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The attachment of positive and negative ions to settling spherical dust particles is studied. A novel 1D numerical model has been developed to parameterize the charging process in the presence of a large-scale electric field. The model is able to self-consistently calculate the modification of atmospheric ion densities in the presence of the dust particles, and the consequent alteration of the atmospheric electrical conductivity and the large-scale electric field. Moreover, the model estimates the acquired electrical charge on the dust particles and calculates the electrical force that is applied on them. Using observed dust size distributions, we find that the particles can acquire electrical charge in the range of 1–1,000 elementary charges depending on their size and number density. The particles become mainly negatively charged, but under specific conditions giant mode particles (larger than 50μm radius) can be positive. Moreover, the large-scale electric field can increase up to 20 times as much as the fair weather value. However, our approach shows that the resultant electrical force is not enough to significantly influence their gravitational settling, as the ratio between the electrical force magnitude and the gravity magnitude does not exceed the value of 0.01. This indicates that the process of ion attachment alone is not sufficient to create strong electrical effects for the modification of particle dynamics. Therefore, other processes, such as the triboelectric effect and updrafts, must be included in the model to fully represent the impact of electricity on particle dynamics.

We assess the impact of advanced heat-mitigation technologies in a coastal temperate city under heatwave conditions. For the first time urban-heating mitigation scenarios that refer to ‘cool/reflective’ roofs and roads, ‘green/living’ roofs and shading by replacing low urban vegetation with deciduous broadleaf trees are considered at the highly dense-populated city of Athens (Greece). Numerical simulations are performed for a typical see-breeze and a heatwave day with the Weather Research and Forecasting (WRF) model coupled to an urban-canopy model. Highresolution data on vegetation and urban land use, derived from satellite image analysis, are considered. All scenarios show a cooling effect, with the maximum mean daytime temperature reduction in the case of ‘cool/reflective’ roofs and roads. During daytime, the mean ambient-temperature reduction reaches up to 1 °C while for the surface-temperature up to 9.5 °C and 11.5 °C, on the see-breeze and heatwave day respectively. In the case of ‘green/living’ roofs, the mean daytime latent-heat flux is increased (e.g. up to 140 W/m2 on the heatwave day) due to increased evapotranspiration while the surface temperature is more affected during nighttime. Both scenarios result in a sea-breeze attenuation of 0.5–1 m/s. The presence of deciduous broadleaf street trees has a minor impact on mean ambient temperature but an evident reduction in surface temperature. The mean urban-heating reduction ranges from 0.1 °C to 0.8 °C and from 0.3 °C to 1.7 °C during the sea breeze and heatwave day respectively, with the maximum reduction shown in ‘cool/reflective’ roofs and roads and the minimum in ‘shading trees’ scenarios.

Three recent Saharan dust outbreaks during different seasons (4–6 days in winter of 2009, late autumn of 2010 and summer of 2011) are selected in order to study the chemical footprint and aging processes of dust intrusions over the Eastern Mediterranean (EM). The applied model system (PMCAMx, WRF and GEOS-CHEM) and methodology are found competent to reproduce dust production, long-range transport and chemical transformations over the EM, with the synergistic use of synoptic patterns analysis, optical depth retrievals, back-trajectories, surface and satellite aerosol measurements. The dust loads were high during the cold period events and much lighter during summertime, when transport was mainly in the free troposphere. In all cases, dust originated from the northwest and/or west Saharan desert and reached the EM from the west/southwest. Sensitivity runs underlie the effect of dust transport on the chemical constituents of aerosols over the EM and show a large impact on calcium (70–90% of maximum daily values 2–5 μg m−3), with its gradient at surface level being around −10% per 100 km along the dust pathway. For the cold period cases, this value can also be considered analogous to the dust dissipation ratio, because the plume is vertically extended down to the surface layers. Interestingly, the surface particulate nitrate concentrations over the EM are reversely affected by the approaching dust loads, exhibiting the highest values (up to 6 μg m−3) and the largest dust fraction (ca. 70%) during summertime. This is attributed to the enhanced nitric acid formation under high atmospheric temperature and insolation, its uptake onto the carbonate dust particles, and their effective accumulation, due to low deposition rates over the sea and scarce precipitation. Sulfate formation onto dust particles is found insignificant (rapid reaction with ammonia and/or sea-salt), while the influence of dust and sea-salt on sodium, when spatio-temporal averages are calculated, is approximately equal.