The Physical Chemistry Properties Of Greenhouse Gases

The fundamental physical property of a greenhouse gas is that it must absorb infra-red radiation via one or more of its vibrational modes in the infra-red range of 5 25 mm. Furthermore, since the primary greenhouse gases of CO2, O3 and H2O absorb in the range 12 17 mm (or 590 830 cm 1), 9.6 mm (1040 cm 1) and l < 8 mm (>1250 cm 1), an effective secondary greenhouse gas is one which absorbs infra-red radiation strongly outside these ranges of wavelengths (or wavenumbers). A molecular vibrational mode is only infra-red active if the motion of the atoms generates a dipole moment. That is, dm/dQ = 0, where m is an instantaneous dipole moment and Q a displacement coordinate representing the vibration of interest. It is worth stating the obvious straightaway, that N2 and O2 which constitute 99% of the earth's atmosphere do not absorb infra-red radiation, their sole vibrational mode is infra-red inactive, so they play no part in the greenhouse effect and global warming. It is only trace gases in the atmosphere (Table 1) such as CO2 (0.038%), CH4 (0.0002%), O3 (3 x 10 6%) and CFCs such as CF2Cl2 (5 x 10 8%) which contribute to the greenhouse effect. Put another way, the earth's atmosphere is particularly fragile if only 1% of the molecules present can have such a major effect on humans living on the planet. Furthermore, the most important molecular trace gas, CO2, absorbs via its n2 bending vibrational mode at 667 cm 1 or 15.0 mm, which coincidentally is very close to the peak of the earth's black-body curve; the spectroscopic properties of CO2 have not been particularly kind to the environment! Thus, infra-red spectroscopy of gas-phase molecules, in particular at what wavelengths and how strongly a molecule absorbs such radiation, will clearly be important properties to determine how effective a trace pollutant will be to the greenhouse effect.

The second property of interest is the lifetime of the pollutant in the earth's atmosphere: the longer the lifetime, the greater contribution a greenhouse gas will make to global warming. The main removal processes in the troposphere and stratosphere are reactions with OH free radicals and electronically excited

TABLE 1 Main constituents of ground-level clean air in the earth's atmosphere

Molecule

Mole fraction

ppmva (2008)

ppmv (1748)

n2

0.78 or 78%

780 900

780 900

O2

0.21

or 21%

209 400

209 400

h2o

0.03 (100% humidity, 298 K)

31 000

31 000

h2o

0.01 (50% humidity, 298 K)

16 000

16 000

Ar

0.01

or 1%

9300

9300

CO2

3.8 ;

10 4 or 0.038%

379

280

Ne

1.8

<10 5 or 0.002%

18

18

ch4

1.77

x 10 6 or 0.0002%

1.77

0.72

N2O

3.2

10 7 or 0.00003%

0.32

0.27

O3b

3.4 ;

10 8 or 0.000003%

0.034

0.025

All CFCsc

.7 8.

<10 10 or 8.7 x 10 8%

0.0009

0

All HCFCsd

1.9 ;

<10 10 or 1.9 x 10 8%

0.0002

0

All PFCse

8.3

10 11 or 8.3 x 10 9%

0.00008

0

All HFCsf

6.1 ;

10 11 or 6.1 x 10 9%

0.00006

0

aparts per million by volume. 1 ppmv is equivalent to a number density of 2.46 x 1013 molecules cm 3 for a pressure of 1 bar and a temperature of 298 K.

bthe concentration level of O3 is very difficult to determine because it is poorly mixed in the troposphere. It shows large variation with both region and altitude.

cchlorofluorocarbons (e.g., CF2G2).

dhydrochlorofluorocarbons (e.g., CHGF2).

eperfluorocarbons (e.g., CF4, C2F6, SF5CF3, SF6).

fhydrofluorocarbons (e.g., CH3CF3).

