Ammonium-Induced Stabilization of Imidazoles in the Particle Phase

MALSHA AMUGODA, James F. Davies, University of California, Riverside

     Abstract Number: 113
     Working Group: Aerosol Chemistry

Abstract
Imidazoles (IMs) are semi-volatile organic compounds (SVOCs) that are emitted into the atmosphere during biomass burning and form through aqueous phase reactions of carbonyl compounds with ammonium species. Recent studies have identified the presence of IMs at various atmospheric sampling sites. However, once these compounds are formed, few studies have reported the further atmospheric processing of IMs. The vapor pressure of pure SVOCs and the volatility of internally mixed SVOCs with inorganic salts can govern the gas-particle partitioning and influence the atmospheric processing of IMs.

In this work, we characterize the vapor pressure of IM, 4-methylimidazole (MeIM) and imidazole-2-carboxyaldehyde and explore the effect of co-solutes on the volatility and lifetime of the particles, using particles levitated in a linear-quadrupole electrodynamic balance coupled with Mie resonance spectroscopy. We estimate pure component liquid phase vapor pressures of these compounds through measurements of evaporation of binary particles at elevated humidity, under the assumption of an ideal solution. We then show that while these compounds evaporate rapidly from pure states, when present in internal mixtures with ammonium salts, they are significantly stabilized in the particle phase. In such ternary 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. At elevated RH, water contributes to the stabilizing effect, allowing particles to persist for much longer than expected based on pure component vapor pressures alone.

Interestingly, under dry humidity, we observe that the presence of ammonium chloride in MeIM particles with mole fraction ≤0.5, causes very slow evaporation of MeIM, resulting in about 48 hrs residence time of MeIM in particle, whereas pure MeIM particle evaporates in few minutes. Based on theoretical studies reported in the literature, ion-molecule interactions stabilize IM due to the formation of hydrogen bonds between the protons of ammonium with the lone pair of the nitrogen atom of IMs. Overall, this work highlights the importance of considering co-solutes and their stabilizing effect on semi-volatile components, with important implications for understanding and predicting the composition of biomass burning aerosol particles.