Coated soot particles with tunable, well-controlled properties generated in the laboratory with a miniCAST BC and a micro smog chamber

Abstract

A Micro Smog Chamber (MSC) was coupled for the first time with a miniCAST 5201 Type BC combustion generator with the aim to produce a series of stable and reproducible model aerosols simulating the physical properties of combustion particles present in ambient air. With this setup it was possible to generate particles ranging from “fresh” soot (single scattering albedo SSA≤0.05, absorption Ångström exponent AAE close to 1, high EC/TC mass fraction (approximately 90%) and mobility diameter typically <100 nm) to “aged” soot with different amounts of organic coating. The “aged” soot particles could grow up to 200 nm and exhibited high SSA (up to 0.7 at λ = 870 nm), an increased AAE (up to 1.7) and low EC/TC mass fraction (down to <10%). The ageing was achieved by coating the soot particles with increasing amounts of secondary organic matter (SOM) formed by the photo-oxidation of α-pinene or mesitylene in the MSC. Thereby, the SSA and AAE increased with coating thickness, while the EC/TC mass fraction decreased. Over the experimental period of 2 h, the generation of the “aged” soot aerosols was stable with a standard deviation in particle size and number concentration of <1% and <6%, respectively. The day-to-day reproducibility was also satisfactory: with α-pinene as SOM precursor the variability (standard deviation) in particle size was <2% and in the AAE and SSA < 6%. Particle number concentrations up to 106 cm−3 and mass concentrations up to 15 mg/m3 (depending on particle size and SOM amount) could be generated, much higher than what has been reported with other oxidation flow reactors. The generated carbonaceous aerosols could find useful applications in the field of aerosol instrument calibration, particularly in the standardization of filter-based absorption photometers under controlled conditions.