Stratospheretroposphere Chemical And Climate Interaction

Although it is convenient to describe the chemistry of the stratosphere and the chemistry of the troposphere separately, both chemical and meteorological processes in one domain play an important role on the chemistry in the other. As previously

Figure 8 Contributions of various trace gases to global warming. Top graph shows the calculations using a model that does not include contributions from tropospheric ozone increases or from feedback relating to photochemical processes; bottom chart shows contributions using a model that includes photochemical feedback mechanisms. See tip site for color image.

Species

COg

Ü

. CHa

NsO

CFCs

|o3

1 NjO (strat)

No Chemical Feedbacks Preiindustrial to 1990

Species

CO?

a

CHa

NiO

CFCs

Figure 8 Contributions of various trace gases to global warming. Top graph shows the calculations using a model that does not include contributions from tropospheric ozone increases or from feedback relating to photochemical processes; bottom chart shows contributions using a model that includes photochemical feedback mechanisms. See tip site for color image.

Wilh Chemical Feedbacks Pre industrial to 1990

mentioned, one important aspect of tropospheric chemistry that impacts stratospheric chemistry is the removal of ozone-depleting chlorine in the troposphere before reaching the stratosphere.

Ozone depiction potentials (ODPs) provide a relative measure of the expected impact on ozone per unit mass emission of a gas compared to that expected from the same mass emission of CFC-11 integrated over time. Their primary purpose is for comparison of relative impacts of different gases upon ozone (e.g., for evaluating the relative c fleets of choices among CFC substitutes upon ozone). The two factors that

8 STRATOSPHERE-TROPOSPHERE CHEMICAL AND CLIMATE INTERACTION 23

contribute to how effectively an anthropogenic compound depletes ozone is how much of it reaches the stratosphere and how much chlorine each molecule contains. CFC-11 (trichlorofluoromethane) contains three chlorine atoms and is not removed in the troposphere through chemical reactions. The primary replacements for many of these compounds contain a hydrogen atom, which can be attacked by OH in the troposphere. The lifetimes of these compounds are also presented in Table 2, which was compiled as part of the International Assessment of Ozone Depletion in 1994 (Albritton et al., 1995). Some of the compounds, such as HFC-134a, the primary choice for refrigerant in many automobile air-conditioning systems, contain no chlorine, and thus, virtually no chance of contributing to ozone depletion.

In addition to ODPs being established, the 1994 ozone assessment also produced a series of global warming potentials (GWPs) to provide a simple representation of the relative radiative forcing resulting from a unit mass emission of a greenhouse gas compared to a reference compound. Because of its central role in concerns about climate change, carbon dioxide has generally been used as the reference gas. The values presented in Table 2 are calculations based on a time horizon of 20 years. Evaluations of GWPs must also take into consideration the radiative property of the atmosphere at some future point, including the concentration of C02, and other major climate altering compounds such as nitrous oxide and methane. In the 1994

TABLE 2 Estimated Lifetime, Ozone Depletion Potential (ODP), and Global Warming Potential (GWP) for Various Anthropogenic Trace Gases

Trace Gas

Chemical Formula

Lifetime (years)

ODP

GWP

CFC-11

CFC13

50

1.0

5000

CFC-12

CF2C12

102

0.82

7900

CFC-113

C2F3C13

85

0.90

5000

CFC-114

C,F4C12

300

0.85

6900

CFC-115

C2F5C1

1700

0.40

6200

Carbon tetrachloride

cc14

42

1.20

2000

Methyl chloroform

CH3CC13

5.4

0.12

360

HCFC-22

CF2HC1

13.3

0.04

4300

HCFC-123

C2F3HC12

1.4

0.014

300

HCFC-124

C2F4HC1

5.9

0.03

1500

HCFC-141b

C2F3H3C1

9.4

0.10

1800

HCFC-142b

C2F3H3C1

19.5

0.05

4200

HCFC-225ca

C3F5HC12

2.5

0.02

550

HCFC-225cb

C3F5HC12

6.6

0.02

1700

HCFC-134a

CH2FCF3

14

<1.5 x 10"5

3300

HCFC-23

chf3

250

<4 x 10"4

9200

HCFC-125

c3hf5

36

<3 x 10~5

4800

Methyl bromide

CH3Br

1.3

0.64

6200

Halon-1301

CF3Br

65

12

Halon-1211

CF2HBr

20

5.1

ozone assessment and the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al., 1996) report, there are also GWPs calculated with time horizons of 100 and 500 years, and such calculations include even more uncertainty than the values presented here because of the assumed scenarios for emissions so far into the future. Thus, although some of the replacement compounds for the CFCs have a negligible impact on the ozone layer, they will make important contributions to the overall greenhouse effect caused by the emission of anthropogenic chemicals released to the atmosphere.

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