- The History of the Network for the Detection of Stratospheric Change and the Network for the Detection of Atmospheric Composition Change
The Network for the Detection of Atmospheric Composition Change (NDACC) and its predecessor Network for the Detection of Stratospheric Change (NDSC) were constituted as international research and measurement programs composed of high-quality, remote-sensing research stations to conduct long-term measurements. Initial program emphases focused on observing and understanding the physical and chemical state of the stratosphere (with a particular emphasis on the Earth’s ozone layer) and have now expanded to include the upper troposphere as well, in order to assess the impacts of stratosphere changes on the underlying troposphere and on global climate. In 2016 the NDACC, comprised of more than 70 research stations, celebrated 25 years of successful network operations in enabling and enhancing global atmospheric research. The results of many past and present measurements, analysis, and exploitation activities have been highlighted in a special issue of Atmospheric Chemistry and Physics (Atmos. Chem. Phys., 18, 4935–4964, 2018; https://doi.org/10.5194/acp-18-4935-2018. These activities and accomplishments include (i) the analysis of long-term data sets from which trends and changes in atmospheric composition have been determined for international ozone and climate assessments, (ii) ground-truth and correlative measurements for U.S. and international satellite investigations, (iii) scientific collaboration in airborne and balloon campaigns for investigating stratospheric and upper tropospheric processes, and (iv) atmospheric model validation and development.
The motivation for making such measurements can be traced back to the 1960s and 1970s when chemical threats to the Earth’s atmosphere became a scientific focus. With the realization of the roles of catalytic reaction cycles in stratospheric ozone chemistry, specific concerns about the ozone layer arose. These concerns were bolstered by early spectroscopic balloon measurements that confirmed the presence of several trace chemical species that had been invoked in the reaction cycles. Thus, a broad research program to understand the physics and chemistry of the Earth's upper atmosphere and its susceptibility to change due to natural and anthropogenic effects was implemented within the U.S. and Europe. Specific details about these international efforts are presented in the aforementioned overview paper in the special issue of Atmospheric Chemistry and Physics. Within the U.S., the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the Chemical Manufacturers Association (CMA) supported a wide range of investigations including field measurements, laboratory kinetics and spectroscopy, atmospheric modeling, and data analysis. The major results and advancements in understanding from the first decade of this effort were summarized in World Meteorological Organization (WMO) Report No. 16, Atmospheric Ozone 1985: Assessment of our Understanding of the Processes Controlling Its Present Distribution and Change; NASA RP 1162, (1986), Present State of Knowledge of the Upper Atmosphere: Processes that Control Ozone and other Climatically Important Gases; and WMO Report No. 20, Scientific Assessment of Stratospheric Ozone: l989.
The total column abundance of ozone in the Earth’s atmosphere had been measured at a few dozen stations worldwide since the International Geophysical Year (IGY) in l957. However, other than these measurements conducted under the Dobson network, no concurrent systematic long-term measurements of other aspects of stratospheric composition (trace gases and chemical species, particles, and physical parameters) existed. The importance of such long-term measurements increased significantly when scientists from the British Antarctic Survey documented severe seasonal depletion of Antarctic ozone in 1985 based on three decades of total ozone measurements at Halley Bay, Antarctica. Due to the paucity of ancillary measurements the cause(s) of this phenomenon, now referred to as the Antarctic Ozone Hole, could not be immediately identified and several candidate theories emerged. Discrimination among various theories required extensive field campaigns using ground-based, balloon-borne, and airborne instruments aimed at measuring not only ozone but also a broad range of stratospheric chemical species and physical parameters. Following two years of such measurements in 1986 and 1987, these unprecedented seasonal losses in stratospheric ozone were linked to the chemical reactions of halogen radicals whose participation in catalytic reaction cycles were enhanced by the unique meteorology of the Antarctic Winter and Spring. This finding added to the already growing concerns about global reductions in the Earth’s protective ozone layer initiated by the photochemical production of halogen radicals from industrial halocarbons released into the atmosphere.
