Chapter 6
Heterogeneous Atmospheric Chemistry of Alternative Halocarbon Oxidation Intermediates Downloaded by NORTH CAROLINA STATE UNIV on October 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/bk-1995-0611.ch006
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C. E. Kolb , Douglas R. Worsnop , M . S. Zahniser , W. J . De Bruyn , Jeffrey A. Shorter , and P. Davidovits 2
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Center for Chemical and Environmental Physics, Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821-3976 Department of Chemistry, Boston College, Chestnut Hill, MA 02167 2
Alternative halocarbons designed to substitute for chlorofluorocarbons and halons banned by the Montreal Protocol are designed to undergo oxidative degradation in the troposphere. Intermediate partially oxygenated products from currently used alternatives include carbonyl halides, haloacetyl halides and haloacetic acids. These intermediates are expected to undergo heterogeneous reactions with aqueous cloud droplets and surface waters. The chemical and physical parameters which govern the heterogeneous uptake of a range of these species by liquid water have been measured in our laboratories using droplet train/flow tube and/or bubble column techniques. The results of these experiments will be presented and compared with available results from other laboratories. Their impact on our ability to predict the atmospheric fate of halocarbon oxidation intermediates will be discussed. Atmospheric Fate of Alternative Halocarbon Oxidation Products Alternative halocarbons planned as substitutes for chlorofluorocarbons and halons banned by the Montreal Protocol are designed to undergo oxidative degradation in the troposphere. Intermediate partially oxygenated products from currently used or proposed alternatives include carbonyl halides, haloacetyl halides and haloacetic acids. A wide variety of atmospheric processes could participate in the removal of relatively stable degradation intermediates, including: transport to the stratosphere followed by photolysis, uptake by both tropospheric and stratospheric aerosols, uptake by cloud droplets, rainout (wet deposition), and dry deposition to vegetation, soil and dew surfaces, and to the oceans. However, recent model analyses indicate that cloud droplet and ocean uptake are the most likely removal rate determining processes. The physicochemical processes which are important in determining the rate of trace atmospheric gas uptake by liquid surfaces include gas phase diffusion, mass accommodation, solvation (Henry's law solubility), liquid phase diffusion and liquid phase reaction. The most effective removal paths and their associated physicochemical parameters are displayed in Table I. Key parameters affecting atmospheric removal rates are the mass accommodation coefficient, a, the Henry's law constant, H, and the first order hydrolysis reaction rate coefficient, k . h y d
0097-6156/95/0611-0050$12.00/0 © 1995 American Chemical Society In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Alternative Halocarbon Oxidation Intermediates
6. KOLB ET AL.
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Table I.
Atmospheric Fate O f Alternative Halocarbon Oxidation Products Halocarbon Oxidation Produces Halocarbonyl Compounds: R X O (g) n
[ R X O = C F 0 , CC1 0, CF C(0)F, CF C(0)C1, CC1 C(0)C1] n
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Atmospheric Fate Of Halocarbonyls hv RX O(g)
> Stratosphere
Downloaded by NORTH CAROLINA STATE UNIV on October 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/bk-1995-0611.ch006
n
> Photolysis Products
Cloud RX O(g)
> RX O(aq) Accommodation by Liquid
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Ocean RX O(aq) n
1 J a,H > RX O(g) n
Evaporation
RX O(g) — > R X _ j 0 - + X- + 2H+ n
n
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Hydrolysis } k
h y d
rain RX .!02"(aq) — > Ocean/Land Deposition n
Experimental uptake data for the halocarbonyl compounds C F 0 , CC1 0, CF C(0)F, CF C(0)C1 and CC1 C(0)C1 will be presented and reviewed. Each of these halocarbonyl compounds is only sparingly soluble in aqueous solutions so their uptake is controlled by both their Henry's law solubility constant, H , and their hydrolysis reaction rate constant, k . Their uptake rate by cloud droplets or ocean surfaces is proportional to the product of these parameters, H k . The trihaloacetly halide compounds hydrolyze to the corresponding trihaloacetic acid in aqueous environments such as cloud droplets. Since most clouds evaporate rather than precipitate, it is necessary to know the uptake coefficients for these species as well, in order to determine how soon they are likely to be reabsorbed into the aqueous phase after vaporization from a dehydrating cloud. Since these compounds are highly soluble in dilute aqueous solutions, their uptake will be controlled by their mass accommodation coefficients rather than their Henry's law constants. 2
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n y d
h y d
Experimental
Procedures
Uptake measurements in our laboratories are made either with a well documented droplet train apparatus (1), which we have used to measure uptake parameters for approximately 40 gaseous atmospheric trace species to date, or with a novel bubble column technique newly developed by our Aerodyne Research, Inc./Boston College (ARI/BC) collaboration (2), In our laboratory uptake coefficients are derived from either technique by measuring the diminution of the gas phase concentration of the trace gas species of interest as the contact time between gas a phase mixture containing that species and the liquid (aqueous) phase is varied. In both techniques the temperature and pH of the aqueous phase are varied to assess the dependence of uptake parameters on these properties. The major difference between the two techniques is the duration of the contact time between the gaseous and liquid phases. In the droplet train technique this ranges from 1 to 20 milliseconds while in the bubble column technique it typically varies from 0.1 to 1 seconds.
