Enhancements in Ammonia and Methane from Agricultural Sources in the Northeastern Colorado Front Range Using Observations from a Small Research Aircraft

Quantifying ammonia (NH3) to methane (CH4) enhancement ratios from agricultural sources is important for understanding air pollution and nitrogen deposition. The northeastern Colorado Front Range is home to concentrated animal feeding operations (CAFOs) that produce large emissions of NH3 and CH4. Isolating enhancements of NH3 and CH4 in this region due to agriculture is complicated because CAFOs are often located within regions of oil and natural gas (O&NG) extraction that are a major source of CH4 and other alkanes. Here, we utilize a small research aircraft to collect in situ 1 Hz measurements of gas-phase NH3, CH4, and ethane (C2H6) downwind of CAFOs during three flights conducted in November 2019. Enhancements in NH3 and CH4 are distinguishable up to 10 km downwind of CAFOs with the most concentrated portions of the plumes typically below 0.25 km AGL. We demonstrate that NH3 and C2H6 can be jointly used to separate near-source enhancements in CH4 from agriculture and O&NG. Molar enhancement ratios of NH3 to CH4 are quantified for individual CAFOs in this region, and they range from 0.8 to 2.7 ppbv ppbv(-1). A multivariate regression model produces enhancement ratios and quantitative regional source contributions that are consistent with prior studies.

Automated summary

Quantifying the enhancement ratios of ammonia (NH3) to methane (CH4) from agricultural sources is crucial for understanding air pollution and nitrogen deposition. A study conducted in northeastern Colorado Front Range reveals that concentrated animal feeding operations (CAFOs) in the region emit significant amounts of NH3 and CH4, which is further complicated by the presence of oil and natural gas extraction activities. Utilizing measurements from a research aircraft, the study demonstrates that NH3 and ethane (C2H6) can be used together to distinguish between near-source enhancements of CH4 from agriculture and oil and gas activities.