First responders need a more or less instant estimate of danger zones resulting from accidental airborne releases of hazardous materials in order to take immediate action, to coordinate rescue teams and to protect the population and critical infrastructure. To fulfill the need for efficient access to reliable results in a first responders environment while maintaining sufficient dispersion modeling accuracy, pre-computed high-resolution CFD modeling can be combined with `physical data reduction' in an emergency assessment tool. This approach, specific to the geometry of the city of Hamburg, has been adopted by the Fire Brigade in Hamburg, Germany.