Revising the retrieval technique of a long-term stratospheric HNO<sub>3</sub> data set: from a constrained matrix inversion to the optimal estimation algorithm

Abstract

Abstract. The Ground-Based Millimeter-wave Spectrometer (GBMS) was designed and built at the State University of New York at Stony Brook in the early 1990s and since then has carried out many measurement campaigns of stratospheric O 3 , HNO 3 , CO and N 2 O at polar and mid-latitudes. Its HNO 3 data set shed light on HNO 3 annual cycles over the Antarctic continent and contributed to the validation of both generations of the satellite-based JPL Microwave Limb Sounder (MLS). Following the increasing need for long-term data sets of stratospheric constituents, we resolved to establish 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 to track the long-and short-term interactions between the changing climate and the 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 the Network for the Detection of Atmospheric Composition Change (NDACC) microwave group, and to provide our retrievals with a set of averaging kernels that allow more straightforward comparisons with other data sets. The new OE algorithm was applied to GBMS HNO 3 data sets from 1993 South Pole observations to date, in order to produce HNO 3 version 2 (v2) profiles. A sample of results obtained at Antarctic latitudes in fall and winter and at mid-latitudes is shown here. In most conditions, v2 inversions show a sensitivity (i.e., sum of column elements of the averaging kernel matrix) of 100 ± 20 % from 20 to 45 km altitude, with somewhat worse (better) sensitivity in the Antarctic winter lower (upper) stratosphere. The 1σ uncertainty on HNO 3 v2 mixing ratio vertical profiles depends Correspondence to: I. Fiorucci (irene.fiorucci@ingv.it) on altitude and is estimated at ∼15 % or 0.3 ppbv, whichever is larger. Comparisons of v2 with former (v1) GBMS HNO 3 vertical profiles, obtained employing the constrained matrix inversion method, show that v1 and v2 profiles are overall consistent. The main difference is at the HNO 3 mixing ratio maximum in the 20-25 km altitude range, which is smaller in v2 than v1 profiles by up to 2 ppbv at mid-latitudes and during the Antarctic fall. This difference suggests a better agreement of GBMS HNO 3 v2 profiles with both UARS/ and EOS Aura/MLS HNO 3 data than previous v1 profiles.

Ground-based stratospheric O<sub>3</sub> and HNO<sub>3</sub> measurements at Thule, Greenland: an intercomparison with Aura MLS observations

“…A detailed description of the OE algorithm is given by Rodgers (1976Rodgers ( , 2000, while the adaptation of the general theory to GBMS measurements has been extensively presented in Fiorucci et al (2011) and Muscari et al (2012). Here we briefly illustrate only those aspects of the GBMS inversion technique necessary to better characterize the retrieved profiles presented in this work.…”

“…The vertical resolution varies from 9 to 14 km in this vertical range (panel f), which is the one recommended for scientific use of the GBMS HNO 3 dataset. The total uncertainty has been assessed following the same criteria used for O 3 and is given by the larger of 15 % or 0.2 ppbv (see also Fiorucci et al, 2011), with the main contribution given by uncertainties due to calibration and to parameters used in the forward model calculation. It is worth stressing that the relative uncertainty becomes very large above ∼ 28 km due to very low HNO 3 concentrations, whereas the absolute error remains as low as 0.2 ppbv throughout the vertical range.…”

“…However, both the GBMS and the MLS retrieval algorithms have been revised since then. Fiorucci et al (2011) compared GBMS profiles retrieved applying the "old" and the "new" techniques to the same set of spectra, finding that the former set of profiles (v1) were systematically larger near the peak by up to 2 ppbv with respect to v2 profiles. On the other hand, as seen in Sect.…”

“…More recently, the GBMS retrieval algorithm has been updated to an Optimal Estimation method in order to conform to the standard of the NDACC microwave group, and to easily provide retrievals with a set of Averaging Kernels that grants more straightforward comparisons with other datasets (Fiorucci et al, 2011). Comparing ground-based measurements with satellite observations is particularly important because, when wellvalidated, the ground-based data may be used to fill gaps in temporal coverage of satellite datasets.…”

Strato‐mesospheric ozone measurements using ground‐based millimeter‐wave spectroscopy at Thule, Greenland

“…Both methods are illustrated in the work of Twomey [1977]. More recently, we updated the retrieval algorithm used for inverting all the observed species adapting the optimal estimation (OE) method [ Rodgers , 1976, 2000; Connor et al , 1995] to GBMS spectral data (for the application to the HNO 3 data set, see Fiorucci et al [2011]). This effort is aimed at conforming GBMS retrievals to the standard of the NDACC microwave group, and providing our retrievals with a set of averaging kernels that allow more straightforward comparisons with other data sets.…”