Solar FTIR measurements of NO$_x$ vertical distributions – Part 2: Experiment-based scaling factors describing the daytime variation in stratospheric NO$_x$

Long-term experimental stratospheric NO$_2$ and NO partial columns measured by means of solar
Fourier-transform infrared (FTIR) spectrometry at Zugspitze (47.42° N, 10.98° E; 2964 m a.s.l.), Germany, were
used to create a set of experiment-based monthly scaling factors (SFNO$_{exp}$). The underlying data set is published in
a companion paper (Nürnberg et al., 2024) and comprises over 25 years of measurements depicting the daytime
variability of stratospheric NO$_x$ and NO partial columns with respect to local solar time (LST). In accordance
with simulation-based scaling factors recently published by Strode et al. (2022), we created SFexp normalized
to SZA = 72° for NO$_2$ and NO for every month of the year as a function of solar zenith angle (SZA). Apart
from a boundary value problem at minimum SZA values originating from averaging over different times of
the month, the obtained scaling factors SFexp(NO$_2$) and SF$_{exp}$(NO) as a function of SZA represent the daytime
behavior already shown in model simulations and experiments in the literature very well. This shows a well-
pronounced increase in the NO$_2$ and NO stratospheric partial column with the time of the day and a flattening
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of this increase after noon. In addition to the discussion of SF$_{exp}$, we validate the simulation-based scaling
factors SF$_{sim}$(NO$_2$) (Strode et al., 2022) and present simulation-based scaling factors for NO SFsim(NO). The
simulation-based scaling factors show excellent agreement with the experiment-based ones; i.e., for NO$_2$ and NO
the mean value of the modulus between the experiment and simulation over all SZAs and months is only 0.02 %.
We show that recently used model simulations can describe the real behavior of nitrogen oxide (NO$_x$ ) variability
in the stratosphere very well. Furthermore, we conclude that ground-based FTIR measurements can be used
for validation of the output of photochemistry models and for creating experiment-based data sets describing the
daytime stratospheric NO$_x$ variability as a function of SZA. This is a contribution to improved satellite validation
and a better understanding of stratospheric photochemistry.