Chemical removal

Preliminary survey

The Earth's atmosphere is oxidizing in nature (Ravishankara, 1988) and therefore CH4 will be photo-oxidized via reaction with free radicals and possibly some closed-shell species in the atmosphere. There are a number of potential species in the atmosphere that may initiate this photo-oxidation, but it emerges that very few of these are important sinks for CH4. Although the dissociation energy for a C-H bond in CH4 is 4.55 eV (~440 kJ/mol), which equates to a wavelength of 272.2 nm, there are no electronic states of CH4 in this region of the spectrum (Yung and DeMore, 1999). Consequently, CH4 can only be broken down by high-energy radiation (100-140 nm) and this is found in the mesosphere (equivalent to an altitude of 50-80 km) and upper regions of the atmosphere. Whilst direct photolysis of CH4 is important in the atmospheres of Jupiter, Saturn, Uranus and Neptune, it is

Table 11.3. Empirical deposition velocities (|m/s) for selected hydrocarbons to various surfaces. (From PORG, 1997.)

Ethene

Ethyne

1,3-Butadiene

n-Hexane

Benzene

Spinach

74

2

78

43

48

Grass

150

65

82

49

56

Soil

27

16

63

22

27

Salt water

1.6

1.6

1.5

3

1.4

not an important loss process in the Earth's atmosphere. Table 11.4 collects data for the enthalpy of reaction for the most likely species that will initiate oxidation of CH4 in the Earth's atmosphere.

This table shows that for the majority of species (both radicals and closed-shell species) the enthalpy of reaction is large and positive, i.e. the reaction is endothermic at temperatures found in the Earth's atmosphere (~200-300 K) and will either proceed very slowly or not proceed at all. The reactions that can be ignored on thermody-namic grounds have been shaded in Table 11.4. Table 11.5 collects the rate coefficient data, where available, for the list of reactions compiled in Table 11.4. A rate coefficient of 1 x 10-10 cm3/molecule/s would be typical of a gas-phase reaction occurring at the kinetic limit (i.e. the reaction occurs every time the reactants collide). A rate coefficient of 1 x 10-11 cm3/molecule/s means that one in ten collisions lead to reaction and so on. All the reactions that were discarded on thermodynamic grounds have rate coefficients between 5 x 10-18 and 1 x 10-52 cm3/ molecule/s, which effectively means that no reaction occurs. Of the reactions that are thermodynamically feasible, only those with excited oxygen atoms, O(1D), and fluorine atoms, F, are very fast. In Table 11.6 the loss rate for CH4 for each of the 11 unshaded gasphase reactions from Table 11.4 is estimated for the troposphere (the lowest 10-15 km of the atmosphere) and the stratosphere (the region encompassing the altitude range of 10-50 km). In each case the loss rate R is determined using Eq. 11.1:

where [X] is the concentration of the co-reactant with CH4 and k(T) is the rate coef-

Table 11.4. Enthalpy of reaction data for the reaction of major atmospheric constituents with methane (CH4). The unshaded data (exothermic or near thermo-neutral enthalpies of reaction) indicate reactions that may take place in the atmosphere, shaded data are considered to be too endothermic to be of importance. (Thermodynamic data from DeMore et al., 1997 and Chase, 1998.)

Table 11.4. Enthalpy of reaction data for the reaction of major atmospheric constituents with methane (CH4). The unshaded data (exothermic or near thermo-neutral enthalpies of reaction) indicate reactions that may take place in the atmosphere, shaded data are considered to be too endothermic to be of importance. (Thermodynamic data from DeMore et al., 1997 and Chase, 1998.)

Reaction

A-H 298 K (kJ/mol)

