Constraining Biomass Burning δ ¹⁵ N(NH ₃ ) in the Western United States Using Field-Based Measurements

Biomass burning is a major global source of atmospheric ammonia (NH3), significantly influencing air quality, aerosol formation, and nitrogen cycling. Nitrogen isotope composition (δ15N) of NH3 has been proposed as a powerful tool for source apportionment, yet values for several emission sources remain poorly constrained. This study presents the first field-based δ15N of total reduced inorganic nitrogen (NHx = NH3 + pNH4) measurements from fresh and aged biomass-burning plumes, collected during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign in the western United States during summer 2019. The NHx concentrations were strongly correlated with carbon monoxide (CO) and fine particulate matter (PM2.5), reflecting elevated emissions during smoldering conditions. The δ15N(NHx) ranged from −9.1‰ to 2.1‰ (x̅ ± σ: −3.3 ± 2.9‰; n = 16). Using a Keeling plot approach, we derived a representative biomass-burning δ15N(NH3) value of −4.7 ± 1.3‰, that integrates measurements across the sampled biomass burning events, while also accounting for background NHx influences. This field-based isotopic signature is clearly distinct from agricultural and vehicular sources and substantially lower than the +12‰ value commonly assumed for biomass burning in δ15N-based source apportionment studies. Overall, this work improves our ability to track NH3 emissions using novel isotopic constraints.