5. Nitrous Oxide and Halocompounds

J.H. Butler (editor), J.W. Elkins, S.A. Montzka, T.M. Thompson, T.H. Swanson, A.D. Clarke, F.L. Moore, D.F. Hurst, P.A. Romashkin, S.A. Yvon-Lewis, J.M. Lobert, M. DiCorleto, G.S. Dutton, L.T. Lock, D.B. King, R.E. Dunn, E.A. Ray, M. Pender, P.R. Wamsley, and C.M. Volk

5.1. Continuing Programs

5.1.1. Introduction

Research conducted by the Nitrous Oxide and Halocompounds Group (NOAH) of CMDL in 1996 and 1997 included: (1) weekly flask sampling and analysis of air from remote locations and regionally influenced sites, (2) operation of instrumentation for hourly, in situ measurement of trace gases at the four CMDL observatories and at four regionally influenced sites, (3) preparation and maintenance of calibration gases for all measurements, (4) participation on campaigns requiring in situ stratospheric measurements from high-altitude aircraft and balloons, (5) investigation of oceanic processes influencing trace gas composition of the atmosphere, (6) in situ monitoring of air at multiple elevations from tall towers, and (7) measurement of gases in air that is archived in consolidated polar snow in Antarctica and Greenland.

The main mission of NOAH is to measure and to evaluate the distribution and trends of nitrous oxide (N₂O) and many halogenated gases in the atmosphere and ocean with the best analytical tools available. The halogenated gases, or halocompounds, include the chlorofluorocarbons (CFCs), chlorinated solvents (CCl₄, CH₃CCl₃, CHCl₃, etc.), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), methyl halides, halons, and sulfur hexafluoride (SF₆). The recent motivations for measuring these compounds are: (1) that many of the brominated and chlorinated compounds that cause ozone depletion are being phased out of production as required by the amended and adjusted Montreal Protocol [United Nations Environmental Programme (UNEP), 1987, 1997], and (2) that these and many of the compounds introduced to replace them contribute to global warming. The Protocol of the Third Conference of Parties to the United Nations (UN) Framework Convention on Climate Change, held in Kyoto, Japan, December 1997, recommended future limits on the emissions of six compounds or compound groups: (1) carbon dioxide (CO₂), (2) methane (CH₄), (3) nitrous oxide (N₂O), (4) HFCs, (5) SF₆, and (6) the perfluorocarbons (PFC). The last four of these six categories are within the domain of NOAH’s research.

Continuing programs within NOAH are based upon in situ or flask air sample measurements from four CMDL observatories and nine cooperative sampling sites (Figure 5.1, Table 5.1. These stations provide a robust network for evaluating global atmospheric change and hemispheric relationships for the various compounds.

One of the more significant discoveries in the past 2 years is the turnover of total anthropogenic chlorine in the atmosphere. This finding came from the combined results of flask sampling, in situ monitoring, and standards programs and is discussed in the following sections.

Fig. 5.1. Geographic locations of old and new stations in the HATS flask (gray circles) and in situ (crosses) networks. The site for the tropical balloon launches, Juazeiro do Norto, Brazil, is noted by an “X.”

TABLE 5.1. Geographic and Network Information on HATS Network Sites

Cooperative sites (F = flasks, I = in situ) with:

*In situ GC: Only N₂O and SF₆; flask sampling for all gases, however.

†Commonwealth Scientific and Industrial Research Organization (CSIRO) and Bureau of Meteorology, Australia

‡Only glass flasks used.

Other significant results include the observed global increases in atmospheric N₂O and SF₆, again in both flask and in situ monitoring, the continued growth of atmospheric HCFCs and HFCs as they increasingly are used as replacements for the ozone-depleting CFCs, and the surprising, protracted growth of the halons in the atmosphere years after the cessation of their production in developed countries.

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