Tropospheric Chemistry Of Halogen Compounds

Halogen compounds arise in the troposphere from the chemical degradation of (partially) halogenated organic compounds that originate from a variety of natural and anthropogenic sources and by the liberation of halogen compounds from seasalt aerosol. Natural sources of gaseous halocarbons include the oceans, which release methyl halides (CH3C1, CH3Br, and CH3I) and polyhalogenated species (CHBr3, CH2Br2). Methyl halides, notably CH3Br, are also produced by biomass burning. Industrial replacements for fully halogenated CFCs that can be chemically degraded in the troposphere are also a source of tropospheric halogen compounds.

Sea salt contains (by weight) 55.7% CI, 0.19% Br, and 0.00002% I. Depletion of the CI and Br content of marine aerosol relative to bulk seawater, as measured by Cl/Na and Br/Na ratios, indicates that there is some net flux of these two halogens to the gas phase. Interestingly, the ratio I/Na in marine aerosol is typically much greater than that in seawater, often by a factor of 1000. The large enrichment for iodine in seasalt aerosols relative to seawater has been attributed, in part, to the enhanced level of organic I compounds in the surface organic layer on the ocean that become incorporated in the aerosol formation mechanism.

6.14.1 Chemical Cycles of Halogen Species

Once in the atmosphere, organic halogen molecules are broken down by direct photolysis or by hydroxyl radical attack. Either path leads to the release of atomic halogen. For example, OH reaction of methyl chloride leads to

CH3C1 + OH- —► -CH2C1 + H20 •CH2C1 + 02 + M —>■ CH2C102- + M CH2C102- + NO —► NOz + HCHO + CI

Halogen atoms are highly reactive toward hydrocarbons, leading to the formation of hydrogen halides through hydrogen abstraction. For CI atoms, for example

F and CI atoms react readily in this manner. Br atoms can abstract hydrogen atoms only from H02 or aldehydes; I atoms are even less reactive. The alternative to the CI + RH reaction is oxidation of the halogen atom (X = CI, Br, I) by ozone:

Because of the decreasing reactivity of the halogens, as they go from F to I, toward H-containing compounds to form HX, the fraction of free halogen atoms that react by path X + 03, as opposed to X + RH, is: F, ~0%; CI, ~50%; Br, ~99%; I, ~100% (Piatt, 1995).

The halogen halide, HX, can itself react with OH

which returns the halogen atom X to the halogen reservoir. The halogen oxide radicals can undergo a number of reactions. These include photolysis (important for X = I, Br, and, to a minor extent, CI)

and reaction with H02:

In airmasses influenced by anthropogenic emissions (NO mixing ratio on the order of 1 ppb), [XO]/[X] ratios are on the order of 10-100 (X = CI) and unity (X = Br, I).

Reactions of the nitrogen oxides N02 or N2O5 with NaX contained in seasalt aerosol can lead to the formation of XNO or XN02, respectively, for example

Hydrogen halides can be liberated from seasalt aerosol by the action of strong acids, such as H2S04 and HN03:

A consequence of the presence of reactive halogen species in the troposphere is that because the rate constants for halogen atom, particularly CI, reactions with hydrocarbons are significantly larger than those for the corresponding OH + HC reactions, hydrocarbons can be effectively removed by reaction with halogen atoms.

6.14.2 Tropospheric Chemistry of CFC Replacements: Hydrofluorocarbons (HFCs) and Hydrochlorofluorocarbons (HCFCs)

Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are replacement compounds for chlorofluorocarbons (CFCs). HCFCs and HFCs contain at least one hydrogen atom and therefore are susceptible to reaction with OH radicals in the troposphere. Since HFCs contain no chlorine they do not give rise to catalytic 03 destruction. Although HCFCs do contain chlorine, scavenging by OH radicals in the troposphere largely prevents these compounds from having a sufficiently long tropospheric lifetime to survive to be transported into the stratosphere. Figure 6.20 depicts the atmospheric degradation of HFCs, HCFCs, and other CFC substitutes.

The dominant loss process for HFCs and HCFCs in the atmosphere is reaction with the OH radical. Rate constants for the OH reaction with a wide variety of HFCs and HCFCs are available (Table B.l). A mechanism for the OH reaction of the generalized HFC or HCFC species

STRATOSPHERE

Temporary reservoirj

Temporary reservoirj

Oxydation Dms Par No3

SURFACE

FIGURE 6.20 Atmospheric degradation of HFCs, HCFCs, and other CFC substitutes. Timescales for different processes are given in italics.

