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

A new, two-channel instrument for simultaneous NO(3) and N(2)O(5) monitoring was used to make the first comprehensive set of nocturnal NO(x) measurements (NO, NO(2), NO(3) and N(2)O(5)) at the Taunus Observatory, a rural mountain site (Kleiner Feldberg) in South-western Germany. In May 2008, NO(3) and N(2)O(5) 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 NO(3), N(2)O(5) and NO(2) were consistent with the equilibrium constant, K(2), defining the rates of formation and thermal dissociation of N(2)O(5). A steady-state lifetime analysis is consistent with the loss of nocturnal NO(x) being dominated by the reaction of NO(3) with volatile organic compounds in this forested region, with N(2)O(5) uptake to aerosols of secondary importance. Analysis of a limited dataset obtained at high relative humidity indicated that the loss of N(2)O(5) by reaction with water vapour is less efficient (> factor 3) than derived using laboratory kinetic data. The fraction of NO(x) present as NO(3) and N(2)O(5) reached similar to 20% on some nights, with night-time losses of NO(x) competing with daytime losses.