Mc Tague, J. P.; Frenkel, D.; Allen, M. P. In Ordering in Two Dimensions; Sinha, S. K., Ed.; North Holland: Amsterdam, 1980. (41) Charych, D. B.; Landau, E. M.; Majda, M. J. Am. Chem. Soc. 1991, 113, 3340. (42) Egberts, E.; Berendsen, H. J. C. J . Chem. Phys. 1988, 89, 3718. (43) Hautman, J.; Klein, M. L. J . Chem. Phys. 1989, 91,4994. (44) Moiler, M. A.; Tddesley, D. J.; Kim, K. S.; Quirke, N. J . Chem. Phys. 1990, 94, 8390. (45) Karaborni, S.; O'Connell, J. P.J . Phys. Chem. 1990, 94, 2624; fungmuir 1990. 6,911. (46) Vilallonga, F.A,; Koftan, R. J.; OConnell, J. P. J. Colloid Interface Sei. 1982, 90, 539. (47) Townsend, R. M.; Rice, S. A. J . Chem. Phys. 1991, 94, 2207.
Measurement of the Mass Accommodation Coefficient of Ozone on Aqueous Surfaces R. G. Utter,+ James B. Burkholder,* Carleton J. Howard, and A. R. Ravishankara*gs NOAAIERL, Aeronomy Laboratory, R/E/AL2, 325 Broadway, Boulder, Colorado 80303 (Received: June 10, 1991; In Final Form: February 17, 1992)
A wetted wall tubular flow reactor was used to measure the reaction probability of ozone with water surfaces. The apparatus and procedure for analyzing the data are described. Use of liquid-phase ozone scavengers, Na2S03,Na2S203,and SnC12, at 276 K. allowed estimation of the true mass accommodation coefficient for ozone on water to be greater than 2 X This result leads toghe conclusion that the uptake of atmospheric ozone by water droplets is not limited by the mass accommodation coeficient. The rate coefficients for the reactions of ozone with the above three scavengerswere also estimated.
Introduction A significant fraction of chemical transformations in the earth's atmaphere takes place in or on condensed matter that is suspended in air. Examples are reactions in cloud droplets'-" and transformations in/on polar stratospheric clouds5 (PSC). Such reactions are important because they augment gas-phase processes such as the oxidation of SO2to ~ulfate'-~*~ and, in the case of HCs, facilitate reactions that do not take place in the gas phase. In the latter example where PSCs are essential for the formation of the Antarctic ozone hole, the effects of condensed-phase reactions are dramatic. Most of the reactants for the condensed-phase reactions are produced in the gas phase and then transported to the condensed phase. Therefore, the importance of the condensed-phase reactions depends not only on the rate of the reactions in droplets or in/on particles, but also on the rate of transport to the condensed pha~e.6.~The rate of transport to the condensed phase can be limited by the rate of diffusion of the molecule through air to the surface and by the rate at which the &aseous molecules are taken up at the surface. If the uptake results in chemical transformation, then the rate of the reaction in the condensed phase may be the rate determining step. If the surface uptake is due to physical incorporation, the solubility of the substance is a critical factor. The mass a m m o d a t i o n coefficient, a,as used here, is defined as the fraction of collisions of a gas-phase species with a surface 'To whom all correspondence should be addressed, at NOAA/ERL, Aeronomy Laboratory. NOAA/NRC Postdoctoral Research Associate. *Also affiliated with the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO. (Also afiliated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO.
