10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Chemical Composition Changes during Secondary Organic Aerosol Particle Evaporation
ANGELA BUCHHOLZ, Arttu Ylisirniö, Claudia Mohr, Celia Faiola, Eetu Kari, Andrew Lambe, Zijun Li, Aki Pajunoja, Sergey Nizkorodov, Siegfried Schobesberger, Douglas Worsnop, Taina Yli-Juuti, Annele Virtanen, University of Eastern Finland
Abstract Number: 815 Working Group: Aerosol Chemistry
Abstract The partitioning of compounds between gas and particle phase can be described with the Volatility Basis Set (VBS), which group them by their saturation vapor pressure. In previous studies, slower evaporation than expected from VBS distributions was observed for α-pinene particles at dry conditions. This could be caused by physical limitations or by chemical processes in the particle phase. It is therefore important to study the changes of the chemical composition of particles during evaporation to gain insights into the processes governing particle evaporation.
We investigated Secondary Organic Aerosol (SOA) from photooxidation of a-pinene with three different O:C ratios (0.55, 0.70, and 0.95). Monodisperse particles were fed into a Residence Time Chamber (RTC), which was at 0%, 40%, or 80% RH, to study evaporation times up to 10 h. The particle size was monitored and the chemical composition of the particles was studied with an Aerosol Mass Spectrometer and a Filter Inlet for Gases and AEROsols coupled with a Chemical Ionization Time-of-Flight Mass Spectrometer (FIGAERO-CIMS).
Particle evaporation was enhanced at higher RH with little change between 40% and 80%. Evaporation strongly depended on the initial particle composition, with lower volatility and a shift towards higher desorption temperatures in the FIGAERO thermograms (total ion count vs desorption temperature) for particles with higher O:C ratios.
Positive Matrix Factorization (PMF) was applied to the thermogram mass spectra data. The identified factors represent volatility classes which can be compared qualitatively to VBS distributions derived from other sources. Four to seven PMF factors were needed to reproduce the measured thermograms. Larger contributions of the low volatility classes were observed with increasing average O:C ratios. As expected from the thermograms, the residual particles after RTC evaporation contained a higher contribution of low volatility classes, but contrary to the expectation the average O:C ratio of the particles did not change with evaporation.
These results have important implications on our understanding of the fate of particles as they are transported and undergo further oxidation in the atmosphere. Higher oxidation levels and dry conditions make particles more resilient against evaporation and this need to be considered when predicting the climate effect of SOA with an equilibrium gas-particle partitioning model.