Effects of Sampling Rate on Estimates of Nitrous Oxide and other Trace Gas Fluxes using Eddy Covariance

Agricultural corn production systems have been identified as significant sources of anthropogenic nitrous oxide (N₂O) emissions. Nitrous oxide is recognized as a potent greenhouse gas with a warming potential nearly 300 times that of carbon dioxide over a 100-year time span. Due to its role as a powerful greenhouse gas and the potential impacts on environmental processes and ecosystem health, there is growing interest in monitoring the surface fluxes of N₂O from corn production systems using an eddy covariance approach. Despite recent technological advances, collecting the necessary high-frequency measurements of N₂O concentrations in synchrony with the wind velocity component measurements from the sonic anemometer can be challenging; the gas analyzer may not be capable of collecting the trace gas concentration data with sufficient speed or regularity to match those of the sonic anemometer when sampling at a nominal 10 Hz rate typical of eddy covariance. While sampling at a lower rate, viz. less than 10 Hz, may overcome these limitations, they can also lead to increased error and uncertainty in the flux measurements. Using a combination of simulations and analyses of high-frequency data collected over different surfaces, the error introduced when the sampling rate was reduced to 1, 2, or 5 Hz was characterized. Regardless of the scalar quantity, e.g., temperature, water vapor, and carbon dioxide, considered, it was found that the flux estimate decreased with frequency following a well-defined exponential decay relationship. The 1 Hz fluxes estimates were approximately 90% of the 10 Hz measurements. Provided the necessary high frequency (10 or 20 Hz) data is available, the well-defined relationships seen here indicate that the underestimate in the N2O or other trace gas flux could be accounted for by invoking Monin-Obukhov Similarity Theory. This provides a viable approach to determine these fluxes despite the limitations of the currently available gas analyzers.