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 O3, HNO3, CO and N2O at polar and mid-latitudes. Its HNO3 data set shed light on HNO3 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 HNO3 data sets from 1993 South Pole observations to date, in order to produce HNO3 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 HNO3 v2 mixing ratio vertical profiles depends on altitude and is estimated at ~15 % or 0.3 ppbv, whichever is larger. Comparisons of v2 with former (v1) GBMS HNO3 vertical profiles, obtained employing the constrained matrix inversion method, show that v1 and v2 profiles are overall consistent. The main difference is at the HNO3 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 HNO3 v2 profiles with both UARS/ and EOS Aura/MLS HNO3 data than previous v1 profiles.
Highlights
Nitric Acid (HNO3) is a major player in processes controlling the springtime depletion of polar ozone, both over short and long time periods
With the aim of a better compliance of Ground-Based Millimeter-wave Spectrometer (GBMS) HNO3 measurements to the NDACC database standards, and in order to establish a long-term stratospheric trace gases observing station in the Arctic, we chose Thule (76.5◦ N, 68.8◦ W) as a permanent site for the GBMS, installed it there in January 2009, and developed an Optimal Estimation method for deconvolving GBMS spectra
We described the main characteristics of the Optimal Estimation (OE) inversions of GBMS HNO3 spectra covering locations and temporal windows that offer a wide variety of spectral features, from the very intense emissions during the Antarctic Fall (Fig. 1a) to the emergence of single resolved lines during the lower stratospheric denitrification and the upper stratospheric HNO3 enhancement processes of Antarctic winter (Fig. 1b)
Summary
Nitric Acid (HNO3) is a major player in processes controlling the springtime depletion of polar ozone, both over short and long time periods It is a primary reservoir for reactive nitrogen in the stratosphere and has a key role in both the activation and the deactivation of chlorine species (Brasseur and Solomon, 1984). GBMS HNO3 spectra were deconvolved to produce stratospheric HNO3 mixing ratio vertical profiles using an algorithm known as constrained matrix inversion (MI) (Twomey, 1977; de Zafra et al, 1997) This technique has its advantages versus other more popular algorithms (e.g., the solution profile is basically independent of the “a priori” concentration profile), profiles retrieved using the MI algorithm cannot be compared to other data sets due to the lengthy process required for determining their corresponding averaging kernels (AKs), AKs being necessary to reduce the reference data sets to the typically lower vertical resolution of the GBMS data. We compare the new HNO3 vertical profiles (hereafter referred to as version 2 or v2) to those obtained in the past using the constrained matrix inversion method (hereafter referred to as version 1 or v1) to evaluate how the change in retrieval algorithm might affect previous comparisons carried out using the GBMS MI-based HNO3 retrievals
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