Self-Calibrating Aerosol Absorption Using TDLAS with Tunable-Laser Photothermal Interferometry
JOEL CORBIN, Alireza Moallemi, Daniel Poitras, Timothy Sipkens, Jalal Norooz Oliaee, National Research Council Canada
Abstract Number: 619
Working Group: Instrumentation and Methods
Abstract
Visible and infrared light absorption by nanoparticles such as soot, brown carbon, tarballs, graphene, metals, and dust underlies key processes and measurements in combustion, materials synthesis, climate, and medicine. Light absorption by black carbon is the measurand of the many photoacoustic or filter-based instruments that are used for monitoring toxic engine emissions during air-quality monitoring and emissions testing of on-road, airborne, and marine transportation. The airborne measurement of nanoparticle light absorption is therefore of broad scientific value, and requires reliable reference measurements for calibration.
Photothermal interferometry (PTI) has the potential to provide such reference measurements. In PTI, the aerosol absorption coefficient ßabn can be retrieved, for use in calculating a sample’s imaginary refractive index, absorption function, mass absorption cross-section, or equivalent black carbon mass (eBC) as reported by most black-carbon sensors. Here, we propose the use of tunable-laser PTI (TL-PTI) as a self-calibrating reference technique for quantifying ßabn. In TL-PTI, a wavelength-tunable laser is used to quantify the narrow absorption lines of a gas such as O2 via direct absorption spectroscopy, providing a reference ßabn,ref in units of Mm-1. The PTI signal is then calibrated to this ßabn,ref. This gas-PTI signal can then be “switched off” by tuning the laser wavelength away from the absorption lines, so that only particulate PTI signals remain in the background-subtracted signal. The TL-PTI signal is directly related to light absorption for both aerosols and gases, since the PTI pump-laser modulation is slower than the thermalization timescales of both. We then show that the 760 nm calibration laser can be used to cross-calibrate a supercontinuum laser across wavelengths 400 – 840 nm. Thus, most instruments can be calibrated at their native wavelengths, and the fundamental spectral properties of nanoparticles can be measured. We present example measurements for NO2 and nigrosine dye.