An online tracer model, based on the fifth-generation Penn State/ NCAR Mesoscale model, was developed. The new model includes full representation of processes for advection, boundary layer mixing, subgrid cumulus convective mixing, and sedimentation of tracers. The model was used in two very different applications to document its potential utility. The first application involves pollutant transport to Istanbul, Turkey, focusing on two high-pollution episodes in January 2002. To better maintain large scale features, model simulations were nudged to reanalysis for this application. Using a semi-idealized approach, it was shown that much of the pollution that affected Istanbul during these events may have come from other highly polluted cities located upstream, rather than just local emission sources. Pollutants from upstream sources were trapped in the boundary layer by statically stable low-level conditions and efficient transport to Istanbul was supported by strong northwesterly flow near the surface. The second application involves the transport of dust from the Sahara Desert to the Atlantic Ocean, and the potential role of this dust and the dry, warm Saharan Air Layer ( SAL) in the genesis and development of Tropical Storm Chantal in 2001. No nudging was applied to this case study since it may degrade small scale features, which were important to dust saltation. The dust uplifting and transport during the earlier period of Chantal's life cycle were simulated to show a potential link between Sahara dust and Chantal's evolution. Results show strong evidence that Chantal started interacting with SAL and dust at a very early stage of storm development after propagating into the eastern Atlantic Ocean. Moreover, it was found that the peak of the averaged surface dust flux occurred in the early morning right before the mixed boundary layer developed, and the mechanism of dust uptake for this event, nocturnal low-level jets, was different from those previously documented.