1834
Anal. Chem. 1983, 55,1834-1836
Interlaboratory Comparison, Preparation, and Stability of Dichlorofluoromethane Samples and Standards R. A. Rasmussen and M. A. K. Khalil* Department of Environmental Science, Oregon Graduate Center, 19600 N . W. Walker Rd., Beaverton, Oregon 97006
G . Crescentini, F. Mangani, A. R. Mastrogiacomo, and F. Bruner Istituto de Scienze Chimiche, Universitci degli Studi di Urbino, Piazza Rinascimento 6-61 029, Urbino, Italy Several years ago Molina and Rowland ( I ) suggested that anthropogenic releases of the extensively used fluorocarbons CC13F (F-11)and CC12F2(F-12)could result in a partial destruction of the ozone layer in the stratosphere. The result would be greater amounts of ultraviolet radiation a t the earth’s surface which could be harmful to human health (2). Even after many years of research, the estimated environmental effects of the increasing concentrations of F-11 and F-12 remain controversial. One central issue has been the possible existence of tropospheric sinks or mechanisms by which F-11 and F-12 may be removed from the troposphere (lower atmosphere). If such mechanisms exist, the lifetimes of F-11 and F-12 could be smaller than currently estimated on the basis of photodissociation in the stratosphere. These sinks could therefore reduce the potential of F-11 or F-12 for destroying stratospheric ozone. Since the removal of F-ll and F-12 from photodissociation in the stratosphere is a slow process, giving these compounds atmospheric lifetimes of 50 years or more, even the mechanisms which remove small amounts of F-11 from the troposphere may be important. Some experimental evidence was reported suggesting that F-11 may be destroyed on suspended atmospheric particles producing F-21 ( 3 , 4 ) . F-21 is removed relatively rapidly (lifetime 2 years) from the troposphere by reacting with hydroxyl (OH) radicals ( 5 ) . Since very little F-21 is produced in industrial processes, its tropospheric background concentration could be used to estimate the possible significance of this removal mechanism of F-11. Calculations show that if the average concentration of F-21 in the troposphere is -6 pptv, and most of it is due to the conversion of F-11 to F-21, then this tropospheric mechanism could remove as much as F-11 from the atmosphere as the stratospheric removal, thus making it a significant component of the fluorocarbon-ozone cycle (5). Atmospheric measurements of F-21have been made by four groups: a t Oregon Graduate Center (OGC); University of Urbino, Italy; Stanford Research Institute (SRI); and A.E.R.E, Harwell, England. The joint work at Harwell and OGC revealed extremely low background concentrations of F-21, generally
