Simulation of biomass burning aerosols and radiative effects in HadGEM3

The impacts of biomass burning aerosol on global and regional climate are highly dependent on the accurate representation of BB emissions, aerosol processses and aerosol optical properties. The simulation of BB aerosol properties has been substantially improved in the Met Office Hadley Centre Global Environment Model (HadGEM3) with the introduction of GLOMAP-mode, a modal aerosol scheme developed as part of United Kingdom Chemistry and Aerosol project (UKCA). The aerosol size distribution, composition, absorption properties and vertical distributions are shown to compare well against aircraft and ground-based observations from the South American Biomass Burning Analysis (SAMBBA). However, as with many models, it is necessary to scale BB aerosol emissions up by a factor of 2 - 4 to gain agreement with observed AODs (MODIS, AERONET) and mass concentrations from ground observations and aircraft profiles. The necessary scaling factor depends on the emission dataset, the assumed injection height and the efficiency of aerosol wet removal processes in the atmospheric model. These findings will be demonstrated with HadGEM3 simulations related to an AEROCOM phase III experiment on biomass burning. Finally, the effective radiative forcing of the BB aerosol (including aerosol-radiation and aerosol-cloud impacts) is shown to be highly dependent assumptions related to black carbon and the absorption of solar radiation.