Several factors were now converging to provide the international impetus for establishing a long-term measurement and analysis network that would provide early detection of changes in stratospheric composition and structure together with the means to understand them. These factors included (i) the observation of Antarctic ozone depletion in Austral Spring together with its established causality, (ii) considerable advances by the international atmospheric research community in the development of technologies required to measure other stratospheric species from the ground, and (iii) the implementation of the 1985 International Vienna Convention for the Protection of the Ozone Layer that gave a political mandate for such comprehensive long-term monitoring. Thus, in March 1986 NASA, NOAA and CMA convened an international workshop in Boulder, Colorado to evaluate the possibility of establishing such an observational network and to begin specifying its goals, measurement priorities and rationale, and operational requirements (such as instrument types and measurement locations). In particular, the workshop organizers wanted to circumvent the need for convening assessment panels specifically tasked with validating the changes and trends in stratospheric composition derived from future network data as was about to be done at that time for re-evaluating ground-based and satellite ozone data (see WMO Report No. 18, Report of the International Ozone Trends Panel — 1988). While the ultimate objective of such a network focused on the long term, it became clear that the proposed network would yield valuable scientific returns over the shorter term for:
- studying the temporal (diurnal, monthly, seasonal and annual) and spatial (latitudinal) variability of atmospheric composition and structure,
- providing the basis for ground truth and complementary measurements for satellite systems such as the NASA Upper Atmosphere Research Satellite, which was then scheduled for launch in the fall of 1991, and
- critically testing multi-dimensional stratospheric models and providing the broad database required for improved model development.
Long-term measurement stability and quality assurance as well as comprehensive measurement and data intercomparisons among various measurement systems (including satellites) were paramount in the construct of such a network. While it was initially envisioned that the network might begin with funding from NASA, NOAA and CMA, it was recognized that a fully implemented network would require collaboration (scientifically, managerially, and financially) among numerous international cosponsors complemented by other WMO and United Nations Environment Programme (UNEP) monitoring activities. In particular, the inclusion of several established European research and monitoring stations such as the Observatoire de Haute Provence in France and the Jungfraujoch Observatory in Switzerland were viewed as being critical to network initiation. Hence, it was concluded that network stations would likely be based at current observatory-type sites where many of the required facilities already existed.
Following the identification of the causes of the Antarctic ozone hole, NASA, NOAA, and WMO convened an international forum in Geneva, Switzerland in 1989 together with several international scientific agency partners to formalize the initial managerial and organizational structure of the network. Discussions of various implementation aspects and international commitments continued at subsequent meetings in Washington, DC in 1990 and in Abingdon, UK in 1991. Thus, following five years of planning, instrument design and development, and implementation, the Network for the Detection of Stratospheric Change began its official operations with endorsements by UNEP, the Global Atmosphere Watch (GAW) Programme of WMO, and the International Ozone Commission (IO3C) of the International Association of Meteorology and Atmospheric Sciences. This rapid implementation could not have been achieved without the several years of instrument developments supported by various international programs. Within the U.S., one of the most focused developmental efforts occurred under the auspices of NASA’s Upper Atmosphere Research Program (UARP), whose establishment stemmed from NASA's FY1976 authorization bill that mandated the Agency to perform research concerned with possible depletion of the ozone layer by "conducting a comprehensive program of research, technology, and monitoring of the phenomena of the upper atmosphere".
At its outset the NDSC embodied a simple and flexible organizational structure that has contributed to the network’s longevity and successful evolution throughout the ensuing years. The principal components of the organization included:
- a Steering Committee (SC) headed by a Chair and Vice-Chair (now two Co-Chairs) and comprised of Representatives from the Instrument Working Groups (IWGs), other Working Groups, Peer Review Scientists, and Ex-Officio Representatives from the sponsoring or partnering international agencies or institutions, and
- a Science Team consisting of the Principal Investigators from all of the network sites.
As the primary managerial body of the Network, the SC is responsible for internal operational and scientific oversight and for recommending implementation and funding actions. The Science Team acts as the actual forum for conducting network research and analysis coordinated through the IWGs, which were organized around the initial instrument types (originally FTIR spectrometers for column abundances of several chemicals; Lidars for aerosols, temperature, and ozone; Microwave radiometers for ClO ozone and water vapor; and UV/Visible spectrometers for column O3 and NO2 measurements). Dobson/ Brewer spectrophotometers for column ozone, Ozone and Aerosol sondes, and UV spectroradiometers were subsequently added. Each IWG has the responsibility of setting actions to maximize internal consistency among the network data, which were archived at a dedicated Data Host Facility (DHF) hosted and supported by NOAA. A Theory and Analysis Working Group and a Satellite Working Group were also established to promote and enhance interactions with the modeling and satellite communities respectively. Measurement sites at which a majority of the aforementioned NDSC-certified instrument types operated were designated as Primary Stations. Sites equipped with a subset of such instruments and/or operating less regularly than the Primary Stations were designated as Complementary Stations and contributed to the global coverage of the network. These sites also provided substantial support during coordinated campaigns targeted at special process studies, at calibration/validation phases of space-based sensors, and at studying more regional / subtle atmospheric characteristics. In the early 1990’s NASA and NOAA were providing support for more than a dozen U.S. investigator teams as well as for the operation of the Primary Station at Mauna Loa, Hawaii and the Complementary Station at Table Mountain, California. International Agencies and Institutions were providing support for measurements and analyses in Europe, Japan, New Zealand, and Antarctica.
