Environ. Sci. Technol. 2009, 43, 2431–2436
Application of Gas Chromatography with a Pulsed Discharge Helium Ionization Detector for Measurements of Molecular Hydrogen in the Atmosphere P. C. NOVELLI,* A. M. CROTWELL, AND B. D. HALL Global Monitoring Laboratory, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA; and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
Received November 11, 2008. Revised manuscript received January 23, 2009. Accepted January 30, 2009.
The Earth’s troposphere contains approximately 160 Tg H2 with an average surface mixing ratio ∼530 nmole mole-1 (ppb) and lifetime of 2 years. Atmospheric H2 is typically measured using gas chromatography (GC) followed by hot mercuric oxide reduction detection (GC-HgO). Here we describe an alternate method using GC with a pulsed-discharge helium ionization detector (HePPD). HePPD is a universal detector; when applied to H2, the GC-HePDD provides a wide linear range (0.3% over a range of 2000 ppb), a detection limit of ∼0.03 pg, high precision (0.12%) and a stable response ((1.6% over nearly one year). HePPD is compared to HgO reduction using a suite of gravimetrically prepared reference gases spanning remote to urban concentrations. The method is excellent for atmospheric measurements as it provides a wide linear range with high precision, stability and reproducibility. We suggest these characteristics will improve the ability to maintain reference gases and improve measurements of atmospheric H2, thus providing better constraints on potential future changes in its sources and sinks.
Introduction Molecular hydrogen (H2) plays an important role in the series of reactions that both produce and destroy atmospheric CH4, CO, and O3 (1). Mixing ratios of H2 in the remote atmosphere currently range from ∼450 to ∼550 ppb depending on location and time of year (2, 3) and in urban areas may reach several part per million (4). Proposals to use hydrogen as an energy carrier have renewed interest in its current atmospheric concentrations, distributions and budget. Anthropogenic processes currently account for about 60% of the global H2 source, and soil uptake accounts for between 70 and 90% of the global sink (2, 5). Multidimensional chemical transport models have been used to predict future H2 levels and their impact on atmospheric chemistry as a result of a global H2-based energy system. These have provided mixed results: some suggested a greater source and atmospheric burden resulting from large inefficiencies in production, transport, storage, and use with subsequent changes in * Corresponding author: Paul Novelli;
[email protected], phone: 303-497-6974. 10.1021/es803180g CCC: $40.75
Published on Web 03/05/2009
2009 American Chemical Society
atmospheric chemistry (6, 7), while others used smaller leak rates in the simulations and concluded a smaller atmospheric impact (8, 9). A major uncertainty in future projections lies in the magnitude of leakage during the fuel supply chain and how the atmosphere and biosphere will respond to this new source. Environmental measurements of hydrogen have largely been made using gas chromatography followed by reaction with hot, solid mercuric oxide (HgOs): H2 + HgOs f H2O + Hgg (10). Reduced analytes such as H2, CO and CH4 are oxidized at the expense of HgO and the resulting Hg gas is detected photometrically at 254 nm. This technique provides the precision and detection limit suitable for atmospheric measurements. However, the detector response is often nonlinear over the narrow range of atmospheric mixing ratios and HgO reaction beds show different response characteristics that can change over time. Nonlinear detectors require multipoint calibration curves with emphasis at near-ambient levels and H2 standards have been found to drift upward over time (from
