Comment on egusphere-2022-968

Abstract

Agriculture emissions, including those from cattle and dairy concentrated animal feeding operations (CAFOs), make up a large portion of the United States’ total greenhouse gas emissions. However, many CAFOs reside in areas where methane (CH4) from oil and natural gas complicates the quantification of CAFO emissions. Traditional approaches to quantify emissions in such regions often relied on inventory subtraction of other known sources. We compare the results of two approaches to attribute a CAFO CH4 emission rate from an aircraft mass-balance derived CH4 emission rate. These methods make use of the CH4, ethane (C2H6) and ammonia (NH3) mixing ratio data collected simultaneously in-flight downwind of CAFOs in northeastern Colorado. The first approach, subtraction method, is similar to inventory subtraction except the amount to be removed is derived from the observed C2H6 to CH4 ratio rather than an inventory estimate. The results from this approach showed high uncertainty, primarily due to how error propagates through subtraction. Alternatively, multivariate regression (MVR) can be used to estimate CAFO CH4 emissions using the NH3 emission rate and an NH3 to CH4 ratio. These results showed significantly less uncertainty. We identified criteria to determine the best attribution method; these criteria can support attribution in other regions. The final emissions estimates for the CAFO presented here were 23 (±5) g CH4 head-1 hr-1 and 22 (±4) g NH3 head-1 hr-1. These estimates are significantly higher than the US EPA inventory and previous studies highlighting the need for more measurements of CH4 and NH3 emission rates.