Long-range transport of wildfire emissions over Zugspitze, Germany: An opportunity to test the field calibration of black carbon absorption photometers

The Schneefernerhaus observatory at Zugspitze, Germany is located at 2650 m a.s.l and provides the opportunity to monitor long-range transport of air pollutants in the free troposphere. In this study, we present aerosol observations performed from July to October 2025, with special focus on aerosol light absorption, i.e. brown and black carbon. Light-absorbing carbonaceous matter (LAC) is relevant to the climate due to its  short atmospheric lifetime and the dynamic behaviour of its optical properties, which change upon aging. Black carbon is included as a metric to be measured in the recent modification of the European Air Quality Directive, underscoring the need for an SI-traceable calibration chain for black carbon. This need is particularly pressing for absorption photometers (e.g., aethalometers), which are widely deployed in air quality monitoring networks. A field calibration has proven challenging given that there is no standard reference material available and that primary measurement methods are not yet ready for straightforward field deployment.

In this study, we have used an Aethalometer AE36s (Aerosol d.o.o., Ljubljana, Slovenia) and a photo-acoustic extinctiometer (PAX, Droplet Measurement Technology, Longmont, USA) to monitor aerosol light absorption during a 10-week field campaign at Schneefernerhaus. Additionally, we have used particle number size distribution (PNSD), and multi-angle absorption photometer (MAAP) data to assess different aerosol physical properties. The primary objective was to use the PAX measurements as a transfer standard to calibrate AE36s measurements in the field. The calibration transfer has proven feasible for the IR wavelength of 880 nm, which is of special interest when the focus is to determine black carbon with the less interference from other LAC components.

Observations in August 2025 show clearly a spike of LAC concentration with different wavelength dependencies (see Fig. 1), indicating a highly variable contribution of brown carbon to the total aerosol mass. HYSPLIT back-trajectory analysis indicate that these aerosol episodes originated from Canadian wildfires, which were highly active during the measurement period.

Overall, the field calibration method using PAX as a transfer standard has proven to be reliable and plausible. However, the method is constrained by the sensitivity and limit of detection of the PAX and also require that the PAX itself is calibrated against a primary method, such as extinction-minus-scattering or photo-thermal interferometry. The development of a robust primary calibration strategy for photo-acoustic spectrometers would significantly improve the traceability chain and would minimize uncertainties for in-field calibration of absorption photometers.

Figure 1. Aerosol absorption coefficient measured by an Aethalometer AE36s at Schneefernerhaus Zugspitze, Germany in August 2025.