Scientific Assessment of Ozone Depletion: 1991

CHAPTER 1

Source Gases: Concentrations, Emissions, and Trends

Authors:

P. Fraser

S. Penkett
R. Harriss
Y. Makide
E. Sanhueza

Additional Contributors:

F. Alyea
D. Blake
D. Cunnold
J. Elkins
M. Hirota
R. Prinn
R. Rasmussen
S. Rowland
T. Sasaki
H. Scheel
W. Seiler
P. Simmonds
P. Steele
R. Weiss

1. 1 INTRODUCTION

Source gases are defined as those gases that influence levels of stratospheric ozone (O3) by transporting species containing halogen, hydrogen, and nitrogen to the stratosphere that are important in O3 destruction. Examples are the CFCs, methane (CH4), an d nitrous oxide (N2O). Other source gases that also come under consideration in an atmospheric O3 context are those that are involved in the O3 or hydroxyl (OH) radical chemistry of the troposphere. Examples are CH4, carbon monoxide (CO) and nonmethane hy drocarbons (NMHCs). Most of the source gases, along with carbon dioxide (CO2) and water vapor (H20), are climatically significant and thus affect stratospheric O3 levels by their influence on stratospheric temperatures. Carbonyl sulphide (COS) could affec t stratospheric O3 through maintenance of the stratospheric sulphate aerosol layer, which may be involved in heterogeneous chlorine-catalyzed O3 destruction.

This chapter updates the previous reviews of trends and emissions of source gases, either from the context of their influence on atmospheric O3 (WMO, 1986; 1990a, b) or global climate change (IPCC, 1990). The current (1989) global abundances and concentra tion trends of the trace gases are given in Table 1-1.

1.2 CFCs AND CARBON TETRACHLORIDE: GLOBAL DISTRIBUTIONS, TRENDS AND CALIBRATION

The CFCs (-11, -12, and -113) and carbon tetrachloride (CCl4) compose 70 percent of the anthropogenic organochlorine loading of the troposphere (CFC-12, 28 percent; CFC-11, 23 percent; CCl4, 13 percent; CFC-113, 6 percent). They are inert in the troposphe re but photodissociate in the stratosphere and hence are a major source of stratospheric reactive chlorine. The CFCs are used as refrigerants, foam blowing agents and solvents. CCl4 is used in the production of CFCs.

1.2.1 CFC-11 and CFC-12

There are several long term measurement programs for CFC-12 and CFC-11. National Oceanic and Atmospheric Administration-Climate Monitoring and Diagnostics Laboratory (NOAA-CMDL) have run a global program since 1977 based on weekly flask measurements at Ba rrow, AL; Niwot Ridge, CO; Mauna Loa, HI; Cape Matatula, Samoa; and the South Pole. The 1989 global mean concentrations for CFC-12 and CFC-ll were 452 and 268 pptv respectively (mean of hemispheric means; Northern Hemisphere: Barrow, Niwot Ridge and Mauna Loa; Southern Hemisphere, Samoa), increasing at 16.9+/-0.2 and 10.1+/-0.1 pptv per year, or 3.7 percent and 3.8 percent per year respectively in 1989, based on linear regressions. The data are reported in the original Oregon Graduate Institute for Scienc e and Technology (OGIST) scale and are shown in Figures 1-1 and 1-2 (Thompson et al., 1990). A possible CFC-12 calibration problem has been identified in the NOAA-CMDL data (J. Elkins, NOAA-CMDL, pe rsonal communication). A recent reevaluation of the flask data indicates that the long-term, linear, global growth of CFC-12 was 16.1+/-0.3 pptv per year, based on data from 1977 to 1991. NOAA-CMDL in situ measurements (12 per day) of CFC-12 and CF C-11 commenced at Barrow, Mauna Loa, and Samoa (1986) and at the South Pole (1987) (Thompson et al., 1990; Hall et al., 1990); preliminary CFC-12 data have been reported in the recently prepared NOAA-CMDL gravimetric scale. The 1989 global m ean CFC-12 concentration was 462 pptv, increasing at about 20 pptv per year, or 4.4 percent per year in 1989. The differences between the global mean derived from flask and in situ measurements presumably reflect differences between the OGIST and N OAA-CMDL CFC-12 calibration scales.

