Automated measurement and calibration of reactive volatile halogenated organic compounds in the atmosphere

Abstract

The automated calibration and analysis of very low mixing ratios of the reactive volatile organic halocarbons CH₃I, CHCl₃, C₂H₅I, 2-C₃H₇I, CH₂Br₂, CH₂ClI, CHBr₂Cl, 1-C₃H₇I, CH₂BrI, CHBr₃ and CH₂I₂ for long term atmospheric field measurements are described. Analytes were pre-concentrated from 3 l of air onto an adsorbent trap cooled to −10 °C using Peltier plates, and rapidly transferred to a gas chromatograph (GC) by resistive heating. A two stage Carboxen 1016/Carbotrap C adsorbent trap allowed good analyte recovery and rapid desorption without the need for post-desorption cryofocussing. Halocarbons were detected using a mass spectrometer (MS) in selective ion mode. Detection limits were between 0.02 and 0.12 pptv (parts per trillion by volume) for approximately hourly samples of CHCl₃, CH₃I, C₂H₅I, 1-C₃H₇I, 2-C₃H₇I, CH₂ClI, CH₂Br₂, CHBr₂Cl, CH₂BrI, CHBr₃ and CH₂I₂ with a precision of 3–8%. A novel calibration system was constructed which utilised fixed volume (50 µl) injections of the output of thermostatted permeation tubes into a stream of nitrogen gas in order to dilute parts per million by volume (ppmv) mixing ratios into pptv. The calibration was completely automated, allowing multi-point calibrations during routine operation. The overall accuracy of the measurements is estimated to be ±15%. The instrument was used continuously for automated atmospheric measurements during a 4-month research cruise from Germany to Antarctica, and a 6 week field campaign at Mace Head, Ireland. The results for CHCl₃ during the latter campaign were within 13% of measurements made by a GC-MS operating continuously at the site within the long term Advanced Global Atmospherics Gases Experiment.