that results in the transport of the species into the condensed phase. Other terms, such as the striking coefficient, are also frequently used for this quantity. The uptake coefficient, y, is the fraction of collisions that removes the species from the gas phase; i.e., it is a measure of the net transport of the species to the condensed phase. The uptake coefficient is also called the reaction probability when the removal of the species in the condensed phase is due to a reactive process. The distinction between a and y becomes important when desorption occurs. For example, in the limit that a surface becomes saturated, the mass accommodation rate is balanced by an equal desorption rate. The net transport rate, and hence y, goes to zero. y is the quantity measured in laboratory studies, and a is the upper limit to y that is measured when the desorption rate is zero. For application to atmospheric problems, knowledge of the mass accommodation coefficient is required to calculate the rate of condensed-phase reactions. In the absence of experimental data on this parameter, calculations have been carried out using a range of value^.^,^ Schwartz6v7showed that when a is above a certain limit, the rate of transport into the condensed phase is controlled by gas-phase diffusion rather than a. For example, in the case of sulfate formation in the troposphere,' the oxidation rate was limited by gas-phase diffusion rather than mass accommodation if the value of a for the oxidant was For values of a I the sulfate formation rate is critically dependent on a. Therefore, it is necessary to have accurate measurements of these quantities. Several determinations of y, and possibly a, for species of importance in atmospheric chemistry have been carried out on liquid drops,I0J1 and aerosol particles.'* Of particular interest to tropospheric chemistry and the acid precipitation problem is uptake onto liquid surfaces. Such studies have been carried out as a function of temperature for many important
0022-3654/92/2096-4973%03.00/0 0 1992 American Chemical Society
4974 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
atmospheric species by the Aerodyne-Boston College group.' I We have developed a method, using a wetted wall flow reactor, which can measure y in the range of 10-5-10-z for gaseous species on liquid water surfaces. This method has the advantage that the liquid surface is constantly renewed and that the well-established mass transport theory for cylindrical flow tube reactors can be applied to the analysis. A major limitation of our method is that the experimental temperatures, using pure liquid water, are restricted to a narrow range near 273 K, because at temperatures much above this the vapor pressure of water is too high for wellcharacterized gas flow and at temperatures below 273 K water freezes. As a fmt application of this apparatus, we measured the uptake coefficient of ozone on water. Ozone is an important oxidant for various species, including SOz,in the liquid pha~e.l-~,'J~ Also, there has been a previous study by Tang and Leei4with which we can compare our results. A study of the removal of OH and HOz radicals on water and sulfuric acid solutions is described in the following paper15 in this issue, which includes details of the analysis and theory of the method.
Experimental Section The approach employed in our study was to measure the first-order loss rate coefficient, k,, for the removal of ozone from the gas phase in a cylindrical flow tube reactor. The uptake coefficientswere calculated from k, as discussed below. The inside wall of the reactor was completely covered with a film of slowly flowing liquid: either pure water or an aqueous solution containing an ozone scavenger. Since first-order loss rate coefficients were measured, the absolute concentration of ozone is not needed; only precise measurements of the relative concentrations as a function of exposure time are needed. Ozone is not very soluble in water; its Henry's law coefficient, H,is 1.3 X M atm-' at 298 K and is, within a factor of 2, the same at 273 K, as discussed later. Therefore, the water surface is saturated very quickly, and the observed ozone loss rate coefficient is not an accurate measure of a. Once the surface is saturated, the uptake rate is limited by the slow liquid-phasediffusion into the bulk solution. Therefore, a scavenger which reacts rapidly with ozone was added to the liquid to enhance the loss of ozone in the liquid phase and hence inhibit the transport of ozone from the liquid back to the gas phase. Under such conditions,one can minimize saturation effects and thus y approaches the true mass accommodation coefficient a. A schematic diagram of the apparatus is shown in Figure 1. It consists of a vertical wetted wall flow tube in which both the liquid film and the gas flow down the tube, a movable injector through which ozone is introduced, a chemiluminescence detector for 03,and the liquid and gas handling components. The Pyrex flow tube reactor was 125 cm long with an internal diameter of 2.54 cm. The tube was maintained at a constant temperature by flowing methanol from a temperature controlled bath through a jacket surrounding the tube. This jacket was surrounded by a vacuum jacket to minimize heat loss and hence to maintain a uniform temperature along the length of the tube. The upper end of the flow tube was fitted with an annular cup so that water entered from a side arm, spilled over the lip of the cup, and flowed evenly down the inside surface of the flow tube wall. The liquid formed a thin uniform film and wetted the entire inside surface, within 10 cm of the top, as it slowly (5-15 cm s-') flowed down the flow tube. The bottom of the flow tube was cut at an angle such that the liquid collected into drops or a thin ribbon at the side that was opposite from the flow of the gas stream at the entrance to the detector region (see Figure 1). The liquid dropped into a dewar that was cooled by an icesalt mixture to approximately 260 K. The vapor pressure of water in this mixture was less than that of the liquid in the flow tube. This arrangement prevented the back-flow of water vapor from the dewar to the flow tube. However, because of the high pressures of He and the low rate of diffusion of HzO through -5 Torr of He, water was not cryopumped faster than He; Le., the flow velocity of He was the same as that of HzO. The inside wall of the flow tube was frequently cleaned with concentrated sulfuric acid, then rinsed
Utter et al. He
[
He
Vessel
Thermoregulating Fluid Return
Thermoregulating Fluid Supply
Flow antrol Valve
\ Thermoregulating Fluid Jacket
J-L
Reactant
T I_ I
Q I
Waste Dewar
F i 1. Schematic diagram of the apparatus used to measure the mass accommodation coefficient of O3on liquid water.
