Long-Term Measurement Records Related to Global Stratospheric Constituent Change III
Presiding: E E Remsberg, NASA Langley Research Center; R S Stolarski, NASA Goddard Space Flight Center
A53B-01 13:30h
Highlights of the SPARC Assessment of Stratospheric Aerosol Properties
The SPARC Assessment of Stratospheric Aerosol Properties (ASAP) is a broad survey of the state of knowledge of aerosol precursors, measurements, and modeling and an extensive report will be released in 2005. This presentation will highlight key findings of this effort that are summarized herein. During the past three decades, aerosol loading in the stratosphere has primarily reflected the effects of a few volcanic eruptions that inject aerosol and its gaseous precursors (primarily SO2) into the stratosphere. The most noteworthy of these eruptions are El Chichon (1982) and Pinatubo (1991). The 1991 Pinatubo eruption likely had the largest impact of any event in the 20th century producing ~30 Tg of aerosol (compared to El Chichon's ~12 Tg) that persisted into at least the late 1990's. Current loading, which is at lowest levels observed, is less than 0.5 Tg and the Pinatubo event represents nearly a factor of 100 enhancement relative to a non-volcanic level. Many parameters that are useful for either scientific or intercomparison purposes are derived indirectly from observations. This is particularly true for space-based measurements where only bulk extinction is measured but also true in degree for most ground-based and in situ systems as well. Space-based and in situ measurements of aerosol parameters tend to be consistent following significant volcanic events. However, during periods of very low aerosol loading, this consistency breaks down and significant differences exist between systems for key parameters including aerosol surface area density and extinction. The source of the non-volcanic aerosol is primarily OCS and there is general agreement between measured OCS and modeling of its transformation to sulfate aerosol, and observed aerosols. However, there is a significant dearth of SO2 measurements, and the role of tropospheric SO2 in the stratospheric aerosol budget - while significant - remains a matter of some guesswork. In addition, it is not well understood whether decreasing global human-derived SO2 emissions or increasing emissions in low latitude developing countries such as China dominate the human component of SO2 transport across the tropical tropopause. While the actual removal of aerosol from the stratosphere to the troposphere is predominately associated with tropopause folds, sedimentation plays a crucial role in the vertical distribution of aerosol throughout the stratosphere including its abundance in the vicinity of the tropopause. Given the high variability of stratospheric aerosol loading, it is difficult to detect trends in the non-volcanic component. Trends derived from the late 1970's to the current period are likely to encompass a value of zero.
A53B-02 13:45h
Trends in Non-Volcanic Stratospheric Aerosol Determined from 30 Years of Aerosol Measurements by Lidar and Balloon-borne Particle Counters
Junge's initial stratospheric aerosol measurements (1959-1960), at the end of a long volcanically quiescent period, and the long term stratospheric aerosol measurements beginning in the 1970s have been investigated for trends in non-volcanic stratospheric aerosol. These investigations have focused on the inter-volcanic "background" periods, which until post Pinatubo, have been rather brief. Since 1959 there have been approximately 30 eruptions with volcanic explosivity indices of 4 or more. The five 30 year records of stratospheric aerosol are comprised of one in situ record [aerosol concentration for 0.15 and 0.25 um radius particles above Laramie, Wyoming, USA (1971-2003, 41N)] and four lidar records [S. J. dos Campos, Brazil (1972-2003, 23S), Mauna Loa, Hawaii, USA (1974-2003, 20N), Hampton, Virginia, USA (1974-2002, 37N), Garmisch-Partenkirchen, Germany (1976-2002, 48N)]. These data capture the three major aerosol-producing eruptions, Fuego, El Chichon and Pinatubo, as well as the three inter-volcanic background periods. These records form the basis for an assessment of the trend in background stratospheric aerosol. The quantities to be investigated are integral number concentrations and integrated backscatter above the tropopause. These records were analyzed following two approaches. First the 3 volcanically quiescent periods are compared using a standard analysis of variance approach. Second an empirical model is used to remove the volcanic signal from the long term records and then the residuals are investigated for trends. The model is a parametric exponential decay model, requiring 4 parameters for each volcano and 2 parameters for background. The optimization procedure uses a priori estimates for each parameter and then standard squared-error residual minimization to obtain the set of parameters providing the minimum in the residuals. The baseline and autocorrelated residuals are then investigated for trend. The results suggest that the trend in background stratospheric aerosol is not significantly different than zero for S. J. dos Campos, Mauna Loa, and Garmisch, with a slight negative trend for the Hampton and Laramie data. This approach does not permit investigations of microphysical changes such as changing size distribtutions. This work forms part of the SPARC assessment of stratospheric aerosol.
