Observational Constraints on Atmospheric Black Carbon in the Climate System

Black Carbon (BC) aerosols are short-lived climate pollutants with uncertain climate impacts. Growing observational records over the past two decades increasingly constrain the dynamics of atmospheric BC. This assessment also utilized the expanding in situ observational records to compare and evaluate emission inventories and global model estimates of BC sources, atmospheric burdens, lifetimes with respect to deposition, solar absorption and radiative effects.Isotopic fingerprinting of atmospheric BC reveals significant regional differences between biomass and fossil fuel combustion sources, with Sub-Saharan Africa (fbiomass-burning 93±3%), South Asia (56±7%) and East Asia (28±5%). These are broadly consistent with a set of commonly used emission inventories.Emissions and columnar measurements indicate recent BC declines in South America and East Asia, continued moderate reductions in Europe and North America, and recent stabilization in Africa and South Asia. The global mean mass absorption coefficient (MAC550) of atmospheric BC is 12.3±5.8 m2/g (151 datasets) and highest in Africa, Europe and South Asia. This is higher than in earlier assessments that focused on near-source measurements. The enhancement (E-MAC550) during long-range transport (ageing) is similar across regions (1.6±0.4).Long-term observations show that models overestimate BC deposition fluxes while underestimating both concentrations and sunlight absorption in high-pollution regions. This has implications for humidity, clouds, precipitation and climate forcing. Model simulations of aerosol absorption optical depth and the direct radiative forcing ratio between surface and top of atmosphere still underestimate observations by factors of 2 and 1.5, respectively.Further progress in understanding BC’s role in the climate system will require more extensive intercomparisons between observations, emission inventories, and climate models. Such advances will also strengthen the scientific basis for mitigation policies.