SURFACE

FIGURE 6.20 Atmospheric degradation of HFCs, HCFCs, and other CFC substitutes. Timescales for different processes are given in italics.

is given in Figure 6.21. The first step is H-atom abstraction to yield a haloalkyl radical, which reacts with 02 to form the corresponding peroxy radical. The peroxy radicals may, under tropospheric conditions, react with NO, N02, and H02 radicals. Background tropospheric abundances of NO, N02, and H02 are quite similar. Since rate constants for reaction of halogenated peroxy radicals with NO are about 3 times larger than for reaction with N02 and H02, the dominant loss process for these radicals is by reaction with NO to give the corresponding alkoxy radicals.

There are several possible reaction paths for the haloalkoxy radicals:

Carbon-halogen bond cleavage:

Carbon-carbon bond cleavage:

02 reaction:

Chemical Reaction Volcano Eruption

FIGURE 6.21 Generalized atmospheric degradation mechanism for CX3CYZH. X, Y, and Z may be F, CI, Br, or H.

ho2-

FIGURE 6.21 Generalized atmospheric degradation mechanism for CX3CYZH. X, Y, and Z may be F, CI, Br, or H.

Sidebottom (1995) has summarized experimental data pertaining to the fate of the haloalkoxy radicals:

1. CX2CIO radicals (X = H, CI, or F) eliminate a CI atom except for CH3C10, where reaction with 02 is the dominant reaction.

2. CH2FO and CHFiO radicals react with 02 to give the corresponding carbonyl fluorides and H02. Fluorine atom elimination or reaction with 02 is unimportant for CF3O radicals. Loss of CF30 is largely determined by reaction with hydrocarbons and nitrogen oxides.

3. CX3CH20 radicals (X = H, CI, or F) react predominantly with 02 to form the aldehyde and H02 radicals.

4. CX3CC120 and CX3CFCIO radicals decompose by CI atom elimination rather than carbon—carbon bond fission.

5. CX3CF20 radicals undergo carbon-carbon bond breaking,

6. CX^CHYO radicals (Y = CI or F) have two important reaction channels. The relative importance of the carbon-carbon bond-breaking process and reaction with 02 is a function of temperature, 02 pressure, and the total pressure and hence varies considerably with altitude. (Laboratory results for the CF3CHFO radical, when used in tropospheric model calculations, indicate that 35—40% of HFC-134a released into the atmosphere will be converted into CFjCFO.)

Halogenated aldehydes, CX3CHO, may further degrade by photolysis or by reaction with OH, with typical lifetimes on the order of several days. Photolysis to form CX3 radicals is the dominant removal process if the quantum yield is close to unity.

Trifiuoromethyl radicals, CF3, are produced from HFCs and HCFCs that contain the CF3 group. As a haloalkyl radical, CF3 will rapidly add 02, then react with NO, to give CF3O radicals. Once present in the stratosphere, the effectiveness of CF302 and CFjO radicals in catalytic 03 destruction will depend on their reactivity wish 03 relative to that with other species. It appears that reactions of these radicals with 03 is slow compared to that with CH4 and NO, leading to a catalytic chain length of less than unity (as compared to catalytic chain lengths of 1000-10,000 for CIO,).

Ozone Destruction in the Polar Tropospheric Boundary Layer A sudden disappearance of O3 in ground-level air at Alert (82.5°N, 62.3°W) at polar sunrise was first noted in 1985. Ozone mixing ratios dropped from 30 to 40ppb to almost zero in the time span of a few hours to a day (Barrie et al. 1988). Continuous, ground-level O3 records between February and June show that, while proceeding from dark winter to sunlit spring, sudden O3 depletion events occur, paralleling the increase of sunlight at polar sunrise. A strong observed anticorrelation between 03 and bromine suggested that bromine is chemically implicated in the ozone removal (Bottenheim et al. 1990; Sturges et al. 1993). The sum of particulate Br and gaseous HBr has been observed to peak in the spring months of March and April throughout the Arctic. It has been suggested that the active component of gas-phase bromine might be bromine oxide (BrO) radicals, possible sources of which (Le Bras and Piatt 1995) include the following:

1. Formation of BrNO or BrNCb by reaction of NOi or N2O5, respectively, with NaBr contained in seasalt particles. BrNO and BrN02 arc rapidly photoiyzed to yield Br atoms.

