AS&T virtual collection: Aerosol particle physical chemistry

The physicochemical properties of aerosol particles, including particle pH, phase, viscosity, surface tension, and hygroscopicity, are critical to our understanding of aerosol dynamics and transformations. These particle properties influence key complex processes such as particle growth and phase transitions, heterogeneous and multiphase chemistry, gas-surface-bulk partitioning, aerosol optical properties, cloud droplet activation, and ice nucleation. Over the past years it has become evident that some barriers to our understanding of aerosol physical chemistry can only be addressed through new fundamental laboratory, theoretical, and modeling studies.

Aerosol physical chemistry is an important area of focus and interest in the aerosol community. In May 2022, we organized the 5th biennial conference entitled “A Molecular-Level Understanding of Atmospheric Aerosols” (MUOAA 2022), bringing together an international community of researchers interested in all aspects of the molecular chemistry and physics of aerosols. The conference provided a unique forum for the participants to exchange ideas on treating aerosol physical chemistry. Support from the National Science Foundation also made it particularly possible for early career aerosol scientists to attend. In MUOAA 2022, discussion points that crossed over many categories included aerosol pH, defining and understanding surfaces, limits of detections and biases for varied measurement techniques, appropriate model/surrogate chemical systems, and how to incorporate measurements into models. Similarly, in October 2022, the AAAR Special Symposium “Aerosol Physical Chemistry and Microphysics”, organized by Cari Dutcher as well as Sarah Petters (Aarhus University) and Miriam Freedman (Pennsylvania State University), sought to stimulate interdisciplinary discussion in fundamental aerosol physical chemistry and microphysical phenomena, bridging scales and using computational, experimental, and observational techniques.

This AS&T special issue was born out of these workshops, seeking to collect a series of papers from participants from MUOAA 2022 and beyond. In particular, motivated by the discussions at the 2022 workshops, we sought topics on (1) Particle surfaces: Defining and understanding surface properties coupled with advanced and emerging measurement and modeling methods, surfactants and surface rheology, bulk-surface-gas partitioning, reactions, and mass transfer, on (2) Particle phase: Particle hygroscopicity, viscosity, glassy systems, liquid-liquid equilibria, new particle formation, droplet nucleation, ice nucleation, and single particle methods and modeling, and on (3) Particle acidity: Aerosol pH, acidity and pKa, role of pH on aerosol processes and reactivity, and advancements in pH modeling and measurement methods. The nine papers highlighted in this special issue have addressed many of these complex topics.

Several papers in this issue explore how different physical properties of the aerosol can influence heterogeneous reaction rates involving atmospheric organics. These papers examine several environments for chemical reaction including particle surface vs the bulk, particle phase state, and particle pH. In AbouHaidar et al. combined classical and quantum methods were used to determine the rate of ozonolysis of aqueous maleic acid at both the surface and in the bulk of the particle, compared to the gas phase. Kaur Kohli et al. used single particle levitation and flow tube methods to explore ozonolysis as a function of phase state using oleic acid that forms a liquid droplet and its trans isomer elaidic acid that is found as either a supercooled liquid or a solid particle. In addition, Jansen et al. used photoacoustic and cavity ring down spectroscopy of aerosols, as well as UV-vis measurement of bulk solutions, to probe the pH dependence of brown carbon formation from mixtures of glyoxal, ammonia and ammonium salts. Each of the above studies found intriguing variations in chemical reactivity as a function of the physical and chemical environment and showcase new molecular level understanding of complex systems.

Experimental methods that seem rather straightforward can often have subtle effects that must be considered to ensure extracted data are reliable. Several papers in this issue employ a mix of experimental and theoretical approaches to better understand the limitations of their respective techniques and to examine what can be done to improve them. In Stollberger et al. the authors explore how the presence of a particle in a resonant reflective cavity changes the properties of that cavity, with experimental observations of single particles in agreement with theoretical results. In Rivera-Adorno et al. the authors used scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM) to estimate the viscosity of substrate deposited particles, establishing a practical correlation between height and total carbon adsorption. Finally, a paper by Schervish et al. investigated how viscosity, together with volatility and non-ideal liquid behavior, affect the timescale τ_mix required for two distinct aerosol populations to mix. The experimental and modeling techniques described in these papers, when applied to lab-generated or field collected aerosol samples, could inform observed and predicted aerosol mixing behavior, thereby improving large scale atmospheric models.

Papers in this issue also highlight some important applications that are strongly influenced by aerosol microphysics, from ice nucleation (IN) to cloud activation to bioaerosols. In House and Dutcher, microfluidic platforms were used to study phase transitions – both particle efflorescence and particle freezing – in Snomax® containing aqueous droplets of varied salinities. The divalent cation played a key role in changing both the particle crystallization and particle ice nucleation, though the residual particle morphology and IN activity were found to be decoupled. In Sengupta and Prisle, the role of aerosol pH due to dissociation of both organic acids and organic bases on cloud activation was explored using an aerosol chemistry climate box model ECHAM6.3-HAM2.3. The dissociation of organic acids resulted in significant changes in cloud droplet number concentration and in short-wave radiative effects in both clean and polluted environments. In Tian et al. the microphysical properties of surrogate exhaled aerosols containing species such as salts, artificial saliva and mucin were studied in a Comparative Kinetics Electrodynamic Balance, to provide new insights into aerosol physicochemical dynamics in exhalation and the role of mucin on water transport, phase transport, and viral infectivity loss. These studies highlight the importance of droplet composition, thermodynamics, and gas-particle partitioning on key applications in climate and health.

Cari Dutcher
Department of Mechanical Engineering, Department of Chemical Engineering and Materials Science, University of Minnesota, Twin Cities, Minneapolis, MN, USA
[email protected]
Margaret Tolbert
Department of Chemistry and CIRES, University of Colorado, Boulder, Colorado, USA
Barbara Wyslouzil
William G. Lowrie Department of Chemical and Biomolecular Engineering, Department of Chemistry and Biochemistry, Ohio State University, Columbus, Ohio, USA

Acknowledgments

We would like to thank the authors of this virtual special issue, and we hope that the collection stimulates continued discussion of these important topics in aerosol physical chemistry.

Disclosure statement

The authors declare there is no Conflict of Interest at this study.

Additional information

Funding

This material is based upon work supported by the National Science Foundation under Grant No. AGS-2021651. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.