Application of the Dual Tandem DMA Method to Study the Amorphous Phase Transition of Organic Aerosols

 

Authors

Markus D Petters — North Carolina State University
Nicholas Rothfuss — NCSU
Sabin Kasparoglu — North Carolina State University
Wyatt Champion — NC State University
Andrew Patrick Grieshop — North Carolina State University
Paul J Ziemann — University of Colorado
Sonia Kreidenweis — Colorado State University
Paul J. DeMott — Colorado State University

Category

Secondary organic aerosol

Description

Atmospheric aerosols can exist in amorphous semi-solid or glassy phase states whose viscosity varies with atmospheric temperature and relative humidity. The particle phase state has important implications for intraparticle mixing time, chemical reactivity, and deposition mode ice nucleation in the upper free troposphere. One method to study the viscosity is to prepare coagulated dimer particles using the Dual Tandem DMA (DTDMA) technique, followed by probing the thermodynamic states that induce the dimer particles to relax into spheres. Here we present new results involving this method. First, we will show new experimental data characterizing the output DTDMA size distribution and the physical processes underlying its apparent modes. Key experimental limitations for both general applications and for viscosity measurements are identified. The primary consideration is the production of an adequate number of particle dimers, which typically requires high mobility‐selected number concentration. Results from the characterization experiments shed light on design considerations for general applications as well as characterization of viscous aerosol phase transitions. Second, we will show new experimental data characterizing the relationship between volatility and viscosity of terpene-derived secondary organic aerosol from flow reactor experiments. We show that there is a robust inverse relationship between viscosity at 30 °C derived from DTDMA measurements and the effective saturation concentration derived from thermodenuder measurements. We also confirm that there is an inverse relationship between viscosity and oxidation state derived from aerosol mass spectrometry measurements that has been reported previously. These results suggest that a unified parameterization of viscosity and volatility based on molecular properties will be possible for complex organic mixtures. Finally, we will show new experimental data extending the experimental characterization of temperature and humidity dependent viscosity toward upper-free tropospheric temperatures using the DTDMA technique. These data are needed to elucidate the conditions and mechanisms of ice nucleation of viscous and glassy particles.