Gap-filling techniques for the annual sums of nitrous oxide fluxes

The full accounting of greenhouse gas emissions is only possible when the three key contributors (CO2, CH4 and N2O) are continuously measured throughout the observation period. While some field techniques such as eddy covariance or automatic closed chamber measurements have attempted near-continuous observations, it is always beyond the control of the experimentalist to ensure perfect continuity. Therefore, the final time series of fluxes will inevitably have periods without reliable values (i.e., gaps) and will need to be gap-filled. While there is abundant literature on methodologies for gap-filling CO2 fluxes, there is no literature on the gap-filling methods for trace gases such as nitrous oxide. We investigate three general approaches for gap-filling nitrous oxide time series: linear extrapolation, moving average and look-up tables. A five-year time-series of eddy-covariance measurements from an intensive grassland site in South-Western Ireland was used as an example. The amount of gaps varied significantly from year to year. The single-year look-up table technique produced consistently good results even when long gaps were present in the time series. In some years, the simpler annual extrapolation technique performed equally well. It is essential that this work be extended with more complex methods, and that methods in this paper are further evaluated under different environmental conditions. We believe that these methods and analysis could also be applied to other trace gas gap-filling (e.g., CH4) which have similar intermittent patterns of emission.