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

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 property of aerosols such as black carbon and brown carbon, the accurate knowledge of which is essential for understanding their climate forcing: 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.