Atmospheric Chemistry Of Carbon Monoxide

We now turn to the tropospheric chemistry of carbon-containing compounds, the simplest of which, in many respects, is CO. The atmospheric oxidation of CO exhibits many of the key features of that of much more complex organic molecules, and so it is the ideal place to begin a study of the chemistry of the troposphere. Carbon monoxide reacts with the hydroxy! radical

and the hydrogen atom formed in reaction 1 combines so quickly with 02 to form the hydroperoxyl radical H02

H + 02 + M —> H02 + M that, for all intents and purposes, we can simply write reaction 1 as

The addition of an H atom to 02 weakens the O—O bond in 02, and the resulting H02 radical reacts much more freely than 02 itself. When NO is present, the most important atmospheric reaction that the H02 radical undergoes is with NO:

We have already encountered this important reaction in the stratosphere.

The hydroperoxyl radical also reacts with itself to produce hydrogen peroxide (H202):1

Hydrogen peroxide is a temporary reservoir for H0X (OH + H02):

Reaction 4 returns two HO* species, whereas in reaction 5 one HO, is lost to H20. The N02 formed in reaction 2 participates in the photochemical NO, (NO + N02) cycle:

(reaction 6) (reaction 7) (reaction 8)

Finally, termination of the chain occurs when OH and N02 react to form nitric acid:

This reaction removes both HO, and NO, from the system.

The CO oxidation is represented in terms of the HO* family in Figure 6.1. Phoa denotes the rate of OH generation from 03 photolysis. For simplicity, we do not show a flux from H202 back into HOt. Within the HO, family, OH and H02 rapidly cycle between

'Reaction 3 has both bimolecular and termolecular channels (Table B.l). At 298 K at the surface ([M] = 2.46 x 1019 molecules cm'3) the second-order rate coefficients for the two channels are 1.7 x 10~12 and 1.2 x 10~12cm3 molecule-1 s_l, respectively.

n02 + hv no + o o + o2 + m-^o3 + m no + o3^no2 + o2

HN03 03 H202

FIGURE 6.1 Reactions involving the HO, (OH + H02) family in CO oxidation.

HN03 03 H202

FIGURE 6.1 Reactions involving the HO, (OH + H02) family in CO oxidation.

themselves, so that a steady-state 0H/H02 partitioning is established, which depends on the NO* level. Figure 6.2 shows CO oxidation from the perspective of the NO* chemical family. The heavy arrows interconnecting NO and N02 in Figure 6.2, which represent the photochemical NO* cycle, indicate that this cycle occurs more frequently than either the NO + H02 or OH + N02 reactions. As a result, the partitioning between NO and N02 within the NO* family is approximately controlled by the photostationary state relation (6.6):

If the ratio of [N02] to [NO] increases, then the steady-state 03 concentration increases.

Each time an H02 + NO reaction occurs, an additional 03 molecule is produced as the resulting N02 molecule photolyzes. In Figure 6.1 and 6.2, this is indicated by the dashed lines leading to 03. This is a "new" 03 molecule because the N02 molecule formed from H02 + NO did not require an 03 molecule in its formation. Thus, the rate of production of 03 is simply equal to the rate of the H02 + NO reaction:

a

n0x

o3

oh

---- hv

'i no, r

FIGURE 6.2 Reactions involving the NO* (NO + N02) family in CO oxidation.

FIGURE 6.2 Reactions involving the NO* (NO + N02) family in CO oxidation.

It turns out that the overall behavior of the CO oxidation system depends critically on the overall level of NO,. (Recall that there is also an effect of NO, level on stratospheric chemistry.) This can be seen most clearly by examining the nature of the system at the limits of low and high NO,.

Let us obtain the expression for the rate of 03 formation in the CO system in the low NO, limit. The overall steady state HO, balance is (see Figure 6.1)

Pnox = 2 kWh, ho2 [H02]2 + &oh+no2 [OH] [N02] (6.10)

The rate of generation of HO, is balanced by loss through both HO, + HO, and HO, + NO, reactions; the two loss terms above can be denoted as

At low NO,, the principal sink of HO, is the H02 + H02 reaction. Neglecting the OH + N02 reaction, we obtain the steady-state concentration of H02 under low NO, conditions:

Substituting (6.13) into (6.9), we obtain the rate of 03 generation in the low NO, limit as

Thus, the rate of 03 production increases linearly with the NO concentration and proportionally to the square root of the HO, generation rate. Note that P0l is independent of the CO level; this is because NO, is the limiting reactant at the low NO, limit.

6.3.2 High NO, Limit

At high NO,, the steady-state HO, balance is

PHOx S* k0H+n02 [OH] [N02] (6.15) so the steady-state concentration of OH is

koh+n02 ijnu2j

In order to compute Po3 from (6.9), we need to know [H02]. Within the HO* family, OH and H02 rapidly interconvert (Figure 6.1), so we can neglect the effect of the OH + N02 and H02 + H02 reactions on their steady-state partitioning kco+on [CO] [OH] = *ho2+no [H02] [NO] (6.17)

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