Cooperative Institute for Research in Environmental Sciences

Three Decades of Continuous Monitoring of Long-lived Halocarbons

Geoff Dutton (1), Brad Hall (2), J. David Nance (1), Debra J. Modeel (1), James W. Elkins (2)

(1) CIRES, (2) NOAA ESRL

In the mid-1970s, the National Oceanic and Atmospheric Administration’s (NOAA) Geophysical Monitoring for Climate Change (GMCC) program made a commitment to measure and monitor trace gases including carbon dioxide, methane, nitrous oxide (N2O) and chlorofluorocarbons (CFCs). Over the next three decades GMCC grew into a division of NOAA/ESRL, and many trace gas measurement programs evolved into separate projects with different instrumentation. Multiple measurements of the same gases at identical locations (e.g., using both in situ instruments and grab samples) can sometimes lead to confusion when determining what measurement to use for analysis. We present a statistical method developed to combine measurements from independent NOAA programs to construct continuous long-term global records for the following ozone-depleting substances: CFC-11, CFC-12, CFC- 113, methyl chloroform (CH3CCl3) and CCl4. The combining technique takes advantage of co-located measurements and accounts for systematic differences between measurement programs. We also use two different statistical approaches to characterize uncertainties in hemispheric and global means. The combined data sets and uncertainties can be used in global growth rate and top down emission estimates of these important gases.