Nocturnal nitrogen oxides at a rural mountain-site in south-western Germany

Abstract. A new, two-channel instrument for simultaneous NO₃ and N₂O₅ monitoring was used to make the first comprehensive set of nocturnal NO_x measurements (NO, NO₂, NO₃ and N₂O₅) at the Taunus Observatory, a rural mountain site (Kleiner Feldberg) in South-western Germany. In May 2008, NO₃ and N₂O₅ mixing ratios were well above the instrumental detection limit (a few ppt) on all nights of the campaign and were characterised by large variability. The concentrations of NO₃, N₂O₅ and NO₂ were consistent with the equilibrium constant, K₂, defining the rates of formation and thermal dissociation of N₂O₅. A steady-state lifetime analysis is consistent with the loss of nocturnal NO_x being dominated by the reaction of NO₃ with volatile organic compounds in this forested region, with N₂O₅ uptake to aerosols of secondary importance. Analysis of a limited dataset obtained at high relative humidity indicated that the loss of N₂O₅ by reaction with water vapour is less efficient (>factor 3) than derived using laboratory kinetic data. The fraction of NO_x present as NO₃ and N₂O₅ reached ~20% on some nights, with night-time losses of NO_x competing with daytime losses.