American Association for Aerosol Research - Abstract Submission

AAAR 39th Annual Conference
October 18 - October 22, 2021

Virtual Conference

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Phase Behaviour of Mixtures of Primary and Secondary Organic Aerosols

FABIAN MAHRT, Elli Newman, Yuanzhou Huang, Julia Zaks, Yiming Qin, Paul Ohno, Scot T. Martin, Markus Ammann, Allan Bertram, University of British Columbia

     Abstract Number: 23
     Working Group: Aerosols, Clouds and Climate

Abstract
Atmospheric aerosol particles play an important role for air quality and climate. Primary organic aerosol (POA) and secondary organic aerosol (SOA) make up a significant mass fraction of these particles. POA denotes aerosols that are directly emitted into the atmosphere. By contrast, SOA mostly forms from oxidation of precursor gases, followed by gas-particle conversion of the oxidation products with lower volatility. In order to describe SOA formation in atmospheric models, and predict their impact on air quality and climate, knowledge on the phase behaviour, i.e. the number and types of phases, in mixtures of POA and SOA is required. For instance, it is often assumed that SOA formation is enhanced in the presence of POA seed particles due to a lowering of the activities in the liquid organic aerosol phase in case of single-phase POA+SOA particles. The presence of POA will have a smaller effect on the formation of SOA mass in case of phase-separated particles.

Here, using optical and fluorescence microscopy, we observed the relative humidity dependent phase behaviour of individual particles containing mixtures of proxies of POA and SOA. Commercially available organic molecules were used as proxies for POA and SOA, covering a range of oxygen-to-carbon ratio between 0 and 1.0. For most of the mixtures investigated, phase-separated particles dominated, and the phase behaviour strongly depends on the oxygen-to-carbon ratio of the two mixed proxies. Results using more complex SOA material derived from environmental chamber studies confirm the dominance of phase separated POA+SOA particles. Our results have important implications for air pollution policies being considered to limit SOA formation in urban environments.