The PM fraction in ambient air due to the formation of secondary inorganic sulfate and nitrate from the emissions of large lignite-fired power plants in Germany is investigated. The power plants are equipped with natural draft cooling towers. The flue gases are fed directly into the cooling towers, giving them an additional lift. The exhausted gas-steam mixture contains the gases CO, CO2, NO, NO2 and SO2, the directly emitted primary particles and additionally an excess of “free” sulfate ions in water solution, which are not neutralized by cations after the desulfurization stages. The precursor gases NO2 and SO2 are capable of forming nitric and sulfuric acids by various routes under supersaturated cooling tower cloud conditions. The acids can be neutralized by ammonia and produce secondary particulate matter by heterogeneous condensation on pre-existing particles. The metal ions released from the ash and the emitted fraction of “free” sulfate ions remaining in excess after the desulfurization steps play an important role. The investigations are carried out with the regional chemical transport model COSMO-MUSCAT as well as with the air parcel model SPACCIM, with which multiphase chemical processes can be described in great detail. Possible formation pathways and dependencies, especially on pH and the meteorological situation, are identified and the maximum particulate matter (PM) load in “worst case” scenarios is estimated.