Distribution of atmospheric SF₆ near a large urban area as recorded in the vadose zone

Local emissions of SF₆ are of interest for studying their impact on the use of SF₆ as a groundwater-dating tool near source regions as well as for investigating the spatial distributions of (inert) gaseous compounds spreading from urban or industrial centers. A precondition for the use of SF₆ in such studies is the capability to document the temporal and spatial evolution of SF₆ in and around source regions with sufficient resolution. Here we present a time series of SF₆ measurements in soil air at a site (Sparkill, NY) about 25 km north of New York City carried out between May 2000 and January 2002. The data show that, below about 2 m depth, the vadose zone integrates atmospheric SF₆ mixing ratios over time scales greater than 1 month. SF₆ mixing ratios in soil air at these depths match averaged high-resolution atmospheric measurements performed at Lamont-Doherty Earth Observatory in Palisades, NY, located about 3 km south of Sparkill. To a first-order approximation, a simple one-dimensional diffusion model reproduces the measured SF₆ profiles in the vadose zone, suggesting that the soil indeed acts as a low-pass filter for inert atmospheric gases. These findings indicate that measurements of soil air can be used to determine the spatial pattern of SF₆ excess relative to the remote atmosphere for a given region. A transect of soil profiles from Manhattan to the tip of Long Island indicates that emissions from sites close to New York City lead to significant SF₆ excesses (ca. 25% or more) above the clean air mixing ratios over distances of the order of 80 km.