Apportioning Sources of Nitrogenous Emissions in California’s Central Valley (Invited Presentation)

Emissions of nitrogen oxides (NO_x = NO + NO₂), a precursor to ozone formation and secondary aerosol formation, have decreased across the U.S. However, the rate of decrease has slowed in the past decade, and recent studies suggest that NO_x emissions associated with agricultural activities, particularly cropland soils, are an increasingly important contribution to the slowing trend in NO_x reductions. California’s Central Valley is a hotspot for agricultural operations, predominantly associated with croplands and livestock operations, and thus nitrogenous emissions associated with agriculture are expected to be prevalent in this region. In this work, we leverage existing datasets from multiple airborne field campaigns conducted in California’s Central Valley to identify sources of reactive nitrogen species from agricultural activities and quantify contributions from various agricultural and non-agricultural sources to the NO_x observations in this region. Datasets include but are not limited to the 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign, the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign, and the 2010 California Research at the Nexus of Air Quality and Climate Change (CalNex) field campaign. The analysis utilizes a common suite of airborne measurements from these field campaigns including NO_x, nitrous oxide (N₂O), ammonia (NH₃), ammonium (NH₄⁺), methane (CH₄), ethane (C₂H₆), carbon monoxide (CO), a suite of speciated volatile organic compounds (VOCs), and aircraft state parameters. Tracer-tracer relationships and a positive matrix factorization (PMF) source apportionment model are used to identify and isolate emissions associated with agricultural operations in this region. 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 a mixture of emissions sources in this area (e.g., highways, croplands, livestock operations, oil and gas, and urban areas). Area-wide emissions ratios will be estimated from tracer-tracer relationships and source contributions to the NO_x observations will be quantified from source factor profiles resulting from the PMF analysis. Results will be compared with national and state emissions inventories and prior reports in the literature.