In situ measurements (4-12 per day) of CFC-12 and CFC-11 have been made in Ireland, Oregon, Barbados, Samoa, and Tasmania since 1978 as part of the ALE-GAGE program (Cunnold et al., 1986). The 1989 global mean concentrations for CFC-12 and C FC-11 were 453 and 255 pptv respectively (mean of hemispheric means; Northern Hemisphere: Ireland, Barbados; Southern Hemisphere, Tasmania), increasing at 18.2+/-0.3 and 9.3+/-0.1 pptv per year, or 4.0 percent and 3.7 percent per year in 1989, based on li near regressions (Cunnold et al., 1986; Prinn et al., 1991b). The data are in the GAGE scale (CFC-ll, GAGE scale = OGIST scale x 0.96;CFC-12, GAGE scale = OGIST scale x 0.95) and are also shown in Figures 1-1 and 1-2. The long term stability of the OGIST CFC-ll scale has been possibly but not absolutely demonstrated by extensive, periodic internal comparisons of several original ALE-GAGE calibration gases and by a comparison of measurements m ade in 1978-1979 in the ALE-GAGE program in Tasmania with modern measurements on air archived from that period. These experiments limit the magnitude of a calibration drift component of the observed trend to about 0.1 percent per year. A possible CFC-12 c alibration problem has been identified in the GAGE data, which are currently being reevaluated. Lower trends will probably result (D. Cunnold, GAGE-GIT, personal communication) although the changes are likely to be small.

The global CFC-11 ratio (GAGE (in situ) NOAA-CMDL (flask) = 0.95) reflects the difference between the two calibration scales involved (GAGE/OGIST = 0.96), whereas the global CFC-12 ratio (GAGE (in situ)/NOAA-CMDL(flask) = 1.00) does not (GAG E/OGIST = 0.95). This requires further investigation. The 1989 mean CFC-12 and CFC11 concentrations at Tasmania (41deg.S) were 441 and 247 pptv, which, when compared to the South Pole observations above, suggest that the NOAA-CMDL and GAGE scales agree to within 2 percent (CFC-12) and 1 percent or better (CFC-11).

In situ CFC-11 measurements (24 per day) have been made at Cape Point, South Africa, since 1980 (Scheel et al., 1990; Brunke and Scheel, 1991). The 1989 annual mean CFC-11 concentration at Cape Point (34deg.S) was 256 pptv, increasing at 9.3 +/-0.1 pptv per year or 3.6 percent per year in 1989 based on a linear regression. The data are reported in the original OGIST scale and are also shown in Figure 1-2. The 1989 mean CFC-11 concentration and trend at Cape Point in t he GAGE scale (OGIST x 0.96) are 246 pptv and 9.0 pptv per year respectively, which compare well with the 1989 mean CFC-11 concentration and trend in Tasmania (GAGE program, 41deg.S), 247 pptv and 9.3+/-0.1 pptv per year respectively.

Flask measurements of CFC-12 and CFC-11 (1 week every 6 months) have been made on Hokkaido, Japan (40deg.-45deg.N), since 1979 (Makide et al., 1987; Makide, 1991) and at Syowa Station, Antarctica (69deg.S), several times per year since 1982 (Makide 1991). On Hokkaido CFC-11 and CFC-12 increases of 18.6+/-0.2 and 9.4+/-0.2 pptv per year respectively have been observed, based on linear regressions. The 1989 mean CFC-12 and CFC-11 concentrations at Hokkaido are 481 and 259 pptv respectively and at Syo wa 441 and 235 pptv respectively. The data are reported in independently prepared University of Tokyo (UT) calibration scales and are shown in Figures 1-1 and 1-2. A direct interlaboratory comparison of th e GAGE and UT CFC-12 and CFC-11 calibration scales and a comparison of CFC-12 and CFC-11 data collected at Hokkaido (UT), Ireland (GAGE), Syowa (UT), and Tasmania (GAGE) has shown that GAGE data agree to within 2 percent for CFC-12 (GAGE lower) and within 3 percent for CFC-11 (GAGE higher) (Y. Makide and P. Fraser, unpublished data).

