Understanding the metrology challenges and variability of black carbon properties from urban to remote environments

Black carbon (BC) is an important short-lived climate forcer, which overall impact is still uncertain in climate models. A better estimate requires the accurate characterization of fundamental properties such as BC size and mixing state, and climate-relevant properties (i.e. BC mass concentration and mass absorption cross-section) in different environments. Here, we integrated field observations from European and South American urban and remote sites with chamber experiments in the CESAM simulation chamber to assess the metrology limits of widely used measurement techniques and the variability of BC properties, both at the emission and after ageing in the atmosphere. Our findings reveal site and technique-dependent metrology limitations and the impact of different correction methods on mass concentration and absorption coefficient measurements. The combined biases from these factors may inflate the estimate of the mass absorption cross-section by up to 90%. Urban environments displayed rapid responses of BC properties to diurnal changes in emissions and meteorology, with traffic sources dominating in summer and biomass burning contributing more in colder months. Mass concentration exhibited significant variations with traffic intensity, leading to sharp increases during rush hours in both European and Bolivian cities. Concurrently, BC particle size increased from urban traffic sites to background locations, reflecting direct source proximity and rapid BC modification. Ventilated conditions promoted dispersion of traffic emissions and dilution with background air, altering BC properties compared to stagnant periods in an urban coastal site (Barcelona). In contrast, BC properties at the remote mountain site of Chacaltaya were influenced by long-range transport, with pollution injection from the boundary layer systematically modifying climate-relevant properties while negligibly impacting particle size. This study contributes to reducing uncertainty in BC observations, offering insights into fundamental and climate-relevant BC properties in urban and remote environments essential for refining BC direct radiative forcing estimates in models.