Increased mass ratio of organic aerosol to organic carbon
Previously, an organic aerosol (OA) to organic carbon (OC) mass ratio of 1.4 was assumed in the model. It has been shown by several studies that this value is outdated and should be increased. The OA composition and thus the OA/OC ratio shows substantial variations depending on emission source (anthropogenic versus biomass burning) and ageing regime. For simplicity, we stick to the original formulation and adopt a single value for the OA/OC ratio, as we also use a single value for the OA density. In this way, the simulated OA mass concentrations can easily be converted to OC. The OA/OC mass ratio has been increased to 1.8. See comments in the text for further details.
The absolute and relative impacts on the annual mean aerosol optical depth (AOD), the contributions from OA and sulfate to the AOD, and the mass concentration of OA are shown in the attached maps.
Results have been obtained at 3x2 degrees with a sub-grid mixing time scale of 12 h to describe scavenging by large-scale clouds and precipitation. In these experiments, the densities of carbonaceous particles were reduced compared to the original values.
For secondary organic aerosols, we have further increased the OA/OC ratio to 2.4. This seems to improve the AOD simulation in regions with a substantial SOA contribution.