Large spread across AeroCom Phase II models in simulating black carbon in melting snow over Arctic sea ice

Over two dozen global atmospheric chemistry models contributing to the Aerosol Comparisons between Observations and Models (AeroCom) project were used in this study to drive the Los Alamos sea ice model to simulate the black carbon (BC) concentration in melting snow on Arctic sea ice. Measurements of BC during the melting season show concentrations in the range 2.8–41.6 ng·g−1 (average: 15.3 ng·g−1) in the central Arctic Ocean and Canada Basin. Most results from models contributing to the Phase I project were within the 25th and 75th percentiles of the observations, and the multimodel mean was slightly lower than that of the observations. In contrast, there was larger divergence among the Phase II model simulations and the mean value of BC was overestimated. The multimodel mean bias was −3.1 (−11.2 to +6.7) ng·g−1 for Phase I models and +3.9 (−9.5 to +21.3) ng·g−1 for Phase II models. The differences between the models of the two phases were probably attributable to the updated aerosol scheme in the new contributions, in which removal processes are parameterized by considering the actual dimensions and chemical compositions of the particles. This means the removal mechanism acts in a way that is more selective and leads to more BC particles being transported to the Arctic. In addition, higher spatial resolution could be another important reason for overestimation of BC concentration in snow in Phase II models