Thermodynamic equilibrium of organic aerosols in the atmosphere

Aerosol particles in the atmosphere directly backscatter incoming sunlight or form reflective clouds by acting as Cloud Condensation Nuclei (CCN), thus having cooling effects on the Earth's climate. Dry aerosol consisting of soluble components undergoes hygroscopic growth by absorbing water from humid air, and increases aerosol mass available for direct scattering. We developed a thermodynamic model to describe the thermodynamic equilibrium of mixtures of electrolytes and organic species in aqueous solutions that exist as aerosol particles, and used it to predict the hygroscopic growth of aerosols (particularly organic aerosols). Our study demonstrated that mixing organic compounds with inorganic salts reduces growth relative to pure salts. In addition, the deliquescence relative humidity (DRH) of pure salts is lowered as a result of mixing with soluble organic components. We also studied the influence of organic components on CCN activation of aerosols by simulating fog/cloud formation in the polluted and clean environments with an aerosol model that includes detailed particle dynamics. The model results showed that organic aerosol increases the average CCN concentration through surface tension and solubility effects. Clean marine cloud has a higher maximum supersaturation than polluted fog due to lower concentration of aerosol and faster cooling rate. Our studies of hygroscopicity and CCN efficiency of aerosols will help to better understand the aerosol-climate interaction, and improve the representation of aerosols and clouds in global climate models.; In a policy analysis, the impact of both climate and health effects of aerosol on climate policy was assessed by using an integrated-assessment model that includes damages from global temperature change and aerosol-induced mortality increases. We found that a policy targeting only health damages significantly lowers sulfur emissions relative to a policy which calls for no controls, resulting in an increase of average temperature.