Ground-based Fourier transform infrared (FTIR) O3 retrievals from the 3040 cm−1 spectral range at Xianghe, China
Abstract. In this study, we present O3 retrievals from ground-based Fourier transform infrared (FTIR) solar absorption measurements between June 2018 and December 2019 at Xianghe, China (39.75 ∘  N, 116.96 ∘  E). The FTIR spectrometer at Xianghe is operated with indium gallium arsenide (InGaAs) and indium antimonide (InSb) detectors, recording the spectra between 1800 and 11 000 cm −1 . As the harmonized FTIR O3 retrieval strategy ( Vigouroux et al. ,  2015 ) within the Network for the Detection of Atmospheric Composition Change (NDACC) uses the 1000 cm −1 spectral range, we apply the O3 retrieval in the 3040 cm −1 spectral range at Xianghe. The retrieved O3 profile is mainly sensitive to the vertical range between 10 and 40 km, and the degrees of freedom for signal is 2.4±0.3 ( 1σ ), indicating that there are two individual pieces of information in partial columns between the surface and 20 km and between 20 and 40 km. According to the optimal estimation method, the systematic and random uncertainties of the FTIR O3 total columns are about 13.6 % and 1.4 %, respectively. The random uncertainty is consistent with the observed daily standard deviation of the FTIR retrievals. To validate the FTIR O3 total and partial columns, we apply the same O3 retrieval strategy at Maido, Reunion (a.k.a. Reunion Island; 21.08 ∘  N, 55.38 ∘  E). The FTIR O3 (3040 cm −1 ) measurements at Xianghe and Maido are then compared with the nearby ozonesondes at Beijing (39.81 ∘  N, 116.47 ∘  E) and at Gillot (20.89 ∘  S, 55.53 ∘  E), respectively, as well as with co-located TROPOspheric Monitoring Instrument (TROPOMI) satellite measurements at both sites. In addition at Maido, we compare the FTIR O3 (3040 cm −1 ) retrievals with the standard NDACC FTIR O3 measurements using the 1000 cm −1 spectral range. It was found that the total columns retrieved from the FTIR O3 3040 cm −1 measurements are underestimated by 5.5 %–9.0 %, which is mainly due to the systematic uncertainty in the partial column between 20 and 40 km (about −10.4  %). The systematic uncertainty in the partial column between surface and 20 km is relatively small (within 2.4 %). By comparison with other measurements, it was found that the FTIR O3 (3040 cm −1 ) retrievals capture the seasonal and synoptic variations of the O3 total and two partial columns very well. Therefore, the ongoing FTIR measurements at Xianghe can provide useful information on the O3 variations and (in the future) long-term trends.