A53B-03 14:00h
Overview of Long-term Stratospheric Measurements at Lauder, New Zealand, and Arrival Heights, Antarctica
Ground-based remote measurements of stratospheric trace species began in Lauder, New Zealand (45 S), in December 1980, and have been made continuously since that time. Initially, only NO2 and O3 were measured, using UV/visible scattered light spectroscopy. A variety of other techniques and species have been added since that time, so that today, ongoing, long-term data series exist for HCl, ClONO2, CO, BrO, H2O, spectral UV irradiances, aerosol, and many other trace species. Balloon-borne ozonesondes have been flown regularly since August 1986. We have measured many of the same species at Arrival Heights, Antarctica (78 S), commencing at different times during the period 1982 to 1998. In addition, a variety of long-term measurements are made at these sites in collaboration with overseas investigators, including ClO, O3, and H2O profiles. The principal institution responsible for measurements at the Lauder and Arrival Heights sites is the New Zealand National Institute for Water and Atmospheric Research (NIWA). This presentation will examine some of the principal results from the NIWA data sets, including the long-term trends and variability of the O3 profile, NO2, Cly, and erythemal UV. We will interpret them in relation to anthropogenic emissions and natural events, and in particular address the question whether reversals are detected in the long-term trends in response to emission controls.
A53B-04 14:15h
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.
http://chloe.ess.sunysb.edu
A53B-05 14:30h
Trace Gas Trends in the Stratosphere: 1991-2005
The first NOAA airborne gas chromatograph measured chlorofluorocarbon-11 (CFC-11) and CFC-113 during the Arctic Airborne Stratospheric Experiment in 1991-1992. In 1994, we added nitrous oxide (N2O), sulfur hexafluoride (SF6), CFC-12, halon-1211, methyl chloroform, carbon tetrachloride, methane, and hydrogen. NOAA scientists have since operated five airborne gas chromatographs on NASA airborne platforms, including the NASA Jet Propulsion Laboratory (JPL) balloon gondola and ER-2, WB-57F, DC-8, and NASA Altair Unmanned Air Vehicle (UAV) aircraft. Using these in situ measurements and tracer-tracer correlations from flask observations for the unmeasured halogen species (HCFCs and methyl halides including methyl chloride and bromide), we have estimated trends of total chlorine and bromine in the stratosphere. The determination of inorganic equivalent chlorine (Cl + 45*Br) requires the trend of tropospheric equivalent chlorine and the mean age of the parcel of stratospheric air. In general, there is good agreement between the mean age of the air mass calculations using carbon dioxide and SF6, except for regions of extreme down welling of mesospheric air where SF6 is consumed. Tropospheric trends of the methyl halides have been compiled against stable standards. We operated a airborne gas chromatograph on the Sage 3 Ozone Loss Validation Experiment (SOLVE-II) mission from Kiruna, Sweden during 2002. It measured the major HCFCs and methyl halides, so that these compounds do not have to be estimated from tracer-tracer correlations in the future. In 2005, we have added a new lightweight airborne instrument (<25 kg) that can measure CFC-11, CFC-12, halon-1211, SF6, N2O, and ozone. This instrument can operate on small or UAV aircraft and will be used for Aura satellite validation. This presentation will show trends for selected trace gases and our estimates of total equivalent chlorine stratospheric trends since 1991.
http://www.cmdl.noaa.gov/hats/insitu/cats/ccly/
A53B-06 14:45h
Evidence for the End of the Decline in the Stratospheric Ozone Layer
Observations indicate that we have seen the end of major declines in the thickness of Earth's protective ozone layer at low and middle latitudes. Time series of stratospheric ozone measured by the SAGE and HALOE satellite instruments are consistent with total ozone columns obtained from the ground-based Dobson/Brewer/filter networks and with the merged TOMS/SBUV satellite measurements. Results derived from the worldwide network of ozonesonde measurements also provide consistent evidence for a slowdown of ozone depletion. Statistical analyses confirm that these changes in ozone loss rates are significant above the 2-sigma confidence level. Regression analyses with Effective Equivalent Stratospheric Chlorine (EESC), and photochemical model calculations, constrained by satellite data, demonstrate that the declining levels of ozone depletion between 18-25 km altitude are consistent with a leveling off of stratospheric abundances of chlorine and bromine due to the Montreal Protocol and its amendments. Ozone changes in the lowest part of the stratosphere (tropopause to 18 km) are also significantly improving, but the dramatic changes in that layer are not primarily a result of halogen decreases. Throughout the entire stratosphere, ozone trend have changed from -7.6~1.0 DU/decade to -0.3~2.0 DU/decade. Recent observations were obtained during a period of unusually low levels of stratospheric aerosol loading. Present understanding suggests that should a major volcanic eruption occur, chemical reactions initiated by volcanic aerosol that reach the stratosphere will likely lead to short periods of decreased ozone due to anthropogenic halogens.