Key to certifying and maintaining the high quality and availability of NDSC data was the establishment of various operational protocols. These included a Data Protocol that provided guidelines for data submission and use, a Measurements Protocol that detailed the application process for network instrument affiliation, a Validation Protocol that gave general guidelines for instrument certification, and an Instrument Intercomparisons Protocol that described the recommended procedures for intercomparison campaigns.
As its operations extended beyond the first decade, the NDSC continued to expand in both its atmospheric measurement capabilities and in its contributions to understanding a broad spectrum of atmospheric science issues. Network activities included numerous instrument validations and intercomparisons (both within the network itself and with external measurement capabilities conducted by aircraft, balloons, and satellites). Of particular note was the pro-active role of the Satellite Working Group in fostering synergy with the satellite community. Workshops within the various IWGs focused on the continued fostering of quality control and quality assurance; developing improved consistency for instrument standard operating procedures, calibration processes, and data retrievals; and expanding measurement capabilities for new species, increased altitudinal coverage, and improved vertical resolution. The more than ten years of high-quality data were now contributing significantly to the international WMO/UNEP Ozone Assessments being conducted under the provisions of the Montreal Protocol on Substances that Deplete the Ozone Layer. NDSC data were also being used by the Theory and Analysis Working Group in 3-D modeling studies in support of such assessments.
With the increase in measurement capabilities and in the breadth of scientific interest in changes in overall atmospheric composition, an expansion of network focus was inevitable. Thus, while retaining a commitment to its stratosphere-oriented goals, network measurement and analysis priorities began to encompass both the stratosphere and free troposphere through an exploration of the interface between changing atmospheric composition and climate. To better reflect the expanded coverage of network measurement, analysis, and modeling activities to the free troposphere as well as the stratosphere, and to convey the linkage to climate change, in 2005 the Steering Committee voted to change the name of the network to the Network for the Detection of Atmospheric Composition Change (NDACC). However, the SC emphasized that NDACC was not intrinsically a climate-monitoring network, but rather an observational network that provided a broad suite of atmospheric data that contributed to understanding the interrelationship between changing atmospheric composition and climate. NDACC’s objectives are now centered on the following enhanced priorities:
- establishing long-term databases for detecting changes and trends in atmospheric composition, and understanding their impacts on the stratosphere and troposphere;
- establishing scientific links and feedbacks between climate change and atmospheric composition;
- calibrating and validating atmospheric measurements from satellites and gap-filling critical satellite data sets;
- providing collaborative support to scientific field campaigns and to other chemistry and climate observing networks; and
- providing validation and development support for atmospheric models.
At this same time, the network scientific community began raising concerns that site designations of Primary and Complementary Stations were creating a possible misconception about the level of network importance or of data quality to institutional and national funding managers, rather than being labels that attempted to clarify the extent of measurement activities at a site. Thus, in order to remove any possibility of such mistaken impressions a decision was made to redesignate all sites (both active and inactive) as NDACC Stations. The Measurements and Analyses Directory available on the web site was then restructured to reflect this change. Two sections in the directory (“Long-Term Measurement Activities” and “Intermittent or Campaign Measurement Activities”) were expanded to detail the measurements at the various stations.
