Numerical simulation and evaluation of global ultrafine particle concentrations at the Earth's surface

Abstract. A new global dataset of annual averaged ultrafine particle (UFP) concentrations at the Earth's surface has been developed through numerical simulations using the ECHAM/MESSy Atmospheric Chemistry model (EMAC). Size distributions of emitted particles from the contributing source sectors have been derived based on literature reports. The model results of UFP concentrations are evaluated using particle size distribution and particle number concentration measurements from available datasets and the literature. While we obtain reasonable agreement between the model results and observations (logarithmic scale correlation of r = 0.76 for non-remote, polluted regions), the highest values of observed, street-level UFP concentrations are systematically underestimated, whereas in rural environments close to urban areas the model generally overestimates observed UFP concentrations. As the relatively coarse global model does not resolve concentration gradients in urban centres and industrial UFP hotspots, high-resolution data of anthropogenic emissions is used to account for such differences in each model grid box, obtaining UFP concentrations with unprecedented 0.1° x 0.1° horizontal resolution at the Earth's surface. This observation-guided downscaling further improves the agreement with observations, leading to an increase of the logarithmic scale correlation between observed and simulated UFP concentrations to r = 0.84 in polluted environments (and 0.95 in all regions), a decrease of the root mean squared logarithmic error (from 0.57 to 0.43), and removes discrepancies associated with air quality and population density gradients within the model grid boxes. Model results are made publicly available for studies on public health and other impacts of atmospheric UFPs, and for intercomparison with other regional and global models and datasets.