Articles | Volume 16, issue 17
Atmos. Chem. Phys., 16, 11163–11176, 2016
Atmos. Chem. Phys., 16, 11163–11176, 2016

Research article 09 Sep 2016

Research article | 09 Sep 2016

Phase state of ambient aerosol linked with water uptake and chemical aging in the southeastern US

Aki Pajunoja1, Weiwei Hu2,3, Yu J. Leong1,4, Nathan F. Taylor5, Pasi Miettinen1, Brett B. Palm2,3, Santtu Mikkonen1, Don R. Collins5, Jose L. Jimenez2,3, and Annele Virtanen1 Aki Pajunoja et al.
  • 1Department of Applied Physics, University of Eastern Finland, Kuopio Campus, P.O. Box 1627, 70211 Kuopio, Finland
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
  • 4Department of Civil and Environmental Engineering, Rice University, 6100 Main St MS-519, Houston, TX, USA
  • 5Department of Atmospheric Sciences, Texas A&M University, College Station, TX, USA

Abstract. During the summer 2013 Southern Aerosol and Oxidant Study (SOAS) field campaign in a rural site in the southeastern United States, the effect of hygroscopicity and composition on the phase state of atmospheric aerosol particles dominated by the organic fraction was studied. The analysis is based on hygroscopicity measurements by a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA), physical phase state investigations by an Aerosol Bounce Instrument (ABI) and composition measurements using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). To study the effect of atmospheric aging on these properties, an OH-radical oxidation flow reactor (OFR) was used to simulate longer atmospheric aging times of up to 3 weeks. Hygroscopicity and bounce behavior of the particles had a clear relationship showing higher bounce at elevated relative humidity (RH) values for less hygroscopic particles, which agrees well with earlier laboratory studies. Additional OH oxidation of the aerosol particles in the OFR increased the O : C and the hygroscopicity resulting in liquefying of the particles at lower RH values. At the highest OH exposures, the inorganic fraction starts to dominate the bounce process due to production of inorganics and concurrent loss of organics in the OFR. Our results indicate that at typical ambient RH and temperature, organic-dominated particles stay mostly liquid in the atmospheric conditions in the southeastern US, but they often turn semisolid when dried below ∼ 50 % RH in the sampling inlets. While the liquid phase state suggests solution behavior and equilibrium partitioning for the SOA particles in ambient air, the possible phase change in the drying process highlights the importance of thoroughly considered sampling techniques of SOA particles.

Short summary
The phase state of ambient particles was inferred from bounce measurements conducted at a rural site in central Alabama during the SOAS campaign. The organic-dominated ambient particles are mostly in the liquid phase at summertime conditions but they turn semisolid when dried in the measurement setup. Bounce humidograms reveal that the hygroscopicity and oxidation of the particles decreases the liquefying RH. The effect of oxidation is emphasized by oxidation flow reactor measurements.
Final-revised paper