Trace gases such as nitrogen dioxide directly affect human health. The concentrations in cities are typically dominated by anthropogenic emissions, primarily from cars, industry and heating systems. But these concentrations also depend on geographical conditions, meteorology and local transport processes. Thus, the precise concentration at a particular location can vary strongly in space and time, and cannot be measured with the existing techniques. However, the distribution is important to quantify the air quality, estimate emission sources and sinks, to study chemical processes and to validate current models. By measuring the average concentrations along 10 to 20 intersecting light paths and applying tomographic inversion techniques, LP DOAS Tomography allows for the determination of trace gas distributions over several km2. Such a setup was applied for the first time from 2005 to 2007 in the city of Heidelberg, Germany. The trace gases studied were nitrogen dioxide, sulphur dioxide, ozone, formaldehyde and nitrous acid, which all play significant roles in the polluted atmosphere. Strong spatial variations of these species were found and were further investigated.