10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Experimental Determination of Aerosol Growth Kinetics via Simultaneous Measurement of Constant Angle Mie Scattering Pattern at Two Different Wavelengths
MIGUEL VAZQUEZ-PUFLEAU, Paul M. Winkler, Universitaet Wien, Vienna, Austria
Abstract Number: 415 Working Group: Instrumentation
Abstract Understanding aerosol formation and growth is essential for better quantification of climate and health effects, the design of catalysts, nano-electronics and novel nanomaterials with tuned morphology. However, the study of the kinetics and mechanism for the initial stages of aerosol growth in the free molecule regime has been limited in the past due to the difficulty to measure aerosol growth in this size regime with sub-millisecond temporal resolution. To tackle these difficulties, the so-called constant-angle Mie scattering (CAMS) instrument was developed (Wagner, J. Colloid Interface Sci., 1985, 105(2): 456-467). This instrument provides growth rate of monodisperse particles, as well as total number concentration based on a laser and optical detectors. The measurement is done, in an expansion chamber where supersaturation is induced by adiabatic expansion of an initial vapor mixture at well-defined conditions. Such features make the CAMS method ideal for in situ measuring growth rates of aerosols from supersaturated vapor. The CAMS principle has been demonstrated in the past using single lasers with visible light (Pinterich, Aerosol Sci. Technol., 2016, 50(9): 947-958.; Winkler, Phys. Rev. Lett., 2004, 93(7): 075701), allowing growth rate measurements in the continuum regime, around a few microns.
The smallest particle size that can be detected using the CAMS method is given by the first Mie maximum in the scattering plot. Subsequent patterns in the signal are used to compute larger size parameters based on Mie scattering theory. Particle growth rate can then be directly computed by matching theoretical peaks to the experimental scattering plot that is measured as a function of time. Since the size parameter given in Mie theory is defined as the ratio of droplet size over wavelength, a direct way to lower the detection limits of the instrument is to use a laser with a smaller wavelength. However, when using adiabatic expansion, smaller particles tend to grow faster than larger particles and determining the first peak as well as obtaining well separated subsequent peaks becomes increasingly difficult.
In this work, we aim to provide aerosol growth rate measurements in the free molecular regime by incorporating an additional laser, perpendicular to the first one and with a smaller wavelength. The narrower wavelength allows the measurement of smaller particles and having two independent particle size measurements allow temporal cross-comparison. This triangulation provides more accuracy and reliability on the whole range of the measurements, even beyond the overlapping region. This research has the potential to open exciting new applications and provide useful evidence for better understanding the initial stages of aerosol growth.