With the broadening of focus and the transition of NDSC to NDACC, the incorporation of new measurement capabilities and the collaboration with other networks whose heritage was developed external to NDSC and NDACC became increasingly important. As the network entered its second decade of operations, increased scientific cooperation between NDACC and such independently operating regional, hemispheric, or global networks of instruments became critical. These other networks typically had comparable quality assurance guidelines, operational requirements, and data-archiving policies, and independent national or international recognition. Thus, NDACC formalized a Cooperating Network affiliation in an attempt to promote and foster the desired collaborative measurement and analysis activities, and developed a Cooperating Networks Protocol to cover the various aspects of such affiliations. There are presently eight NDACC Cooperating Networks: the Aerosol Robotic Network (AERONET), the Advanced Global Atmospheric Gases Experiment (AGAGE), the Baseline Surface Radiation Network (BSRN), the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN), the Halocarbon and other Trace Species (HATS) Network operated by NOAA, the Micro Pulse Lidar Network (MPLNET), the Southern Hemisphere Additional Ozonesondes (SHADOZ) Network, and the Total Carbon Column Observing Network (TCCON). Representatives from the Cooperating Network are appointed to the NDACC SC where they work to promote inter-network scientific collaboration. Further details can be found in the Cooperating Network Section on the NDACC web site.
As the network continued to further mature during its second decade of operations, its intercomparison campaigns became more comprehensive and involved multiple instrument types in order to better understand the measurement synergies. In addition, the capabilities of FTIR and UV/Vis instruments were expanding to provide vertically resolved data. Current NDACC observational capabilities are depicted in a chart on the NDACC web site. The increased recognition of NDACC as a source of long-term high quality data for multiple species and parameters of atmospheric interest resulted in a significant increase in network citations in subsequent WMO/UNEP Ozone Assessments. Long-term UV data records were now becoming available for the Polar Regions where appreciable stratospheric ozone depletions were observed and NDACC data were also being used extensively in the evaluation of Chemistry-Climate Models (CCMs). In addition, the now-extensive database (accessible by anonymous ftp at ftp.cpc.ncep.noaa.gov/ndacc) was gaining increased recognition by space agencies (such as NASA, ESA, JAXA, etc.) and by the European Union (EU) as an important fiducial measurement resource for satellite validation.
On the international stage NDACC had now become an important contributor to initiatives organized by the Stratosphere-troposphere Processes And their Role in Climate (SPARC) project of the World Climate Research Program, to WMO-GAW and IO3C projects, to working groups within GRUAN, and to the recommendations drafted by representatives at the Meetings of Ozone Research Managers of the Parties to the Vienna Convention for the Protection of the Ozone Layer. Of particular note was the use of NDSC/NDACC data in the SPARC/IO3C/IGACO-O3/NDACC Initiative on Past Changes in the Vertical Distribution of Ozone (SI2N), which was undertaken to study and document those long-term changes and possibly enable attribution for observed ozone layer recovery. Network organizational aspects also advanced to the structure now shown in the NDACC System Chart on the NDACC web site. The web site also continued to undergo revisions to include (i) a section for “Hot News” items; (ii) links to new web sites for the various Working Groups; and (iii) download capability for all sections of an improved Measurements and Analyses Directory, for PDF files of all network Newsletters (2003, 2005, 2008, 2011, 2013, and 2015), and for other operational aspects. As was the case for the scientific highlights from the first decade of NDSC operations, comprehensive details about accomplishments from the first decade of NDACC operations can be found in the corresponding Newsletters.
In November 2011, NDACC commemorated its first decade of scientific successes and two decades of combined NDSC and NDACC contributions to atmospheric science with a 20-Year Anniversary Symposium in Saint Paul, Île de la Reunion. This four-day symposium featured more than 125 presentations focused on:
- Polar Chemistry and Ozone Loss
- Stratospheric Composition & Long-Term Trends
- Short-Term Ozone Variability
- Tropospheric Observations and Analyses
- Tropical & Subtropical Observations and Analyses
- Interactions between Atmospheric Composition & Climate
- Water Vapor
- Aerosols, Radiation, & Spectral UV
- Satellite Calibration, Validation, & Intercomparisons
Symposium attendees were also given the opportunity to visit the new Mäido Observatory that subsequently opened in 2013.
Continued successful contributions to worldwide atmospheric research has required constant Network evolution, and the oversight of such growth has been and continues to be a major responsibility of the NDACC Steering Committee. While the permanent NDACC Working Groups were organized around specific instrument types and other relevant activities (Satellites and Theory and Analysis) as described earlier, the SC identified the need for new Theme Groups that could be of more limited duration and/or be organized around specific foci. The initial Theme Groups were focused around Combining Trace-Gas Data from Various Networks, Measurement Strategies and Emphases, Ozone, and a network Water Vapor Measurement Strategy. Representatives from each Theme Group also serve on the NDACC SC.
