Effect of the tropospheric trend on the stratospheric tracer-tracer correlations: Methyl chloroform

In situ measurements in the lower stratosphere in 1997 produced distinct intersecting correlations between methyl chloroform (CH3CCl3) and chlorofluorocarbon-11 (CFC-11) for air parcels sampled during spring and summer in the same physical space. The disagreement between spring and summer correlations in the stratosphere below 225 ppt of CFC-11 is in the opposite direction than is expected from the documented tropospheric trend of -15 ppt yr(-1). Summertime air parcels were enriched in CH3CCl3 compared to spring parcels. Mean ages based on sulfur hexafluoride (SF6) for CH3CCl3-rich air masses observed during summer were older by about a year than mean ages of CH3CCl3-depleted air parcels sampled during spring. The ages of stratospheric air masses and the documented history of methyl chloroform mixing ratios in the troposphere were used to normalize the stratospheric input of methyl chloroform, as if it had no tropospheric trend. The normalized correlations with and without parameterization for phyotoylic loss on the age distribution are more compact and revealed hidden anomalous mixing lines between polar vortex and midlatitude air. Higher mixing ratios of CH3CCl3, which the older air had at the time of the stratospheric entry, caused the summer correlation to be elevated on the tracer-tracer plot below 225 ppt of CFC-11. The ongoing tropospheric decrease of CH3CCl3 caused an inflection point and crossing of spring and summer correlations at 225 ppt of CFC-11. These results suggest that the tropospheric history is especially important for correlations of species that were increasing in the 1980s and are now decreasing (some CFCs, CCl4, and CH3CCl3). Distinct correlations between CH3CCl3 and CFC-11 observed for spring and summer 1997 measurements are the result of the 1991 tropospheric maximum of CH3CCl3 [Prinn et al., 1995] that has propagated into the stratosphere, where air parcels with high CH3CCl3 were sampled at 20 km altitude and 60 degrees-90 degrees N.