Measurements of CFC-12 and CFC-11 have been made in the free troposphere via aircraft over Europe and the North Atlantic Ocean since 1976 (Scheel et al., 1988; Seiler and Scheel, 1991). CFC-12 and CFC-11 increases of 16.3+/-0.5 and 10.6+/-0.3 pptv per year have been measured over the period 1976-1987, similar to the trends observed from ground-based observations in the Northern Hemisphere. These aircraft data are based on a commercially available Scott-Marrin standard.

Free tropospheric measurements of CFC-12 and CFC-11 have been made via aircraft over Japan (33deg.-38deg.N) since 1978 (Hirota et al, 1988; Hirota and Sasaki, 1991). CFC-12 and CFC-11 increases of 16.2+/-0.7 and 10.3+/-0.4 pptv per year have been measured over the period 1978-1990, similar to the trends observed over Europe. These aircraft data are based on a commercially available Seitetsu Kagaku and Nihon Sanso standards, whose absolute concentration is cenified to +/-5 percent.

A comparison of shipboard measurements of CFC-12 and CFC-11 from 1981 to 1984 on the North and South Atlantic (Penkett, l991a) to GAGE data from corresponding latitudes shows that the GAGE and Penkett data agree to witbin 3 percent for CFC-12 (GAGE higher ) and to within 1 percent for CFC-11 (GAGE higher). Similarly, a comparison of shipboard measurements of CFC-12 and CFC-11 from 1983 to 1990 on the North and South Atlantic (Weiss, 1991) to GAGE data from corresponding latitudes shows that the GAGE and We iss data agree to within 2 percent for CFC-12 (GAGE lower) and to within 2 percent for CFC-11 (GAGE higher).

1.2.2 CFC-113

Flask measurements of CFC-113 have been made on Hokkaido, Japan (40deg.-45deg.N) since 1980 and at Syowa Station (69deg.S) (10 days each winter) since 1987 (Makide et al., 1987; Makide, 1991). On Hokkaido the mean CFC-113 increase over the period 1 979-1990 was 5.5+/-0.2 pptv per year and 7.9+/-0.3 pptv over the period 1987-1990, based on linear regressions. The data are reported in an independently prepared UT calibration scale and are shown in Figure 1-3.

Real-time measurements (12 per day) of CFC113 have been made in Ireland, Oregon, Barbados, Samoa, and Tasmania since 1982 as part of the GAGE program (Fraser et al., 1991). The 1989 global mean CFC-113 concentration was 64 pptv (mean of hemispheric means; Northem Hemisphere: Ireland, Barbados; Southem Hemisphere, Tasmania), increasing at 5.8+/-0.4 pptv per year, or 9.1 percent per year in 1989, based on linear regressions (Fraser et al., 1991). The GAGE CFC-113 data (Fig ure 1-3) are obtained relative to OGIST calibration gases, but are reported in the GAGE scale, which is based on an interlaboratory comparison to the UT CFC-113 scale (Makide et al., 1987). This comparison showed that the UT/OGIST ratio is 1.4- 1.5 (Y. Makide and P. Fraser, unpublished data).

In situ measurements of CFC-113 at Cape Point, South Africa (Brunke and Scheel, 1991) show a lower growth for CFC-113 (2.4+/-0.2 pptv per year; 7.3 percent per year in 1989). The data are reported in the OGIST scale, so the increase would translate to ~3.5 pptv per year in the UT scale. Measurements of CFC-113 in the free troposphere over Europe and the North Atlantic during the period 1982-1987 show an increase of 6.6+/-0.3 pptv per year (Seiler and Scheel, 1991). The data are based on a Scott-Mar rin calibration standard, which gives CFC-113 concentrations that are a factor of ~2 higher than data obtained using OGIST calibration.

