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

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 (NOx) 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 NOx 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 NOx scaling factors describing the diurnal increase out of the retrieved partial columns and validating recently published model-based scaling factors.