Measurements of MCF

Since 1978, MCF has been monitored at 5 remote atmospheric stations within the ALE/GAGE/AGAGE program (ALE=atmospheric lifetime experiment, black in the figures; GAGE= global atmospheric gases experiment, blue in the figures; AGAGE, advanced global atmospheric gases experiment, green in the figures).

Figures: Monthly mean MCF concentrations in parts per trillion (ppt) as a function of time. The vertical bars represent the standard deviation during the month. Black: ALE, blue: GAGE, green: AGAGE. From left to right: Mace Head, Oregon, Barbados, Samoa, and Tasmania (see Prinn et al., 2000).

Inspection of the ALE/GAGE/AGAGE MCF measurements reveals that

• The MCF concentrations were higher on the northern hemisphere (NH) than on the southern hemisphere (SH) up to the late 1990s
• The variability (the vertical bars represent the standard deviation during the month) before the late 1990s was higher on the NH than on the SH.
• The concentrations follow the global emissions closely, but with a delay that corresponds to the tropospheric lifetime (which is about 5 years).
• Recent years show the lowest concentrations close to the tropics, which corresponds to low emissions, and breakdown of MCF by OH. Breakdown of MCF is favored when the temperatures are high, and the OH concentrations are high.

Apart from the ALE/GAGE/AGAGE measurements, other observations confirm the global MCF distribution and the changes in this distribution during the 1990s (see Montzka et al., 2000).

References

1. Prinn, R. G. et al. A history of chemically and radiatively important gases in air deduced from ALE/
   GAGE/AGAGE. J. Geophys. Res. 105, 17751–17792 (2000)

2. Singh, H. B. Preliminary estimation of average tropospheric HO concentrations in the Northern and
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3. Prinn, R. G. et al. Atmospheric trends in methylchloroform and the global average for the hydroxyl
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4. Prinn, R. G. et al. Atmospheric trends and lifetime of CH3CCl3 and global OH concentrations. Science
   269, 187–192 (1995)

5. Prinn, R. G. et al. Evidence for substantial variations of atmospheric hydroxyl radicals in the past two
   decades. Science 229, 1882–1888 (2001)

6. Krol, M., van Leeuwen, P. J. & Lelieveld, J. Global OH trend inferred from methylchloroform
   measurements. J. Geophys. Res. 103, 10697–10711 (1998)

7. Krol, M. & Lelieveld, J. Can the variability in tropospheric OH be deduced from measurements of
   1,1,1-trichloroethane (methyl chloroform)? J. Geophys. Res. (accepted for publication, pdf)

8. Montzka, S. A. et al. New observational constraints for atmospheric hydroxyl on global and
   hemispheric scales. Science 288, 500–503 (2000)

9. Krol, M., van Leeuwen, P. J. & Lelieveld, J. Comment on “Global OH trend inferred from
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   (2001)