CMDL FY98 4th Quarter Milestones

Provide an initial estimate as to when the beginning of the recovery of the Antarctic ozone layer could be first detected. (CMDL, D. Hofmann)

Depletion of the Earth’s stratospheric ozone layer is considered a major environmental problem as it enables the increase of ultraviolet radiation harmful to humans and other biological systems. The global annual average total column ozone has decreased by an average of about 6% during the past 15 years, while springtime Antarctic ozone (the "ozone hole") is now reduced to one third the value it had prior to the 1970's during the months of September and October. In response to this situation, the Montreal Protocol of 1987 and subsequent amendments regulated the production of the long-lived chlorofluorocarbons (CFCs) and other chlorine- and bromine-containing compounds (such as halons). NOAA/CMDL measurements indicated that the total effective ozone-destroying chlorine in all these compounds reached a maximum and began a slow decrease at the surface in 1994. This scenario is expected to be repeated in the stratosphere in the next one or two years. All else being equal, the recovery of the ozone layer can then begin. The observation of the beginning of the recovery of the Antarctic ozone hole was estimated to occur as early as 2010.

New information has changed these estimates. The final draft of the 1998 WMO/UNEP Scientific Assessment of the Ozone Layer was submitted in August 1998. Following review by the parties to the Montreal Protocol on substances that deplete the ozone layer, the assessment will be released to the public, in early 1999. The Executive Summary appeared in July 1998. Chapter 12 of the full document is entitled "Predicting Future Ozone Changes and Detection of Recovery" (D. Hofmann and J. Pyle, Lead Co-Authors). Because of the increase in greenhouse gases during the next 50 years, a conclusion of the Assessment is that the atmosphere will not return to it’s pre-1970 state but to a new climatic and chemical state which will affect ozone levels. Unfortunately, the climatic effects (a colder stratosphere) and the chemical effects (increased nitrous oxide and a slowdown in the growth rate of methane) all exacerbate ozone depletion and will cause a delay in the beginning of recovery. While total effective stratospheric chlorine will remain high for the next 10-20 years, when it begins to decline, ozone will not follow lock-step because of these atmospheric changes. It is now estimated that the unambiguous detection of the beginning of the closing of the Antarctic ozone hole will not occur before about 2030. The figure shows total column ozone as measured at the US station at the South Pole and the UK station at Halley. The smooth curves are a typical two-dimensional dynamical-chemical model prediction of the expected recovery, normalized to 1980 initial conditions, with uncertainty limits. The low values in the early 1990’s were associated with the additional aerosol particles from the Pinatubo eruption which augmented the chlorine-catalyzed chemistry of the normal polar stratospheric cloud particles.

Full recovery of global ozone is estimated to occur in the decade after 2050 assuming that the future phaseouts of compounds such as methyl bromide, halons and the CFC replacements take place as projected.

Test automatic air sampling system for use on military and commercial aircraft. (CMDL, D. Guenther, P. Tans)

The objective of monitoring atmospheric greenhouse gas concentrations is not only to track their rates of increase but also to deduce, from the atmospheric concentration patterns, large-scale changes in their sources and sinks at the surface of the earth. This information will be important in defining policies aimed at lowering the human contribution to the greenhouse effect and climate change. The largest gap in the greenhouse gas global monitoring system is the lack of regular measurements of the vertical profile of such gases over continental areas. In order to make such measurements economically possible we have developed an automated system for obtaining air samples from aircraft platforms. The system consists of two suitcases, one with batteries and compressors, and one with 20 sampling flasks, a gas manifold, dryer, and controller/datalogger (see photo in figure). The aircraft needs only to provide a clean air intake for outside air. In the laboratory we analyze the air samples for CO₂, CH₄, N₂O, CO, H₂, SF₆, and the isotopic ratios of CO₂, also in automated fashion.

The system has operated for some years aboard small privately owned aircraft, but is now undergoing further engineering development to make it rugged and fail-safe enough for extensive use aboard commercial and military aircraft. Some results on the vertical distribution of CO₂ from the surface to an altitude of 8 km, from a recent test flight over Carr, Colorado, are shown in the right of the figure. The enhanced database, including the vertical profiles, will be used in inverse models developed by researchers in the U.S., Europe, Australia, and Asia. These models are used to "translate" the observed concentrations into estimates of the greenhouse gas sources and sinks that produce the observations. The new observations will play a critical role in verifying and refining a recent result, published in Science, Vol. 282, 16 October 1998, that the vegetation and soils of the North American continent are currently an enormous sink of carbon dioxide.

Complete a balloon-borne observation of ozone-related gases in the POLARIS and OMS-Brazil missions. (CMDL, J. Elkins, S. Oltmans).

The purpose of the POLARIS (Photochemistry of Ozone Loss in Arctic Regions in Summer) mission conducted from Fairbanks, Alaska was to study the natural loss of stratospheric ozone that occurs in the Arctic during summertime. The purpose of the OMS (Observations of the Middle Stratosphere) mission in Brazil and New Mexico was to study transport in the tropical and midlatitude stratosphere and thus aid the study of ozone depletion. The two missions involving over six scientific balloon flights were completed in May 1998. Two instruments from NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL) were involved. The Lightweight Airborne Chromatograph Experiment (LACE) measures chlorofluorocarbons (CFCs), CFC-11, CFC-12, and CFC-113, and halon-1211 and SF₆ once every 70 seconds to altitudes as high as 32 km. LACE is mounted on a NASA Jet Propulsion Laboratory (JPL) gondola (400 kg) containing five other instruments measuring O₃, CH₄, N₂O, CO₂, and meteorological parameters. CMDL scientists also launched a separate free-flying balloon at about the same time to record water vapor (H₂O) with a cooled mirror frost-point hygrometer.

