Optimized low-level liquid scintillation spectroscopy of 35S for atmospheric and biogeochemical chemistry applications.

Anthropogenic activities, dominated by emissions of sulfur dioxide (SO(2)), have perturbed the global sulfur (S) cycle. Uncertainties in timescales of S transport and chemistry in the atmosphere lead to uncertainties in the predicted impact of S emissions. Measurements of cosmogenic (35)S may potentially be used to resolve existing uncertainties in the photochemical and chemical transformation of S in the environment. The lack of a simple, effective, and highly sensitive technique to measure (35)S activity in samples with low activities may explain the scarcity of published measurements. We present a set of new sample handling and measurement procedures optimized for the measurement of (35)S in natural samples with activities as low as 0.20 dpm above background (2sigma, integration time = 2 hr). We also report simultaneous measurements of aerosol (35SO4 ) and gas phase (35SO2) collected at inland and coastal locations; the range of observed activities corresponds to SO(2) residence lifetimes of 0.2 +/- 0.04 (coastal) - 22.3 d +/- 0.04 (inland). These optimized techniques offer the potential for resolving atmospheric processes that occur on 6-12-hour timescales as well as resolving transport phenomena such as stratospheric mixing into the troposphere.