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

Abstract

Abstract. The reactivity of NO3 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 NOX losses via formation and uptake of N2O5 to particles. We present direct NO3 reactivity measurements (kNO3) 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. kNO3 was on average ~ 0.035 s-1 during the daytime, ~ 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 kNO3, with monoterpenes representing 84 % of the total reactivity. The fractional contribution F of kNO3 to the overall NO3 loss rate (which includes additional reaction of NO3 with NO and photolysis) were on average ~16 % during the daytime and ~50–60 % during the nighttime. The relatively low nighttime value of F is related to the presence of several tens of pptv of NO on several nights. NO3 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 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 O3 present at levels of mostly 25 to 60 ppbv, NO is oxidised to NO2 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-1 (into a shallow surface layer of 10 m depth). This in turn requires rapid deposition of NO2 to the surface (vdNO2 ~ 0.15 cm s-1) to reduce nocturnal NO2 levels to match the observations.

Determination of Temporal Stability and Instrument Performance of an airborne QCLAS via Allan-Werle-plots

“…Establishing a PC-based user interface enables remote operation via internet and reduces the required training time for airborne operators in aircraft measurement campaigns. Autonomous and remote operation has been demonstrated during the groundbased field experiment at the German Taunus Observatory in 2021 [23], requiring only maintenance and calibration gas source exchange as on-site activities.…”