One of the major discoveries of the POLARIS mission was the observation of very low values for many trace gases at 20-25 km. Typically, vertical profiles of CFC-12 (CCl₂F₂) are identical for midlatitude and polar regions (green and blue curves in the figure, panel a). However, there was a major ozone depletion event formed in a strong and stable vortex over the Arctic in late March 1997. The remnants of that event were still visible in the mixing ratios of many trace gases by late June. It is believed that these trace gases had descended earlier from higher altitudes (blue arrow in panel a). Chemical tracer models were able to point to this event as the most probable cause of the low values in late June. The vertical profiles of CFC-12 in the tropics (red curves in panel a) are not identical to those of the midlatitudes because of the large average upward air flow resulting in higher mixing ratios in the upper tropical stratosphere. The water vapor measurements (panel b) show the seasonal variation of transport of water vapor into the lower tropical stratosphere. The upward movement of the water vapor maximum (A -> A’) and minimum (B -> B’) into the stratosphere (red arrows), from February to November, allows one to obtain the mean vertical advection ascent rate of air in the tropics. These results are used by modelers that estimate the effects of emissions from aircraft and pollutants on stratospheric ozone depletion. Different transport rates can significantly change the results of the models and affect estimated ozone predictions. The observations reported here will be part of an upcoming NASA Assessment of the Effects of Aviation on the Atmosphere. Three papers are in preparation for journal publication. The next step is to examine transport during the formation of the polar vortex in wintertime during the SAGE-III Ozone Loss Validation Experiment (SOLVE) in 1999-2000.

Report on the test predictions of radiative forcing by aerosols by evaluating theory and observations at a well-measured site. (CMDL, J. Ogren)

Calculations of aerosol radiative forcing rely on many assumptions about aerosol properties. A key "checkpoint" in these calculations is the aerosol optical depth (AOD), which is a measure of the attenuation of the direct solar beam by particles in the atmosphere. The objective of this milestone was to determine how well direct measurements of aerosol optical depth, which are sensitive to aerosols throughout the entire vertical column, agree with values calculated from independent, surface-based measurements of aerosol radiative properties. The extent of agreement between the two values can be interpreted as a measure of the applicability of surface-based measurements, such as are obtained at NOAA's baseline observatories and regional aerosol monitoring stations, for testing model predictions of aerosol radiative forcing.

One year of measurements from a highly-instrumented site in Oklahoma, operated by the U.S. Department of Energy's "ARM" program (Atmospheric Radiation Measurements), were evaluated. A new and unique aspect of the analysis was the integration of in-situ measurements near the surface with a remote-sensing technique (LIDAR) for determining the vertical profile of aerosols, in order to estimate aerosol properties that neither approach can determine alone. The results (summarized in the figure) show that if four key pieces of information are available, the aerosol optical depth can be calculated from surface measurements to within about 20%. Three of these are routinely being measured at the ARM site: aerosol light scattering and absorption coefficients near the surface, and the vertical profile of aerosol backscattering. The missing piece, which must be included if good agreement is to be obtained, describes the dependence of aerosol light scattering on relative humidity. The implication of this result is that aerosol hygroscopic growth must be measured in order to relate surface-based measurements to the column optical depth, and the ARM program has subsequently funded NOAA/CMDL to implement this measurement at their site in Oklahoma. A report of the results from this study was submitted to the Journal of Geophysical Research in August, 1998, and preliminary results were presented at the 1997 ARM Science Team meeting.

Improve the instrumentation of two North American "baseline" observation stations for the physical and chemical characteristics of aerosols (CMDL, J. Ogren)

NOAA/CMDL operates one baseline observatory in North America, located at Point Barrow, Alaska. The aerosol sampling system at this station was upgraded in October, 1997, in order to enhance the utility of the data for calculation of aerosol radiative forcing. The original instrumentation for determining aerosol light scattering (integrating nephelometer) and light absorption coefficients (Aethalometer) was left in place, and newer instruments offering higher sensitivity, additional measurement parameters, and more reliable calibration were installed. A new sample inlet system, which allows control of sample relative humidity and the ability to sample two different size ranges of particles, was also installed. The upgrade brings the Barrow observatory, which has been measuring aerosols since 1976, up to the standard of CMDL's regional aerosol monitoring network, which started in 1992. The next step, planned for 1999, will be a similar upgrade of the aerosol sampling system at the Mauna Loa observatory.

The primary customers for these measurements are researchers trying to quantify aerosol radiative forcing. The ARM (Atmospheric Radiation Measurements) program of the U.S. Department of Energy, which provided the funding to purchase and deploy the new instruments, is particularly interested in using the data to improve their ability to test model-based predictions of radiative fluxes at the surface with direct measurements. Data from the new system are quality-checked within one working day of collection, and made available to the ARM program (as well as the general public) on CMDL's web site. A preliminary evaluation of the results from the two systems (shown in the figure) illustrates the high degree of correlation between the old and new methods for determining aerosol light absorption coefficient, and suggests the need for re-evaluation of the old data record in light of the improved calibration of the new method.

Upgrade UV Measuring Instruments at Barrow Observatory. (CMDL, B. Bodhaine, R. Tatusko)

This activity could also contribute to the next phase of the Arctic Monitoring and Assessment Program (AMAP), which has placed a high-priority on climate change and UVB. The results of this activity will be reported in articles submitted to peer-reviewed journals and in presentations at appropriate scientific conferences. Discussions are also taking place with members of the Alaskan native communities to inform them of this effort and solicit their advice and local expertise. The next step will be to deploy two additional multi-wavelength instruments at sites in and near the Bering Sea.