Physical removal wet and dry deposition

Henry's law constant, ^H(T0), for CH4 is very small and it is therefore unlikely that CH4 will partition into the aqueous phase. Hence, physical removal by wet deposition (i.e. removal from the atmosphere by uptake into rain or aerosol species) will not be a significant sink for CH4. Table 11.2 summarizes some Henry's law constants for a range of C1 compounds found in the atmosphere for comparison, from which it

Table 11.1. Sources of methane (CH4) in the atmosphere. (From IPCC, 1996.)

Likely (Tg/year)

Range (Tg/year)

Natural sources

_

-

Wetlands

-

-

Tropics

65

30-80

Northern latitudes

40

20-60

Termites

20

10-50

Ocean

10

5 -5 0

Freshwater

5

1 -2 5

Geological

10

5 -1 5

Others

10

5 -1 5

Total

160

-

Anthropogenic sources

-

-

Fossil fuel-related

-

-

Coal mines

30

15-45

Natural gas

40

25-50

Petroleum industry

15

5 -3 0

Coal combustion

15

5 -3 0

Waste management system

-

Landfills

40

20-70

Animal waste

25

20-30

Domestic waste

25

15-80

treatment

Enteric fermentation

85

65-100

Biomass burning

40

20-80

Rice paddies

60

20-100

Total

3 75

-

Total sources

535

-

emerges that methanoic acid (formic acid) is very soluble and partitions preferentially into the aqueous phase, but as one progresses from carbonyl to alcohol to alkane, the partitioning into the aqueous phase becomes smaller. For comparison, hydrogen peroxide, a highly soluble species, and nitric oxide, a highly insoluble species, are included in the list.

The dry deposition velocities for hydrocarbons have not been measured extensively, but data suggest that they are rather small (PORG, 1997). Table 11.3 summarizes the data from a series of chamber experiments covering a range of hydrocarbons that span molecular masses and molecule type. Surface types were chosen to give a variation in plant canopy type and leaf area index. Although it is not measured explicitly in this study, owing to the particu-

Table 11.2. Henry's Law constants, KH(T0), at 298 K for some C1 compounds. (From Sander, 1999.)

Gas Symbol ^H(70)/mol/atm

Table 11.2. Henry's Law constants, KH(T0), at 298 K for some C1 compounds. (From Sander, 1999.)

Gas Symbol ^H(70)/mol/atm

Methane

CH4

1.4

X

10-3

Methanol

CH3OH

2.2

X

10-2

Methanal

CH2(O)

1.3

Methanoic acid

CH(O)OH

5.5

X

103

Hydrogen peroxide

HOOH

8.3

X

104

Nitric oxide

NO

1.9

X

10-3

larly non-polar nature of CH4, removal by dry deposition to the surface is assumed to be small as well. As discussed by Dunfield (Chapter 10, this volume), the soil sink for CH4 is not insignificant, and is estimated to be between 10 and 44 Tg CH4/year. CH4 is consumed in soils by methanotrophic bacteria and archaea (e.g. Pancost et al., 2000; Aloisi et al., 2002). Hence, it is concluded that wet and dry depositions are not significant sinks for CH4 from the atmosphere, but consumption of CH4 within the soil is indeed a significant atmospheric sink.

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