Long-Term Trend of Stratospheric Chlorine Monoxide Over Mauna Kea, Hawaii, 1982-2004

We report on the long-term trend in stratospheric ClO concentration and column over Mauna Kea, Hawaii (latitude 19.8 N, altitude 4200 meters). We present altitude profiles, trends in mixing ratio and trends in total ClO column. At the meeting, results up to December 2004 will be presented. The mixing ratio at the peak of the altitude distribution is determined more precisely than the total column, and is the best parameter for tracing the long-term trend of active chlorine. At the peak, essentially all of the active chlorine is ClO. The ClO mixing ratio near the peak at 35--39~km (~4~hPa) increased by 38% from 1982.8 to 1995, when it reached a maximum and declined from 1994--95 to 2003. The trend is consistent with that expected from the increase of total tropospheric chlorine which reached a maximum in 1992. The trend of ClO at 37~km appears to have a time lag of 3 to 4 years with respect to total tropospheric ClO. Most important, the data since 1994--95 show clear evidence of declining active chlorine. A linear fit to 74 measurements gives a decline of MR(35-39~km) = 0.560 - 0.006 ± 0.0014( DATE - 1994) ppbv which corresponds to a decline of 0.95% ± 0.26%, or slightly less than 1% per year. A fit to yearly averages of the data gives essentially the same decline. Measured over the period of 1994 to 2003 the total decline is 8.5%, consistent with the trend expected from total tropospheric chlorine. We have been making ground-based millimeter-wave measurements of stratospheric ClO from Mauna Kea in Hawaii since October, 1982. Before 1989, measurements were made several times a year with a portable instrument. Since 1992, an improved, automated instrument has been permanently stationed at the site, as part of the Network for the Detection of Stratospheric Change (NDSC), and takes data continuously. The instrument is a spectrometer tuned to the thermally-excited emission line of ClO at 278.3 GHz. Its bandwidth permits the measurement of the pressure-broadened line shape, from which the altitude profile of ClO between 15 and 45 km can be retrieved. Our extensive data from Mauna Kea are the only long-term continuous set of observations of ClO. We plan to continue these measurements to follow the expected decline and to test these results.