10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Gas-particle Partitioning and Temperature: Competition Between Vapor Pressure Dependence and Phase-state
CHEN LE, Weihan Peng, Mary Kacarab, David R. Cocker III, University of California, Riverside
Abstract Number: 1510 Working Group: Aerosol Chemistry
Abstract Secondary Organic Aerosol(SOA) formation potentials from Volatile Organic Compounds(VOC) precursors are often described assuming that partitioning species are at thermodynamic equilibrium as is the case in the classic two-product SOA partitioning model and in the volatility basis-set model. Temperature dependence on SOA formation following these models can then be estimated using the Clausius-Clapeyron vapor-pressure temperature dependence relationship and an assumed enthalpy for the condensable species. Describing gas-particle partitioning using this model assumes: 1) temperature change in the system does not alter the composition in the system; 2) that equilibrium within the system is rapidly achieved. Therefore, it should be expected that the temperature of the system will determine gas-particle partitioning equilibrium regardless of the temperature that oxidation occurred at.
Observations in the CE-CERT environmental chamber over the past decade demonstrate: 1) far more aerosol is formed in systems operated at 5ºC and then warmed to 25ºC than in systems operated at 40ºC and cooled to 25ºC, even after accounting for chamber wall effects; 2) gas-particle partitioning cycling between temperatures for a single experiment regardless of initial temperature, however, is reversible and can be described applying an either empirical or semi-empirical enthalpy of vaporization.
This study discusses these apparent observational anomalies. The effect of aerosol-phase state and viscosity is evaluated using the dual-differential mobility analyzer technique described by Rothfuss and Petters (2016) in the dual-Teflon indoor chamber at CE-CERT. The technique induces coagulation between two oppositely charged particles and relates viscosity to the relaxation time for the dimer to return to a spherical shape. The effect of phase-state along with chemical kinetics at the different experimental temperatures are used to explain the apparent non-equilibrium temperature effects observed. Strong relaxation time temperature dependencies were observed during environmental chamber temperature shifts that suggested that mass-transfer related kinetic effects were responsible for the irreversibility observed between gas-particle partitioning of SOA formed at 5ºC and 40ºC. This work focuses on observations of temperature effects from the α-pinene dark ozonolysis and m-xylene – NOx photooxidation system including gas-particle partitioning and changes to chemical composition.