\_y aparts per million by volume. 1 ppmv is equivalent to a number density of 2.46 x 1013 molecules cm 3 for a pressure of 1 bar and a temperature of 298 K.

bthe concentration level of O3 is very difficult to determine because it is poorly mixed in the troposphere. It shows large variation with both region and altitude.

cchlorofluorocarbons (e.g., CF2G2).

dhydrochlorofluorocarbons (e.g., CHGF2).

eperfluorocarbons (e.g., CF4, C2F6, SF5CF3, SF6).

fhydrofluorocarbons (e.g., CH3CF3).

\_y oxygen atoms, O* (1D), and photodissociation in the range 200 300 nm (in the stratosphere) or 300 500 nm (in the troposphere). Thus, the reaction kinetics of pollutant gases with OH and O* (1D) and their photochemical properties in the UV/visible will yield important parameters to determine their effectiveness as greenhouse gases. All these data are incorportated into a dimensionless number, the global warming potential (GWP) or greenhouse potential (GHP) of a greenhouse gas. All values are calibrated with respect to CO2 whose GWP value is 1. A molecule with a large GWP is one with strong infra-red absorption in the windows where the primary greenhouse gases such as CO2, etc., do not absorb, long lifetimes, and concentrations rising rapidly due to human presence on the planet. GWP values of some of the most important secondary greenhouse gases are given in the bottom row of Table 2. Note that CO2 has the lowest GWP value of the seven greenhouse gases shown.

TABLE 2 Examples of greenhouse gases and their contribution to global warming [2,20]

Greenhouse gas

CO2

O3

[all CFCs]

SF6

SF5CF3

Concentration (2008)/ppmv

379

0.034a

1.77

0.32

0.0005 [0.0009]

5.6 x 10 6

1.2 x 10 7

AConcentration (1748 2008)/ppmv

99

0.009a

1.05

0.05

0.0005 [0.0009]

5.6 x 10 6

1.2 x 10 7

Radiative efficiency, ao/Wm 2ppbv 1

1.68 x 10 5

3.33 x 10 2

4.59 x 10 4

[0.18 0.32]

0.52

0.60

Total radiative forcing b/Wm 2

1.66

ca. 0.30c

0.48

0.16

0.17 [0.27]

2.9 x 10 3

7.2 x 10 5

Contribution from long lived greenhouse gases excluding ozone to overall greenhouse effect/%d

63 (57)

(10)

18 (16)

6 (5)

6 [10] (6 [9])

0.1 (0.1)

0.003 (0.003)

Lifetime, te/a

ca. 50 200f

ca. days weeksg

12

120

100 [45 1700]

3200

800

Global warming potential (100 a projection)

1

h

[6130 14 400]

22 800

17 700

areference [20].

bdue to change in concentration of long-lived greenhouse gas from the pre-lndustrial era to the present time.

can estimated positive radiative forcing of 0.35 Wm 2 in the troposphere is partially cancelled by a negative forcing of 0.05 Wm 2 in the stratosphere [2]. dassumes the latest value for the total radiative forcing of 2.63 ± 0.26 Wm 2 [2].

The values in brackets show the percentage contributions when the estimated radiative forcing for ozone is included in the value for the total radiative forcing. eassumes a single-exponential decay for removal of greenhouse gas from the atmosphere. fCO2 does not show a single-exponential decay [4].

gO3 is poorly mixed in the troposphere, so a single value for the lifetime is difficult to estimate. It is removed by the reaction, OH + O3^HO2 + O2. Its concentration shows large variations both with region and altitude.

hGWP values are generally not applied to short-lived pollutants in the atmosphere, due to serious inhomogeneous changes in their concentration.

