N2O fluxes over a corn field from an open-path, laser-based eddy covariance system and static chambers

Abstract:

Nitrous oxide (N₂O) is important greenhouse and ozone-depleting gase. Although many efforts have been paid to N₂O emissions, the spatial and temporal variability of N₂O emissions still subject to large uncertainty. Application of the eddy covariance method for N₂O emissions research would allow continuous ecosystem level flux measurements. The caveat, however, is need for high precision and high frequency measurements in field. In this study, an open-path, quantum cascade-laser-based eddy covariance N₂O sensor has been deployed nearly continuously since May 2015 over a corn field at the W.K. Kellogg Biological Station site in SW Michigan. The field precision of the N₂O sensor was assessed to be 0.1 ppbv at 10 Hz, and the total consumption was ~ 40 W, allowing the system to be powered solely by solar panels. The stability of the sensor under different temperature and humidity was tested within an environmental chamber. Spectroscopic experiments and cospectra analyses were carried out to study specific corrections associated with the sensor for eddy covariance techniques, including the line broadening effect due to water vapor and high frequency flux attenuation owning to sample path averaging. Ogive analyses indicated that the high-frequency N₂O flux loss due to various damping effects was comparable to those of the CO₂ flux. The detection limit of flux was estimated to be 0.3 ng N s^-1 m^-2 with a flux averaging interval of 30 minutes. The results from the EC system were also compared with ground measurements by standard static chambers (SC). Overall, more than 150 individual chamber measurements were taken within the footprint of the EC system. We found good correlation between the EC and SC methods given the spatiotemporal differences between the two techniques (R² = 0.75). Both methods detected increased emissions during afternoon as compared to morning and night hours. Differences between EC and SC were also studied by investigating spatial variability with a footprint model. Our results indicate diurnality of ecosystem level N₂O emissions may have important consequences for both field and global scale budgets and highlight the need of more continuous measurements for future investigation.