with a detergent solution, and finally rinsed with deionized water. This procedure kept the surface clean, which was found to be essential for uniformly wetting the flow tube wall. When not in use, the flow tube was filled with a dilute detergent solution to prevent contamination. Deionized water (1 8 Mil cm) and prepared solutions containing ozone scavengers were stored in large glass reservoirs above the flow tube. These solutions were precooled to a temperature close to that of the flow tube and deaerated by bubbling He through them. The solution passed through a Teflon valve, a flow meter, and a heat exchanger maintained at the same temperature as the flow tube, before entering the flow tube. The gas exited the flow tube through the detector where ozone was monitored using the chemiluminescent NO O3 reaction. The detector consisted of a cooled red-sensitive photomultiplier (PM) tube mounted on one port of a six way cross. The gas flowed through the cross in a direction orthogonal to the viewing region of the PM tube. Just prior to the viewing region, an excess of NO was added to the flow. The reaction of NO with O3forms electronically excited NOz (NO**) which emits light in the red and the near infrared region. The chemiluminescencewas detected by the PM tube, the signal from it was passed through an amplifier/discriminator and counted using a fast counter. This method for detecting ozone (by adding NO) or NO (by adding 0 3 )is a well-established analytical technique16 and has been routinely used for laboratory and field measurements. The concentration of NO was adjusted to maximize the conversion of O3 to NOz* in front of the PM tube while the quenching of NOz* by NO was minimized. The detection sensitivity of this arrangement was measured by flowing known concentrations of
+
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4975
Mass Accommodation Coefficient of 0,on H 2 0 ozone into the detector under the same conditions used for the ozone loss measurements. The detection limit for ozone (Le., signaI/noise = 1) was approximately 3 X IO8 molecule cm-, for a 10-s integration. Most measurements were carried out with an initial ozone concentration of approximately 1 X 10" molecule cm-,. Flow rates were measured using either calibrated electronic mass flow meters or by measuring the time rate of change of pressure in a calibrated volume. The electronic flow meters were calibrated using a wet test meter. Dilute mixtures of ozone in He were used. The concentrations of ozone in these mixtures were measured by UV absorption at 253.7 nm. The concentration of ozone in the flow tube was calculated from the flow rates and the total pressure, measured using a capacitance manometer. The dry chemicals used in the present experiments were analytical grade reagents from the Mallinkrodt or Baker companies. Helium (99.0%) was from Scientific Gases. These were used as supplied. Ozone was generated by passing dried ultrahigh purity oxygen (Scientific Gases, >99.99%) through an ac-discharge, trapped and stored on silica gel at dry-ice temperatures. Ozone was degassed at 77 K prior to the preparation of a dilute mixture in He in a 12-L glass bulb. Tap water was deionized with a series of ion-exchange columns to a resistivity of 18 Mil cm. Characterization of the Liquid F h . The fluid dynamics of a liquid film flowing down a cylindrical surface are presented by Danckwerts.I7 The thickness of the liquid layer on the wall of a cylindrical flow tube is given byI7 6 = [3r)V/7rgdp]1/3
C =
760FT 2 7 3 ~ $ ( P- PH20)
(4)
where F [cm,(STP) s-l (STP, 273 K and 1 atm)] is the sum of the measured flow rates for all of the gases (except H20), T is the reactor temperature, r is the reactor radius (cm), P is the measured operating pressure in the reactor (Torr), and PH is the vapor pressure of water (Torr) at T. As pointed out eariier, the H 2 0vapor and He were well mixed and have a common linear flow velocity. Even though the vapor pressure of H 2 0 at the temperature of the experiment was not measured, its value cannot be different from the assumed value by more than 5%. Therefore, we are confident that our measured gas velocity is accurate to at least 5% and is one of the most accurate parameters in our measurements (i.e., the error in the measured value of 7 is not determined by the error in the knowledge of the gas flow rates). The Reynold's number of the resulting helium-water vapor flow was calculated usingI9
(1)
g cm-I s-l for where r) is the viscosity of the liquid (1.62 X wateri8 at 276 K), Vis the liquid volume flow rate (cm3 S-I), g is the acceleration due to gravity (980 cm s-~),d is the flow tube diameter (2.54 cm), and p is the liquid density (1 .OO g cm-3 for wateri8at 276 K). For a typical flow rate of 1.6 cm3s-', the liquid film was 0.02 cm thick. It is worth noting that even when the liquid flow rate was varied by a factor of 5 , as was done to check for flow rate effects on the uptake, the thickness of the film changed by a factor of only 1.7. The average velocity of the liquid film down the flow tube, u, is given byI7si9 = [Pg62/3r)l
