Eddy covariance flux measurements of reactive nitrogen compounds using a chemiluminescence analyser with different converter types

For ecosystems, the availability of (reactive) nitrogen is a key limiting factors for the productivity and the competition success of individual species. On the other hand, certain nitrogenous compounds can also be emitted from natural or managed ecosystems. Thus the quantification of the reactive nitrogen (N_r) exchange can be essential for the interpretation of ecosystem behavior. For the observation of N_r dry deposition and emission the eddy covariance (EC) method is preferable since it does not modify the environmental conditions of the ecosystem, is less prone to wall effects than chamber methods, and is less affected by gas phase chemical reactions than gradient methods. Since the various Nr compounds can undergo fast chemical reactions and have differing chemical and physical characteristics, a variety of detection techniques is usually necessary that often cannot meet the fast response requirements of the EC technique.

Here we show applications of a fast response chemiluminescence detector for NO suitable for EC measurements. In combination with different inlet converters (photolytic converter, gold catalyst converter, and high-temperature steel converter), the system could alternatively be used for flux measurements of NO₂, NO_y, and total N_r. The quantification of NO₂ (or NO_X) fluxes required a 2-channel analyzer for simultaneous detection of the unmodified NO signal, since the photolytic converter showed a limited conversion efficiency. By combining the 2-channel analyzer with the NO_y and total N_r converter simultaneously, the NH₃ flux could be determined from the difference between the two channels. Concentration and flux measurements of the system were verified by inter-comparison with other methods. Potential problems include the damping of high-frequency fluctuations in the inlet system. It is therefore important to place the converter close to the sampling inlet and to quantify and correct the damping effects. Moreover, like most other flux measurement techniques, the EC system is susceptible to non-stationary trace gas concentrations that often occur near pollution sources.