GEOPHYSICAL RESEARCH LETITERS, VOL. 19, NO. 19, PAGES 1939-1942, OCTOBER 2, 1992 TROPOSPHERIC HETEROGENEOUS CHEMISTRYOF HALOACETYL AND CARBONYL HALIDES W. J. De Bruyn, S. X. Duan, X. Q. Shi and P. Davidovits Department of Chemistry, Boston College, Chesmut Hill, MA 02167 D. R. Worsnop, M. S. Zahniser, andC. E. Kolb Aerodyne Research, Inc., Billerica,MA 01821 Abstract. A series of uptake studies havebeen completed for the gas phase carbony! halides CC120 and CF20 and the haloacetyl halides CC13CC10, CF3CFOand CF3CC10 which areoxidation intermediates of volatile fluorocarbon, chlorocarbon and hydroflu- orocarbon (HCFC) species in the atmosphere. Theexperimental apparatus in these studies employs a fastmoving monodispersed train of droplets (50 to 200 gm in diameter) passed through a low pressure flow reactor. Experiments weredone as a function of droplet pH andgas/droplet interaction time. Themeasured uptake coefficients (¾) for all these molecules areless than our experimen- tal detection limit of5 x 10 -4 for pH7 droplets at300 K. From the results of the measurements andotheravailable data,the uptake coefficients forthese species are estimated to be 10 -6< T < 10-4. Thelower limit is sufficientlylarge so that the tropospheric removal rate of the species is not expected to affectthe ozone depletion potential values of theHCFC parent species for CF20, CF3CFO and CF3CC10.However with such uncertainty it is not possible to determine the partitioning of the ocean/land deposition ofthe species. Introduction The currentlyusedchlorofluorocarbon (CFC) gases(most commonly CFC-11, CFC-12 andCFC-113) are highly stable with tropospheric lifetimesof 65, 130 and 90 years respectively [IPCC, 1990]. As a result they do notreact in thetroposphere but are transported to the stratosphere where they aredissociated by ultraviolet solar radiation releasing chlorine atoms. The chlorine atoms thenreactwith ozone. initiatingthe now well documented depletion of stratospheric ozone [WMO, 1985; 1989]. As a response to thisproblem, several morereactive hydro- chlorofluorocarbon (HCFC), and hydrofluorocarbon compounds with tropospheric lifetimes in the range of 1 to 5 years are being considered as CFC substitutes [WMO, 1989]. Thefn'st degrada- tion step forthese species inthe troposphere isH atom abstraction bythe OH radical [Atkinson et al., 1990]. Finaltropospheric removal of degradation products depends onformation of gaseous species withaqueous solubility large enough for efficient rain out or for dissolution in theoceans [Wine and Chameides, 1990]. Recent studies have identified halogenated carbonyl com- pounds askey gaseous degradation intermediates resulting from the oxidation of HCFCs as well as other organic halogen compounds [WMO, 1989; Edney et al., 1991] . Important examples of these species include CF3CFO, CF20 and CF3CC10, which aredegradation products of proposed HCFCs [WMO, Copyright 1992 bythe American Geophysical Union. Paper number 92GL02!99 0094-8534/92/92GL-02199503.00 1989;STEP-HALOCSIDE/AFEAS, !991], CC120 whichis a possible degradation product of chloroethylenes (C2C1• ' and C2HC! 3 [Singh et al., 1975; Singh, 1976]; CHC13 [Ryan and Plumb, 1984; Spence eta!., 1976]; andCC13CH 3 [Nelson et al., 1990]) and CC13CC10 which isa potential degradation product of perchloroethylene [Singh et al., 1975; Tuazon et al., 1988]. They appear to be relatively inertto further homogeneous gas phase reactions andtherefore, if their precursors are to be short-Iived in the troposphere, they must have significant solubility and/or reactivity in aqueous media so that their aqueous removalin the troposphere can berapid compared to theirdiffusionaI transport to the stratosphere. In thiscontext a knowledge of themagnitude of theuptake coefficients T for these degradation products is essential for theevaluation of the overall ozone depletion potentials (ODP) of proposed CFC substitutes and is important in assessing the environmental fateof other organic halogen compounds. The uptake coefficient takes into account the various factors whichlimit theuptake of gaseous species by a liquid[Worsnop et al. 1989,Jayne et al. 1991]. Thusthe netflux (J) of the species intotheliquid is: J= ng•/4 (1) Here ng isthe density ofthe gas molecules and e isthe average thermal velocity. Values of ¾ > 10 -5have been reported forHCFC degradation products onwater[Edney etal., 1992] and ¾= 5 x !0-6 for CF20oncold sulfuric acid (40 wt%,230 K) solution [Hanson andRavishankara, 199 !]. Recent estimates by Rodriguez and Sze [1992] indicate that the actual value of ¾is crucial in determining the aanospheric fate and environmental effect of chloro(fluoro)carbon degradation products. The fate of the gases isbest understood interms of their tropospheric lifetimes. If this lifetime is greater than 10 to 20 years then most of the species will be transported to the strato- sphere and will add to the ozone depletion potential of the parent compound. For ¾ greater than 10 -4,gas will enter tropospheric cloud drops on a time scale of about one week, andassuming effective and irreversible liquidphase hydration or oxidation, the degradation species' ODP is negligible. For ¾ inthe region of 10 -4 to 10 '6 , the species are partitioned between cloud drops and the oceans witha lifetime of up to about 2 months. Again,assuming irreversible liquid phase reaction, the ODP of the degradation product is still negligible. With ¾ on theorder of 10 -7the tropospheric lifetime is I to2 years and the ODP for C1 containing species becomes significant. A ¾ smaller than 10 -8 results innear complete transport ofthe degradation species tothe stratosphere. Even in the region of ¾ between 10 -4and 10 -6,where, for example, the direct impact of the alternate CFC degradation product would likelyto be negligible, theexact value of ¾is significant since it determines the partition factor between !and and ocean for the deposition ofthespecies. This partitioning in turn willdetermine the chemistry ofthe deposited species w•ch inlight 1939