AbstractWildfires are a major source of gas‐phase ammonia (NH₃) to the atmosphere. Quantifying the evolution and fate of this NH₃ is important to understanding the formation of secondary aerosol in smoke and its accompanying effects on radiative balance and nitrogen deposition. Here, we use data from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) to add new empirical constraints on the e‐folding loss timescale of NH₃ and its relationship with particulate ammonium (pNH₄) within wildfire smoke plumes in the western U.S. during summer 2018. We show that the e‐folding loss timescale of NH₃ with respect to particle‐phase partitioning ranges from ∼24 to ∼4000 min (median of 55 min). Within these same plumes, oxidation of nitrogen oxides is observed concurrent with increases in the fraction of pNH₄ in each plume sampled, suggesting that formation of ammonium nitrate (NH₄NO₃) is likely. We find wide variability in how close our in situ measurements of NH₄NO₃ are to those expected in a dry thermodynamic equilibrium, and find that NH₄NO₃ is most likely to form in fresh, dense smoke plumes injected at higher altitudes and colder temperatures. In chemically older smoke we observe correlations between both the fraction of pNH₄ and the fraction of particulate nitrate (pNO₃) in the aerosol with temperature, providing additional evidence of the presence of NH₄NO₃ and the influence of injection height on gas‐particle partitioning of NH₃.