In 1996, in an attempt to resolve an inconsistency between ozone profile trends obtained from satellite vs. ground-based measurements, the Stratospheric Processes and their Role in Climate (SPARC) project of the World Climate Research Programme (WCRP) initiated a collaboration with the International Ozone Commission (IO3C) to carefully re-evaluate the ground-based and satellite ozone data. This study, headed by the SPARC Panel on Understanding Ozone Trends, did not simply review the published literature but conducted a critical re-analysis and interpretation of ozone vertical profiles. A significant focus of the study included validation of the data quality and quantification of the errors to determine if such aspects limited trends determinations as a function of altitude or latitude. The resulting SPARC/IO3C/GAW Assessment of Trends in the Vertical Distribution of Ozone was published in 1998 (SPARC Report No. 1) and was used extensively in the 1998 WMO/UNEP Ozone Assessment. This complete report can be downloaded from the SPARC web site at https://www.sparc-climate.org/publications/sparc-reports/sparc-report-no-1/.
Since this 1998 trends assessment, the end of certain satellite records (the SAGE instruments in particular) has limited our observations of global changes in the vertical distribution of ozone. More specifically, while a number of new satellite instruments have been launched since 2000, no thorough assessment of how well these new measurements agree with each other or with the SAGE record has been conducted. In addition, ozone profile measurements from various ground-based networks have matured through an additional 15 years of operations and through improved geographical coverage. Thus, in an effort to improve our knowledge and understanding of the past changes in the vertical distribution of ozone, a new SPARC/IO3C/IGACO/NDACC (SI2N) initiative has been organized (see http://igaco-o3.fmi.fi/VDO/index.html and recent SPARC Newsletter articles (Harris et al., 2011 and Harris et al., 2012) appearing in SPARC Newsletters 37 and 39 respectively, which can be accessed at http://www.sparc-climate.org/publications/newsletter/).
Under this initiative satellite, ground-based, and sonde measurements of ozone are being critically analyzed, as are methods of preparing combined data sets in an effort to provide input to the next WMO/UNEP Scientific Assessment of Ozone Depletion anticipated for 2014. Within the SI2N initiative, an essential aim is to better understand all instrumental records and to improve the methods for combining them. Finally the SI2N initiative highlights the need for, and challenges associated with, keeping high quality profiling operational for at least the next two decades or so. It should be noted, however, that while the SI2N initiative will hopefully make broad gains in our abilities to utilize and combine data from diverse measurement suites for ozone profiles, temperature profiles, and even trace gas abundances and profiles, it cannot fill all the gaps created by measurement cessation.
Such an SI2N focus on the quality assurance of long-term data records, verification of instrument performance, and intercomparisons of data sets from a broad spectrum of instruments has been a corner stone of the NDACC since its inception as the NDSC. Three NDACC instrument working groups (led by S. Godin-Beekman, T. Leblanc, and W. Steinbrecht; N. Kampfer and G. Nedoluha; and J. Hannigan and M. De Maziere) for lidar, microwave and FTIR respectively are coordinating the NDACC contributions by these instruments to the SI2N initiative. NDACC scientists are also contributing to the efforts of two additional SI2N working groups (Ozonesondes led by S. Oltmans and H. Smit, and Umkehr led by T. McElroy and I. Petropavlovskikh). A broad range of SI2N-focused activities is now underway within the NDACC and the associated publications are in preparation. These include studies dealing specifically with lidar, microwave, or FTIR measurements as well as those that combine data from multiple instrument types including both ozonesondes and Umkehr. The full details of these activities are summarized in the latest NDACC Newsletter to be released in early 2013.