10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Molecularly Resolved Atmospheric Aerosol Processes Studied in Single Levitated Particles Using Electrodynamic Balance Mass Spectrometry
ADAM BIRDSALL, John Hensley, Paige Kotowitz, Andrew Huisman, Frank Keutsch, Harvard University
Abstract Number: 860 Working Group: Aerosol Chemistry
Abstract A thorough representation of how atmospheric aerosol particle composition changes over their multiday lifetimes, and the consequent effects on climate and human health, requires a detailed fundamental understanding of the physicochemical system coupling the gas and condensed phases. For instance, individual compounds partition between the gas and condensed phase or can undergo particle-phase reactions.
Laboratory studies of aerosol particles have provided an opportunity to isolate systems of interest in a highly controlled environment and study them with specialized instrumentation. One approach in laboratory studies has been to analyze single levitated particles, largely with optical analytical techniques. We have extended the utility of this technique by coupling an electrodynamic balance, a method of suspending single charged particles (radius ~10 μm) in an electric field, to mass spectrometry. With this electrodynamic balance-mass spectrometry (EDB-MS) approach, we can understand with greater molecular specificity changes in particle composition under controlled laboratory conditions. Previous proof-of-concept work demonstrated the ability of our setup to quantitatively analyze the changing molecular composition of a model system.
Here we present the results of experiments that use EDB-MS to track the molecular evolution of individual aerosol particles, over timescales comparable to those of atmospheric particles. We have implemented instrumental modifications to improve our control of the environment surrounding the levitated particle, our particle sizing measurements, and our ionization source. With this refined setup, we have studied processes with atmospheric relevance.
First, we have measured the effect of interactions between organic and inorganic compounds on activity coefficients. The extent to which interactions between organic and inorganic compounds modulate particle-phase activities, and hence properties such as effective vapor pressures, remain poorly constrained. Activity coefficients can be extracted from EDB-MS data by fitting measured evaporation rates to a previously validated kinetic model of particle evaporation. Compared to levitated single particle experiments that track particle evaporation via change in particle radius, with EDB-MS we are able to use the mass spectra to monitor the evaporation of chemically speciated compounds within a multicomponent particle.
Second, we have studied nonradical, condensed-phase chemical transformations of organic compounds in single levitated particles. With EDB-MS, it is possible to identify with chemical detail the products of particle-phase chemistry as well as quantify the rates at which the chemistry takes place, as a function of parameters such as temperature and relative humidity. Compared to studies that take place in the bulk condensed phase, single-particle studies of chemical transformations represent a more realistic sample matrix in which the coupling of chemical and physical processes (e.g., partitioning between the gas and condensed phases) can be studied and highly concentrated conditions can be accessed.
The fundamental parameters extracted from these EDB-MS experiments (activity coefficients, chemical rates and equilibria) can be implemented in models to improve our representations of processes taking place in atmospheric aerosol particles.