The vapor pressure of water at the operating temperature was about 5 Torr, which was sometimes more than half of the gas in the flow tube. Since the water vapor flow rate was not measured, the gas flow velocity in the reactor, c, was calculated by assuming that the water vapor was at equilibrium with the liquid at the temperature of the experiment and that there was no temperature gradient along the flow tube. Hanson et al.I5 show that the dry He carrier gas entering the top of the flow tube is uniformly saturated with water vapor within approximately 10 cm. Under these conditions, the following equation applies:
(2)
For the liquid volume flow rates of 0.5-3.5 cm3 s-I, u was 4-16 cm s-I, with a nominal value of 10 cm s-l with the typical liquid flow rate of 1.6 cm3 s-l. The Reynold's number, which is an indicator of possible turbulence in the flowing liquid film, is given byL9 NR,(film) = 4p6U/r) (3) Reynold's numbers 2500 indicate turbulent flow while NR,(film) I10 indicate laminar flow, i.e. flow with a smooth velocity gradient along 6 and parallel to the flow tube axis. In our experiments the Reynold's number was approximately 50. Therefore the liquid flow was not completely laminar, and some rippling was observed. Rippling of the liquid film can cause turbulence in the gas flow and augment, via a convective process, the diffusive transport of ozone to the wall. A more detailed discussion of the effects of rippling is given in Hanson et al.15 Because of rippling, the measured values of 7 may be larger than the actual coefficients, as discussed later. Characteristics of the Gas Flow. The flow tube reactor used in this study was operated under conditions quite different from the plug flow conditions used for most kinetics studies. Higher gas pressures were used, and the liquid on the reactor wall had a significant vapor pressure. Because of slower diffusion rates at the high He and H 2 0 partial pressures, both at - 5 Torr, there was a significant radial concentration gradient in 0,when the values of y were greater than -1 X lo-,. These conditions required an analysis to correct for the non-plug-flow conditions in the flow tube. The method developed by BrownZowas used in our study.
pg and qB are the density and viscosity of the He-H20 vapor
mixture at 276 K. For a typical value for c of 500 cm s-l, NRc was -50. Measurement Procedure. All experiments were performed between 276.0 and 276.6 K and between 10.2 and 11.0 Torr total pressure. The gas flow velocities ranged from 300 to 1000 cm si. The ozone wall loss rate coefficient was determined by measuring the ozone signal as a function of the injector position, i.e., the extent of exposure to the liquid surface. The bottom 5 cm of the flow tube was not used to determine k, because the water film was not uniform. The length of the usable reaction zone was about 50 cm. Above this region about 40 cm was allowed for the laminar gas flow profile to establish and for water vapor to saturate the He stream. The PM tube signal was recorded at injector positions spaced at 1@cmintervals along the reaction m e . Data were collected both as the injector was pulled out of the flow tube and as it was pushed into the flow tube. A background signal, due to stray light and dark counts from the PM tube, was measured by turning off the ozone flow. It was subtracted from the signal to obtain the chemiluminescencesignal. The data analysis is based upon the assumption that all the ozone loss processes are first order. The ozone loss is described by [O,] = [O3lOexp[-k,z/c - k't] where [O,] is the O3concentration with the injector at a position z (cm), [OJOis the 0,concentration for some reference injector position z,, k, is the first-order loss rate coefficient for Ojrand the term k't accounts for all other ozone loss processes including diffusion into the water reservoir and wall loss between the bottom of the reaction zone (z = 0) and the detector. A plot of In [O,] vs z yields a straight line with a slope of -k,/c, while the other loss terms contribute to the intercept. In practice the chemiluminescence signal was plotted in lieu of [O,]. The uptake of 0,on a clean water surface is reversible as there are no significant loss processes for 0,in pure water. Because the solubility of O3in water is relatively low, the liquid surface rapidly saturates and comes to equilibrium with the gas-phase 03. As saturation occurs the flux of O3from the surface equals the flux of 0,to the surface and there is no net loss of 03.In our reactor the saturation process occurred so rapidly that no O3loss
4976 The Journal of Physical Chemistry, Vol. 96, No. 12, I992
Utter et al.