NOAA-CMDL have produced a new gravimetric CGC-113 scale, which is about 40 percent higher than the OGIST scale (NOAA-CMDL/OGIST = 1.37) (Thompson et al., 1990) and therefore presumably about 3-5 percent lower than the UT (=GAGE) scale. Measurements of CFC-113 at Cape Grim, Tasmania (41deg.S), have been compared to CFC-113 measurements on the South Atlantic at similar latitudes and times, which were obtained using an independent calibration scale University of East Anglia (UEA) (Penkett, 1991). The Atlantic data were approximately 10-20 percent lower, which probably reflects the difference in calibration scales (i.e. GAGE/UEA ~1.1-1.2; thus UEA/OGIST ~1.2-1.4). It would appear that three independent laboratories (UT, NOAA-CMDL and UEA agree t o within +/-10 percent on CFC-113 calibration.

1.2.3 CFC-114 and CFC-114a

Rasmussen et al., (1990) has reported growth rates for CFC-114 (CCIF2CCIF2) and CFC-114a (CCI2FCF3) from the OGIST global flask sampling network from 1979 to 1990 of approxirnately 6 percent per year. Absolute concentrations were not reported. Ther e have been no new data reported for CFC-115.

1.2.4 Carbon Tetrachloride

In situ CCl4 measurements have been made at the GAGE stations since 1978 (Simmonds et al., 1988). The data (Figure 1-4) show a global mean 1989 concentration of 134 pptv, based on data from Ireland, Barbados, and Tas mania, and an increase over the entire record of 1.6+/-0.3 pptv per year, or 1.2 percent per year in 1989.

A similar CCl4 increase has been observed from in situ measurements at Cape Point (1.7+/-0.1 pptv per year) over the period 1980-1990 (Figure 1-4). This program employs the same calibration scale as the GAGE program. The 19 89 mean concentration (128 pptv) at Cape Point (32deg.S) is very similar to that observed (130 pptv) at Cape Grim, Tasmania (41deg.S). However in situ measurements in 1989 at Samoa (14deg.S) and the South Pole using the new NOAA-CMDL gravimetric ca libration scale gave mean concentrations of 104 and 106 pptv respectively (Thompson et al., 1990; Hall et al., 1990), whereas GAGE measurements at Samoa in 1989 average about 132 pptv, suggesting that concentrations in the NOAA-CMDL scale ar e ~20 percent lower than those in the GAGE scale.

Flask measurements of CCl4 on Hokkaido, (40deg.$5deg.N) over the period 1979-1990 show an increase of 1.4+/-0.2 pptv per year (Makide et al., 1987; Makide, 1991). A direct interlaboratory comparison between GAGE and Makide indicate that the GAGE CC 14 scale is ~20 percent higher than Makide (Y. Makide and P. Fraser, unpublished data).

Measurements of CCl4 in the free troposphere over Europe and the North Atlantic during the period 1976-1987 show an increase of 2.0+/-0.3 pptv per year (Seiler and Scheel, 1991). These data are based on a Scott-Marrin calibration standard, which gives CCl 4 concentrations that are a factor of ~1.15 lower than those based on the GAGE calibration.

Measurernents of CCl4 at Cape Grim (41deg.S) have been compared to CCl4 measurements on the South Atlantic at similar latitudes and times, which were obtained using an independent calibration scale (UEA) (Penkett,199la). The Atlantic data were approximate ly 15 percent lower, which probably reflects the difference in calibration scales (i.e., GAGE/UEA ~1.15). A direct comparison of National Institute of Standards and Technology (NIST) and GAGE CCl4 standards indicates that the GAGE/NIST ratio is 1.3 5 (Fraser~, personal communication, l991).

