Solar FTIR measurements of NO_x vertical distributions: Part I) First observational evidence for a seasonal variation in the diurnal increasing rates of stratospheric NO₂ and NO

Abstract. Observations of nitrogen dioxide (NO₂) and nitrogen oxide (NO) in the stratosphere are relevant to understand long-term changes and variabilities in stratospheric nitrogen oxide (NO_x) and ozone (O₃) concentrations. Due to the versatile role of NO₂ and NO in stratospheric O₃ photochemistry they are important for recovery and build-up of O₃ holes in the stratosphere, and therefore can indirectly affect the human life. Thus, we present in this work the evaluation of NO₂ and NO stratospheric partial columns (> 16 km altitude) retrieved from ground-based Fourier-transform infrared (FTIR) measurements from over 25 years at Zugspitze (47.42° N, 10.98° E, 2964 m a.s.l.) and 18 years at Garmisch (47.47° N, 11.06° E, 745 m a.s.l.), Germany. The obtained stratospheric columns are only weakly influenced by tropospheric pollution and show only a very small bias of (2.5±0.2) % when comparing NO₂ above Zugspitze and Garmisch. Stratospheric columns of both NO₂ and NO show a diurnal increase in dependence of local solar time (LST). We quantified this behavior by calculating diurnal increasing rates. Here, we find mean values for the NO₂ diurnal increasing rate of (0.89±0.14)·10¹⁴ cm^-2 h^-1 and (0.94±0.14)·10¹⁴ cm^-2 h^-1 at Zugspitze and Garmisch, respectively. The mean NO a.m. diurnal increasing rate above Zugspitze can be found to be (1.42±0.12)·10¹⁴ cm^-2 h^-1. Regarding the seasonal dependency of these increasing rates, for the first time, we were able to detect a significant seasonal variation of both NO₂ diurnal increasing rates and NO a.m. diurnal increasing rates experimentally with a maximum of (1.13±0.04)·10¹⁴ cm^-1 h^-1 for NO₂ and (1.76±0.25)·10¹⁴ cm^-1 h^-1 for NO in September and a minimum of (0.71±0.18)·10¹⁴ cm^-1 h^-1 in December for NO₂ and a minimum of (1.18±0.41)·10¹⁴ cm^-1 h^-1 in November for NO. This similar behavior may be explained by the interconnection of both species in stratospheric photochemistry. The outcome of this work is a retrieval and analyzation strategy of FTIR data for NO_x stratospheric columns, which can help to further validate photochemical models or improve satellite validations. The first use of this data set is shown in a companion paper (Nürnberg et al., 2023) extracting experiment-based NO_x scaling factors describing the diurnal increase out of the retrieved partial columns and validating recently published model-based scaling factors.