TABLE I: Ex~eri~nental Panmetem .ad Measured V a l w of y with N E- W -*as the Licluid-FluseOzone Scavenger c/(cm s-l) k,/s-‘ PHe/Torr k,-/s-l T/K Ptd/Torr Y 20.1 10.41 4.56 462 13.6 1.5 x 10-3 276.6 4.43 434 9.38 12.0 8.7 x 10-4 276.6 10.28 4.85 8.84 11.1 8.1 x 10-4 433 276.6 10.70 4.85 8.75 11.0 8.0 x 10-4 276.6 10.70 425 4.39 4.77 439 5.35 3.9 x 10-4 276.6 10.24 4.59 10.48 423 1.72 1.3 X lo4 1.66 276.7 4.77 1.45 276.8 10.70 406 1.50 1.1 x 10-4 4.24 440 0.55 4.0 X 0.54 276.8 10.17 4.61 0.11 8.2 x 10” 10.46 428 0.1 1 276.6 TABLE 11: Experimental Panmeters and Measured Values of y with Na&03 PB the Liquid-Phase Ozone Scavenger c/(cm s-I) k,/s-l T/K Pmd/Torr P,,/Torr kwanr/s-l Y 16.4 27.1 2.0 x 10-3 4.95 393 276.6 10.80 10.4 7.6 X lo4 276.4 10.85 4.89 457 8.40 276.5 10.40 4.59 443 7.27 8.74 6.4 X lo4 276.5 10.82 5.01 382 5.15 6.63 4.8 x 10-4 276.5 10.70 4.89 382 5.50 6.30 4.6 x 10-4 276.5 10.72 4.91 413 4.30 4.77 3.5 x 10-4 276.5 10.56 4.15 439 3.37 3.65 2.7 X lo4 1.18 1.21 8.8 x 10-5 276.5 10.85 5.04 402 276.5 10.75 4.94 410 0.50 0.51 3.1 x 10-5
[NazSO3I/M 1.8 x 10-1 8.4 x 3.7 x 4.5 x 8.0 x 1.3 X 2.0 x 5.0 X 2.1 x
10-2 10-2
10-2 10-3
lo-’
10-4 10-5
[Na2S,O3I/M 1.7 X 8.0 x 4.0 X 1.5 X 2.4 X 7.0 x 8.0 x 2.0 x 2.0 x
10-1 10-2
1C2 10-3 10-3 loJ 10-5
TABLE III: Experimental Parameters and Measured Values of y with SnC12 118 the Liquid-Phase Ozone Scavenger
T/K 275.9 216.5 276.5 216.5 276.4 276.5 276.5 276.5 276.5 276.5 276.5
PtOdTO~ 10.60 10.60 10.40 10.35 10.70 10.75 10.80 10.40 10.56 10.50 10.47
PHe/Torr 5.03 4.79 4.59 4.54 4.93 4.94 4.99 4.59 4.75 4.69 4.66
c/(cm
SI)
483 403 515 521 433 430 408 443 428 434 427
was observed when a clean water surface was employed. To overcome this problem, we added to the liquid some reagents which are known to react with O3in solution to scavenge the adsorbed gas and to effectively eliminate the flux from the surface. Thus the uptake coefficient we measure is a reaction probability if it is not limited by a. In other words, the measured uptake coefficient is a lower bound for a. We used sodium sulfite (Na2S03),sodium thiosulfate (Na2S203),and S ~ ~ M O chloride W (SnC12)as scavengers as these species are oxidized by ozone. The concentrations of sodium sulfite, sodium bisulfate, and stannous chloride were varied between 5 X and 1.8 X 10-1 M, 2 X and 1.7 X 10-1 M, and 9.6 X and 6.4 X 10-1 M, respectively. The upper limits of the concentrations were determined by the solubility of the reagents in water. The stannous chloride solutions were prepared by acidifying the water with HCl to assist in dissolving the salt.
Data A ~ l y s i and s Results The primary quantity measured in these experiments was the first-order loss rate coefficient, k,, for gas-phase ozone due to the uptake at the wall. The relatively high gas pressure, containing a large amount of H 2 0vapor, and the efficient removal of O3on the wall contribute to the development of significant radial gradients in the O3concentration. Using the method developed by Brown,2owe calculated the corrected wall loss rate coefficient, k,””, from the measured value of k,. More details of this procedure are given in the following paperI5 in this issue. The pressure-independent, binary diffusion coefficients for O3at 276 K were estimated to be 110 Torr cm2 s-’ in H 2 0 and 400 Torr cm2 s-l in He, yielding a value of 16 cm2 s-l for the diffusion coefficient of O3 in our mixture of water and He at 276 K. In the limit that the radial concentration gradient becomes large, the measured wall loss rate is limited by the diffusion of O3 to the wall and the uncertainty in y becomes very large. This occurs when y is greater than 0.02, where upon the measured k, cor-
-
k,/s-’ 43.1 33.8 29.0 16.9 14.3 6.74 4.29 2.92 2.24 0.68 0.26
kWmm/s-l
468 298 112 28.5 21.7 7.98 4.76 3.13 2.36 0.69 0.26
[SnC121/M
7
3.4 x 2.2 x 8.1 x 2.1 x 1.6 x 5.8 x 3.5 x 2.3 X 1.7 X 5.0 x 1.9 x
10-2 10-2 10-3 10-3 10-3 10-4
lo4 lo4 lo4
10-5 10-5
6.4 X 1.9X 1.2 x 4.0 X 3.1 X 1.4 X 7.4 x 3.8 x 1.8 x 2.5 X 9.6 X
10-1
lo-’ 10-1
10-3 10-3 10-3
lo4
responds to a gaseous diffusion rate and is relatively insensitive to the value of the uptake coefficient.15 The value of y is extracted from the corrected first-order rate coefficient using an analysis based on gas kinetic theory. The appropriate relation for a cylindrical reactor2I is
