Direct measurements of aerosol absorption coefficient using photo-thermal interferometry – methodology, traceable calibration and laboratory and ambient measurements

Measuring the aerosol absorption coefficient is central for the determination of the aerosol influence on the climate and for the determination of sources of carbonaceous aerosol, especially black carbon (BC). Different methods can be used to determine or measure the aerosol absorption coefficient. Filter absorption photometers (FPs) are most easy to deploy but feature artefacts and require sophisticated calibration with a reference method. Few direct methods for in-situ aerosol absorption measurement exist, with photothermal interferometry (PTI) showing several advantages. We present how the PTI can be traceably calibrated, used as a reference method in the laboratory and in the field. 

A photothermal interferometer measures the change of the refractive index caused by light absorption in (and the subsequent heating of) the sample – the change of phase in the interferometer is proportional to the aerosol absorption coefficient. The detection is linear and can be traced to first principles. Since the laser-sample interaction region is monitored continuously, the method does not suffer from artefacts. The interferometric measurement can be calibrated to first principles. 

In-situ absorption instruments use different calibration schemes. NO2 is used for calibration in the visible range and can be traceable to SI units. Particles, on the other hand, allow calibration without wavelength limitations. We compare different calibration schemes in light of their measurement uncertainty and ease of implementation. The experimental results and a comparison with the Mie model monodisperse nigrosin aerosols show that mass-based parameters are more suitable for the modelling rather than mobility-based ones. The resulting aerosol absorption coefficient uncertainty is smaller than 5%.

Laboratory and ambient campaigns have shown similar FP calibration parameter values for ambient aerosols and laboratory experiments. We have also determined the absorption enhancement by coating BC with non-absorbing secondary organic matter in laboratory and ambient campaigns in contrasted environments (Slovenia, France). Mass absorption cross-section increase due to coatings were determined using different mass metrics – elemental carbon or refractory black carbon.

We present measurements with calibrated FPs at the Virtual Alpine Observatory station Otlica, where we determined the BC emission rates, and airplane measurements, where the atmospheric heating rate was measured separately for BC and Saharan desert dust. 

This work was supported by EURAMET (22NRM02 stanBC), ARIS (P1-0385, I-0033, L2-4485) and ESA (4000131931/20/NL/FF/an).