O(1 D) + CH4

^OH + CH3

-1 78.7

F + CH4

^HF + CH3

-130.6

CN + CH4

^HCN + CH3

-78.7

cf3o + ch4

^cf3oh + ch3

-63.3

OH + CH4

^h2o + ch3

-59.5

H + CH4

^h2 + ch3

+3.3

ch3o +ch4

^ch3oh + ch3

+3.7

Cl + ch4

^HCl + CH3

+8.0

O (3P) + CH4

^OH + CH3

+10.9

NO3 + ch4

^hno3 + ch3

+12.5

FO + CH4

^HOF + CH3

+14.6

BrO + CH4

^HOBr + CH3

+37.2

ClO + ch4

^HOCl + CH3

+43.8

ho2 + ch4

^h2o2 +ch3

+73.1

Br + CH4

^HBr + CH3

+73.3

ch3o2 + ch4

^ch3ooh + ch3

+73.6

N + CH4

^NH + CH3

+105.4

n2o5 + ch4

^HNO3 + NO2 + CH

3 +1 07.9

no2 + ch4

^HONO + CH3

+1 08.7

o3 + ch4

^OH + O2 + CH3

+1 17.5

I + ch4

^HI + CH3

+1 40.8

o2 + ch4

^ch3 + ho2

+233.0

NO + CH4

^HNO + CH3

+238.1

CO + ch4

^HCO + CH3

+3 73.6

ficient at a temperature T, taken nominally to be 300 K for the troposphere (surface) and 250 K for the stratosphere (30 km). [CH4] is the concentration of CH4. The concentration of X has been derived from the modelling studies of Lary and Toumi (1997) and Lary and Shallcross (2000), at the Earth's surface for the troposphere and at 30 km for

Table 11.5. Gas-phase rate coefficients for the reaction of major atmospheric constituents with methane (CH4). k(T) = A exp(-fa/RT)(T/300)n.

Arrhenius A

Reaction molecule/s) (kJ/mol) (T/298)n (cm3/molecule/s) Reference

O(1 D) + CH4 ^ OH + CH3

1.5 x10"

10

0.0

0.0

1.5

x

0"

10

Atkinson et al.

(2001)

F + CH4 ^ HF + CH3

1.3 x

10"

-10

1.8

0.0

6.4

x

0"

11

Atkinson et al. (2001)

CN + CH4 ^ HCN + CH3

5.1 x

10-

13

2.64

0.0

8.6

x

0"

13

Baulch et al. (1994)

cf3o + CH4 ^ CF3OH + ch3

2.5 x

10"

12

11.8

0.0

2.2

x

0"

14

Atkinson et al. (2001)

OH + CH4 ^ H2O + CH3

2.5 x

10"

12

14.8

0.0

6.5

x

0"

-15

DeMore et al. (1997)

H + CH4 ^ H2 + CH3

5.8 x

10"

13

33.6

3.0

8.4

x

0"

19

Baulch et al. (1 992)

ch3o + ch4 ^ ch3oh + ch3

2.6 x

10"

13

37.0

0.0

9.4

x

0"

20

Tsang and Hampson (1986)

Cl + CH4 ^ HCl + CH3

6.6 x

10"

12

10.3

0.0

1.1

x

0"

13

Atkinson et al. (2001)

O(3P) + CH4 ^ OH + CH3

8.3 x

10"

12

35.5

1.56

5.5

x

0"

-18

Baulch et al. (1 992)

no3 + ch4 ^ hno3 + ch3

<8.0 x

10"

-19

-

-

-

Boyd et al. (1991)

ClO + CH4 ^ HOCl + CH3

1.0 x

10"

12

30.8

0.0

4.4

x

0"

-18

DeMore et al. (1 997)

ho2 + ch4 ^ h2o2 + ch3

1.5 x

10"

11

103.1

0.0

1.7

x

0"

29

Baulch et al. (1 992)

Br + CH4 ^ HBr + CH3

50 x

10"

10

73.9

0.0

6.8

x

0"

23

Russell et al. (1988)

CH3O2 + CH4 ^ CH3O2H + ch3

3.0 x

10"

13

77.3

0.0

1.0

x

0"

-26

Tsang and Hampson (1 986)

N + CH4 ^ NH + CH3

-

-

-

<3.0

x

0"

18

Aleksandrov et al. (1990)

n2o5 + CH4 ^ hno3

-

-

-

<2.0 x 10-23

Cantrell et al. + NO2 + CH3

(1987)

NO2 + CH4 ^ HONO + CH3

2.0 x

10"

11

125.6

0.0

2.7

x

0"

33

Slack et al. (1981)

O3 + CH4 ^ OH + O2 + CH3

-

-

<1.2

x

0"

21

Stedman and Niki (1 973)

I + CH4 ^ HI + CH3

2.5 x

10"

10

138.0

0.0

2.3

x

0"

34

Pardini and Martin (1983)

O2 + CH4 ^ CH3 + ho2

6.6 x

10"

11

237.8

0.0

2.6

x

0"

52

Baulch et al. (1 992)

NO + CH4 ^ HNO + CH3

<4.3

x

0"

30

Yamaguchi et al. (1999)

Table 11.6. Gas-phase rate coefficients for the reaction of major atmospheric constituents with methane (CH4) under tropospheric and stratospheric conditions, see text for more details. (From Lary and Toumi,1997 and Lary and Shallcross, 2000.)

