10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Laboratory-Generated Coated Soot Aerosols with Tunable Physical, Chemical and Optical Properties Using a Cast Generator and a Portable Micro Smog Chamber
Michaela N. Ess, Alejandro Keller, Adam Kimak, Heinz Burtscher, KONSTANTINA VASILATOU, METAS
Abstract Number: 116 Working Group: Carbonaceous Aerosol
Abstract Particles in ambient air usually consist of a solid fraction, such as elemental carbon or minerals, and a more or less volatile fraction. Depending on temperature and volatility, this fraction may condense on the surface of preexisting particles, remain in the gas phase or nucleate and form new particles. This is strongly influenced by chemical reactions occurring in the atmosphere (mainly atmospheric oxidation known as aging) that lead to the formation of secondary organic aerosol (SOA; i.e. organic matter formed from organic gaseous precursors which -upon atmospheric oxidation- condense on preexisting particles or nucleate to form new particles).
In our study, we show that coated soot particles mimicking the properties of aged carbonaceous aerosols in the atmosphere can be synthesized in the laboratory. The soot core particles were generated with a CAST burner and subsequently mixed with vapors of a preselected gaseous precursor in a photo-oxidation reactor (Micro Smog Chamber, [1]). Experiments were performed with precursors of both biogenic and anthropogenic origin, such as α-pinene and 1,3,5-trimethylbenzene (TMB), respectively.
The Micro Smog Chamber has been used in the past to simulate the SOA formation potential of wood stove emissions [2,3] and has been compared to other common tools aiming to simulate atmospheric aging in the lab, including large-scale smog chambers [4]. It has been shown that MSC generates SOA of realistic chemical composition with yields within the range expected from previous smog-chamber studies. Compared to smog chambers the MSC offers the following advantages: i) the device is compact, portable and easy to use; ii) the generation of SOA and coating of particles is much faster due to the small size of the reactor and the high power of the UV radiation.
We have found that with MSC combustion particles can be coated with organic matter of biogenic/anthropogenic origin in a matter of seconds. As example, for 200 nm soot particles the thickness of the coating can be easily varied between a few nm to about 150 nm. Upon coating, the fractal-like structure of fresh soot collapses to give more compact soot core particles, a process known to occur often in the atmosphere. Moreover, the EC/OC ratio of the coated particles can be readily adjusted between 98:2 and 20:80. Measurements are under way in order to determine the optical properties of the coated particles, such as the absorption coefficient and single scattering albedo (SSA).
Thanks to their controlled and tunable properties these lab-generated carbonaceous aerosols have the potential to serve as reference aerosols to challenge common field instruments measuring equivalent Black carbon (EBC), such as aethalometers. These filter-based instruments suffer from complex measuring artefacts and high measurement uncertainties. Joint efforts are undertaken within the EMPIR-Black Carbon European research project to develop a validated method for their calibration [5].
Moreover, the stable and reproducible generation of coated carbonaceous aerosols in the lab is the first step towards more complex synthetic ambient aerosols as described in the EMPIR-Aeromet project [6].
Acknowledgments: This work is part of the 16ENV02 Black Carbon and 16ENV07 Aeromet projects funded by the European Union through the European Metrology Programme for Innovation and Research (EMPIR).
[1] A. Keller and H. Burtscher, J. Aerosol Sci. 49, 9-20 (2012). [2] J. C. Corbin et al., Atmos. Chem. Phys., 14, 2591-2603 (2014). [3] J. C. Corbin et al., Atmos. Chem. Phys., 15, 11885–11907 (2015). [4] E. A. Bruns et al., Atmos. Meas. Tech., 8, 2315-2332 (2015). [5] www.empirblackcarbon.com [6] www.aerometproject.com