Atmospheric Chemistry Of Methane

The principal oxidation reaction of methane, CH4, is with the hydroxyl radical:

As in the case of the hydrogen atom, the methyl radical, CH3, reacts virtually instantaneously with 02 to yield the methyl peroxy radical, CH302

CH302 M

so that the CH4-OH reaction may be written concisely as

The rate coefficient for reaction 1 is k\ = 2.45 x 10~12 exp (-1775/T) cm3 molecule"1 s_1. At T = 273 K and [OH] = 106 molecules cm"3, the lifetime of CH4 against OH reaction is about 9 years. Despite its long lifetime, because of its high concentration (about 1750 ppb), CH4 exerts a dominant effect on the chemistry of the background troposphere.

Under tropospheric conditions, the methyl peroxy radical can react with NO, N02, H02 radicals, and itself; the reactions with NO and H02 are the most important. The reaction with NO leads to the methoxy radical, CH30, and N02:

The only important reaction for the methoxy radical under tropospheric conditions is with 02 to form formaldehyde (HCHO) and the H02 radical:

The CH30 + 02 reaction is so rapid that reaction 2 can be written as

The reaction of CH302 with H02 leads to the formation of methyl hydroperoxide, CH3OOH

which can photolyze or react with OH,

H20 + CH2OOH (reaction 5b)

The lifetime of CH3OOH in the troposphere resulting from photolysis and reaction with OH is ~ 2 days.

The fate of CH3OOH has a significant impact on the methane oxidation chain. Its formation by reaction 3 removes two radicals, CH302 and H02. The photolysis of CH3OOH returns two radicals, CH30 and OH. The reaction of CH3OOH with OH leads to an overall loss of radicals, however. In reaction 5a, OH abstracts the hydrogen atom from the —OOH group; the net result is that one OH radical is destroyed and one CH302 radical is regenerated. Some fraction of the CH302 radicals generated in reaction 5a can react with H02 by reaction 3 to reform CH3OOH; in this manner, reactions 5a and 3 constitute a catalytic cycle for the removal of OH and H02. Reaction 5b proceeds by abstraction of an H atom from the —CH3 group by OH, followed by breaking of the weak O—O bond in CH2OOH. The OH is regenerated, and a molecule of formaldehyde is formed.

Formaldehyde is a first-generation oxidation product of CH4 and, it turns out, of many other hydrocarbons. Indeed, the chemistry of formaldehyde is common to virtually all mechanisms of tropospheric chemistry. Formaldehyde undergoes two main reactions in the atmosphere, photolysis fia

and reaction with OH,

As we have already noted, the hydrogen atom combines immediately with 02 to yield H02. The formyl radical, HCO, also reacts very rapidly with 02 to yield the hydroperoxyl radical and CO:

Because of the rapidity of this reaction, the formaldehyde reactions may be written concisely as

Approximate atmospheric photolysis rates of HCHO are

HCHO + hv^ 2 H02 + CO jBCuoa ~ 3 x 1(T5 s"1 ^H2+CO jhcho, ~4x lO-v1

Thus, the lifetimes against these two photolysis pathways are thcho„ ~ 9 h and tHCHO,, ~ 7h. The rate coefficient for reaction 7, k7 = 9 x 10 12 cm3 molecule-1 s-1, gives an HCHO lifetime against OH reaction ([OH] = 106 molecules cm"3) of x7 ~ 31 h. While the rates of the two photolysis channels, 6a and 6b, are roughly comparable, their effect on the HO* radical pool is quite different; reaction 6a is a source of two HO* radicals, whereas reaction 6b leads to permanent loss of radicals.

Virtually every CH4 molecule is converted to formaldehyde, and the HCHO lifetime is relatively short.2 Both photolysis and OH reaction of HCHO lead to formation of CO. Thus, CO is the principal product of methane oxidation. As a result, the CO oxidation chemistry of the previous section automatically becomes part of the CH4 oxidation chain. Rewritten here, this includes reaction of H02 with NO to regenerate the OH radical

HO,-NO, termination by nitric acid formation

and HO,-HO, termination to form hydrogen peroxide:

Eventually CO is converted to C02 on a several-month timescale to complete the CH4 oxidation chain. Table 6.1 summarizes the tropospheric methane oxidation chain. The overall reaction sequence leading eventually to C02, through the HCHO and CO intermediate "stable" products, is shown in Figure 6.5.

