Impact of mineral dust on the global nitrate aerosol direct and indirect radiative effect

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 NO_x and especially NH₃ emissions. Thus, the radiative effect (RE) of NO₃^- aerosol may become more important than that of SO₄^2- 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 (RE_ari) and cloud interactions (RE_aci) at the top of the atmosphere (TOA). For this purpose, multi-year 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/m², driven by coarse-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/m². 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.