The Water Vapor Measurement Strategy Theme Group was specifically established to develop a network-wide measurement strategy for atmospheric water vapor and was initially focused on frost point sondes (an newly accepted NDACC measurement capability). However, the strategy rapidly expanded to coordinate all NDACC water vapor measurements (lidar, microwave, FTIR, as well as frost point sondes) recognizing that these other NDACC instruments produce either integrated column values or low-resolution vertical profiles. The high-resolution sonde profiles could be vertically averaged for comparison with the low-resolution profiles or integrated to compare with the column measurements. The development of this strategy continues to examine a number of strategic considerations and aims to enhance the relationship between NDACC and climate-focused networks such as GRUAN.
The Combining Trace-Gas Data Theme Group is focusing on an assessment of how data from NDACC and some of its Cooperating Networks can be combined to lower the uncertainty of trace-gas budget estimations. This is a joint effort with the Theory and Analysis Working Group and is another area in which the collaboration between NDACC and its Cooperating Networks is being enhanced.
With the increased network maturity, the time scale for the archiving of verifiable network data at the DHF and for their public release was reduced to one year, although most PIs actually release data publicly upon submittal to the database. These data are available at https://www-air.larc.nasa.gov/missions/ndacc/data.html. The use of NDACC data prior to its being made publicly available (i.e., for field campaigns, satellite validation, etc.) is possible via collaborative arrangement with the appropriate Principal Investigators. The DHF, itself, continues to undergo improvements in an effort to provide greater clarity regarding data file versions and data processing, more thorough data quality checks, and a dedicated directory for submission of and access to Rapid Delivery Data. These data, which may be revised before entry in the full database, are available for some instruments at https://www-air.larc.nasa.gov/missions/ndacc/data.html?RapidDelivery=rd-list.
In addition to the earlier mentioned web site changes, more recent changes have focused on improved data access and the inclusion of model data generated by the Theory and Analysis Group. This Working Group is producing simulated station data using GMI-MERRA (categorized by instrument type: Dobson, FTIR, lidar, and sondes), which can be accessed at https://www-air.larc.nasa.gov/missions/ndacc/data.html?GMI_MODEL=gmi-list. These model-generated data are aimed at providing a better understanding of station data variability and representativeness (a bridge between individual stations and the global perspective) and a context for interpreting station observations. Model simulations produced by the group will help set priorities for network expansion and/or instrument relocation.
As the network approaches the end of its third decade of operation, each of the Instrument Working Groups is engaged in data reprocessing and in enhancing data consistency and stability through the homogenization of data processing procedures. Pertinent to these issues is the consideration of the degree to which centralized data processing might be appropriate and achievable within the individual instrument types. Long-term quality assurance, together with up-to-date data archiving and availability, are critical for continued international network recognition and data use and for maintaining NDACC’s identity in providing fiducial reference measurements for characterizing satellite observations. The IWGs are participating in several European projects focused on ensuring measurement traceability, harmonizing and quantifying measurement uncertainties, and harmonizing and documenting the traceability of retrieval methods. For example, under the GAIA-CLIM activity (Gap Analysis for Integrated Atmospheric ECV CLImate Monitoring) the maturity and quality of NDACC data are being assessed by such attributes as traceability, documentation, metadata retention, uncertainty quantification, etc. (in an effort to better use ground-based and sub-orbital measurements to characterize satellite observations for a number of atmospheric Essential Climate Variables, ECVs).
The IWGs are also evaluating the potential for using new instruments within the network as recommended in the Report of the Ninth Meeting of Ozone Research Managers of the Parties to the Vienna Convention for the Protection of the Ozone Layer (WMO Global Research and Monitoring Project Report No. 54 (2014)).
As a major component of the international atmospheric research effort, NDACC stands as a shining example of what can be achieved through international cooperation and quality assurance in measurements and their analyses. NDACC looks forward to continuing its important role for many years to come.
NDACC Historical Documents
- NDACC pamphlet
A two-sided tri-fold pamphlet containing general information about NDACC and its activities is useful for conferences and general public distribution.
- NDACC brochure
A 24 page (33Mb) historical booklet documents the goals, organization and implementation of the original Network for the Detection of Stratospheric Change (NDSC). Published in 2001.
- 2001 Symposium report
In celebration of the first 10 years of NDSC operations, an international scientific symposium was held at the Palais des Congres 'Le Palatium' in Arcachon, near Bordeaux, France on 24–27 September 2001.