Ice nucleating particles are rare types of atmospheric aerosol particles that heterogeneously initiate the formation of ice crystals from supercooled liquid water droplets above –35 ºC where an ice nucleant is required to catalyze the freezing of water. Many types of atmospheric particles can nucleate ice, including biological macromolecules from bacteria and fungi, mineral dusts, and combustion-produced particles. The cloud glaciation induced by ice nucleating particles can greatly alter a cloud’s microphysical properties, change a cloud’s optical properties and radiative forcing, as well as alter the frequency and intensity of precipitation. The ice crystals quickly steal water from unfrozen droplets, growing rapidly so they precipitate out and often produce intense storms. However, there is a large uncertainty pertaining to which physicochemical properties of a particle contribute to its ability to nucleate ice, and the role that some particles emitted by wildfires play in introducing ice nucleants to the atmosphere. As ice nucleating particles are one-in-a-million particles in the atmosphere, there is also a need for improved methods and instruments for measuring these important but rare particles.

Our research focuses on coupling physical and chemical analysis techniques to connect ice nucleation activity to the properties of individual and aggregate particles. We focus on understanding how atmospheric chemical processing can alter a particle’s freezing activity through changing surface chemical features of ice nucleating particles. Our experiments on biomass-burning aerosol emitted by wildfires revealed the ice nucleants to largely be from new mineral phases produced by the combustion itself. Removal of organic carbon coatings that conceal these ice-active mineral surfaces through evaporation or oxidation then dissolution during atmospheric aging can enhance the freezing activity. These findings have resulted in an entirely new framework for considering the sources, evolution, and cloud effects of ice nucleating particles emitted from wildfires, and how they should be properly represented in atmospheric models to understand their climate impacts.

We have also developed a new framework to model and predict the heterogeneous ice nucleation properties of different types of particles, and can accurately predict the complex temperature freezing spectrum for varying mixtures of different particle types. This framework is also used to interpret our experimental results, and revealed an important unaccounted for effect of particle concentration that explains the persistent disagreement between methods that study large droplets containing many particles versus single-particle analysis techniques. To experimentally determine at what temperature a droplet freezes and the ice nucleation activity of the particle(s) it contains we use a trio of complementary techniques: 

1. A droplet-on-substrate assay to obtain freezing properties of a dilute sample of particles.‌

2. A novel microfluidics-based freezing assay to obtain high-resolution freezing temperature spectra from hundreds of nanoliter microdroplets over almost the entire mixed-phase cloud temperature regime down to –33 ºC.‌

3. A custom temperature-controlled aerosol optical tweezers system where supercooled droplets are levitated in a laser beam in a subzero environment to study the immersion freezing or contact freezing properties of particles added to the tweezed droplet. Particle physicochemical characteristics are determined using a variety of techniques, including Raman spectroscopy, scanning and transmission electron microscopies, X-ray diffraction, and laser-ablation single-particle mass spectrometry in conjunction with our ice nucleation experiments.‌

Selected Publications:

Brubaker, T.; Polen, M.; Cheng, P.; Ekambaram, V.; Somers, J.; Anna, S. L.; Sullivan, R. C. Development and Characterization of a “Store and Create” Microfluidic Device to Determine the Heterogeneous Freezing Properties of Ice Nucleating Particles. Aerosol Sci. Technol. 2020, 54 (1), 79–93.

Jahn, L. G.; Fahy, W. D.; Williams, D. B.; Sullivan, R. C. Role of Feldspar and Pyroxene Minerals in the Ice Nucleating Ability of Three Volcanic Ashes. ACS Earth Sp. Chem. 2019, 3 (4), 626–636.

Polen, M.; Brubaker, T.; Somers, J.; Sullivan, R. C. Cleaning up Our Water: Reducing Interferences from Nonhomogeneous Freezing of “Pure” Water in Droplet Freezing Assays of Ice-Nucleating Particles. Atmos. Meas. Tech. 2018, 11 (9), 5315–5334.

Beydoun, H.; Polen, M.; Sullivan, R. C. A New Multicomponent Heterogeneous Ice Nucleation Model and Its Application to Snomax Bacterial Particles and a Snomax–Illite Mineral Particle Mixture. Atmos. Chem. Phys. 2017, 17 (22), 13545–13557.

