Atmospheric SF₆ near a large urban area

Sulfur hexafluoride (SF₆) has the potential to be a valuable transient tracer for dating of young ground water. However, near urban areas, there are typically numerous point sources of SF₆ which have a significant impact on the temporal evolution of its atmospheric mixing ratio, thereby complicating its use as an age-dating tool. Here, we present and discuss a 12-month record of atmospheric SF₆ from a location near New York City. The data were obtained by gas chromatographic analyses performed at intervals of approximately 10 minutes yielding about 40,000 data points for the time series. Nearly all measured SF₆ values are in excess of remote Northern Hemisphere (NH) atmospheric mixing ratios. Temporal trends in the baseline data from LDEO indicate that atmospheric SF₆ at LDEO decreased at a rate of 0.4 ppt yr^-1 over the 12-month period. The SF₆ data are compared to records of CFCs obtained during the same period. Whereas the CFCs show daily, weekly, and seasonal patterns of variability near New York City, the SF₆ data exhibit only a daily cycle. The observed SF₆ excesses are far greater than those found for CFCs during the same time period. This indicates that in order to use SF₆ as an age dating tool of groundwater near source regions, its input function, i.e., its concentration in soil air above the groundwater table, needs to be defined explicitly.