Description
Understanding aerosol formation and growth is essentialfor better estimation of climate and health effects, and the
design of novel nanomaterials with tuned morphology
including nano-electronics, catalysts and functionalized
surfaces. 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
(1985).
The CAMS instrument provides growth rate of
monodisperse particles, as well as total number
concentration based on a laser and optical detectors with
sub-µs response time. The measurement is done in an
expansion chamber where supersaturation is induced by
adiabatic expansion of an initial vapor mixture
conditioned into a well-defined state. Such features make
the CAMS method ideal for measuring condensational
growth of aerosols from supersaturated vapor. The CAMS
principle has been demonstrated in the past using single
lasers with visible light (Pinterich et al. (2016), Winkler
et al. (2004)) allowing growth rate measurements in the
continuum regime, around a few micrometers.
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
patterns 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 shorter
wavelength. However, when using adiabatic expansion,
super-saturation reaches a maximum immediately after
the expansion and slows down as the available vapor
condenses. Therefore, the growth rate of particles is
maximum at the beginning, when the particles are still
small and the growth rate progressively slows down as the
vapor gets depleted. Therefore, determining the first peak
as well as obtaining well separated subsequent peaks
becomes increasingly difficult as shorter wavelengths are
used.
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 shorter wavelength. This permits the
measurement of smaller particles and having two
independent particle size measurements allow temporal
cross-comparison. Such triangulation provides more
accuracy and reliability on the whole range of the
measurements, even beyond the overlapping region. This
is particularly useful for cases on which the first
experimental Mie peak of the lowest wavelength laser is
not known with accuracy. This research has the potential
to open exciting new applications and provide useful
evidence for better understanding the initial stages of
aerosol growth.
| Period | 19 Jun 2018 |
|---|---|
| Event title | Conference Aerosol Technology 2018 |
| Event type | Conference |
| Location | Bilbao, SpainShow on map |