Ammonium-Induced Stabilization of Imidazoles in Aerosol Particles

The chemical evolution of biomass burning aerosol occurs through reactive and nonreactive pathways, with both involving the partitioning of semivolatile organic compounds (SVOCs) between the gas and particle phase. Here, we explore the vapor pressure of imidazoles, a class of compounds characterized by an aromatic N-containing five-membered ring and commonly found in atmospheric particles. We estimate liquid phase vapor pressures of these compounds to be greater than 0.2 Pa, indicating that these compounds are highly volatile SVOCs. Despite this, ambient measurements identified imidazoles in the particle phase. In this work, we show that when imidazoles are internally mixed with certain inorganic salts, they are stabilized in the particle phase. In these mixed particles, we measure two distinct phases of evaporation, characterized by fast and slow changes. We analyze these regions separately, allowing the evolving composition of the particle to be determined from an evaporation model and identifying the characteristic composition at which stabilization occurs. Based on these observations, further supported by water uptake behavior and optical properties, we determine that the stabilization is driven by ammonium depletion due to protonation of the imidazole and evaporation of ammonia. This work highlights the importance of considering cosolutes and their stabilizing effect on SVOCs, with important implications for understanding and predicting the composition of biomass burning aerosol particles.