Environ. Sci. Techno/. 1995,29, 1179- 1185
Uptake of Haloacetyl and Carbonyl Halides by Water WARREN J. DE BRUYN, JEFFREY A . S H O R T E R , + A N D PAUL D A V I D O V I T S * Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02167
D O U G L A S R . WORSNOP, M A R K S . ZAHNISER, A N D CHARLES E . K O L B Center for Chemical and Environmental Physics, Aerodyne Research, Inc., Billerica, Massachusetts 01821
Gas-liquid uptake studies have been completed for the carbonyl halides CCIZO and CFZO and the haloacetyl halides CC13CCI0, CF3CF0, and CF3CCI0, which are intermediate products from gas phase oxidation of volatile halogen-containing species in the atmosphere. The fluorine-containing species result from the degradation of hydrochlorofluorocarbons (HCFCs) (proposed substitutes for chlorofluorocarbons, CFCs). The tropospheric lifetime of the halides depends on their dissolution in the aqueous phase, determined by Henry's law solubility (H) and hydrolysis rate (khyd). Using a bubble column apparatus, time-resolved gas-liquid interaction experiments measured the product Hkhyd1lZ.Studies were performed at 278 K and pH = 1-13; for CC13CCIO and CC120, temperature was varied from 278 to 298 K. From this work and results from other laboratories, limits on values of the product Hkhyd were established and used to estimate a 'global' cloud processing rate of these halides. A -30-day upper limit to their tropospheric lifetime implies that tropospheric removal of the halide degradation products is fast enough not to contribute to the ozone depletion potential of the parent HCFCs.
Introduction The currently used chlorofluorocarbon (CFC) gases (most commonly CFC-11, CFC-12, andCFC-113)arehighlystable with tropospheric lifetimes of 65, 130, and 90 years, respectively (1). As a result, they are transported to the stratosphere where photodissociation by ultraviolet solar radiation releases chlorine atoms, which then can react with ozone, initiating the now well-documented ozone depletion (2-4). Hydrochlorofluorocarbons (HCFCs)have been proposed as substitutes for CFCs. This choice is based on HCFC degradation in the troposphere initiated by H atom abstraction by the OH radical. Efficient removal of HCFCs in the troposphere minimizes the amount of chlorine transported to the stratosphere. Overall HCFC troposphericlifetimes depend on the rates of both OH reaction and the subsequent removal of the degradation products. Mechanistic studies of HCFC degradation chemistry have identified haloacetyl and carbonyl halides as gas phase products that are expected to be stable against further homogeneous reaction in the gas phase (5). Examples of these species included in the current study are CF20, CF3CF0, and CF3CClO. Also included in this work are CC120, a possible degradation product of chloroethylenes ( C Z Cand ~ C2HCl3) (6,3 , and CCbCClO, a potential degradation product of perchloroethylene (6, 8). Because of their low gas phase reactivity, the tropospheric removal of these halide species occurs via dissolution in the aqueous phase, i.e., on rain-out from clouds or on deposition to the oceans. Aqueous removal rates depend primarily on the species solubility and rate of hydrolysis to form their corresponding acids. Three parameters are needed to evaluate heterogeneous uptake rates: the Henry's law coefficient, H (M atm-l), which describes solubility; the hydrolysis rate coefficient, khyd (s-l); and the liquid phase diffusion coefficient Q (cm2s-l). The processes are represented by
H
RX,O(g) RX,O(aq)
+ H,O khyd
RX,O(aq)
+ X- + 2H'
For the carbonyl halides, the hydrolysis products are the acid halide (HX) and CO2. The hydrolysis step is assumed to be irreversible, which is to be expected for the acid products (e.g.,HXor trifluoroaceticacid, TFA). While liquid phase diffusion coefficientscan be calculatedwith adequate reliability by techniques such as the Wilke-Chang method (91, values for H and khyd generally have to be obtained from laboratory measurements. In experiments designed to study heterogeneous interactions, the uptake of the trace gas of interest can be expressed in terms of an uptake coefficient, ymeas,defined as
- no. of molecules lost to surface (molecule s-3 Ymeas -
t
no. of gas-surface collisions (collision s-l) (1)
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VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1 1179
TABLE 1
Solubility and Hydrolysis of Haloacetyl and Carbonyl Halides CF3CFO this worka George et at. (27P CF3CCIO
this worka George et at. CCIZO this worka
(27P
Manogue and Pigford ( 78) U g i and Beck ( 79)
Behnke et al. (76, 77) Mertens et al. (27) CFzO this worka George et al. (27P CC13CCIO this worka
George et at. (20la George et al. (20)f Wine a n d Chameides (298
278 284
0.96b 3.0
3.8b 37,60
4.3-96 450
1.5 x IO6
150
278 284
0.27b 2.0
1.2b 30,60
1.2-27 440
8.9 x 105
220
278 298 298 253 296 298
0.15b 0.06b 0.07
278 284
1 .Ob
278 274-294 288
2.0b 2.0
6
0.66b 0.2gb 0.17
0.68- 15
0.17c
3.8d
4.6 x 1 0 3
0.4
, . -100
5.38
>IO
DeBruyn et al. (7418
4.3b 350 500 -150 210-3
6.gb 45 20.3