In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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HALON REPLACEMENTS Both experiments measure an effective uptake coefficient, y m e a s , defined as : 1
_ Number of molecules lost to the surface (s" ) meas
1
Number of gas - surface collisions (s" )
^
The difference in gas-liquid contact times makes allows the droplet train technique to determine y m e a s values in the range of 1 to 5 x IO while the bubble column technique is sensitive to y m e a s values of order IO to IO . Mirabel and co-workers have recently developed a version of the droplet train method which measure the build-up of trace gases or their reaction products in the liquid phase by trapping and pooling the exposed droplets. This method extends the sensitivity of the droplet train technique to somewhat smaller values of y m e a s allowing them to use this method to measure uptake parameters for several of the compounds of interest to this study (3,4). Measurements of y m e a s as a function of contact time and other experimental parameters can be analyzed to yield a variety of fundamental physical and chemical parameters controlling uptake, including mass accommodation coefficients, Henry's law constants, reaction rate constants and liquid phase diffusion coefficients, depending on the actual parameter or parameters which control uptake for a given trace gas/liquid surface combination. This analysis process has recently been reviewed by Kolb et al. (5); this review also discusses the two experimental techniques noted above and compares their capabilities with other recently developed methods for measuring gas/aqueous surface and gas/ice kinetics. Further experimental details will not be presented here, but can be found in references 1-7. -4
Downloaded by NORTH CAROLINA STATE UNIV on October 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/bk-1995-0611.ch006
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Halocarbonyl Uptake Results For the moderately soluble halocarbonyl compounds noted above both the droplet train and the bubble column technique directly measure the product of H k with the possibility of separately determining H from the temporal dependence of the trace gas uptake at short times. Most laboratory heterogeneous chemistry experiments are sensitive to H k not H k , which is often the more atmospherically relevant parameter product (see discussion below). Some time ago we published droplet train studies of these compounds (6) showing that for the atmospherically relevant droplet pH range of 3-7 their y m e a s values were below 5 x IO" , although larger uptakes were measured for droplets with pH levels above 12.These results led us to develop the bubble column technique described in reference 2 and then to apply it to the five halocarbonyl compounds ( C F 0 , CC1 0, CF C(0)F, CF C(0)C1 and CC1 C(0)C1) studied previously (7). Results from this study (labeled DeBruyn et al.), based on analyses of y m e a s , are displayed in Table II. These analyses confirmed that the time dependent uptake data are not sensitive enough to clearly determine separate values of H, so Table II contains H k values as well as an entry for H , which is the Henry's law constant needed to explain the observed uptake if the hydrolysis rate constant is zero (7). Also shown in Table II are the results, in the form of upper limits, from our previous, less sensitive droplet train uptake experiment (6). Table II also presents results for four of the target carbonyl compounds derived from droplet train uptake experiments performed by Mirabel and coworkers (3,4) as well as other measurements of H and k available in the literature. Uptake parameter estimates prepared by Wine and Chameides (8) are also included. The results from Mirabel's group are derived from an analysis of droplet train uptake data that is essentially the same as that used in the Aerodyne Research, Inc./Boston College effort. Additional literature values of k and H for CC1 0 (which has not yet been studied by the Mirabel group) are also listea (9-12). 1 / 2
n y d
1 / 2
h y d
n y d
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1 / 2
h y d
m a x
h y d
h y d
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In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
6. KOLB ET AL.
Alternative Halocarbon Oxidation Intermediates
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Table II.
Solubility and Hydrolysis of Haloacetyl and Carbonyl Halides (PH 4-7) Hk Temp. H Hyd Hk l/2 (Matm- s" ) (M/atm) (S") Species/Reference (K) CF C(0)F a * De Bruyn et al. (7) 278 0.96 3.8 4.3-96 George et al.(4) 150 450 273 3.0 60,37 CF C(0)C1 ' De Bruyn et al. (7) 278 0.27 1.2 1.2-27 George et al.(4) 273 2.0 220 60,30 440 CC1 0 De Bruyn et al. (7) 278 0.15 0.66 0.68-15 298 0.29 0.06 0.17 0.4 6 Manogue/Pigford (9) 0.07 298 Ugi/Beck(10) 253 -100 0.17 Behnke etal. (11) 296 3.8 d Mertensetal. (12) 298 5.3 298 CF 0 De Bruyn et al. (7) 278 1.0 4.3 4.7-97 George et al.(4) 273 350 CC1 C(0)C1 b e Bruyn et al. (7) 278 2.0 6.9 9.1-150 45 1000 274-294 2.0 George et al. (3) 500 George et al. (3) 288 -1508 >10^ >0.3 >10" Wine/Chameides (8) >10 (all 5 compounds) which equation 3 reduces to: 1
h y d
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t
m
x=
(HRTk
h y d
L f )-l c
~ 15(Hk )-l
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(6)
hyd
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For the conservative estimate of H k > 0.5 M a t n r V , this gives an upper limit to the tropospheric lifetime of the halide species of x < 30 days. h y d
Previous Work
This Work C(0)CI CF C(0)CI CF C(0)F C(0)F CCI C(0)CI
1000
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•g
100
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(208K) (278K) (278K) (278K) (278K)
O ref11 (295K) 4 ref4(284K) • ref 4 (284K) •
ref3(284K)
o E • o 'SZ
£ a g
a. o
101
10*
10*
10°
Figure 2, Tropospheric lifetimes (x) estimated from Eq. 5 versus k for selected studies as summerized in Table II. The highlighted lines represent the range 5
