Fate of the nitrate radical at the summit of a semi-rural mountain site in Germany assessed with direct reactivity measurements

The reactivity of NO3 plays an important role in modifying the fate of reactive nitrogen species at nighttime. High reactivity (e.g. towards unsaturated volatile organic compounds - VOCs) can lead to formation of organic nitrates and secondary organic aerosol, whereas low reactivity opens the possibility of heterogeneous NOx losses via the formation and uptake of N2O5 to particles. We present direct NO3 reactivity measurements (k(NO3)) that quantify the VOC-induced losses of NO3 during the TO2021 campaign at the summit of the Kleiner Feldberg mountain (825 m, Germany) in July 2021. k(NO3) was on average similar to 0.035 s(-1) during the daytime, similar to 0.015 s(-1) for almost half of the nights and below the detection limit of 0.006 s(-1) for the other half, which may be linked to sampling from above the nocturnal surface layer. NO3 reactivities derived from VOC measurements and the corresponding rate coefficient were in good agreement with k(NO3), with monoterpenes representing 84% of the total reactivity. The fractional contribution F of k(NO3) to the overall NO3 loss rate (which includes an additional reaction of NO3 with NO and photolysis) were on average similar to 16% during the daytime and similar to 50 %-60% during the nighttime. The relatively low nighttime value of F is related to the presence of several tens of parts per trillion by volume (pptv) of NO on several nights. NO3 mixing ratios were not measured, but steady-state calculations resulted in nighttime values between <1 and 12 pptv. A comparison of results from TO2021 with direct measurements of NO3 during previous campaigns between 2008 and 2015 at this site revealed that NO3 loss rates were remarkably high during TO2021, while NO3 production rates were low. 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-3 present at levels of mostly 25 to 60 ppbv (parts per billion by volume), NO is oxidized to NO2 on a timescale of a few minutes. We find that maintaining NO mixing ratios of, e.g., 40 pptv requires a ground-level NO emission rate of 0.33 pptv s(-1) (into a shallow surface layer of 10m depth). This in turn requires a rapid deposition of NO2 to the surface (vd(NO2) similar to 0.15 cm s(-1)) to reduce nocturnal NO2 levels to match the observations.