Polen, M.; Lawlis, E.; Sullivan, R. C. The Unstable Ice Nucleation Properties of Snomax® Bacterial Particles. J. Geophys. Res. Atmos. 2016, 121 (19), 11,666-11,678.

Beydoun, H.; Polen, M.; Sullivan, R. C. Effect of Particle Surface Area on Ice Active Site Densities Retrieved from Droplet Freezing Spectra. Atmos. Chem. Phys. 2016, 16 (20), 13359–13378.

The major type of combustion aerosol studied in the Sullivan group is biomass-burning aerosol representative of the emissions from wildfires and prescribed burns. Biomass burning is a significant source of aerosol particles and gases that influence our climate, air quality, and atmospheric chemistry, and as the climate warms, the frequency and severity of biomass-burning events are expected to increase. Combustion can also produce highly toxic chemicals, whose sources are still not well understood. In the CMU Air Quality Lab, we have the ability to burn authentic biomass fuels and study how their emissions evolve in an environmental chamber reactor.

Much of the Sullivan Groups’ research lies in determining how properties of the biomass-burning aerosol change as the aerosol is subject to atmospheric aging. We can perturb the biomass-burning aerosol using the chamber reactor to simulate atmospheric aging and other chemical reactions that occur in the atmosphere that influence particle composition, morphology, reaction products, and ice-nucleating activity. Recent projects include studying how the ice-nucleating activities of biomass-burning aerosol change as they undergo atmospheric aging, studying the formation of important trace reactants such as N2O5 and ClNO2 in biomass-burning plumes that influence the atmospheric oxidant budget and ozone production, and investigating changes in gas–particle reactivity as we chemically perturb the biomass-burning aerosol, using electron and x-ray microscopy to understand differences in chemical reactivity between individual particles that have very different composition in the complex highly heterogeneous aerosol.  

We are also interested in how combustion processes introduce persistent organic pollutants (POPs) to the environment, both as primary emissions and secondary pollutants formed through chemical transformations of the emissions. We study this question both in the context of biomass burning with a focus on the chemistry of halogens and nitrogen oxides, as well as studying the combustion of synthetic materials used in consumer products such as furniture foams. For these experiments, we combine offline and online mass spectrometry techniques to allow for molecular-level identification of compounds emitted during combustion and their transformation products. From this, we can elucidate mechanisms of chemical aging as well as understand how the composition of the combusted material gives rise to different emission profiles and chemical transformation pathways and products.

Selected Publications:

Goldberger, L. A.; Jahl, L. G.; Thornton, J. A.; Sullivan, R. C. N2O5 Reactive Uptake Kinetics and Chlorine Activation on Authentic Biomass-Burning Aerosol. Environ. Sci. Process. Impacts 2019, 21 (10), 1684–1698.

Ahern, A. T.; Robinson, E. S.; Tkacik, D. S.; Saleh, R.; Hatch, L. E.; Barsanti, K. C.; Stockwell, C. E.; Yokelson, R. J.; Presto, A. A.; Robinson, A. L.; Sullivan, R. C.; Donahue, N. M. Production of Secondary Organic Aerosol During Aging of Biomass Burning Smoke From Fresh Fuels and Its Relationship to VOC Precursors. J. Geophys. Res. Atmos. 2019, 124 (6), 3583–3606.

Ahern, A. T.; Goldberger, L.; Jahl, L.; Thornton, J.; Sullivan, R. C. Production of N2O5 and ClNO2 through Nocturnal Processing of Biomass-Burning Aerosol. Environ. Sci. Technol. 2018, 52 (2), 550–559.

Tkacik, D. S.; Robinson, E. S.; Ahern, A.; Saleh, R.; Stockwell, C.; Veres, P.; Simpson, I. J.; Meinardi, S.; Blake, D. R.; Yokelson, R. J.; Presto, A. A.; Sullivan, R. C.; Donahue, N. M.; Robinson, A. L. A Dual-Chamber Method for Quantifying the Effects of Atmospheric Perturbations on Secondary Organic Aerosol Formation from Biomass Burning Emissions. J. Geophys. Res. Atmos. 2017, 122 (11), 6043–6058.

Saleh, R.; Robinson, E. S.; Tkacik, D. S.; Ahern, A. T.; Liu, S.; Aiken, A. C.; Sullivan, R. C.; Presto, A. A.; Dubey, M. K.; Yokelson, R. J.; et al. Brownness of Organics in Aerosols from Biomass Burning Linked to Their Black Carbon Content. Nat. Geosci. 2014, 7 (9), 647–650.

