Self-calibrating aerosol absorption measurements using co-located TDLAS and tunable-wavelength photothermal interferometry

Visible and infrared light absorption by nanoparticles such as soot, graphene, metals, and dust underlies key processes and measurements in combustion, materials synthesis, climate, and medicine. The calibration of such light absorption is essential for quantifying soot and brown-carbon climate forcing; for understanding synthetic nanoparticles including graphene and reduced graphene oxide; and for understanding synthetic nanoparticles for industrial applications. The airborne measurement of nanoparticle light absorption is therefore of broad scientific value.

Photothermal spectroscopy, and photothermal interferometry (PTI) in particular, has the potential to provide reference measurements of aerosol absorption coefficients βabn. Such βabn measurements represent a fundamental aerosol property. Also, βabn can be used in calculating the imaginary refractive index, absorption function, or mass absorption cross-section of a sample. Furthermore, βabn is commonly expressed in units of equivalent black carbon mass (eBC) in the many photoacoustic or filter-based instruments that are used for air-quality monitoring and emissions testing of on-road, airborne, and marine transportation.

Here, we propose the use of tunable-laser PTI (TL-PTI) as a self-calibrating reference technique for quantifying βabn. In TL-PTI, a wavelength-tunable laser is used to quantify the narrow absorption lines of a gas via direct absorption spectroscopy, providing a reference βabn,ref in units of Mm-1. The A-band absorption lines of ambient oxygen at about 760 nm provide a convenient reference. The PTI signal is then calibrated to this βabn,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 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 demonstrate the accuracy of our PTI prototype by retrieving the refractive index of nigrosine dye.