Sub-grid process analysis of the sensitivity of black carbon aging to particle microphysical properties at emission

The microphysical properties of black carbon-containing particles change due to interaction with other aerosol components after emission, altering their radiative properties and cloud condensation nuclei activity. Understanding these aging processes is essential in assessing black carbon???s direct and indirect climate forcing. The initial black carbon becomes a complex distribution of distinct multi-component particles, which is not adequately represented by conventional aerosol microphysical schemes. Most models simulate a bulk aerosol population, assuming that each particle contains only a single species or a homogeneous composition for all particles of a given mode or size. In this study, we use the particle-resolved aerosol model PartMC-MOSAIC to assess the aging mechanisms of black carbon-containing particles of varied initial size. PartMC-MOSAIC explicitly tracks the mass composition of each particle and is uniquely capable of resolving changes in the particle mixing state owing to coagulation and condensation. We simulated black carbon-containing particles of varied size emitted into an idealized urban plume of varied local pollution and environmental conditions. We assessed the sensitivity of properties of the processed black carbon population, such as geometric mean diameter, hygroscopicity, and cloud condensation nuclei activity, to changes in local ambient conditions. The simulation results indicate that the evolution of the black carbon mixing state is most sensitive to the formation of secondary inorganic aerosol, a???ecting condensation, and the total aerosol number concentration, a???ecting condensation and coagulation. Because small particles tend to age by coagulation and large particles tend to age by condensation, the relative importance of condensation or coagulation as aging mechanisms depends on the initial size of the black carbon particles