As part of a cooperative venture with scientists from the University of Rhode Island, Pennsylvania State University, and CCG, NOAH analyzed the contents of flasks filled with firn air from the SPO in early 1995 [Battle et al., 1996].

The sampling system was designed so that large quantities of air could be pulled from discrete depths in the firn down to the firn-ice transition depth of 122 m. Because it was possible to obtain large amounts of air, flasks could be flushed adequately and filled for nearly routine air analyses in Boulder and elsewhere. Air at the bottom of the firn had a CO₂ age of about 100 years [Battle et al., 1996].

N₂O in these samples analyzed by NOAH forms a bridge between ice-core data, which typically are much less precise owing to sample handling procedures and small samples, and real-time, present-day measurements (Figure 5.32). These results suggest that preindustrial levels of N₂O in the atmosphere had to be about 280 ppb and that N₂O was increasing steadily through the latter part of the 20th century. The growth rate of atmospheric N₂O from 1904 through 1958 was 0.06 0.01% yr^-1 (95% confidence level); thereafter, it has increased at a rate of 0.22 0.02% yr^-1 (95% C.L.). N₂O covaried well with CO₂ throughout the profile, although the smoothness of the fit could be attributable to subsurface diffusion of the gases. Nevertheless, the overall trend of N₂O as a function of CO₂ was 0.50 0.03 ppb N₂O ppm^-1 CO₂^-1 (95% confidence level, r² = 0.98).

Fig. 5.32. History of atmospheric N₂O over the past century, derived from antarctic ice-core measurements [Machida et al., 1995], real-time air measurements in the southern hemisphere (NOAH), and analyses of South Pole firn air (NOAH). Firn air ages are determined from correlation of CO₂ in the samples with the atmospheric CO₂ history of Etheridge et al. [1996]. Diffusivities of N₂O and CO₂ are assumed to be identical and ice core data of Machida et al. [1995] have been lowered by 1 ppb to conform to the CMDL scale.

Surprisingly these flasks, which were sealed with Teflon o-rings, did not cause significant contamination of most halocarbons. Consequently, depth profiles were obtained of CFCs, chlorocarbons, and bromocarbons representing air as far back as the late 19th century. (Dates assigned to halocarbons will be older than CO₂ in the same bolus of air owing to their slower rates of diffusion.) As shown in Figure 5.33, which is a close-up of the lower portion of the CFC-11 profile, the sampling and analytical precisions are on the order of tenths of a ppt. Small amounts of contamination are suggested in that the lowest values were still 2 ppt (<1% of today's atmosphere) and that two pairs of flasks showed higher levels of CFC-11 in some of the deepest firn. This latter contamination was probably caused by stress on the pump near the firn-ice transition zone where less air was available to pull, thus increasing the probability of sucking in unrepresentative air. This feature showed up in all of the gases, further suggesting contamination with modern air. Nevertheless, this level of contamination is not representative of the rest of the profile. Thus, we were able to obtain precise, but probably accurate, measurements at sub-ppt levels throughout most of the profile.

Fig. 5.33. CFC-11 in the lower portion of the firn at the South Pole. The high degree of precision is shown in the actual measurements (triangles), where two flasks were collected at each depth and plotted separately. Only at 108 m depth are these symbols distinguishable and there only because two pairs of flasks from this depth in two separate holes were analyzed. Flasks at 120 and 122 m were subjected to some contamination with modern air during sampling, probably owing to stress on the pump as the firn layers began to turn to ice. Also shown here is the effect of the gravitational correction for settling of CFC-11, which is a gas heavier than air. Again, the error is small.

These results yield entire atmospheric histories for CFCs, halons, and other halocarbons of purely anthropogenic origin (Figures 5.33 through 5.37). They also showed atmospheric trends for gases of both natural and anthropogenic origin, such as CH3Br, during a time when the human population, its agricultural output, and its industrial activity increased dramatically. These data demonstrate that natural sources of CFCs and halons are minimal at best and most likely nonexistent. Models of anthropogenic CFC emissions are supported by these findings, confirming the predominance of anthropogenic activity in the atmospheric budget of refractory, organic chlorine. The data for CFCs and the longer-lived organic chlorocarbons ultimately will be useful in dating oceanic water masses and isolated ground waters, providing atmospheric data where none existed before. Atmospheric CH3Br in the southern hemisphere appears to have been about 25% lower at the turn of the century than it is today (Figure 5.36).

Fig. 5.34. Depth profiles of CFC -11 and -12 in South Pole firn air. Mole fractions of both gases near the bottom of the firn are less than 1% of the present day values, suggesting that natural sources are minimal or non-existent.

Fig. 5.35. Depth profiles of CCl₄ and CH₃CCl₃ in South Pole firn. There is some evidence of contamination of a few ppt in the CH₃CCl₃ data, although mole fractions of this compound came very close to zero near the bottom of the profile. Mole fractions of CCl₄ never fell below 10 ppt, suggesting either significant, specific contamination of this compound, a very early history of significant anthropogenic release, or a natural source.

Fig. 5.36. Brominated gases in South Pole firn air. The anthropogenic halon mole fractions both drop to zero early in the profile. These gases were not introduced into the atmosphere in significant amounts until the 1970s. CH₃Br is about 6.5 ppt in air nominally dating back to about 1880.

Fig. 5.37. SF6 in South Pole firn air. Because of its small analytical peak on the GC, the detection limit for SF6 in real air is around a few tenths of a ppt. Like the halons, this gas was not introduced into the atmosphere until the 1970s [Maiss et al., 1996].

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