Improvements of organic aerosol representations and their effects in large-scale atmospheric models

Organics dominate the composition of the atmospheric aerosol, especially in the fine mode, influencing some of its characteristics such as the hygroscopicity, which is of climatic relevance for the Earth system. This study targets an improvement in the description of organic aerosols suitable for large-scale modelling, making use of recent developments based on laboratory and field measurements. In addition to the organic mass and particle number distribution, the proposed method keeps track of the oxidation state of the aerosol based on the OH exposure time, describing some of its chemical characteristics. This study presents the application of the method in a global chemistry climate model, investigates the sensitivity to process formulations and emission assignments, provides a comparison with observations and analyses the climate impact. Even though the organic aerosol mass distribution is hardly affected by the new formulation, it shows impacts (regionally of the order of 10% to 20%) on parameters directly influencing climate via the direct and indirect aerosol effects. Furthermore, the global distribution of the organic O:C ratio is analysed in detail, leading to different regimes in the oxidation state: low O: C ratios over the tropical continents due to small OH concentrations caused by OH depletion in chemical reactions, and enhanced oxidation states over the tropical oceans based on less OH scavengers and at high altitudes due to longer atmospheric residence time. Due to the relation between O: C ratio and the aerosol hygroscopicity the ageing results in a more physically and chemically consistent description of aerosol water uptake by the organic aerosol. In comparison with observations reasonable agreement for the O: C ratio within the limits of a global model of the simulations is achieved.