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The Ground-Based Millimeter-wave Spectrometer (GBMS) was designed and builtat the State University of New York at Stony Brook in the early 1990s andsince then has carried out many measurement campaigns of stratosphericO3, HNO3, CO and N2O at polar and mid-latitudes. ItsHNO3 data set shed light on HNO3 annual cycles over the Antarcticcontinent and contributed to the validation of both generations of thesatellite-based JPL Microwave Limb Sounder (MLS). Following the increasingneed for long-term data sets of stratospheric constituents, we resolved toestablish a long-term GMBS observation site at the Arctic station of Thule(76.5° N, 68.8° W), Greenland, beginning in January 2009, in order totrack the long- and short-term interactions between the changing climate andthe seasonal processes tied to the ozone depletion phenomenon. Furthermore,we updated the retrieval algorithm adapting the Optimal Estimation (OE)method to GBMS spectral data in order to conform to the standard of theNetwork for the Detection of Atmospheric Composition Change (NDACC)microwave group, and to provide our retrievals with a set of averagingkernels that allow more straightforward comparisons with other data sets.The new OE algorithm was applied to GBMS HNO3 data sets from 1993 SouthPole observations to date, in order to produce HNO3 version 2 (v2)profiles. A sample of results obtained at Antarctic latitudes in fall andwinter and at mid-latitudes is shown here. In most conditions, v2 inversionsshow a sensitivity (i.e., sum of column elements of the averaging kernelmatrix) of 100 ± 20 % from 20 to 45 km altitude, with somewhat worse(better) sensitivity in the Antarctic winter lower (upper) stratosphere. The1σ uncertainty on HNO3 v2 mixing ratio vertical profilesdepends on altitude and is estimated at ~15 % or 0.3 ppbv, whicheveris larger. Comparisons of v2 with former (v1) GBMS HNO3 verticalprofiles, obtained employing the constrained matrix inversion method, showthat v1 and v2 profiles are overall consistent. The main difference is atthe HNO3 mixing ratio maximum in the 20–25 km altitude range, which issmaller in v2 than v1 profiles by up to 2 ppbv at mid-latitudes and duringthe Antarctic fall. This difference suggests a better agreement of GBMSHNO3 v2 profiles with both UARS/ and EOS Aura/MLS HNO3 data thanprevious v1 profiles

annales geophysicae

Revising the retrieval technique of a long-term stratospheric HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; data set: from a constrained matrix inversion to the optimal estimation algorithm