Tropospheric

CH4 loss rate

k (300 K)

concentration

troposphere

Relative CH4

Reaction

(cm3/molecule/s)

(molecule/cm3)

(molecule/cm3/s)

loss rate

O(1 D) + CH4 ^ OH + CH3

1.5 x 10-10

2.5 x 10-1

1.7 x 106

5.8 x 10-1

F + CH4 ^ HF + CH3

6.4 x 10-11

2.5 x 10-10

7.2 x 10-4

2.5 x 10-10

CN + CH4 ^ HCN + CH3

8.6 x 10-13

2.5 x 10-13

9.7 x 10-9

3.3 x 10-16

cf3o +ch4 ^ CF3OH + ch3

2.2 x 10-14

2.5 x 10-8

2.5 x 10-5

8.5 x 10-13

OH + CH4 H2O + CH3

6.5 x 10-15

1.0 x 106

2.9 x 1011

100.0

H + CH4 ^ H2 + CH3

8.4 x 10-19

2.5 x 10-2

9.5 x 100

3.2 x 10-6

ch3o + ch4 ^ ch3oh + ch3

9.4 x 10-20

2.5 x 102

1.1 x 100

3.6 x 10-7

Cl -i- CH4 ^ HCl + CH3

1.1 x 10-13

2.5 x 103

1.2 x 107

4.2

O(3P) + CH4 ^ OH + CH3

5.5 x 10-18

2.5 x 103

6.2 x 102

2.1 x 10-4

no3 + ch4 ^ hno3 + ch3

<8.0 x 10-19

2.5 x 108

9.0 x 106

3.1

Stratospheric

concentration

k (250 K)

(molecule/

CH4 loss rate

Relative CH4

Reaction

(cm3/molecule/s)

cm3) (/s)

stratosphere

loss rate

O(1 D) + CH4 ^ OH + CH3

1.5 x 10-10

2.9 x 101

1.9 x 106

37.0

F + CH4 ^ HF + CH3

5.5 x 10-11

2.5 x 10-10

6.0 x 10-6

1.2 x 10-10

CN + CH4 ^ HCN + CH3

3.2 x 10-13

2.9 x 10-4

4.1 x 10-2

7.9 x 10-7

cf3o + CH4 ^ CF3OH + ch3

8.6 x 10-15

2.5 x 10-8

9.5 x 10-8

1.8 x 10-12

OH + CH4 ^ H2O + CH3

2.0 x 10-15

5.9 x 106

5.2 x 106

1 00.0

H + CH4 ^ H2 + CH3

3.3 x 10-20

3.3 x 100

4.8 x 10-5

9.2 x 10-10

ch3o + ch4 ^ ch3oh + ch3

4.8 x 10-21

2.5 x 102

5.3 x 10-4

1.0 x 10-8

Cl + CH4 ^ HCl + CH3

4.6 x 10-14

1.7 x 105

3.4 x 106

66.1

O(3P) + CH4 ^ OH + CH3

2.4 x 10-19

3.3 x 109

3.5 x 105

6.7

no3 + ch4 ^ hno3 + ch3

<8.0 x 10-19

2.5 x 108

8.8 x 104

1.7

the stratosphere; the level of CH4 assumed is 1800 ppb and 1500 ppb for the troposphere and stratosphere, respectively. The combination of rate coefficient and concentration of species leads to the conclusion that in the troposphere the OH radical is the major sink for CH4, and chlorine atoms and possibly nitrate radicals also play a minor role. In the stratosphere, the OH radical is still the dominant sink, whilst the role of O(1D) as a sink becomes more prominent with altitude, and at ~30 km is a significant sink, as are chlorine atoms.

The accurate measurement of gas-phase rate coefficients is non-trivial and a source of error in any estimation of CH4 atmospheric sinks. However, for many of the species proposed in Table 11.4, the atmospheric concentration can only be inferred from the models. The concentration of radical (open-shell) species in the troposphere and stratosphere is usually very small and poses a significant measurement challenge. In the troposphere there have been several measurements of the OH radical and, whilst these have not been extensive in time and space, there is reasonable agreement between models and measurements (e.g. Heard and Pilling, 2003). Several measurements have been made of the nitrate radical (NO3) in the troposphere (e.g. Brown et al., 2003; Stutz et al., 2004) that are in keeping with the model estimates used. In contrast, the measurements of Aliwell and Jones (1998) suggest that if NO3 concentrations in urban environments could be as much as 40 times larger than those used in this assessment, such a level of NO3 would become an important sink for CH4 locally. However, since the rate coefficient measured for the reaction between NO3 and CH4 is an upper limit, it would be judicious to view NO3 as a potentially minor sink only for CH4 in and near urban areas, and unimportant elsewhere. There are no direct measurements of chlorine atom concentrations in the atmosphere, but they have been inferred from measurements of other species (e.g. Wingenter et al., 2001; Hopkins et al., 2002). These studies suggest that in marine environments the model-derived concentration used in Table 11.6 for the troposphere may be up to a factor of ten or lower. Therefore, chlorine atoms (generated from the oxidation of sea salt) could be an important sink for CH4 in the marine boundary layer. In conclusion to this preliminary survey, reaction with OH radicals is the major sink for CH4 in the lower atmosphere (stratosphere and troposphere), with chlorine atoms making a minor contribution globally in the troposphere but being potentially important in marine environments. In the stratosphere both chlorine atoms and excited oxygen atoms are non-negligible sinks for CH4.

