The Importance of Ammonia and Nitrous Oxide in Receptor Models of Agriculturally Impacted Regions: A Case Study Using Airborne Observations in California’s Central Valley During AEROMMA

A positive matrix factorization (PMF) receptor model is one method for identifying and isolating regional emissions sources of nitrogen oxides (NO_x = NO + NO₂) associated with agricultural operations. However, source identification can be difficult if there are insufficient tracers to attribute source factor profiles. NO_x is often co-emitted with ammonia (NH₃) and nitrous oxide (N₂O) during select agricultural activities and management practices. Thus, the inclusion of these species in receptor models may significantly improve regional source factor identification and attribution. In this case study, a positive matrix factorization source apportionment model is used to identify and isolate emissions associated with agricultural operations in this region. We test differences in the output of a PMF source apportionment analysis when NH₃ and N₂O are individually excluded from the model input. California’s Central Valley provides an excellent region for this case study since it contains a mixture of agricultural and non-agricultural sources. The area is well-known as a hotspot for agricultural activities with emissions predominantly associated with livestock operations, croplands, and fertilizer production/application. There is also substantial influence from non-agricultural sources associated with interspersed urban centers and industry that are connected by major roadways. The valley is also surrounded by forests and grasslands. For this case study, we utilize airborne observation collected aboard the NASA DC-8 aircraft during the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign conducted in summer 2023. Measurements include NO_x, N₂O, NH₃, ammonium (NH₄⁺), methane (CH₄), ethane (C₂H₆), carbon monoxide (CO), a suite of speciated volatile organic compounds (VOCs), and aircraft state parameters. Observations are used from two flights conducted in California’s Central Valley during AEROMMA. The flight patterns during these campaigns typically included west-east transects in the mixed boundary layer between Sacramento and Bakersfield providing a broad regional sampling of the mixture of emissions sources in this area (e.g., highways, croplands, livestock operations, oil and gas, and urban areas). Results from PMF model runs with and without NH_x and N₂O in the input file will be compared with each other and to national and state emissions inventories.