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Accurate representation of black carbon (BC) concentrations inclimate models is a key prerequisite for understanding its net climateimpact. BC aging and scavenging are treated very differently in currentmodels. Here, we examine the sensitivity of three-dimensional (3-D),temporally resolved BC concentrations to perturbations to individual modelprocesses in the chemistry transport model OsloCTM2–M7. The main goals areto identify processes related to aerosol aging and scavenging whereadditional observational constraints may most effectively improve modelperformance, in particular for BC vertical profiles, and to give anindication of how model uncertainties in the BC life cycle propagate intouncertainties in climate impacts. Coupling OsloCTM2 with the microphysicalaerosol module M7 allows us to investigate aging processes in more detailthan possible with a simpler bulk parameterization. Here we include, for thefirst time in this model, a treatment of condensation of nitric acid on BC.Using kernels, we also estimate the range of radiative forcing and globalsurface temperature responses that may result from perturbations to keytunable parameters in the model. We find that BC concentrations inOsloCTM2–M7 are particularly sensitive to convective scavenging and theinclusion of condensation by nitric acid. The largest changes are found athigher altitudes around the Equator and at low altitudes over the Arctic.Convective scavenging of hydrophobic BC, and the amount of sulfate requiredfor BC aging, are found to be key parameters, potentially reducing biasagainst HIAPER Pole-to-Pole Observations (HIPPO) flight-based measurements by60 to 90  %. Even for extensive tuning, however, the total impact onglobal-mean surface temperature is estimated to less than 0.04 K. Similarresults are found when nitric acid is allowed to condense on the BC aerosols.We conclude, in line with previous studies, that a shorter atmospheric BClifetime broadly improves the comparison with measurements over the Pacific.However, we also find that the model–measurement discrepancies can not beuniquely attributed to uncertainties in a single process or parameter. Modeldevelopment therefore needs to be focused on improvements to individualprocesses, supported by a broad range of observational and experimental data,rather than tuning of individual, effective parameters such as the global BClifetime

Sensitivity of black carbon concentrations and climate impact to aging and scavenging in OsloCTM2–M7