Correcting high-frequency losses of reactive nitrogen flux measurements

Abstract

Abstract. The eddy-covariance (EC) technique is nowadays widely used in experimental field studies to measure land surface-atmosphere exchange of a variety of trace gases. In recent years applying the EC technique to reactive nitrogen compounds has become more important since atmospheric nitrogen deposition influences the productivity and biodiversity of (semi-)natural ecosystems and its carbon dioxide (CO2) exchange. Fluxes which are calculated by EC have to be corrected for setup-specific effects like attenuation in the high-frequency range. However, common methods for correcting such flux losses are mainly optimized for inert greenhouse gases like CO2 and methane or water vapor. In this study, we applied a selection of correction methods to measurements of total reactive nitrogen (ΣNr) conducted in different ecosystems using the Total Reactive Atmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescence dectector (CLD). Average flux losses calculated by methods using measured cospectra and ogives were about 26–38 % for a semi-natural peatland and about 16–22 % for a mixed forest. The investigation of the different methods showed that damping factors calculated with measured heat and gas flux cospectra using an empirical spectral transfer function were most reliable. Flux losses of ΣNr with this method were on the upper end of the median damping range, i.e. 38 % for the peatland site and 22 % for the forest site. Using modified Kaimal cospectra for damping estimation worked well for the forest site, but underestimates damping for the peatland site by about 12 %. Correction factors of methods based on power spectra or on site-specific and instrumental parameters were mostly less than 10 %. Power spectra of ΣNr were heavily affected likely by white noise and deviated substantially at lower frequencies from the temperature (power) spectrum. Our study suggests using an empirical method for estimating flux losses of ΣNr or any reactive nitrogen compound and locally measured cospectra.