Development of open-path sensors for eddy covariance (EC) flux measurements – The EC method is the most direct way to measure surface-atmosphere fluxes continuously. However, existing instrumentation for EC fluxes for reactive nitrogen species is too power-hungry (~ kW) and cumbersome for widespread deployment in remote ecosystems. An open-path sensor, where the air is sampled passively as opposed to an inlet/sample cell, solves the power/size issues. An open-path design also mitigates sampling artifacts for sticky gases like NH3. Such an approach was previously argued to be impossible for N2O and NH3 flux measurements. In brief, the changes in concentrations from a nominal flux would be overwhelmed by density (ideal gas law) and spectroscopic effects from temperature fluctuations for an open-path design. We overcame this challenge by developing a N2O sensor such that the spectroscopic temperature effects of the absorption lines nearly canceled the temperature-driven density effect. In the 2nd Edition of Fundamentals of Boundary-Layer Meteorology, Xuhui Lee considered this work a significant advancement in laser spectroscopy that raised hope for N2O open-path flux measurements.
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