The theoretical maximum yield of O3 from CH4 oxidation would occur when NO, levels are sufficiently high that the peroxy radicals H02 and CH302 react exclusively with NO and all the formaldehyde formed photolyzes by the radical path. The theoretical maximum yield is 4 03 molecules per each CH4 molecule oxidized:

HCHO + h\ —+ CO + 2H02 3(HOz + NO —> N02 + OH)

4(N02 + hv NO + O3) Net: CH4 + 8 02 —> CO + H20 + 2 OH + 4 03

Since the theoretical maximum yield of 03 from oxidation of CO is one more molecule of 03, the theoretical maximum yield of 03 from one CH4 molecule, all the way to C02 and H20, is

Such a maximum yield is, of course, not realized in the actual atmosphere because of all the competing reactions that have been neglected.

2Since thcho t6 a t6b x7

the overall lifetime of HCHO for the conditions described above is 3.6 h.

TABLE 6.1 Methane Oxidation Mechanism

Rate Coefficient,

Reaction (cm3 molecule-1 s~')a

TABLE 6.1 Methane Oxidation Mechanism

Rate Coefficient,

Reaction (cm3 molecule-1 s~')a

1.

ch4 + oh

ch3o2 + h2o

2.45 x 10"12 exp (-1775/7)

2.

ch302 + no

~ch

hcho + ho2 + no2

2.8 x 10"12 exp(300/7)

3.

ch3o2 + ho2

—>

CH3OOH + o2

4.1 x 10"13 exp(750IT)

4.

ch3ooh + hv

hcho + ho2 + oh

Depends on light intensity

5a.

chjooh + oh

—>

h2o + ch3o2

3.6 x 10-12 exp(200/7)fc

5b.

—>

h20 + hcho + oh

1.9 x 10-12"

6a.

hcho + hv

~Ch

2h02 + co

Depends on light intensity

6b.

—>

h2 + co

7.

hcho + oh

~<h

ho2 + co + h2o

9.0 x 10"12

8.

ho2 + no

n02 + oh

3.5 x 10-12 exp(250/7)

9.

oh + no2

M

hno3

See Table B.2

10.

ho2 + ho2

—>

h2o2 + 02

2.3 x 10"13 exp(600/7)

+ 4.3 x 10-14 exp(1000/7)

11.

co + oh

~<h

co2 + ho2

1.5 x 10"13(1 +0.6patm)

12.

h202 + hv

oh + oh

Depends on light intensity

13.

h202 + oh

—>

ho2 + h2o

2.9 x 10"12 exp(-160/7)

14.

no2 + hv

~Ch

no + o3

Depends on light intensity

15.

no + o3

no2 + 02

3.0 x 10"12 exp(-1500/7)

16.

ho2 + o3

—»

ho + 2O2

1.0 x 10"14 expM90/7)

17.

oh + o3

—»

ho2 + 02

1.7 x 10"12 exp(-940/7)

"Values from Sander et al. (2003) unless noted otherwise. Atkinson et al. (2004)

"Values from Sander et al. (2003) unless noted otherwise. Atkinson et al. (2004)

oh ch3+ h2o o2

Methane Oxidation Atmosphere

co oh c02

FIGURE 6.5 Atmospheric methane oxidation chain.

The chemistry of the background troposphere is dominated by that of CO and CH4. In continental regions where human emissions significantly influence the volatile organic compound composition of the atmosphere, the chemistry is more complex than that of CO and CH4 alone. Despite the complexities that arise as a result of the larger molecules, the basic elements of the chemistry are similar to those of CO and CH4: initiation by OH attack, formation of O3 as a result of peroxy radical-NO reactions, and termination by HO* + HO* and HO* + NO* reactions.

6.5 THE NO* AND NOv FAMILIES 6.5.1 Daytime Behavior

The NO* family comprises NO and N02: NO* = NO + N02. During daytime, NO and N02 interconvert by the photochemical NO* cycle, which is shown in Figure 6.2 by the heavy lines. The steady-state N0/N02 ratio from this cycle is given by (6.6)

Let us estimate this ratio at 298 K under typical urban conditions and noontime Sun: NO* = lOOppb, 03 = lOOppb, ;'no2 = 0.015 s \ [M] = 2.5 x 1019 molecules cm"3, and &NO+O3 = 1-9 x 10"14 cm3 molecules"1. One finds that [N0]/[N02] ^ 0.32, so NO =s 24ppb and N02 = 76ppb. At noon, j'no, is at a maximum, and NO constitutes the largest possible fraction of NO*. A typical 12-h daytime average ratio is [N0]/[N02] = 0.1. The principal daytime removal path for NO* is

At surface (300K, 1 atm) conditions, &oh+no2 ~ 1 x 10 "cm3 molecule 1 s 1. At [OH] 106 molecules cm , the lifetime of N02 during daytime is about 1 day. The lifetime of the NO* chemical family under daytime conditions is given by applying (3.34):

[N02

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Responses

  • jasper
    How is ch4 oxidized in the atmosphere?
    10 months ago
  • anja
    Does methane react with anything in the atmosphere?
    7 months ago

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