The chemical and physical properties of aerosol droplets are a complex web of co-evolving feedbacks; a change in one property can cause a cascade of changes in anything from reactive gas uptake to acidity to droplet morphology and viscosity. Understanding these feedbacks provides critical information regarding the lifecycle of atmospheric particles and how they alter atmospheric chemistry and radical oxidant budgets, particulate matter formation, and their effects on radiation, clouds, and climate. The aerosol optical tweezers (AOT) enable us to study these chemical and physical transformations through the collection of real-time observations in a microdroplet’s Raman spectrum. The AOT levitates a droplet in a focused laser beam while simultaneously collecting a cavity-enhanced Raman spectrum that provides real-time characterization of the chemical and physical properties of the droplet. Analysis of the whispering gallery modes in the cavity-enhanced Raman spectrum that resonate with the droplet’s surface allows us to directly determine critical properties of biphasic particles and SOA systems that other techniques cannot.

We have advanced the analytical capabilities of the aerosol optical tweezers (AOT) technique such that we can now directly determine the phase-separated morphology, size, and acidity of both the core and shell phases of a complex biphasic levitated droplet. Our new analysis algorithm enables measurements of the properties of both the core and shell phases in liquid–liquid phase separated droplets. We can detect the formation of a new phase and its morphology, observe how the biphasic droplet evolves, and determine critical physicochemical properties of the two phases as reactions occur in or between these phases. Our ability to measure the pH of a picoliter droplet to ±0.03 addresses long-standing aerosol pH measurement needs to understand the critical role that acidity plays in chemical reactivity and phase separations of atmospheric particulate matter.

By performing the first AOT experiments on authentic secondary organic aerosol (SOA) produced through terpene oxidation directly in the custom tweezers chamber, we have produced important new constraints on the atmospheric evolution and impacts of SOA. The first framework to predict the evolution of aerosol morphology during atmospheric oxidation was developed using unique constraints the AOT provides on the interfacial tension of SOA. The core–shell morphology observed with SOA forming the core around both polar and nonpolar cores exists over a wide range of thermodynamic space. A stable emulsified state of SOA in an aqueous salt core was also discovered. These findings have important implications for multiphase chemistry and the cloud-nucleation ability of atmospheric aerosol. The multidimensional aerosol characterization we have achieved now enables novel experiments where we can directly observe the co-evolution of and feedbacks between aerosol morphology, acidity, and composition during hours of exposure to trace reactants, and the interplay between these properties that alter the propensity for ongoing multiphase chemistry  

Selected Publications:

Gorkowski, K.; Donahue, N. M.; Sullivan, R. C. Aerosol Optical Tweezers Constrain the Morphology Evolution of Liquid-Liquid Phase-Separated Atmospheric Particles. Chem 2020, 6 (1), 204–220.

Boyer, H. C.; Gorkowski, K.; Sullivan, R. C. In Situ PH Measurements of Individual Levitated Microdroplets Using Aerosol Optical Tweezers. Anal. Chem. 2020, 92 (1), 1089–1096.

Gorkowski, K.; Donahue, N. M.; Sullivan, R. C. Emerging Investigator Series: Determination of Biphasic Core–Shell Droplet Properties Using Aerosol Optical Tweezers. Environ. Sci. Process. Impacts 2018, 20 (11), 1512–1523.

Gorkowski, K.; Donahue, N. M.; Sullivan, R. C. Emulsified and Liquid–Liquid Phase-Separated States of α-Pinene Secondary Organic Aerosol Determined Using Aerosol Optical Tweezers. Environ. Sci. Technol. 2017, 51 (21), 12154–12163.

Gorkowski, K.; Beydoun, H.; Aboff, M.; Walker, J. S.; Reid, J. P.; Sullivan, R. C. Advanced Aerosol Optical Tweezers Chamber Design to Facilitate Phase-Separation and Equilibration Timescale Experiments on Complex Droplets. Aerosol Sci. Technol. 2016, 50 (12), 1327–1341.