kWcorr = yw/2r
(7)
where w is the average molecular speed of the O3 reactant (cm s-l) and the other parameters have been defined earlier. Figure 2 shows a plot of In [O,] vs the exposure distance (time). From the slope of such plots the wall loss rate coefficient k, was as described calculated. The value of k, was corrected to kWCOrT earlier; the decay corresponding to the corrected slope is also shown in Figure 2. k, was measured at various concentrations of liquid-phase scavenger shown in Tables 1-111. The corrected wall loss rate coefficients along with the calculated values of y are also shown in the tables. The major sources of error in the measurement of k, (all at the 1u level) are uncertainties in pressure including the pressure drop across the length of the flow tube (14%), gas flow rates (S2%), statistical error in determining the relative [O,] as a function of distance ( the high concentration points, since they deviate slightly from the expected line in a log-log plot, we can very conservatively place a lower limit of 2 X lo-' for a. Schwartz' has shown that when the processes that control the transfer of atmospheric a1 O3 into water are the rate of gas-phase diffusion to the liquid surface and the rate of reaction in the liquid phase. Our measured value of y shows that a is indeed greater than for the case of ozone loss to water. In our study the increased surface loss of ozone was due to chemical destruction of ozone reactant in the liquid phase. The relationship6J'J7 between the measured uptake coefficient and reactive loss in the liquid phase is l/y = l / a
+ U / ~ R T H ( ~ ~ D ~ ) ~ / (8) ~
where R is the gas constant (1 atm K-'mol-'), Tis the temperature (K),H is the Henry's law constant (M atm-I), Dl is the diffusion coefficient of ozone in the liquid phase (cm2 SI), and kl is the pseudo-first-order rate coefficient for the reaction of ozone with the scavenger in the liquid phase; Le., kl = kl*[scavenger]. Here k" is the second-order reaction rate coefficient for the reaction of ozone with the scavenger. If the uptake is limited by reactive loss in the bulk, i.e. w/4RTH(klD1)'l2 >> l / a , a plot of log y vs log [scavenger] yields a straight line with slope equal to 0.5. As seen in Figures 4 4 ,the plots of log y vs log [scavenger] are indeed linear. The slopes are 0.49 f 0.19 and 0.39 f 0.1 1 for the sodium sulfite and sodium bisulfate, respectively, where the errors are 2a (statistical error only). These values are 0.5 within experimental error, as expected from the equation. The values for the
rate coefficients for the reactions of sulfite and thiosulfate with ozone of 3.92;; X 10" and 2.2%; X 10" M-'s-l, respectively, at 276 K are calculated from the extrapolated value of y at 1 M solute concentration. The error bars are the 95% confidence limits and are obtained from the uncertainty in y. Similar values of the rate coefficients were obtained when calculated from individual measured values of y and eq 8. In these calculations, H = 2.1 X M atm-' (extrapolated from the data in ref 24) and D = 2.0 X lo-' cm2 s-' (based on values for SOz and C 0 2 in ref 7). The derived rate coefficient for sulfite compares very favorably with the value recommended by Hoffmann13 ((4.5 f 1.8) X lo8 M-'S-I) at 276 K. The rate coefficient for the reaction of thiosulfate with ozone has not been previously reported. When a similar analysis is attempted for the case of SnC12,the slope of the log-log plot is 0.87 f 0.12 and the rate coefficient for the reaction between O3and Sn2+is 4.32;; X lo9 M-I s-'. The slope is significantly different from 0.5 even when the error bars are considered. The reason for this deviation is not clear, but could be due to the reaction of SnZ+with ozone not being a simple bimolecular reaction, or due to errors in measuring the high values of y in this system. If we neglect the data for y > Le., those values for which the correction due to the radial concentration gradient is more than a factor of 1.5, the slope of the log y vs log [SnCl,] is 0.65 f 0.06 and the rate coefficient for the reaction is (8.5??