Working Group I: The Scientific Basis


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4.2.3 Reactive Gases

4.2.3.1 Carbon monoxide (CO) and hydrogen (H2)

Carbon monoxide (CO) does not absorb terrestrial infrared radiation strongly enough to be counted as a direct greenhouse gas, but its role in determining tropospheric OH indirectly affects the atmospheric burden of CH4 (Isaksen and Hov, 1987) and can lead to the formation of O3. More than half of atmospheric CO emissions today are caused by human activities, and as a result the Northern Hemisphere contains about twice as much CO as the Southern Hemisphere. Because of its relatively short lifetime and distinct emission patterns, CO has large gradients in the atmosphere, and its global burden of about 360 Tg is more uncertain than those of CH4 or N2O. In the high northern latitudes, CO abundances vary from about 60 ppb during summer to 200 ppb during winter. At the South Pole, CO varies between about 30 ppb in summer and 65 ppb in winter. Observed abundances, supported by column density measurements, suggest that globally, CO was slowly increasing until the late 1980s, but has started to decrease since then (Zander et al. 1989; Khalil and Rasmussen, 1994), possibly due to decreased automobile emissions as a result of catalytic converters (Bakwin et al., 1994). Measurements from a globally distributed network of sampling sites indicate that CO decreased globally by about 2 %/yr from 1991 to 1997 (Novelli et al., 1998) but then increased in 1998. In the Southern Hemisphere, no long-term trend has been detected in CO measurements from Cape Point, South Africa for the period 1978 to 1998 (Labuschagne et al., 1999).

Some recent evaluations of the global CO budget are presented in Table 4.6. The emissions presented by Hauglustaine et al. (1998) were used in a forward, i.e., top-down, modelling study of the CO budget; whereas Bergamasschi et al. (2000) used a model inversion to derive CO sources. These varied approaches do not yet lead to a consistent picture of the CO budget. Anthropogenic sources (deforestation, savanna and waste burning, fossil and domestic fuel use) dominate the direct emissions of CO, emitting 1,350 out of 1,550 Tg(CO)/yr. A source of 1,230 Tg(CO)/yr is estimated from in situ oxidation of CH4 and other hydrocarbons, and about half of this source can be attributed to anthropogenic emissions. Fossil sources of CO have already been accounted for as release of fossil C in the CO2 budget, and thus we do not double-count this CO as a source of CO2.

Table 4.6: Estimates of the global tropospheric carbon monoxide budget (in Tg(CO)/yr) from different sources compared with the values adopted for this report (TAR).
Reference:
Hauglustaine et al.
Bergamasschi et al.
WMO
SAR
TARa
 
(1998)
(2000)
(1999)
(1996)
 
Sources
 
 
 
 
 
  Oxidation of CH4
 
795
 
400 - 1000
800
  Oxidation of Isoprene
 
268
 
200 - 600b
270
  Oxidation of Terpene
 
136
 
 
~0
  Oxidation of industrial NMHC
 
203
 
 
110
  Oxidation of biomass NMHC
 
-
 
 
30
  Oxidation of Acetone
 
-
 
 
20
Sub-total in situ oxidation
881
1402
 
 
1230
  Vegetation
 
-
100
60 - 160
150
  Oceans
 
49
50
20 - 200
50
  Biomass burningc
 
768
500
300 - 700
700
  Fossil & domestic fuel
 
641
500
300 - 550
650
Sub-total direct emissions
1219
1458
1150
 
1550
Total sources
2100
2860
 
1800 - 2700
2780
Sinks
  
 
 
 
 
  Surface deposition
190
 
 
250 - 640
 
  OH reaction
1920
 
 
1500 - 2700
 
Anthropogenic emissions by continent/region
Y2000
Y2100(A2p)
 
 
 
  Africa
80
480
 
 
 
  South America
36
233
 
 
 
  Southeast Asia
44
203
 
 
 
  India
64
282
 
 
 
  North America
137
218
 
 
 
  Europe
109
217
 
 
 
  East Asia
158
424
 
 
 
  Australia
8
20
 
 
 
  Other
400
407
 
 
 
  Sum
1036
2484
 
 
 
a Recommended for OxComp model calculations for year 2000.
b Includes all VOC oxidation.
c From deforestation, savannah and waste burning.

It has been proposed that CO emissions should have a GWP because of their effects on the lifetimes of other greenhouse gases (Shine et al., 1990; Fuglesvedt et al., 1996; Prather, 1996). Daniel and Solomon (1998) estimate that the cumulative indirect radiative forcing due to anthropogenic CO emissions may be larger than that of N2O. Combining these early box models with 3-D global CTM studies using models from OxComp (Wild and Prather, 2000; Derwent et al., 2001) suggests that emitting 100 Tg(CO) is equivalent to emitting 5 Tg(CH4): the resulting CH4 perturbation appears after a few months and lasts 12 years as would a CH4 perturbation; and further, the resulting tropospheric O3 increase is global, the same as for a direct CH4 perturbation. Effectively the CO emission excites the global 12-year chemical mode that is associated with CH4 perturbations. This equivalency is not unique as the impact of CO appears to vary by as much as 20% with latitude of emission. Further, this equivalency systematically underestimates the impact of CO on greenhouse gases because it does not include the short-term tropospheric O3 increase during the early period of very high CO abundances (< 6 months). Such O3 increases are regional, however, and their magnitude depends on local conditions.

Molecular hydrogen (H2) is not a direct greenhouse gas. But it can reduce OH and thus indirectly increase CH4 and HFCs. Its atmospheric abundance is about 500 ppb. Simmonds et al. (2000) report a trend of +1.2 ± 0.8 ppb/yr for background air at Mace Head, Ireland between 1994 and 1998; but, in contrast, Novelli et al. (1999) report a trend of 2.3 ± 0.1 ppb/yr based on a global network of sampling sites. H2 is produced in many of the same processes that produce CO (e.g., combustion of fossil fuel and atmospheric oxidation of CH4), and its atmospheric measurements can be used to constrain CO and CH4 budgets. Ehhalt (1999) estimates global annual emissions of about 70 Tg(H2)/yr, of which half are anthropogenic. About one third of atmospheric H2 is removed by reaction with tropospheric OH, and the remainder, by microbial uptake in soils. Due to the larger land area in the Northern Hemisphere than in the Southern Hemisphere, most H2 is lost in the Northern Hemisphere. As a result, H2 abundances are on average greater in the Southern Hemisphere despite 70% of emissions being in the Northern Hemisphere (Novelli et al., 1999; Simmonds et al., 2000). Currently the impact of H2 on tropospheric OH is small, comparable to some of the VOC. No scenarios for changing H2 emissions are considered here; however, in a possible fuel-cell economy, future emissions may need to be considered as a potential climate perturbation.


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