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Retrieving high-precision concentrations of atmospheric trace gases from FTIR (Fourier transform infrared) spectrometry requires a precise knowledge of the instrumental performance. In this context, this paper examines the impact on the ozone (O 3) retrievals of several approaches used to characterize the instrumental line shape (ILS) function of ground-based FTIR spectrometers within NDACC (Network for the Detection of Atmospheric Composition Change). The analysis has been carried out at the subtropical Izaña Observatory (IZO, Spain) by using the 20-year time series of the high-resolution FTIR solar absorption spectra acquired between 1999 and 2018. The theoretical quality assessment and the comparison to independent O 3 observations available at IZO (Brewer O 3 total columns and electrochemical concentration cell, ECC, sondes) reveal consistent findings. The inclusion of a simultaneous retrieval of the ILS parameters in the O 3 retrieval strategy allows, on the one hand, a rough instrumental characterization to be obtained and, on the other hand, the precision of the FTIR O 3 products to be slightly improved. The improvement is of special relevance above the lower stratosphere, where the cross-interference between the O 3 vertical distribution and the instrumental performance is more significant. However, it has been found that the simultaneous ILS retrieval leads to a misinterpretation of the O 3 variations on daily and seasonal scales. Therefore, in order to ensure the independence of the O 3 retrievals and the instrumental response, the optimal approach to deal with the FTIR instrumental characterization is found to be the continuous monitoring of the ILS function by means of independent observations, such as gas cell measurements. [ABSTRACT FROM AUTHOR]

Accurate observations of atmospheric ozone (O 3) are essential to monitor in detail its key role in atmospheric chemistry. The present paper examines the performance of different O 3 retrieval strategies from FTIR (Fourier transform infrared) spectrometry by using the 20-year time series of the high-resolution solar spectra acquired from 1999 to 2018 at the subtropical Izaña Observatory (IZO, Spain) within NDACC (Network for the Detection of Atmospheric Composition Change). In particular, the effects of two of the most influential factors have been investigated: the inclusion of a simultaneous atmospheric temperature profile fit and the spectral O 3 absorption lines used for the retrievals (the broad spectral region of 1000–1005 cm -1 and single micro-windows between 991 and 1014 cm -1). Additionally, the water vapour (H 2 O) interference in O 3 retrievals has been evaluated, with the aim of providing an improved O 3 strategy that minimises its impact and, therefore, could be applied at any NDACC FTIR station under different humidity conditions. The theoretical and experimental quality assessments of the different FTIR O 3 products (total column (TC) amounts and volume mixing ratio (VMR) profiles) provide consistent results. Combining a simultaneous temperature retrieval with the optimal selection of single O 3 micro-windows results in superior FTIR O 3 products, with a precision of better than 0.6 %–0.7 % for O 3 TCs as compared to coincident NDACC Brewer observations taken as a reference. However, this improvement can only be achieved provided the FTIR spectrometer is properly characterised and stable over time. For unstable instruments, the temperature fit is found to exhibit a strong negative influence on O 3 retrievals due to the increase in the cross-interference between the temperature retrieval and instrumental performance (given by the instrumental line shape function and measurement noise), which leads to a worsening of the precision of FTIR O 3 TCs of up to 2 %. This cross-interference becomes especially noticeable beyond the upper troposphere/lower stratosphere, as documented theoretically as well as experimentally by comparing FTIR O 3 profiles to those measured using electrochemical concentration cell (ECC) sondes within NDACC. Consequently, it should be taken into account for the reliable monitoring of the O 3 vertical distribution, especially over long-term timescales. [ABSTRACT FROM AUTHOR]

Accurate observations of atmospheric ozone (O3) are essential to monitor in detail the key role of O3 in the atmospheric chemistry. The present paper examines the performance of different O3 retrieval strategies from FTIR (Fourier Transform InfraRed) spectrometry by using the 20-year time series of the high-resolution solar spectra acquired from 1999 to 2018 at the subtropical Izaña Observatory (IZO, Spain) within NDACC (Network for the Detection of Atmospheric Composition Change). In particular, the effect of two of the most influential factors have been investigated: the spectral region used for O3 retrievals and inclusion of an atmospheric temperature profile fit. The theoretical and experimental quality assessments of the different FTIR O3 products (total column, TC, amounts and volume mixing ratio, VMR, profiles) provide consistent results. Combining an optimal selection of spectral O3 absorption lines and a simultaneous temperature retrieval results in superior FTIR O3 products, with a precision greater than 0.6-0.7% for O3 TCs as compared to coincident NDACC Brewer observations used as reference. However, this improvement can be only achieved provided the FTIR spectrometer is properly characterised and stable over time. For unstable instruments, the temperature fit has been found to exhibit a strong negative influence on O3 retrievals by increasing the cross-interference between instrumental performance and temperature retrieval. This cross-interference becomes especially noticeable beyond the upper troposphere/lower stratosphere as documented theoretically, as well as experimentally by comparing FTIR O3 profiles to those measured using Electrochemical Concentration Cell (ECC) sondes within NDACC. Consequently, it should be taken into account for the reliable monitoring of O3 vertical distribution, especially on long-term timescales. [ABSTRACT FROM AUTHOR]