Effect of relative humidity on soot - secondary organic aerosol mixing: A case study from the Soot Aerosol Aging Study (PNNL-SAAS)

Friday, 19 December 2014
Noopur Sharma1, Swarup China2, Rahul A Zaveri3, John E Shilling4, Mikhail Pekour4, Shang Liu5, Allison C Aiken5, Manvendra Krishna Dubey5, Jacqueline Mary Wilson6, Alla Zelenyuk4, Rachel E OBrien7, Ryan Moffet8, Mary Kathleen Gilles9, Kulkarni Gourihar4, Duli Chand4, Arthur J Sedlacek III10, R Subramanian11, Timothy Bruce Onasch12, Alexander Laskin4 and Claudio Mazzoleni2, (1)Michigan Technological Univ, Houghton, MI, United States, (2)Michigan Technological University, Houghton, MI, United States, (3)Pacific Northwest Natl Lab, Richland, WA, United States, (4)Pacific Northwest National Laboratory, Richland, WA, United States, (5)Los Alamos National Laboratory, Los Alamos, NM, United States, (6)Battelle Pacific Northwest, Richland, WA, United States, (7)University of the Pacific, Department of Chemistry, Stockton, CA, United States, (8)University of the Pacific, Stockton, CA, United States, (9)Lawrence Berkeley National Lab, Berkeley, CA, United States, (10)Brookhaven National Lab, Upton, NY, United States, (11)Carnegie Mellon University, Department of Mechanical Engineering, Pittsburgh, PA, United States, (12)Aerodyne Research, Inc., Billerica, MA, United States
Atmospheric processing of fresh soot particles emitted by anthropogenic as well as natural sources alters their physical and chemical properties. For example, fresh and aged soot particles interact differently with incident solar radiation, resulting in different overall radiation budgets. Varying atmospheric chemical and meteorological conditions can result in complex soot mixing states. The Soot Aerosol Aging Study (SAAS) was conducted at the Pacific Northwest National Laboratory in November 2013 and January 2014 as a step towards understanding the evolution of mixing state of soot and its impact on climate-relevant properties. Aging experiments on diesel soot were carried out in a controlled laboratory chamber, and the effects of condensation and coagulation processes were systematically explored in separate sets of experiments. In addition to online measurement of aerosol properties, aerosol samples were collected for offline single particle analysis to investigate the evolution of the morphology, elemental composition and fine structure of sample particles from different experiments.

Condensation experiments focused on the formation of α-pinene secondary organic aerosol on diesel soot aerosol seeds. Experiments were conducted to study the aging of soot under dry (RH < 2%) and humid conditions (RH ~ 80%). We present an analysis of the morphology of soot, its evolution, and its correlation with optical properties, as the condensation of α-pinene SOA is carried out for the two different RH conditions. The analysis was performed by using scanning electron microscopy, transmission electron microscopy, scanning transmission x-ray microscopy and atomic force microscopy for single particle characterization. In addition, particle size, mass, composition, shape, and density were characterized in-situ, as a function of organics condensed on soot seeds, using single particle mass spectrometer.