Collaborative Aerosol Research: Laboratory studies of the chemical and physical properties of atmospherically relevant secondary organic aerosol

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Description

Secondary Organic Aerosols (SOAs) are formed through oxidation of volatile organic compounds (VOCs) in the atmosphere and dominate the particle mass loadings over large portions of the globe. Recent ambient and laboratory observations show that biogenic and anthropogenic VOCs can rapidly oxidize into extremely low-volatility vapors (ELVOCs), which condense irreversibly onto pre-existing particles and, possibly, nucleate new particles. These ELVOCs enhance the growth of particles, driving new particles into the active size range for cloud condensation nuclei (CCN) and ice nuclei (IN). Furthermore, condensed-phase SOA particles have been observed to exist in liquid, semi-solid, and solid phases in the atmosphere. How well these organic-dominated particles may activate as CCN or IN depends upon their chemical compositions and phase states. Therefore, to understand how SOA influences climate through acting as CCN and IN, we must understand SOA particle growth mechanisms and particle phase states in the atmosphere. We are developing new techniques for studying the formation, chemical composition, and phase states of atmospherically relevant SOA. (1) In collaboration with Prof. Brune from Pennsylvania State University, we are continuing to develop and apply the Potential Aerosol Mass (PAM) oxidation flow tube reactor to study SOA formation. Specifically, we will describe results obtained with a new method for controlling NOx concentrations in the PAM under high oxidant (i.e., O3 and OH) concentrations to study anthropogenic-specific, NOx-dependent SOA formation pathways. (2) We are applying a new technique, broadband dielectric spectroscopy (BDS), to study the phase behavior, including measuring the glass transition temperatures, of individual organic compounds and PAM-generated atmospherically relevant SOA. Understanding the phase state of SOA-dominated particles during temperature changes in updrafts in the atmosphere will help provide insights into the CCN/IN activity of SOA. Finally, (3) we are characterizing the molecular-level chemical composition of the gas and particle phases of the SOA generated in the PAM using Chemical Ionization Mass Spectrometry (CIMS) techniques, with direct applications to similar measurements conducted in the ambient. We present recent experimental results for these three projects.