$) X lo8 M-'s-l. This slope is closer to 0.5. In the above calculations we have used a Henry's law constant for O3of 2.1 X M atm-' at 276 K. The value of H is affected by both ionic strength and pH of the solution. On the basis of the measured salting out effect for the case of the effect of ionic solutes on H in our experiments should be less than 15%. Thus this effect is negligible compared to the other errors in our calculated rate coefficients. The Henry's law constant for O3 decreases at lower pH.2' However, the pH of the solutions flowing down our apparatus were not measured and could be different from that in the stock solution. Therefore, this uncertainty in the value of pH introduces an uncertainty of a factor of 1.4 in the calculated rate coefficients. As discussed by Hanson et al.,'' a small uncertainty in the gas-phase diffusion coefficient propagates to a substantial uncertainty in the calculated value of y. To ascertain whether the choice of the gas-phase diffusion coefficient for O3led to the large slope in the plot of log y vs log [Sn2+],its value was varied to see if the slope could be reduced to 0.5. The value of the diffusion coeffcient needed is unrealistically high, approximately twice the value used here. Furthermore, this value of the diffusion coefficient will yield much lower slopes for the sulfite and thiosulfate data. Therefore, we believe that the reason for the break in the data at y > 2 X is due either to unknown solution reactions in this system or to a difficulty in our system for measuring large values of y. In spite of this uncertainty in the SnC12 data, we believe that our measurements do show that the model for the transport of ozone to the liquid phase is indeed correct and that a 2 2 x 10-3. In conclusion, we report that the mass accommodation coefficient for ozone on water is greater than 2 X l C 3and is very likely to be close to unity. In any case the mass accommodation coefficient is not the limiting factor for atmospheric ozone to oxidize substances dissolved in the liquid phase.
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Acknowledgment. We thank D. R. Hanson for his many comments and great help in preparing the manuscript, M. Mozurkewich for bringing to our attention the relationship between y and the concentration of the scavenger, M. Trolier for help with the parabolic velocity profile corrections, D. W. Worsnop for helpful discussions regarding the variation of y with scavenger concentrations, and Allen Fuller of Corning, Inc. for helpful discussions on cleaning and wetting glass. R.G.U. thanks Prof. Allen J. Bard, University of Texas, for helpful discussions on chemical equilibria. This work was supported by NOAA as a part of the National Acid Precipitation Assessment Program. Registry No. 03,10028-15-6; HzO,7732-18-5; Na2S03,7757-83-7; Na2S20,, 7772-98-7; SnClZ,7772-99-8.
J. Phys. Chem. 1992,96,4979-4985
References and Notes (1) Erickson, R. E.; Yates, L. M.; Clark, R. L.; McEwen, D. Armos. Enuiron. . . 1977. 11. 813-817. (2) Penkett; S. A.; Jones, B. M. R.; Brice, K. A.; Eggleton, A. E. J. Armos. Emiron. 1979, 13, 123-137. (31 Calvert. J. G.: Lazrus. A.: Kok. G. L.; Heikes, B. G.; Waleaa, - J. G.; Lind,'J.; Cantrell, C. A. Nature 1985, 317, 27-38. (4) Chameides, W.L.; Davis, D. D. J . Geophys. Res. 1982,87,4863-4877. (5) Solomon, S . Reu. Geophys. 1988, 26, 131-148. (6) Schwartz, S . E. In Chemistry of Multiphase Atmospheric Systems; Jaeschke, W.,Ed.; NATO AS1 Series; Springer-Verlag: Berlin, 1986; Vol. G6. p 415. (7) Schwartz, S. E. Atmos. Enuiron. 1988, 22, 2491-2499. (8) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Molina, M. J.; H a m p son, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, Evaluation 9 JPL Publication 90-1; California Institute of Technology: Pasadena, $A: 1990. (9) Hanson, D. R.; Ravishankara, A. R. J . Geophys. Res. 1991, 96, 5081-5090. (10) Gardner, J. A.; Watson, L. R.; Adewuyi, Y. G.; Davidovits, P.; Zahniser, M. S.; Worsnop, D. R.; Kolb, C. E. J . Geophys. Res. 1987, 92, 10887-10895. (1 1) Jayne, J. T.; Gardner, J. A.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E. J. Geophys. Res. 1990, 95, 20559-20563 and references cited therein.