2. Photochemical reaction of sea salt Br on the surface of aerosol particles to form Br2, followed by

3. Cycling of inorganic bromine compounds, whereby HBr, HOBr, and Br0N02 are converted back to Br2 by aqueous-phase chemistry on sulfuric acid aerosol,

4. Free-radicai-induced release of Br2 from sea sail aerosols by aqueous-phase oxidation of Br".

The O.i-depletion cycle might involve photolysis of Br2, followed by the Br + O? reaction and

PROBLEMS

6.1a a. Compute the number of OH radicals produced per O('D) generated from O3 photolysis for lower stratospheric conditions (240K: [H20]/[M] — 6 x 10~6) and for planetary boundary layer conditions (298 K; RH = 0.5). b. Calculate the concentration of 0(*D) and the production rate of OH in the lower stratosphere ([03] = 3 x molecules cm ) and in the boundary layer ([Oj] = 8 x Î011 molecules cm"3).

6.2A Compute the ozone production efficiency for CO oxidation at the Earth's surface (298 K) assuming a CO level of 200ppb, an O3 ievei of 50ppb, and a NO level of 40ppb. Assume /no, = 0.0 ! 5 1, You may use the rapid cycling approximation for [HOa]/[OH].

6.3 A Compute the daytime average [N0]/1N02| ratio (12 h) at the Earth's surface for a fixed 03 mixing ratio of lOOppb. Assume that the peak noontime value of /no, = 0.015 s"1 and that o2 is a parabolic function of time, rising from zero at 6:00 a.m. and returning to zero at 6:00 p.m.

6.4b We wish to estimate the effect of jet aircraft emissions of NO* on ozone production in the mid- and upper troposphere. Consider the following conditions: [M] = 7 x 1018 molecules cm-3, T = 230K, [03]/[M] = 70ppb, and [CO]/[M] = 100 ppb.

a. Assuming that the ratio of [N0]/[N02] is determined by the photostationary state, with /'\,o, = 0.01 s 1, calculate the ratio under these conditions.

b. The oxidation of CO will dominate the behavior of OH and H02. Using &oh+co = 1-8 x 10-13cm3 molecule-1 s"1 and &ho2+no = 1-0 x 10~u cm3 molecule-1 s 1, determine the [H02]/[0H] ratio for

NO* = 10, 100, 1000, and 104ppt c. Show that for these conditions the production rate of 03 equals the loss rate of CO.

d. In this airmass, HO* is being produced at a rate of 104 molecules cm-3 s' 1. Assuming that the only significant loss processes for HO* are

OH + H02 —> H20 + 02 fc(230K) = 1.4 x 10"10 cm3 molecule-1 s-1

OH + N02 + M —> HN03 + M ¿(230 K, [M] = 7 x 1018)

calculate for the different NO* levels above the number of 03 molecules produced for each HO* produced.

e. Assuming this chemistry occurs for 12 h each day, what is the production rate of 03 per day for the different NO* levels above?

6.5a Determine the lifetime of N02 at night against the reaction N02 + 03 —> N03 + 02 at z = 0, 5, and 10 km. Assume an 03 mixing ratio of 50 ppb.

6.6b We consider here the formation of N205 at night. Assume at the beginning of nighttime, the levels of N02 and 03 are 20 ppb and 50 ppb, respectively. As N03 forms from the reaction

N02 + 03 —> N03 + 02 k= 1.2 x 10"13 exp(—2450/r)

N02 is converted to N03. The N03 that forms reacts with N02 to form N205 and the following equilibrium can be assumed to be established:

N02 + N03 + M ^ N2Os + M K — 3.0 x 10-27 exp(10990/r) cm3 molecule"1

Assume that [03] remains constant at its initial value throughout the night (This is an approximation because 03 reacts with NO and is converted to N02.)

Compute [N02], [N03], and [N2O5] over a 12-h nighttime. Consider conditions at the Earth's surface at temperatures of 298 and 273 K. To obtain an upper-limit estimate of the amount of N2O5 that forms, assume that the concentration of N02 is determined only from the first reaction above, that is, do not account for the loss of N02 by conversion to N2O5 via the equilibrium. (A more accurate estimate of the amount of N2O5 formed is obtained by solving the coupled reaction rate equations for N02, NO3, and N205 using the forward and reverse rate coefficients of the N205-forming reaction.)

