Atmospheric SF6 near a large urban area

Abstract

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.