We developed a flux measurement system for the determination of biosphere-atmosphere exchange fluxes of PAN using both the hyperbolic relaxed eddy accumulation (HREA) method and the gradient method. The system consists of a modified, commercially available gas chromatograph with electron capture detection (GC-ECD, Meteorologie Consult GmbH, Germany). Sampling for the HREA method was realized by trapping PAN onto two pre-concentration columns, one for updraft and one for downdraft events. Subsequent analysis of PAN was made online by the GC-ECD located downstream of the splitter valves. The design ensured constant flow and pressure conditions in the inlet tubes. The wind vector was adjusted online using the double rotation method and special emphasis was given to the online determination of the lag time between the vertical wind speed signal and the splitter valves. High frequency O3 measurements were used as a proxy for calculating the hyperbolic dead band (H=1.1) and the b-coefficient (~0.2). The setup of the system additionally allowed the application of the gradient method by simultaneous sampling at two different inlet heights (0.8 m and 4 m a.g.l.) and subsequent online analysis of PAN.
The PAN flux measurements were conducted over the largest continuous nutrient-poor steppe-like grassland ecosystem in Rhine Hessen (region of Rhineland-Palatinate, Germany) in August and September 2011. Maximal daytime PAN deposition fluxes over the whole period were on average 0.1 nmol m-2 s-1. The minimum detectable flux (defined as 1&sigma of side-by-side measurements) was 0.04 nmol m-2 s-1 for the REA system and 0.02 nmol m-2 s-1 for the gradient system. Since the measured fluxes were comparatively small, a detailed quality assessment was required. We will present the performance and applicability of the HREA and the gradient method for PAN flux measurements under different environmental conditions.