An Automated System for Near-Real-Time Monitoring of Trace Atmospheric Halocarbons

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Abstract
A new gas chromatographic method developed to quantitatively determine important atmospheric halocarbons is described. Target compounds include replacement CFCs, chlorinated solvents, and biosynthesized (naturally produced) organohalogens, all trace gases in the atmosphere at concentrations ranging from 0.1 to 600 pptv (where pptv = 1 part in 10-12 by volume). A combination of ultralow concentrations and relatively small electron attachment cross sections renders these compounds very difficult to routinely measure in the background air typical of remote atmospheric monitoring stations. Detection is achieved by preconcentration of a 200-mL air sample using an adsorbent-filled microtrap and enhancement of electron capture detector response by oxygen doping one of two detectors connected in series. Oxygen doping specifically targets halocarbons with relatively poor electron attachment rate coefficients. The work described here details construction of a novel analytical system, laboratory trials, and optimization followed by an extended field campaign at a remote atmospheric monitoring station, Mace Head, Ireland. A calibration standard or ambient air sample was acquired every hour using a cyclic, automated procedure without employing cryogenic preconcentration or refocusing. Overall precision of the analytical method for the target compounds is between 0.3 and 1.5%.