CALCULATED TRENDS AND THE ATMOSPHERIC ABUNDANCE OF 1,1,1,2-TETRAFLUOROETHANE, 1,1-DICHLORO-1-FLUOROETHANE, AND 1-CHLORO-1,1-DIFLUOROETHANE USING AUTOMATED IN-SITU GAS-CHROMATOGRAPHY MASS-SPECTROMETRY MEASUREMENTS RECORDED AT MACE HEAD, IRELAND, FROM OCTOBER 1994 TO MARCH 1997

The first in-situ measurements by automated gas chromatograph-mass spe ctrometer are reported for 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1-d ichloro-1-fluoroethane, (HCFC-141b), and 1-chloro-1,1-difluoroethane, (HCFC-142b). These compounds are steadily replacing the chlorofluoroca rbons (CFCs) as refrigerants, foam-blowing agents, and solvents. The c oncentrations of all three compounds are shown to be rapidly increasin g in the atmosphere, with 134a increasing at a rate of 2.05 +/- 0.02 p pt yr(-1) over the 30 months of observations. Similarly, 141b and 142b increased at rates of 2.49 +/- 0.03 and 1.24 +/- 0.02 ppt yr(-1), res pectively, over the same period, The concentrations recorded at the at mospheric research station at Mace Head, Ireland, on January 1, 1996, the midpoint of the time series, were 3.67 ppt (134a), 7.38 ppt (141b) , and 8.78 ppt (142b). From these observations we optimally estimate t he HCFC and HFC emissions using a 12-box global model and OH concentra tions derived from global 1,1,1-trichloroethane (CCl3CH3) measurements . Comparing two methods of estimating emissions with independent indus try estimates shows satisfactory agreement for 134a and 141b, while fo r 142b, industry estimates are less than half those required to explai n our observations.