<p>The reactivity of NO<sub>3</sub> plays an important role in modifying the fate of reactive nitrogen species at nighttime. High reactivity (e.g. towards unsaturated VOCs) can lead to formation of organic nitrates and secondary organic aerosol, whereas low reactivity opens the possibility of heterogeneous NO<sub>X</sub> losses via formation and uptake of N<sub>2</sub>O<sub>5</sub> to particles.</p> <p>We present direct NO<sub>3</sub> reactivity measurements (<em>k</em><sup><em>NO</em><sub>3</sub></sup>) that quantify the VOC-induced losses of NO<sub>3</sub> during the TO2021 campaign at the summit of the Kleiner Feldberg mountain (825 m, Germany) in July 2021. <em>k</em><sup><em>NO</em><sub>3</sub></sup> was on average ~ 0.035 s<sup>-1</sup> during the daytime, ~ 0.015 s<sup>-1</sup> for almost half of the nights and below the detection limit of 0.006 s<sup>-1</sup> for the other half, which may be linked to sampling from above the nocturnal surface layer. NO<sub>3</sub> reactivities derived from VOC measurements and the corresponding rate coefficient were in good agreement with <em>k</em><sup><em>NO</em><sub>3</sub></sup>, with monoterpenes representing 84 % of the total reactivity. The fractional contribution <em>F</em> of <em>k</em><sup><em>NO</em><sub>3</sub></sup> to the overall NO<sub>3</sub> loss rate (which includes additional reaction of NO<sub>3</sub> with NO and photolysis) were on average ~16 % during the daytime and ~50–60 % during the nighttime. The relatively low nighttime value of <em>F</em> is related to the presence of several tens of pptv of NO on several nights. NO<sub>3</sub> mixing ratios were not measured but steady-state calculations resulted in nighttime values between < 1 pptv and 12 pptv. A comparison of results from TO2021 with direct measurements of NO<sub>3</sub> during previous campaigns between 2008 and 2015 at this site revealed that NO<sub>3</sub> loss rates were remarkably high during TO2021, while NO<sub>3</sub> production rates were low.</p> <p>We observed NO mixing ratios of up to 80 pptv at night which has implications for the cycling of reactive nitrogen at this site. With O<sub>3</sub> present at levels of mostly 25 to 60 ppbv, NO is oxidised to NO<sub>2</sub> on a time-scale of a few minutes. We find that to maintain NO mixing ratios of e.g. 40 pptv requires a ground-level NO emission rate of 0.33 pptv s<sup>-1</sup> (into a shallow surface layer of 10 m depth). This in turn requires rapid deposition of NO<sub>2</sub> to the surface (<em>vd</em><sub>NO2</sub> ~ 0.15 cm s<sup>-1</sup>) to reduce nocturnal NO<sub>2</sub> levels to match the observations.</p>