2.4.2. Flask Measurements of Carbon Monoxide

The expansion of the number of sites in the CMDL cooperative air sampling network from which CO was determined continued to increase during the past few years.  Only glass flasks fitted with glass pistons and Teflon O-rings were used for CO analysis.  These flasks have been gradually introduced to the network by replacing the previously used glass flasks, which had greased ground glass stopcocks, and by providing all new sites with the greaseless flasks.  By 1996 nearly all network sites used flasks suitable for CO measurements.  The annual mean CO mole fractions for 1996 and 1997 are given in Tables 2.8 and 2.9.  With many more sites measuring CO than in previous years, its global distribution is now better determined.  In particular, more locations in the high southern latitudes are being sampled.  Mole fractions measured at the antarctic sites are typically the lowest found on Earth.  As observed in the high latitudes of the northern hemisphere, the time series from individual sites in the high southern hemisphere also show very similar features (Figure 2.17b).  The seasonal cycles in both the arctic and antarctic are thought to reflect the effects of both transport of polluted air from lower latitudes plus photochemical production and destruction of CO.    

The seasonal CO maximum in the southern hemisphere was quite depressed in 1996.  However, many sites in the tropics and extratropics showed a strong recovery during 1997 (Figure 2.18).   The relatively large annual mean CO values reported for 1997 in the low latitudes of the South China Sea (Table 2.9) reflect very high CO levels during August, September, and October.  The magnitude of the seasonal maximum in the southern hemisphere is believed to be driven largely by seasonal biomass burning [Novelli et al., 1998].  Widespread fires, fostered by dry weather typical of El Niño conditions, occurred in Indonesia during June to November 1997.  The enhanced CO levels found in the south during spring 1997 most likely resulted from these fires.  Samples of air collected from a commercial jet over Indochina during this period show elevated CO in the middle troposphere, presumably resulting from the vertical transport of gases from the polluted boundary layer [Matsueda et al., 1998, H. Matsueda, personal communication, 1998]. 

TABLE 2.8.   Preliminary 1996 and 1997 Annual Mean CO Mole Fractions from Land Sites  

 TABLE 2.9.   Provisional Annual Mean CO From Shipboard Programs

   *Annual mean values are not reported for 1997 as no samples were collected during December 1997.

It is clear that there is considerable variability in levels of tropospheric CO as is evident, for example, in the 15-year record from Cape Point, South Africa [Scheel et al., 1996].  The shorter record of global measurements also shows periods of CO increase and decrease.  To study global trends in CO, the time series from all marine background sampling locations were combined and smoothed to provide a surface representing tropospheric CO as a function time and latitude.  From this surface, global trends were extracted. This surface also provides averaged time series for the two hemispheres and four semi-hemispheres, from which zonal trends were also calculated.  After a sharp decline in global CO levels during 1992-1993  [Novelli et al., 1994], there was some recovery during 1994-1995 [Novelli et al., 1998].  However, the recovery in the southern hemisphere was short-lived, and another decline in CO occurred during 1996.  The temporally averaged global rate of change was -2.1 ppb CO yr^-1 from 1991 through 1996.  This is equivalent to a decrease of approximately 2.4% yr^-1 relative to average mole fractions in 1991.  The zonally-averaged decrease in the northern hemisphere (-2.3 ppb yr^-1) was greater than that in the southern hemisphere (-1.9 ppb yr^-1).  These decreases are believed to be due primarily to changes in emissions from fossil fuel emissions and biomass burning [Novelli et al., 1994, 1998].

Fig. 2.18.   Time series from the Seychelles (4ºS), Ascension Island (8ºS), and American Samoa (14ºS).

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