Information in the previous two paragraphs is described in qualitative and descriptive terms. However, all the data can be quantified, and a mathematical description is now presented. The term that characterises the infra-red absorption properties of a greenhouse gas is the radiative efficiency, ao. It measures the strength of the absorption bands of the greenhouse gas, x, integrated over the infra-red black-body region of ca. 400 2000 cm 1. It is a (per molecule) microscopic property and is usually expressed in units of Wm 2 ppbv 1. If this value is multiplied by the change in concentration of pollutant over a defined time window, usually the 260 a from the start of the Industrial Revolution to the current day, the macroscopic radiative forcing in units of W m 2 is obtained. (Clearly, a pollutant whose concentration has not changed over this long time window will have a macroscopic radiative forcing of zero.) One may then compare the radiative forcing of different pollutant molecules over this time window, showing the current contribution of different greenhouse gases to the total greenhouse effect. Thus, the IPCC 2007 report [2] quotes the radiative forcing for CO2 and CH4 in 2005 as 1.66 and 0.48 W m 2, respectively, out of a total for long-lived greenhouse gases of 2.63 W m 2. These two molecules, therefore, contribute 81% in total (63% and 18%, individually) to the global warming effect. Effectively, the radiative forcing value gives a current-day estimate of how serious a greenhouse gas is to the environment, using concentration data from the past.

The overall effect in the future of one molecule of pollutant on the earth's climate is described by its GWP (or GHP) value. It measures the radiative forcing, Ax, of a pulse emission of the greenhouse gas over a defined time period, t, usually 100 a, relative to the time-integrated radiative forcing of a pulse emission of an equal mass of CO2:

GWPx(f)

Ax(t)dt

The GWP value therefore informs how important one molecule of pollutant x is to global warming via the greenhouse effect compared to one molecule of CO2, which is defined to have a GWP value of unity. It is an attempt to project into the future how serious the presence of a long-lived greenhouse gas will be in the atmosphere. (Thus, when the media state that CH4 is 25 times as serious as CO2 for global warming, what they are saying is that the GWP value of CH4, looking 100 a into the future, is 25; one molecule of CH4 is expected to cause 25 times as much 'damage' as one molecule of CO2.) For most greenhouse gases, the radiative forcing following an emission at t = 0, takes a simple exponential form:

where tx is the lifetime for removal of species x from the atmosphere. For CO2, a single-exponential decay is not appropriate since the lifetime ranges from 50 to 200 a, and we can write:

where the response function, the bracket in the right-hand side of Eq. (4), is derived from more complete carbon cycles. Values for bi (i = 0 4) and ti (i = 14) have been given by Shine et al. [4]. It is important to note that the radiative forcing, Ao, in Eqs. (2) (4) has units of Wm 2 kg 1. For this reason, it is given a different symbol to the microscopic radiative efficiency, ao, with units of W m 2ppbv 1. Conversion between the two units is simple [4]. The time integral of the large bracket on the right-hand side of Eq. (4), defined KCO2, has dimensions of time, and takes values of 13.4 and 45.7 a for a time period of 20 and 100 a, respectively, the values of t for which GWP values are most often quoted. Within the approximation that the greenhouse gas, x, follows a single-exponential time decay in the atmosphere, it is then possible to parameterise Eq. (2) to give an exact analytical expression for the GWP of x over a time period t:

In this simple form, the GWP only incorporates values for the radiative efficiency of greenhouse gases x and CO2, ao, x and ao CO ; the molecular weights of x and CO2; the lifetime of x in the atmosphere, tx; the time period into the future over which the effect of the pollutant is determined; and the constant KCO2 which can easily be determined for any value of t. Thus the GWP value scales with both the lifetime and the microscopic radiative forcing of the greenhouse gas, but it remains a microscopic property of one molecule of the pollutant. The recent rate of increase in concentration of a pollutant (e.g., the rise in concentration per annum over the last decade), one of the factors of most concern to policymakers, does not contribute directly to the GWP value. This and other factors [4] have caused criticism of the use of GWPs in policy formulation.

