Development and application of an eddy covariance system based on amplitude-modulated cavity-enhanced absorption spectroscopy for NO2 flux measurement in a wheat field

This is the authors' abstract. We don't add key points for this paper.

Long-term measurements of nitrogen dioxide (NO 2 ) flux are essential for improving understanding of the mechanisms underlying biosphere-atmosphere NO 2 exchange and elucidating its role in secondary pollutant formation. However, accurately quantifying NO 2 flux remains a challenge due to the absence of a robust fast response gas analyzer. In this work, we report the development of an eddy covariance (EC) NO 2 flux observation system based on amplitude-modulated cavity-enhanced absorption spectroscopy (AM-CEAS) technique, along with its application in a wheat field. Based on a NO 2 detection precision of 78 pptv (1 σ ) at 10 Hz, the NO 2 flux detection limit was estimated to be 4.65 µg N m −2 h −1 . An analysis of spectral damping showed median flux losses of 24.96 % in the high-frequency range and 0.37 % at low frequencies. The median random error was estimated at 39.64 %. During the observation period, the wheat field acted as a net source of atmospheric NO 2 , with a median NO 2 flux of 11.47 (−138.59 − 200.75) µg N m −2 h −1 . Relative dominance of fertilizer, soil temperature, and soil moisture in controlling NO 2 emissions shifted competitively with varying soil conditions. This indicated a key gap in current NOₓ estimates, where simplified driver constraints diverge from observed emissions. A strong dependence of NO 2 exchange rates on their atmospheric concentrations was observed, indicating the existence of a compensation point or a similar mechanism. The compensation point was further estimated to range from 4 to 5 ppbv. Based on the advantages of high sensitivity, and minimal maintenance of AM-CEAS, the flux observation system demonstrated good performance, and provided the potential for investigating NO 2 flux using the EC method across diverse temporal scales.