Continuous Spatiotemporal Sensing of N2O through an Optical Web

Emissions of greenhouse gases from the agricultural sector vary in space and time, leading to hot spots and hot moments with large variability between farms. Large hot spots can exhibit enhancements of only a few ppbv above background and last on time scales of hours to days. For constraining N₂O emissions and developing reduction strategies, detailed source characterization on emissions (impacts of fertilization type and timing, agricultural practices, soil conditions, etc.) is required. To this end, we developed and deployed in a soybean field a laser tomographic imaging system for N₂O mapping and associated emissions quantification. A pair of continuous wave quantum cascade lasers scan across a field to an array of inexpensive mid-IR reflectors lining its perimeter, casting an optical web over an agricultural field. Each laser is tower mounted with a gimbal to aim the beams at various retroreflectors spread around the perimeter of the agricultural field. Each scan takes ~37 minutes and operates autonomously. Multiple retroreflectors are needed to create the high-resolution optical web, but traditional retroreflector cubic prisms are too bulky, expensive, and delicate for such field use. Consequently, we developed custom, thin (4 mm thick) plastic retroreflectors for the mid-IR with reflectivities reaching ~86% which are broadband across the entire infrared and perform better than traditional corner cubes. FPGA electronics ensure a low power (25 W/tower) system for remote field use. In the deployed configuration, 32 path-integrated overlapping measurements from two separate laser towers are combined for full mapping of N₂O over the field through a computed tomographic reconstruction. A Monte Carlo approach is used for inversion modeling to locate the plume location to within two meters and estimate the emission rate to within 10% on acre sized fields. The same techniques and tools developed can readily be adapted to other gases like methane and ammonia for other applications in agricultural and industrial settings.