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Abstract. Nitrate (NO3-) aerosol is projected to increase dramatically in the coming decades and may become the dominant inorganic particle species. This is due to the continued strong decrease in SO2 emissions, which is not accompanied by a corresponding decrease in NOx and especially NH3 emissions. Thus, the radiative effect (RE) of NO3- aerosol may become more important than that of SO42- aerosol in the future. The physicochemical interactions of mineral dust particles with gas and aerosol tracers play an important role in influencing the overall RE of dust and non-dust aerosols but can be a major source of uncertainty due to their lack of representation in many global climate models. Therefore, this study investigates how and to what extent dust affects the current global NO3- aerosol radiative effect through both radiation (REari) and cloud interactions (REaci) at the top of the atmosphere (TOA). For this purpose, multiyear simulations nudged towards the observed atmospheric circulation were performed with the global atmospheric chemistry and climate model EMAC, while the thermodynamics of the interactions between inorganic aerosols and mineral dust were simulated with the thermodynamic equilibrium model ISORROPIA-lite. The emission flux of the mineral cations Na+, Ca2+, K+, and Mg2+ is calculated as a fraction of the total aeolian dust emission based on the unique chemical composition of the major deserts worldwide. Our results reveal positive and negative shortwave and longwave radiative effects in different regions of the world via aerosol–radiation interactions and cloud adjustments. Overall, the NO3- aerosol direct effect contributes a global cooling of −0.11 W m−2, driven by fine-mode particle cooling at short wavelengths. Regarding the indirect effect, it is noteworthy that NO3- aerosol exerts a global mean warming of +0.17 W m−2. While the presence of NO3- aerosol enhances the ability of mineral dust particles to act as cloud condensation nuclei (CCN), it simultaneously inhibits the formation of cloud droplets from the smaller anthropogenic particles. This is due to the coagulation of fine anthropogenic CCN particles with the larger nitrate-coated mineral dust particles, which leads to a reduction in total aerosol number concentration. This mechanism results in an overall reduced cloud albedo effect and is thus attributed as warming. 

Abstract. Information on the rate of diffusion of organic moleculeswithin secondary organic aerosol (SOA) is needed to accurately predict theeffects of SOA on climate and air quality. Diffusion can be important forpredicting the growth, evaporation, and reaction rates of SOA under certainatmospheric conditions. Often, researchers have predicted diffusion rates oforganic molecules within SOA using measurements of viscosity and theStokes–Einstein relation (D∝1/η, where D is the diffusioncoefficient and η is viscosity). However, the accuracy of thisrelation for predicting diffusion in SOA remains uncertain. Usingrectangular area fluorescence recovery after photobleaching (rFRAP), wedetermined diffusion coefficients of fluorescent organic molecules over8 orders in magnitude in proxies of SOA including citric acid, sorbitol,and a sucrose–citric acid mixture. These results were combined withliterature data to evaluate the Stokes–Einstein relation for predictingthe diffusion of organic molecules in SOA. Although almost all the data agreewith the Stokes–Einstein relation within a factor of 10, a fractionalStokes–Einstein relation (D∝1/ηξ) with ξ=0.93is a better model for predicting the diffusion of organic molecules in the SOAproxies studied. In addition, based on the output from a chemical transportmodel, the Stokes–Einstein relation can overpredict mixing times of organicmolecules within SOA by as much as 1 order of magnitude at an altitudeof ∼3 km compared to the fractional Stokes–Einstein relation with ξ=0.93. These results also have implications for other areas such as infood sciences and the preservation of biomolecules.