. Measurement capabilities of the CU SOF Instrument: Separation of Methane Emissions from Agricultural and Natural Gas Sources & Developing Techniques to Quantify Wildfire Emissions

Abstract
This proof-of-concept study demonstrates that methane (CH4) emissions from natural gas (NG) and agriculture can be disentangled using the concept of excess column observations. The University of Colorado Solar Occultation Flux (CU SOF) instrument and three COCCON (Collaborative Carbon Column Observing Network) instruments were set up across Colorado's Front Range. This network of cost-effective sensors measured excess column-averaged dry-air mole fractions for CH4 (dXCH4; measured by COCCON), ethane (dXC2H6 as NG tracer; measured by CU SOF), and ammonia (dXNH3 from agriculture; measured by CU SOF) in the Denver-Julesburg Basin during March 2015. dXCH4 varied up to 17 ppb, and was >3 times higher with winds from directions where NG is produced. The dXCH4 variance is explained by variations in the C2H6-NH3 tracer pair, attributing 63+-17% to NG, 25+-10% to agriculture, and 12+-12% to other sources. The ratios dXC2H6/dXCH4 (16+-2%; indicates wet NG), and dXNH3/dXCH4 (43+-12%) were compatible with in-situ measured ratios. Excess columns are independent of boundary layer height, characterize gases in the open atmosphere, are inherently calibrated, average over extended spatial scales, and provide a complementary perspective to quantify and attribute CH4 emissions on regional scales. An airborne version of the CU SOF instrument was deployed on research aircraft to quantify wildfire emissions during the 2018 wildfire season in the Pacific Northwest (BB-Flux project). We show that column measurements can be used to quantify emission fluxes from wildfires as well.