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(12) Mozurkewich, M.; McMurry, P. H.; Gupta, A.; Calvert, J. G. J. Geophys. Res. 1987, 92, 4163-4170 and references cited therein. (13) Hoffmann, M. R. Armos. Ewiron. 1986, 20, 1145-1154. (14) Tang, I. N.; h, J. H. In The Chemistry of Acid Rain; Sources and Atmospheric Processes; Johnson, R. W., Gordon, G. E., Eds.; ACS Symposium Series 349; American Chemical Society: Washington, DC, 1987; pp 109-1 17. (15) Hanson, D. R.; Burkholder, J. B.; Howard, C. J.; Ravishankara, A. R. J . Phys. Chem., following paper in this issue. (16) Ridley, B. A. Armos. Ewiron. 1978, 9, 27-34. (17) Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970. (18) Handbook of Chemistry and Physics, 60th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1979. (19) Geankoplii, C. J. Transport Processes and Unit Operations, 2nd Ed.; Allyn and Bacon: Boston, 1983. (20) Brown, R. L. J. Res. Natl. Bur. Stand. (US.)1978, 83, 1-8. (21) Howard, C. J. J. Phys. Chem. 1979,83, 3-9. (22) Mozurkewich, M. Private communication. (23) Goldberg, R. N.; Parker, V. R. J. Res. Narl. Bur. Srand. (US.)1985, 90, 341-358. (24) Kosak-Channing, L. F.; Helz, G. R. Ewiron. Sci. Technol. 1983, 17, 145-1 49. (25) Roth, J. A.; Sullivan, D. E. Ind. Eng. Chem. Fundam. 1981, 20, 137-140.
Measurement of OH and H02 Radical Uptake Coefficients on Water and Sulfuric Acid Surfaces David R. Hamon,*.+ James B. Burkholder,t Carleton J. Howard, and A. R. Ravishankaras NOAAIERL. Aeronomy Laboratory, RIElAL2, 325 Broadway, Boulder, Colorado 80303 (Received: June 10. 1991; In Final Form: February 17, 1992)
A wetted wall flow tube reactor was used to measure the uptake coefficients, y, of OH and H 0 2 on pure water at 275 K and 28% w/w sulfuric acid at 249 K. The uptake coefficients are lower limits to the mass accommodation coefficients, a, and the y were determined to be 0.0035 for OH and >0.01 for HOz on liquid water and >0.08 for OH and s0.05 for HOz on the sulfuric acid solution. In addition, the binary diffusion coefficients for H 0 2 and OH in water vapor were estimated to be 79 f 8 and 116 f 20 Torr cm28,respectively, at 275 K. The determination of uptake coefficients using wetted wall flow tube reactors is described including the effect upon y of gas-phase diffusion, solvation, and reaction within the liquid.
Introduction In the preceding paper in this issue, Utter et al.' explained the motivation for measuring the mass accommodation coefficients of trace atmospheric species onto liquids and solids. To summarize briefly, some reactions which are negligibly slow in the gas phase occur much more rapidly in/on condensed media. As the condensed-phase reactants are made in the gas phase and then transported into the liquid or solid phase, transport of gases into atmospheric particles can be the rate limiting step for these reactions. The mass accommodation coefficient, a,is the parameter that can limit transport at the gas-condensate interface. Utter et al.' described our experimental approach, where a wetted wall flow tube2 was used to measure the mass accommodation coefficient of ozone onto liquid surfaces. Using much the same apparatus, we report measurements of the uptake coefficientsof OH and H 0 2 on liquid surfaces. These species could play important roles in the liquid-phase oxidation of acid precursors. Their transport into the condensed phase could account for a significant loss for these radicals from the gas phase and could lead to formation of peroxide^.^ No measurements of a To whom correspondence should be addressed. 'NOAA/NRC Post-Doctoral Research Associate. *Also affiliated with the Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO. 1 Also affiliated with the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO.
for OH and H 0 2radicals on liquid water have been reported, and such measurements would improve our understanding of the transport process. If a surface becomes partially saturated with a species, a measured loss rate may not be representative of the gross transport that occurs into the liquid. To avoid this, a chemical scavenger is dissolved in the liquid so that the species is rapidly removed and transport back from liquid to gas is minimized. The value obtained under these conditions represents the reaction probability or uptake coefficient, 7,for that species and is taken to be a lower limit to the mass accommodation coefficient. Although measurements of the mass accommodation coefficient of OH on aqueous surfaces have not been previously reported, Baldwin and Golden4 measured y = 5 X lo4 for OH on -96% sulfuric acid at 298 K. Gershenzon et al.s report very high values for y of OH on ice at 253 K and on -96% sulfuric acid at 298 K, 0.4 and 1, respectively. The loss of H 0 2 on small NH4HS04 and LiN03 aerosols (-0.1-pm diameters) has been investigated by Mozurkowich et a1.6 who report a 1 0.2. Their aerosols were concentrated solutions and doped with CuS04 in order to scavenge H 0 2 in the liquid. A critical process which can limit transport in our system, as well as in the atmosphere, is gas-phase diffusion. Therefore, knowledge of the diffusion coefficients of gases is important. The diffusion coefficients of polar gases, such as OH and HOz, can be used to test the calculated transport properties of systems that
0022-365419212096-4979$03.00/00 1992 American Chemical Society
