Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 11 2009 10.5194/bg-6-2575-2009 http://www.biogeosciences.net/6/2575/2009/ http://www.biogeosciences.net/6/2575/2009/bg-6-2575-2009.html http://www.biogeosciences.net/6/2575/2009/bg-6-2575-2009.pdf 2575 2588 2009-11-16 Inter-comparison of ammonia fluxes obtained using the Relaxed Eddy Accumulation technique A. Hensen hensen@ecn.nl E. Nemitz M. J. Flynn A. Blatter S. K. Jones L. L. Sørensen B. Hensen S. C. Pryor B. Jensen R. P. Otjes J. Cobussen B. Loubet J. W. Erisman M. W. Gallagher A. Neftel M. A. Sutton Energy research Centre of the Netherlands (ECN), Petten, The Netherlands Centre for Ecology and Hydrology (CEH), Edinburgh, UK University of Manchester, Centre for Atmospheric Science, Manchester, UK Agroscope Reckenholz-Tänikon Research Station ART, Zürich, Switzerland RISOE National Laboratory, Roskilde, Denmark National Environmental Research Institute, Roskilde, Denmark Indiana University, Bloomington, Indiana, USA Institut National de la Recherche Agronomique (INRA), Thiverval-Grignon, France The exchange of Ammonia (NH₃) between grassland and the atmosphere was determined using Relaxed Eddy Accumulation (REA) measurements. The use of REA is of special interest for NH₃, since the determination of fluxes at one height permits multiple systems to be deployed to quantify vertical flux divergence (either due to effects of chemical production or advection). During the Braunschweig integrated experiment four different continuous-sampling REA systems were operated during a period of about 10 days and were compared against a reference provided by independent application of the Aerodynamic Gradient Method (AGM). The experiment covered episodes before and after both cutting and fertilizing and provided a wide range of fluxes &minus;60–3600 ng NH₃ m^&minus;2 s^&minus;1 for testing the REA systems. The REA systems showed moderate to good correlation with the AGM estimates, with <i>r</i>² values for the linear regressions between 0.3 and 0.82. For the period immediately after fertilization, the REA systems showed average fluxes 20% to 70% lower than the reference. At periods with low fluxes REA and AGM can agree within a few %. Overall, the results show that the continuous REA technique can now be used to measure NH₃ surface exchange fluxes. 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