Open-path Dual-comb Spectroscopy in the 4.5 to 4.9 μm Region for Measurements of CO2, N2O, CO, and O3

Abstract:

The 4.5-5 µm spectral region can be used to measure a number of important atmospheric trace gases including the greenhouse gases CO₂ and N₂O and criteria air pollutants O₃ and CO. In addition, this spectral region is relatively clear of strong water absorption features, which allows for long open-path measurements. Existing sensing technologies typically only measure one or two species; however, multispecies detection can be very beneficial for understanding sources. Open-path Fourier-transform infrared spectroscopy (OP-FTIR) can measure multiple gases, but the low spectral resolution leads to potential biases and very long open paths are challenging due to the source divergence. Dual-comb spectroscopy (DCS) is an attractive solution for several reasons. Like FTIR, the broad spectral coverage of DCS allows for simultaneous quantification of many trace gas species as well as the path-averaged temperature. However, DCS has higher spectral resolution and negligible instrument lineshape compared to OP-FTIR instruments allowing for concentration measurements at higher precision. In addition, the combs emit a bright, single mode beam which can be propagated long distances, day or night, enabling observation of gas fluxes with high precision and over large areas.

Here, we demonstrate open-path DCS covering the 4.5-4.9 µm spectral region to measure H₂O, N₂O, CO₂, CO, and O₃ across 600-m and 2-km long paths. We present 5 days of measurements and compare the results with several in situ point sensors. Overall, we show excellent agreement between DCS data and co-located reference data with a mean offset of 1.8 ppb for CO and 3 ppb for N₂O. These offsets could arise from the spectral databases used to fit the DCS data. We also show reasonable agreement with an O₃ monitor located ~10 km away. In addition, we determine a ratio of excess CO to CO₂ of 5.7 ppb/ppm, which is significantly lower than that predicted from the EPA 2017 National Emissions Inventory. This indicates that CO sources are likely overestimated for this area. In the future, accurate open path measurements in this wavelength region will enable applications such as quantification of N₂O emissions from agricultural sources, monitoring of urban greenhouse gas emissions, tracking the influence of wildfires on urban air quality, and updating emissions inventories.