Long-lived Halogenated Compounds in the Stratosphere

Abstract

Stratospheric air was collected using a balloon-borne cryogenic whole air sampler at both mid-latitudes and in the Arctic. This device collects up to fifteen samples from the tropopause to above 30 km. Analyses of a wide range of halocarbons were made using a gas chromatograph-high resolution sector mass spectrometer. In this abstract we present selected results for some very long-lived halocarbons measured in the stratosphere. In the poster these are compared with tropospheric trends obtained from air archived at Cape Grim, Tasmania.

Figure 1 shows the altitidinal profiles from one of the balloon flights over France. The concentrations in a high-pressure standard collected by NOAACMDL at 3100 m in the Colorado Mountains in 1994 are shown for comparison (except for CHF₃, which is anomalously high in the standard). The lower-most balloon samples closely resemble the concentrations in the NOAA standard, reflecting the essentially tropospheric nature of the bottom of the profile. Declining concentrations with height arise both from stratospheric loss processes (where these exist) and from tropospheric growth, bearing in mind that the mean age of stratospheric air generally increases with height.

The shortest-lived compound of those shown is CFC-114a, a less abundant isomer of the much longer-lived (300 y) CFC-114, as clearly demonstrated by their very different gradients. CFC-115 is longer lived in the atmosphere at 1,700 y. SF₆, which is now quite widely used for dating stratospheric air, has a lifetime of 3,200 y. Fluoroform (CHF₃), despite an only moderately long tropospheric lifetime (230 y) has a steep gradient in the stratosphere, which is itself largely accounted for by its tropospheric growth rate.

CFC-13, c-C₄F₈, and C₂F₆ have lifetimes of 640, 3,200, and >10,000 y respectively. The latter compound is of interest as a potential alternative age tracer to SF₆, due to its longer lifetime, absence of sinks in the upper atmosphere, and essentially linear tropospheric growth as shown by the Cape Grim record. The poster will show that the SF₆ trend deduced from C₂F₆dated stratospheric air from four balloon flights matches the observed tropospheric trend well, although there is a slight tendency towards higher SF₆ values in mid-latitudinal profiles compared with Arctic profiles for equivalent C₂F₆ ages.

An interesting ‘new’ compound has been tentatively identified as SF₅CF₃. The concentrations shown in Figure 1 are highly approximate from comparison with SF₆ peak areas. This compound appears to be extremely long-lived and to be growing in the atmosphere. Also measured in more recent profiles, as shown in the poster, is C₃F₈. This also shows a steep stratospheric gradient (long lifetime) and a positive tropospheric trend.