Breakout Summary Report

ARM/ASR User and PI Meeting

10 - 13 June 2019

Session Title: Phase State of Organic Aerosol and Its Impact on Gas-Particle Interactions and Cloud Formation
Session Date: 13 June 2019
Session Time: 10:30 AM - 12:30 PM
Number of Attendees: 30
Summary Authors: Manabu Shiraiwa, Daniel Knopf

Breakout Description

The particle phase state of organic aerosol particles can vary among liquid, amorphous semisolid, and glassy solid states depending on chemical composition, water content, relative humidity, and temperature. The occurrence of glassy and amorphous (semi-)solid states can pose kinetic limitations to mass transport, affecting various gas-particle interactions including chemical transformation, secondary organic aerosol (SOA) formation and partitioning, hygroscopic growth, and cloud activation. It challenges the treatment of condensed-phase organics in aerosol and cloud models. There is still substantial lack of understanding of the role and impact of phase state on gas-particle interactions. The goals of this breakout session were to identify gaps in our knowledge and to continue to foster collaborative research within the DOE ASR/ARM community (and beyond).

Main Discussion

We had nine presentations as given below:

1. Manabu Shiraiwa: "Impacts of phase state on gas-particle interactions"


2. Daniel Knopf: "The role of the phase state of organic aerosol in cold cloud formation."


3. Yue Zhang: "Examining the Effects of Aerosol Phase State on Aerosol Processes and Cloud Interactions: from Laboratory Studies to Modeling and Field Measurements"


4. Gourihar Kulkarni: "Implications of phase state towards ice nucleation at cirrus temperatures"


5. Markus Petters: "How well do we understand the phase state of secondary organic aerosols?"


6. Manish Shrivastava: "Synergistic interactions between PAHs and SOA particles"


7. Alla Zelenyuk: "The effect of the SOA phase on particle morphology and on reactive and non-reactive mass transfer processes"


8. Rahul Zaveri: "Growth kinetics of secondary organic aerosol as a function of relative humidity"


9. Joel Thornton: "Constraints on SOA viscosity from molecular composition observations during room temperature evaporation experiments"

Key Findings

All participants agreed that particle phase state is important for improved description of climate via cloud formation and aerosol processes. Prediction of ice formation by organic aerosols necessitates knowledge about their phase state or viscosity, as glassy SOA particle can nucleate ice and phase state affects onset temperatures and relative humidity of heterogeneous ice nucleation and pathways of ice nucleation (e.g., homogeneous versus heterogeneous; deposition versus immersion freezing). Glassy particles may promote multigenerational oxidation of particle surfaces, leading to an increase of hygroscopicity and thus may impact concentrations of cloud condensation nuclei (CCN). Glass transition temperatures depend on cooling rates, which impacts the altitude at which glass transition occurs and subsequent cloud formation. Recent field measurements suggest that biogenic SOA may increase the number of ice nuclei. New laboratory experiments show that OA morphology (e.g., porous structure) would promote heterogeneous ice nucleation. New experimental techniques have emerged to measure viscosity and glass transition temperatures of organic compounds. A method has been developed to predict viscosity based on elemental composition of organic compounds. Viscosity is shown to have an inverse correlation with volatility and empirical methods have been developed to relate these two quantities. Phase state drives heterogeneous and multiphase chemistry, including OH, NO3, NH3, and IEPOX uptake as well as ozonolysis of polycyclic aromatic hydrocarbons (PAHs). Formation of surface crusts (i.e., viscous layer) is found to control heterogeneous ozonolysis and also affects particle evaporation kinetics. Trapping and shielding of PAHs by solid organic coatings can lead to long-range transport of PAHs, significantly enhancing their chemical lifetimes. During the GoAmazon campaign, the evolution of particle size distribution and particle growth was only reproduced with semisolid states of organic particles.

Issues

We identified three issues:

1. Discrepancy in laboratory viscosity measurements


2. Limited field observations


3. Little efforts on implementing particle phase state in large-scale models

For point 1, thanks to several emerging new laboratory techniques, viscosity of SOA has been measured or inferred by kinetic experiments, but not all experiments are yielding consistent results. This may depend on SOA formation conditions (concentration or particle loadings) or measurement methods and there are also some contradictions regarding viscosity and kinetic limitations associated with SOA formation and partitioning. Heterogeneous and multiphase kinetics under low temperatures (where particles are most likely glassy) are largely unexplored.

Regarding point 2, there are only a few phase state observations available from field campaigns, mostly through bounce measurements. Geographical, seasonal, and diurnal variations of particle phase state are mostly unknown. Variations within convective-eddy, boundar- layer, and drying-wetting processes in the real atmosphere are also unexplored.

For point 3, most of current large-scale atmospheric models do not resolve phase state and its effects on aerosol processes and cloud formation, except a few case studies. While laboratory measurements and experiments are ahead, there are gaps between lab measurements and field observations, preventing implementation in large-scale models.

Needs

There are strong needs to look for field evidence of particle phase state effects on aerosol processes and cloud formation in ambient measurements that would allow a connection between laboratory measurements, model predictions, and field observations. A particle phase state-targeted field campaign could yield novel insights. For this, it is necessary to develop or translate laboratory viscosity measurement techniques into the field. Though viewed with criticism, particle bounce measurements provide qualitative phase information in the most cost-effective way and have been already deployed in the field. Development of models that describe and treat phase state effects on aerosol processes and cloud formation are crucial to assess the importance of phase state for climate prediction. Models need to be evaluated against field observations.