It would appear that five independent laboratories (UT, NOAA-CMDL, UEA, Fraunhofer Institute for Atmospheric Environmental Research (FIAER) (Scott-Marrin), and NIST) agree to within +/-10 percent on CCl4 calibration, and are ~20 percent lower than GAGE.

1.3 METHYL CHLOROFORM AND HCFC-22

Methyl chloroform (CH3CCl3) and HCFC-22 (CHClF2) are important trace gases in the global atmosphere. They constitute about 17 percent of the tropospheric anthropogenic organochlorine loading (CH3CCl3, 14 percent; CHClF2, 3 percent) and both are partially removed from the atmosphere by reaction with OH. Assuming emissions and absolute abundances of these species are known, they can be used to calculate average tropospheric OH levels (for CH3CCl3, see Prinn et al., 1987, 1992). Methyl chloroform is u sed as an industrial solvent and HCFC-22 is being increasingly used as a substitute for CFCs.

1.3.1 Global distributions and trends

Long-term, high-frequency measurements (4-12 per day) of CH3CCl3 have been made in Ireland, Oregon, Barbados, Samoa and Tasmania (Prinn et al., 1987, l991a) since 1978 as part of the ALE-GAGE program and on Hokkaido twice a year (10 days every 6 mo nths) since 1979 and at Syowa Station in Antarctica (several times per year) (Makide et al., 1987; Makide, 1991). January measurements made in the Pacific North West (PNW) of the United States and at the South Pole since 1975 have been published (R asmussen and Khalil, 1986; Khalil and Rasmussen, 1990b). Mid-tropospheric CH3CCl3 data have been obtained by aircraft air sampling over Europe and the North Atlantic by FIAER since 1978 (Scheel et al., 1988).

The available data are shown in Figure 1-5. Twelve years of ALE-GAGE CH3CCl3 data (July 1978 to June 1990) have recently been analyzed (Prinn et al., 1992) showing a global trend of 5.5+/-0.2 pptv per year or 4.4+/-0.2 perc ent per year (mid 1984). The 1989 global mean concentration was 150.0 pptv (based on data from Ireland, Barbados, and Tasmania), and the 1989 increase, based on a linear regression, was 3.7 percent. The free tropospheric data over Europe and the North Atl antic (1978-1987) show a trend of 5.2+/-0.8 pptv per year or 4.5 percent per year in 1984 (Scheel et al., 1988; Seiler and Scheel, 1991). The Hokkaido data (Makide et al., 1987; Makide, 1991) show an increase of 4 pptv per year over the peri od 1980-1990 (2.7 percent in 1989) and 5 pptv per year over the period 1987-1990 (3.4 percent in 1989). The concentrations of CH3CCl3 observed on Hokkaido are about 15 percent lower than in Ireland or Oregon, although these differences could be due to dif ferent calibration scales (see 1.3.2).

The available data on the global distribution and trends of HCFC-22 are limited, reflecting the relative difficulty in making atmospheric HCFC-22 measurements, which can be achieved by spectroscopy (total column) or gas chromatographic techniques involvin g large volume air samples.

HCFC-22 data have been regularly obtained from the PNW region of the U.S. and from the South Pole (Rasmussen et al., 1980; Khalil and Rasmussen, 1981; Rasmussen and Khalil, 1982, 1983). The PNW data from 1976 to 1981 showed concentrations increasin g by about 12+/-1 percent per year, with an absolute concentration uncertainty of +/-10 percent. Combined PNW-South Pole data for 1979-1987 have recently been reported (Khalil and Rasmussen, 1990b), which show a concentration in January 1987 of 105 pptv i ncreasing by 6.4+/-0.3 pptv per year or 6.1 percent in January 1987. Observations from Cape Grim (1984-1987), show a mean concentration and increase in 1987 of 91 pptv and 6.5+/-0.3 pptv per year, or 7.1+/-0.3 percent per year (Fraser et al., 1989) . These data are all in the same scale (OGIST)

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