AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
The Effect of Hygroscopicity and Oxidation on the Phase State of Ambient SOA Particles in the Southeastern US
AKI PAJUNOJA, Weiwei Hu, Yu Jun Leong, Nathan Taylor, Pasi Miettinen, Don Collins, Jose-Luis Jimenez, Annele Virtanen, University of Eastern Finland
Abstract Number: 297 Working Group: Aerosol Physics
Abstract During the summer 2013 SOAS field campaign in a rural site in the Southeastern United States, a set of measurements including data from a hygroscopic tandem differential mobility analyzer (HTDMA), aerosol bounce instrument (ABI), and a high resolution time-of-flight aerosol mass spectrometer (AMS) were used to interpret an effect of hygroscopicity and composition on the phase state of the secondary organic aerosol (SOA) particles. In addition to sampling directly from ambient through a silica gel dryer, particles were sent through an OH-radical potential aerosol mass (PAM) flow reactor to simulate longer atmospheric aging times, up to two weeks.
Hygroscopicity and bounce behaviour of the SOA particles were found to have a clear relationship which agrees well with earlier laboratory studies; a bounce of less hygroscopic semisolid particles stays high even at high relative humidity (RH). Based on the HTDMA and AMS results, hygroscopicity parameter K was derived for organic fraction of the SOA particles assuming K-mixing rule for organic and inorganic fractions. At typical ambient relative humidities and temperatures, organic-dominated SOA particles stay mostly liquid in the atmospheric conditions in Southeastern US, but they often turn semisolid when dried below 50%RH in the sampling. While the liquid phase state suggests ideal solution behavior and instantaneous equilibrium partitioning for the SOA particles in the ambient, phase change in the drying process highlights the importance of thoroughly considered sampling techniques of SOA particles. Additional UV-oxidation of the SOA particles in the PAM chamber was found to increase the oxidation level and decrease the viscosity of the particles as seen earlier in laboratory studies.