Tsokankunku, A*; Zhu, Z; Meixner, FX; Trebs, I; Foken, T; Welling, M; Plake, D; Andreae, MO: Eddy covariance fluxes of the NO-NO2-O3 triad above a spruce forest canopy in south-eastern Germany
Vortrag, Atmospheric Transport and Chemistry in Forest Ecosystems, Thurnau: 05.10.2009 - 08.10.2009

We investigated the diel variability of measured eddy covariance fluxes of the NO-NO2-O3 triad above a spruce forest canopy at the "Weidenbrunnen" research site (Fichtelgebirge, Germany). Measurements were part of the EGER project (ExchanGE processes in mountainous Regions), which focuses on the role of process interactions among the different scales of soil, in-canopy and atmospheric exchange processes of reactive and non-reactive trace gases and energy. The eddy covariance platform was at the top of a 32 m high tower (50˚ 08’31” N, 11˚ 52’1”E, elevation 755 m.a.s.l). The eddy covariance system consisted of a CSAT3 sonic anemometer and a high speed, high resolution NO-NO2 two channel chemiluminescence analyzer (Ecophysics CLD 790 SR2). Two solid-state blue-light photolytic converters in series were connected to the NO2 channel of the analyzer just behind the sample inlet for specific conversion of NO2 to NO. Ambient air was sampled via 52 m long tubes with the instrument located in a temperature-controlled container at the ground. The NO-NO2 analyzer was operated at 5 Hz. A fast solid-phase chemiluminescence ozone analyzer (GFAS) was deployed to measure O3 eddy covariance fluxes. All trace gas inlets were situated at 32.5 m, 20 cm below the path of the sonic anemometer. The mixing ratio measured at 32m by an independent NO, NO2, and O3 profile system was used as reference for the fast ozone analyzer and the two channel NO-NO2 chemiluminescence analyzer. Preliminary results showed that NO and NO2 advection played a substantial role in the magnitude and direction of the fluxes at the site. The main source of the advection was a busy country road situated about 2 km west of the site. Extended periods of fluxes unaffected by advection of anthropogenic NOx usually occurred on Sundays when the amount of traffic was significantly lower. During the "golden days period" (29 June – 3 July 2008), NO and O3 fluxes were mainly downward (within the margin of error), and net emission of NO2 from the spruce forest canopy was observed. This is in contrast to what is typically found under ideal micrometeorological conditions such as short grassland. Due to decreased turbulence within the taller forest canopy O3 reacted with NO to form NO2, depleting NO in the canopy to such extent that the canopy became a sink for NO above the canopy. In this respect, the ratio of chemical reaction timescales to turbulent transport timescales plays an important role in determining whether the forest canopy is a source or sink of reactive gases. The critical Damköhler number was used to determine periods when the contribution of chemical production and loss to the net eddy covariance flux was high. It was found to be largest during night time and periods of stationarity. NO and NO2 fluxes ranged between +1.5 and -1.5 nmol m-2 s-1 and maximal O3 deposition fluxes were up to -25 nmol m-2 s-1 during daytime.

Letzte Änderung 03.11.2009