The presence and persistence of synthetic contaminants in the environment, such as pharmaceuticals, pesticides, and endocrine disrupting compounds, can have serious deleterious effects on human health and the health of ecosystems worldwide. These “everyday, everywhere chemicals” often have harmful effects even at low doses, so it is imperative to develop and implement technology that can efficiently remove these compounds from water, air, and soil while avoiding the production of toxic disinfection byproducts. We also seek to understand new “emerging” environmental contaminants and their sources. We often collaborate with other research groups at Carnegie Mellon who have developed such technologies, both to contribute to a better understanding of the chemical mechanisms and kinetics involved in the degradation of synthetic chemicals, and to explore using their technologies in new applications. The major methods we use for these purposes are high-resolution mass spectrometry techniques and our home-built aerosol optical tweezers system. A major focus of the Institute for Green Science at CMU that we are part of is to use our chemistry knowledge and approaches to correct and prevent the environmental threats caused by unsafe and unsustainable use of synthetic chemicals.

The Sullivan group participates in select field campaigns where we provide our expertise performing comprehensive single-particle analysis using online mass spectrometry and offline microspectroscopy (Raman, T/SEM/EDX), along with ice nucleating particle analysis using microfluidics. Our techniques are applied to understand the sources, chemical reactivity and evolution, and resulting climate-relevant properties of atmospheric particulate matter. This has included aircraft-based campaigns such as the CalWater study of how local and long-range transported aerosols influence clouds, precipitation, and storms over the Sierra Nevada mountains in California. We have even visited wildlife refuges in the Southeastern US to collect authentic biomass fuel for laboratory experiments. We also participate in large collaborative experimental campaigns in facilities such as the Missoula Fire Sciences Laboratory to study biomass-burning aerosol during the FLAME-4 campaign, and the Center for Aerosol Impacts on Chemistry and Climate (CAICE) to explore the chemistry and effects of sea spray aerosol. Most recently in the fall of 2019, the Sullivan group participated in a pilot field campaign to perform the first closure between model predictions and measurements of ice nucleating particle concentrations in rural environments at the DOE ARM Southern Great Plains site in Oklahoma.

Selected Publications:

Levin, E. J. T.; DeMott, P. J.; Suski, K. J.; Boose, Y.; Hill, T. C. J.; McCluskey, C. S.; Schill, G. P.; Rocci, K.; Al‐Mashat, H.; Kristensen, L. J.; et al. Characteristics of Ice Nucleating Particles in and Around California Winter Storms. J. Geophys. Res. Atmos. 2019, 124 (21), 11530–11551.

Stockwell, C. E.; Yokelson, R. J.; Kreidenweis, S. M.; Robinson, A. L.; DeMott, P. J.; Sullivan, R. C.; Reardon, J.; Ryan, K. C.; Griffith, D. W. T.; Stevens, L. Trace Gas Emissions from Combustion of Peat, Crop Residue, Domestic Biofuels, Grasses, and Other Fuels: Configuration and Fourier Transform Infrared (FTIR) Component of the Fourth Fire Lab at Missoula Experiment (FLAME-4). Atmos. Chem. Phys. 2014, 14 (18), 9727–9754.

Rosenfeld, D.; Chemke, R.; DeMott, P.; Sullivan, R. C.; Rasmussen, R.; McDonough, F.; Comstock, J.; Schmid, B.; Tomlinson, J.; Jonsson, H.; et al. The Common Occurrence of Highly Supercooled Drizzle and Rain near the Coastal Regions of the Western United States. J. Geophys. Res. Atmos. 2013, 118 (17), 9819–9833.

Prather, K. A.; Bertram, T. H.; Grassian, V. H.; Deane, G. B.; Stokes, M. D.; Demott, P. J.; Aluwihare, L. I.; Palenik, B. P.; Azam, F.; Seinfeld, J. H.; et al. Bringing the Ocean into the Laboratory to Probe the Chemical Complexity of Sea Spray Aerosol. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (19), 7550–7555.

Creamean, J. M.; Suski, K. J.; Rosenfeld, D.; Cazorla, A.; Demott, P. J.; Sullivan, R. C.; White, A. B.; Ralph, F. M.; Minnis, P.; Comstock, J. M.; et al. Dust and Biological Aerosols from the Sahara and Asia Influence Precipitation in the Western U.S. Science 2013, 339 (6127), 1572–1578.

Prenni, A. J.; Demott, P. J.; Sullivan, A. P.; Sullivan, R. C.; Kreidenweis, S. M.; Rogers, D. C. Biomass Burning as a Potential Source for Atmospheric Ice Nuclei: Western Wildfires and Prescribed Burns. Geophys. Res. Lett. 2012, 39(11).