Tropospheric Trends

An excess of sources relative to sinks leads to accumulation of gases in the atmosphere. Increasing sources through the industrial era have enhanced atmospheric burdens of C02 and CH4.36 Similar long-term increases in CO could be expected; however, the few analyses of CO in firn and ice samples have not produced convincing evidence of such a change.39

Time series of CO mixing ratios often show periods of increase and decrease.17'24 Spectroscopic measurements made at Jungfrauhoch, Switzerland, during 1950-1951 and again in 1985-1987 suggested an average rate of increase of ~1% per year in the total column abundance of CO above the European boundary layer.12 A similar rate of increase was seen in column measurements made over western Russia.40 Surface measurements made at six sites (evenly distributed between the Northern and Southern Hemispheres) during 1981-1986 suggested a similar rate of increase.41 In contrast, no significant trend could be identified at Cape Point, South Africa, during the period from the early 1970s through the mid-1980s.18'42 Trends largely reflect imbalances in its sources and sinks. The reported long-term CO increase in the Northern Hemisphere has been attributed to increasing CO emissions from industrial and transportation-related sources.12'39'43 However, a quantitative study relating CO emissions and increased atmospheric mixing ratios is still needed.

The long-term increase in CO may have slowed, then reversed in the late 1980s. Khalil and Rasmussen41'43 present time series determined at six sites beginning 1981 that show an increase in CO over the period 1981-1986, followed by a decrease during 1987-1992 (Fig. 2). The absolute decline in the Northern Hemisphere was about twice that in the south; in the Southern Hemisphere the relative rate of decrease was four times that in the Northern Hemisphere. Novelli et al.44 reported results from the NOAA/CMDL air sampling network showing a 10% decrease in global average CO mixing ratios during 1992-1993. And while CO declined in both hemispheres, the absolute rate of decrease in the Northern Hemisphere was nearly twice that in the south, while the relative rates were the same (approximately 6 to 7% per year). After 1993, CO levels showed short periods of increase and decrease with some recovery toward pre-1992 levels.24

CO time series determined in the north show a significant decrease over the past 10 years; in contrast, a trend in the Southern Hemisphere is more difficult to discern, due in part to the high level of interannual variability. This high variability is likely

J 10

1978 1981 1984 1987 1990 19

Time (months)

Khalil and Rasmussen (1994)

Figure 2 Time series of deseasonalized hemispheric and global mean CO mixing ratios. (Reprinted with permission from Khalil and Rasmussen.43)

related to yearly variations in emissions from biomass burning. The short-term increases and decreases seen in the recent CO time series17'24 may be related to interannual variability in biomass burning43'44 and a short-term increase in OH related to the eruption of Mt. Pinatubo in June 1991.39'46 Decreased emissions from anthropogenic sources in the Northern Hemisphere have contributed to the observed decrease44'45.

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