Abstract/Summary

The effects of aerosols on the climate system are a major source of uncertainty in past and future simulations of climate. The role played by carbonaceous aerosols is particularly uncertain. Black carbon, which absorbs shortwave radiation, exerts a positive radiative forcing of the climate system, warming the Earth. This warming is offset by co-emitted organic carbon, which scatters shortwave radiation back to space. However, a subset of organic carbon aerosols, called brown carbon, may be absorbing, further complicating the issue. Carbonaceous aerosols also influence cloud formation and properties, potentially contributing an additional cooling of Earth’s surface temperatures. This work aims at improving the simulation of carbonaceous aerosols in the UK Met Office’s HadGEM3-UKCA model with the GLOMAP-mode aerosol scheme. That model does not always compare well with regional multi-variable observations, often underestimating surface concentrations and aerosol optical depth. Although single scattering albedo, a measure of aerosol absorption, is well replicated in biomass burning regions, it is overestimated in regions where fossil- and bio-fuel emissions are dominant. To improve the comparison, we propose a range of changes to modelled carbonaceous aerosol emissions, refractive index and density. An analysis of the sensitivity of the model to these changesshows that emissions alone cannot be responsible for the discrepancies between observations and models. The analysis also shows that while the black carbon mass absorption coefficient needs to be high in fossil fuel combustion regions, black carbon absorption must be reduced in biomass burning areas to leave room for brown carbon absorption. A selected combination of changes leads to improvements in the simulation of carbonaceous aerosols in most regions compared with observations. Those improvements do not however lead to a significant change in the total aerosol effective radiative forcing, but reduces the contribution of aerosol-radiation interactions. Using an internally-mixed aerosol scheme means that changes in one aerosol species affects the simulation of other aerosol types. Therefore, aerosol model improvements require an integrated consideration of all aerosol species.