Data for seven greenhouse gases are shown in Table 2. CO2 and O3 constitute naturally occurring greenhouse gases whose concentration levels ideally would have remained constant at pre-industrial revolution levels. Although H2O vapour is the most abundant greenhouse gas in the atmosphere, it is neither long-lived nor well mixed: concentrations range 0 3% (i.e., 0 30 000 ppmv) over the planet, and the average lifetime is only a few days. Its average global concentration has not changed significantly in the

last 260 a, and it therefore has zero radiative forcing. CH4 and N2O constitute naturally occurring greenhouse gases with larger ao values than that of CO2. The CH4 concentration, although small, has increased by ca. 150% since pre-industrial times. After CO2, it is the second most important greenhouse gas, and its current total radiative forcing is ca. 29% that of CO2. N2O concentration has increased only by ca. 16% over this same time period. It has the fourth highest total radiative forcing of all the naturally occurring greenhouse gases, following CO2, CH4 and O3. Dichlorofluoromethane, CF2Cl2, is one of the most common of CFCs. These are man-made chemicals that have grown in concentration from zero in pre-industrial times to a current total concentration of 0.9 ppbv (1 ppbv is equivalent to 1 part per 109 (billion) by volume, or a number density of 2.46 x 1010 molecules-cm 3 at 1 bar pressure and a temperature of 298 K). Their concentration is now decreasing due to the 1987 Montreal and later International Protocols, introduced to halt stratospheric ozone destruction and (ironically) nothing to do with global warming! SF6 and SF5CF3 are two long-lived halocarbons with currently very low concentration levels, but with high annual percentage increases and exceptionally long lifetimes in the atmosphere. They have very high ao and GWP values, essentially because of their large number of strong infra-red-active vibrational modes and their long lifetimes.

It is noted that CO2 and CH4 have the lowest GWP values of all greenhouse gases. Why, then, is there such concern about levels of CO2 in the atmosphere, and with the possible exception of CH4 no other greenhouse gas is hardly ever mentioned in the media? The answer is that the overall contribution of a pollutant to the greenhouse effect, present and future, involves a convolution of its concentration with the GWP value. Thus CO2 and CH4 currently contribute most to the greenhouse effect (third bottom row of Table 2) simply due to their high change in atmospheric concentration since the Industrial Revolution; note, however, that the ao and GWP values of both gases are relatively low. Indeed, the n2 bending mode of CO2 at 15.0 mm, which is the vibrational mode most responsible for greenhouse activity in CO2, is close to saturation. By contrast, SF5CF3 is a perfluorocarbon molecule with the highest microscopic radiative forcing of any known greenhouse gas (earning it the title 'super' greenhouse gas [5,6]), even higher than that of SF6. SF6 is an anthropogenic chemical used extensively as a dielectric insulator in high-voltage industrial applications, and the variations of concentration levels of SF6 and SF5CF3 with time in the last 50 a have tracked each other very closely [7]. The GWP of these two molecules is very high, SF6 being slightly higher because of its atmospheric lifetime, ca. 3200 a [8], is about four times greater than that of SF5CF3. However, the contribution of these two molecules to the overall greenhouse effect is still very small because their atmospheric concentrations, despite rising rapidly at the rate of ca. 6 7% per annum, are still very low, at the level of parts per 1012 (trillion) by volume; 1 pptv is equivalent to a number density of 2.46 x 107 molecules-cm 3 at 1 bar and 298 K).

In conclusion, the macroscopic properties of greenhouse gases, such as their method of production, their concentration and their annual rate of increase or decrease, are mainly controlled by environmental and sociological factors, such as industrial and agricultural methods, and ultimately population levels on the planet. The microscopic properties of these compounds, however, are controlled by factors that undergraduates world-wide learn about in science degree courses: infra-red spectroscopy, reaction kinetics and photochemistry. Data from such lab-based studies determine values for two of the most important parameters for determining the effectiveness of a greenhouse gas: the microscopic radiative efficiency, ao, and the atmospheric lifetime, t.

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