6.7B Formaldehyde (HCHO) and acetaldehyde (CH3CHO) are important carbonyl species in the atmosphere. Both photodissociate and react with OH:

HCHO + OH H02 + CO + H20 CH3CHO + hv ^ CH3O2 + H02 + CO CH3CHO + OH —- CH3C(0) + H20

a. Assume that initial concentrations of HCHO and CH3CHO are added to a mixture of NO* and air in a laboratory reactor and that at t = 0 photolysis begins. Write out a mechanism for the major chemical reactions that take place in the system.

b. Assuming that the NO* concentration is sufficiently high that HO*-HO* termination reactions can be neglected, reduce the mechanism in (a). (On the timescale of the laboratory reactor, oxidation of CO can be neglected.)

c. Write an expression for the rate of 03 formation in this system, Pq, , for the mechanism of (b). Making any reasonable assumptions, express Pq,, in terms of the concentrations of stable compounds in the system.

d. Give an expression for the ozone production efficiency in this system.

e. If this experiment were repeated at a temperature of, say, 20° C higher, how would you expect Pq, to change? Why?

6.8b The effectiveness of PAN formation depends on the rate constant ratio, a = £ch3c(0)02+n02 Ach3c(o)o2+no- This ratio can be measured in the laboratory by producing peroxyacetyl radicals, CH3C(0)02-, by photolyzing biacetyl, CH3C(0)C(0)CH3 in the presence of 02. Show that in such a system, with NO and N02 both initially present at concentrations [NO]0 and [NO2]0, if [PAN(/)| and [C02(f)] are measured as a function of time, and if temperatures are used at which PAN decomposition is slow, a can be determined from

If it is possible to measure PAN only as a function of time, then show that

where [PAN(i)]NO=0 is the PAN concentration at time t in the absence of initial NO.

Regions of the troposphere can be ozone-producing or ozone-depleting depending on the local level of NO,. The principal chemical sink of 03 is 03 photolysis followed by O('D) + H20. For example, at 10°S at the surface, ofo, ~ 7 x 10""6 s_1, leading to a photolysis lifetime of 03 of about 11 days. The chemical sink of 03 next in importance to photolysis is the reaction,

The principal chemical source of 03 in the troposphere is production through the methane oxidation chain. The level of NO is critical in this chain in dictating the fate of the H02 and CH302 radicals. The reactions H02 + NO and CH302 + NO lead to 03 production, whereas H02 + 03 leads to 03 removal. Consider conditions at the surface at 298 K.

a. Determine the mixing ratio of NO at which the rate of the H02 + NO reaction just equals that of the H02 + 03 reaction if 03 = 20 ppb. Assume 298 K.

b. To what level of NO, does the NO level determined in (a) correspond under noontime conditions? Assume yNO, = 0.015 s 1.

c. A competition also exists between NO and H02 for the CH302 radical, and the path depends on the concentration of H02 and NO. Assume that the local OH concentration is 106 molecules cm-3. Compute the local H02 concentration assuming that reaction with CO is the main sink of OH and H02-H02 self-reaction is the main sink of H02. Assume a CO mixing ratio of 100 ppb.

d. Determine the NO mixing ratio for the conditions of (c) at which the reaction of CH302 with NO just equals that with H02. To what NO, level does this NO value correspond?

e. It is desired to prepare a plot of the rates of both 03 production, Pq, , and loss, ¿o3> (molecules cm~3 s"1) as a function of NO. Consider NO mixing ratios from 1 to lOOppt. Ozone production occurs as a result of HOz and CH302 reacting with NO, whereas 03 loss occurs by photolysis and reaction with H02. Plot Pq, and Lo3 as a function of NO. To estimate the photolysis term in L0;, assume 50% RH and a local 03 mixing ratio of 20 ppb. Note any assumptions you make in estimating [CH302].

f. Based on the NO mixing at which Pq, = L0s, would you judge the following regions of the troposphere to be 03-producing or 03-destroying?

Remote Pacific Ocean marine boundary layer

Downtown Los Angeles

Rural Southeastern United States

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