Nitrous Oxide Emission Flux Measurements for Ecological Systems with an Open-Path Quantum Cascade Laser-Based Sensor

The ambient concentration of nitrous oxide (N₂O), the fourth most abundant greenhouse gas, is rapidly increasing with emissions from both natural and anthropogenic sources [1]. Soil and aquatic areas are important sources and sinks for N₂O due to complicated biogenic processes. However, N₂O emissions are poorly constrained in space and time, despite its importance to global climate change and ozone depletion. We report our recent N₂O emission measurements with an open-path quantum cascade laser (QCL)-based sensor for ecological systems. The newly emergent QCLs have been used to build compact, sensitive trace gas sensors in the mid-IR spectral region. A compact open-path QCL based sensor was developed to detect atmospheric N2O and CO at ~ 4.5 μm using wavelength modulation spectroscopy (WMS) to achieve a sensitivity of 0.26 ppbv of N2O and 0.24 ppbv of CO in 1 swith a power consumption of ~50 W [2]. This portable sensor system has been used to perform N2O emission flux measurement both with a static flux chamber and on an eddy covariance (EC) flux tower. In the flux chamber measurements, custom chambers were used to host the laser sensor, while gas samples for gas chromatograph (GC) were collected at the same time in the same chamber for validation and comparison. Different soil treatments have been applied in different chambers to study the relationship between N2O emission and the amount of fertilizer(and water) addition. Measurements from two methods agreed with each other (95% or higher confidence interval) for emission flux results, while laser sensor gave measurements with a much high temporalre solution. We have also performed the first open-path eddy covarianceN2O flux measurement at Kellogg research station, Michigan State University for a month in June, 2012. Our sensor was placed on a 4-metertower in a corn field and powered by batteries (connected with solar panels). We have observed the diurnal cycle of N2O flux. During this deployment, an inter-comparison between our sensor and a commercial gas sensor was done to check the sensor's performance. Overall, our sensor showed a good performance with both static chamber measurement and EC flux measurement of N2O. Its open-path, compact and portable design with low power consumption provides lots of advantages for N2O emission flux measurement in the ecological systems. [1] S. A. Montzka, E. J. Dlugokencky, and J. H. Butler, "Non-CO2 greenhouse gases and climatechange," Nature 476, 43-50 (2011). [2] L. Tao, K, Sun, D. J. Miller, M.A. Khan and M.A. Zondlo, "Optimizations for simultaneous detection of atmospheric N2O and CO with a quantum cascade laser," CLEO, 2012