CH2O hv CO h2

Table 7.6 Model global mean surface emission and deposition rates together with global mean mixing ratios for CH4, CO and H2.

rate

ch4

co

h2

Mass emission (Tg year-1)

1038

3295

47

Molecular emission (cm-2 s-1)

2.5x 1011

4.5x 1011

9.0x 1010

Mass deposition (Tg year-1)

30

198

51

Deposition rate (s-1)

2.0x10-9

1.0x 10-7

9.0x 10-8

Soil resistance (s m-1)

4.9x 104

2000

2500

Mixing ratio (ppmv)

1.70

0.10

0.54

estimated global emission rate of 51 Tg year-1. estimates for the various sources and sinks of tropospheric hydrogen have varied over the years, with the emission rate varying within a range of 30-70 Tg year-1. Hydrogen sources, beyond methane oxidation, include oxidation of non-methane hydrocarbons, fossil-fuel combustion and biomass burning. Additional to the soil sink of H2 is oxidation by oH and reaction with Ho2, o(1D), o and cl. A significant potential source of hydrogen in the troposphere, is the reaction of co with oH, however, in the present oxygen-rich atmosphere this is rapidly converted to hydrogen peroxide, H2o2, via the reactions co + oH ^ co2 +H, (7.136)

Hydrogen peroxide is highly water soluble and is removed by rain from the atmosphere. In an anoxic atmosphere, one that has little oxygen, the production of H, from the reaction of co and oH, may become a significant source of atmospheric hydrogen.

In Table 7.6 are shown the emission and deposition rates of cH4, co and H2 that give near-surface tropospheric mixing ratios close to the observed values, based on 1D radiative/convective-photochemical model calculations (see chapter 11). The soil resistance corresponds to a soil cover fraction 0.21 of the earth's total surface area, so the global mean is reduced by this fraction.

The vertical variation of molecular hydrogen is shown in Fig. 7.13, where we see that it remains fairly constant near 540 ppbv within the troposphere but gradually rises to 6000 ppbv near 140 km. The escape rate to space is 2.2x108 molecules cm-2 s-1. Atomic hydrogen becomes significant above 80 km.

7.9.2 Effects of increasing CH4 emission

Molecular hydrogen, like carbon monoxide and methane, is oxidized by atmospheric oH

flG. 7.16. The variation of methane, H2 and CO surface mixing ratios with methane surface emission (1 PR = 2.5X1011 molecules cm~2 s_1). Also shown is the rise in global mean stratospheric H2O mixing ratio.

and its atmospheric mixing ratio is thus dependent on the efficiency of methane and CO oxidation by OH. As methane emission increases the OH that is available to oxidize it is reduced resulting in a non-linear rise in methane surface mixing ratio with increasing methane surface emission. In Fig. 7.16 is shown the non-linear rise in the tropospheric methane mixing ratio with methane surface emission measured in units of a model present rate (PR) of 1 PR = 2.5X1011 molecules cm-2 s-1) equivalent to 1080 Tg year-1. Also shown are the rate of increase in the H2 and CO surface mixing ratios with increasing methane surface emission. Also shown is the resultant stratospheric peak H2O mixing ratio. The rise in the global mean surface temperature corresponding to double methane surface emission is about 1.0 K (see §11.3.5).

7.9.3 Effects of increasing CO2 levels

In Table 7.7 are shown the effects of increasing CO2 from its present atmospheric level (1 PAL = 365 ppmv) on various climatic parameters of the atmosphere.

Table 7.7 Effects of increasing atmospheric CO2 levels (1, 2, 4 and 8 PAL) on methane, carbon monoxide and hydrogen levels (ppmv). Water-vapour column Wh2o is in g cmT2. Stratospheric peak water vapour mixing ratio H2O St is in ppmv.

parameter

1

2

4

8

ch4

1.70

1.46

1.22

0.99

co

0.101

0.093

0.085

0.076

h2

0.539

0.496

0.447

0.389

Ts

288.15

289.43

290.90

292.50

Wh2 o

2.09

2.35

2.65

3.02

h2o St

5.75

5.43

4.79

3.19

A radiative-convective model (see §11.3) was run in conjunction with a photochemistry model where the surface emission rates of methane, co and H2 are fixed to their present levels. As the carbon-dioxide level increases, the surface temperature increases and so does the water vapour amount in the atmosphere. The effect is that the oxidation of cH4, co and H2 by oH increases and so the atmospheric content of these three gases decreases. Thus methane levels in the atmosphere are linked to co2 levels through the water-vapour content. The stratospheric water vapour mixing ratio decreases because of the cooling of the tropospheric-stratospheric region (§11.3) and also by the decrease in atmospheric methane.

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