Ambient black carbon monitoring in urban environments using photoacoustic spectroscopy

Black carbon (BC) is a key indicator of combustion-related urban air pollution, strongly associated with road traffic, industrial emissions and residential heating, and is addressed within the European Ambient Air Quality Directive (EU) 2024/2881 as a parameter for air quality assessment. Reliable BC measurements at low ambient concentrations are therefore required for air quality studies and regulatory monitoring. The Aethalometer is currently the instrument of choice for monitoring ambient air quality and for scientific studies of BC. One issue with the Aethalometer is that it does not use a direct absorption measurement principle; it measures light attenuation and calculates black carbon (BC) absorption using several empirically determined factors. Photoacoustic spectroscopy (PAS) is a filter-free measurement principle that directly measures the absorption of BC. The PAS-based AVL Micro Soot Sensor (MSS) was originally developed for exhaust soot measurements in the automotive sector. In this study, the MSS was adapted for ambient air monitoring by modifying it to improve sensitivity and accuracy, resulting in the AVL Black Carbon Monitor. In the laboratory,  a first benchmark was performed to compare the sensitivity, stability and applicability for air quality measurements of different PAS-based BC instruments against the Aethalometer. The results demonstrate that the AVL Black Carbon Monitor enables time-resolved black carbon (BC) measurements in the nanogram-per-cubic-meter range, making it suitable for long-term urban use. Field measurements were carried out at multiple urban monitoring sites in Graz, Austria, representing locations with varying traffic exposure and residential surroundings. The observed BC time series show characteristic diurnal variability associated with traffic activity. Spatial differences between sites reflect varying local influences on emissions. The study illustrates the applicability of PAS-based BC monitoring for urban air quality assessment and transport-related emission characterization.