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Impact of Nitrate on Aerosol Composition and Radiative Effects

Final Report Summary - NACRE (Impact of Nitrate on Aerosol Composition and Radiative Effects)

During NACRE, the researcher contributed on the development of a revised mineral dust emission scheme that improved the dust budget calculations by the global CCM EMAC. The emission scheme computes the chemical composition of the emitted dust, since mineral cations are important for the phase partitioning of semivolatile species (i.e. HNO3). Gas-aerosol partitioning is simulated using the ISORROPIA-II thermodynamic equilibrium model. EMAC simulates high fine aerosol nitrate concentrations over urban and industrialized areas while coarse aerosol nitrate is highest close to deserts. The presence of metallic ions resulted in an increase of nitrate aerosol tropospheric burden by 44%. Sensitivity tests show that nitrate is most sensitive to the chemical composition of mineral dust, followed its size distribution and emission load. Results from the NACRE project contribute on an assessment of global nitrate aerosol by the AeroCom Phase III study. During this study, a budget analyses was conducted to understand the typical magnitude, distribution, and diversity of nitrate and its precursors among nine models.
The researcher has implemented into EMAC a new cloud droplet formation parameterization, the "unified dust activation framework", which considers the inherent hydrophilicity from adsorption and acquired hygroscopicity from soluble salts during dust aging. The presence of dust increases cloud droplet number concentration (CDNC) over the major deserts due to the activation of freshly emitted dust particles and decreases CDNC over polluted areas. The adsorption activation of insoluble aerosols and the mineral dust chemistry are shown to be equally important for the cloud droplet formation over the main desert. Remote from deserts the application of adsorption theory is critically important since the increased water uptake by the large aged dust particles reduce the maximum supersaturation and thus CDNC from the smaller anthropogenic particles. Furthermore, a new ice crystal formation parameterization has been implemented into EMAC improving the ice crystal number concentration estimates. The parameterization allows for a better treatment of ice nucleation taking into account the water vapor competition between homogeneous and heterogeneous nucleation.
Overall, the research undertaken under the NACRE project enhanced the state of the art in nitrate aerosol modeling by improving its representation in a CCM. The developed modeling framework takes into account the nitrate thermodynamic interactions with crustal species and accurately predicts the aerosol size distribution by considering the kinetic limitations of gas/aerosol partitioning in coarse mode. Furthermore, by using an explicit geographical representation of the emitted dust chemical composition the researcher created a novel representation of the mineral cation emissions. In addition, a novel approach is used to account for the CCN activity of mineral dust coated with soluble aerosols (like nitrates) by considering both the water adsorption and absorption effects. The developed modeling framework serves as an ideal tool for providing unprecedented understanding of the increased role of nitrate on the future global climate and air quality.
The excellent profile of the researcher and his expertise in aerosol modeling and cloud microphysics makes him a natural favorite for recruitment as a senior researcher at the host institute or other European research organizations.