Climate change creates new risks and amplifies existing ones, both for natural systems and for society. The goals of the UNFCCC Paris Agreement require limits on emissions of short-lived climate forcers, which include black carbon (BC). By measuring aerosol absorption and its wavelength dependence, it is possible to determine the heating rate and direct radiative contribution of BC and other absorbing aerosols, as well as systematic uncertainties in climate models.

This requires reliable instruments with traceable, artifact-free measurements. A photothermal interferometer (PTI) measures aerosol absorption linearly and traceably from first principles. Air containing aerosols enters the measurement chamber, where a modulated pump laser irradiates the sample. The aerosols absorb a portion of the light, heat up, and transfer heat to the surrounding air, and the resulting decrease in air density causes a change in the refractive index, which the interferometer detects as a phase shift in the optical path. This phase shift is linearly proportional to the absorption coefficient, and the reference arm of the interferometer automatically corrects for the contribution of gaseous absorbers (NO₂, O₃).

Within the project, a fiber-based photothermal interferometer in a Mach-Zehnder configuration was developed and tested. The fiber-based design substantially simplified the experimental arrangement and made the instrument more robust and less susceptible to external influences. A thermal phase stabilization method was developed without mechanical moving parts, exploiting the thermo-optic effect and thermal expansion of the optical fiber to maintain operation at the quadrature point, and a multi-wavelength pump system was developed with laser modules at 450 nm, 525 nm, and 785 nm. Multi-wavelength excitation was validated through photothermal measurements of nanoparticles, demonstrating simultaneous determination of particle size and concentration without prior knowledge of either parameter.

A systematic comparison of calibration methods for in-situ aerosol absorption instruments was carried out. It was found that calibration with NO₂ provides traceability to SI units, while calibration with monodisperse aerosol particles yields lower measurement uncertainties. The PTAAM-2λ instrument achieved comparable or better measurement uncertainty than the photoacoustic spectrometer PAX and the extinction-minus-scattering method. Field testing in Granada confirmed instrument performance under diverse aerosol conditions, including urban emissions, Saharan dust intrusions, and wildfire smoke, and key fiber-optic solutions were implemented in the portable PTAAM-3λ prototype.

The project results contribute to more accurate quantification of the contribution of BC and other absorbing aerosols to regional and global climate change, and provide a basis for evaluating emission reduction measures targeting short-lived climate forcers.

WP1 Project Management – Project finished

WP 2 Fiber Interferometer – Realization: 100%

Investigations of a fiber-integrated low-noise interferometer based on singlemode optical fibers and with a stabilized (fiber) laser as an interferometric source.

WP 3 Fiber laser and its pumping scheme – Realization: 100%

Research on a fiber laser suitable for pumping, exciting the measurement sample

WP 4 The Laboratory prototype – Realization: 100%

Setup of a laboratory prototype with a combined fiber interferometer and pump laser.

WP5 Field Testing – Realization: 100%

Testing a fiber interferometer in a relevant environment.