JO URNA L O F C HRO MA TO G RA PHY A ELSEVIER Journal of Chromatography A, 676 (1994) 389-398 Fully automated gas situ determination of atmospheric non- methane hydrocarbons at low parts per trillion concentration J. P. Greenberg*, B. Lee, D. Helmig, P.R. Zimmerman zyxwvutsrqponmlkjihgfed National Center for Atmospheric Research, Boulder, CO, USA First received 2 February 1994; revised manuscript received 31 March 1994 zyxwvutsrqponmlkjihgfedcbaZY Abstract A completely automated gas chromatography-flame ionization detector system with cryogenic sample freeze-out for measuring atmospheric non-methane hydrocarbons was deployed at the Mauna Loa Observatory, Hawaii during the MLOPEX II experiment, September 1991 through August 1992. The system was designed to (1) rapidly trap air samples of up to 4 litres volume to allow for sub-parts per trillion detection limits, (2) eliminate interferences from ambient ozone, water vapor and carbon dioxide, (3) reduce to negligible levels any contamination in the analytical systems, and (4) allow for continuous, unattended operation. The instrumentation consisted of two parallel analytical systems, employing packed and capillary chromatographic columns, which allowed quantification of C,-C,, non-methane hydrocarbons from sub-parts per trillion to parts per million concentrations. A dynamic dilution system was used to calibrate the analytical system over the range of concentrations measured (low parts per trillion to parts per billion) at this site. 1. Introduction Atmospheric non-methane hydrocarbons (NMHCs) play an important role in the atmos- pheric chemistry of the troposphere. They are central to the production and destruction of atmospheric oxidants and oxidant precursors, such as OH, O,, organic peroxides and peroxy radicals [l]. The annual global emissions of NMHCs into the atmosphere exceed that of methane [2], and, therefore, represent a major component of the atmospheric carbon budget. Because of their higher chemical reactivity, how- ever, NMHCs are often found at a few parts per * Corresponding author. billion (ppb) to low parts per trillion (ppt) levels in the remote troposphere, compared to approxi- mately 1.7 parts per million (ppm) for methane PI. Atmospheric NMHCs have been measured both by zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON in situ analysis and by sample collection in canisters, bags, or adsorbent cartridges, with subsequent laboratory analysis. The latter strategy has disadvantages, which may include artifact formation in the storage medium, storage losses or transformations, sample recovery, and sample size constraints [2]. More recently, mea- surements have been reported which utilized in situ methods [4-61. The in situ techniques also have revealed analytical problems, which include chemical or chromatographic interferences as a 0021-9673/94/$07.00 0 1994 El sevier Science B.V. All rights reserved SSDI 0021-9673(94)0321-Y