Isotopic analysis of the chemical sinks for methane

An emerging field in atmospheric science is the measurement of isotopic abundances.

Such analyses have been used frequently in Earth sciences but have only recently been possible in atmospheric studies due to improvements in sensitivity of instrumentation and pre-concentration techniques. Isotopic analysis is proving to be a useful technique to further refine an assessment of atmospheric CH4 sinks. Table 11.7 collects gas-phase rate coefficients for the reactions of OH radicals and chlorine atoms with 12CH4, 12CH3D, 12CH2D2, 12CHD3 and 12CD4. The kinetic isotope effect is particularly pronounced when comparing hydrogen with deuterium, and Table 11.7 exemplifies this, where substitution of the heavier isotope slows down the rate of reaction dramatically. Less pronounced is the effect of substituting 12C with 13C. Recently, measurement of 12CH4 (98.8% abundant), 13CH4 (1.1% abundant) and 12CH3D (0.06% abundant) in the atmosphere has become possible. Using inverse modelling it is possible to use these data to study the atmospheric sinks for CH4. Bergamaschi et al. (2000) have made measurements of these three species at Izana on Tenerife over a period of 2 years. Interestingly, the seasonal cycle observed for 12CH3D is shifted by several months with respect to that observed for 12CH4. By using an inverse model they were able to account for the seasonal changes in CH4 with just two main sinks in the troposphere: (i) loss by reaction with OH radicals;

Table 11.7. Gas-phase rate coefficients for the reaction between OH and various isotopomers of methane (CH4) (from Gierczak et al., 1997) and chlorine (Cl) atoms and various isotopomers of CH4 (from Boone et al., 2001).

OH

A (cm3/molecule/s)

Ea (kJ/mol)

k298 (cm3/molecule/s)

12CH4

2.50 x 10-12

14.8

6.50 x 10-15

12ch3d

3.11 x 10-12

15.9

5.12 x 10-15

12ch2d2

2.30 x 10-12

16.0

3.54 x 10-15

12chd3

1.46 x 10-12

16.4

1.94 x 10-15

12CD4

1.00 x 10-12

17.5

0.87 x 10-15

Cl

k298 (cm3/molecule/s)

1.10 x 10-13 0.65 x 10-13 0.42 x 10-13 0.19 x 10-13 0.05 x 10-13

12CH4

12ch3d

12ch2d2

12chd3

12CD4

1.10 x 10-13 0.65 x 10-13 0.42 x 10-13 0.19 x 10-13 0.05 x 10-13

and (ii) loss due to soil uptake. Under certain wind directions, characterized by air that had spent many days over the sea, an additional loss due to chlorine atoms was consistent with these observations.

Combining models and measurements in this way allowed these researchers to estimate that the OH sink was responsible for ~95% of the atmospheric loss and that chlorine atoms contributed up to 5% of the total loss. More extensive measurements of isotope data across the globe will be of considerable benefit in determining more accurately the role of chlorine atoms as a CH4 sink in the troposphere, particularly in remote marine environments.

Saueressig et al. (2001) have investigated the kinetic isotope effects in the reaction between O(1D) atoms and the three CH4 isotopomers discussed earlier. In addition to studies on the reaction of OH and chlorine with these species, they have used a 2D time-dependent chemical transport model to estimate the strengths of these three main sinks in the stratosphere. In the middle and upper stratosphere (p < 10 hPa or an altitude above 30 km) the sink, due to reaction with chlorine atoms, approaches 20% of the total CH4 sink at low and middle latitudes. At high latitudes, ~30% of CH4 is destroyed by chlorine atoms above 30 km. O(1D) atoms also contribute to the total CH4 sink, accounting for 30% of the loss in the tropics and middle latitudes at ~30 km. The OH radical is the remaining (and dominant) sink for CH4.

In conclusion, a very simple preliminary analysis shows that OH radicals, chlorine atoms and O(1D) atoms are the only significant atmospheric sinks for CH4 in the stratosphere and that OH radicals (with a minor contribution from chlorine atoms) comprise the main sink in the troposphere.

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