The performance of an aerosol module of the three-dimensional Eulerian model CAMx (Comprehensive Air Quality Model with Extensions) was evaluated in a domain covering the Po Basin in northern Italy. Concentrations of secondary aerosol species such as particulate NO3-, NH4+, SO42-, and SOC (secondary organic carbon) were calculated for the particle size below 2.5 ¿m and compared with the data available from a field experiment, which took place in May--June 1998. Model results for the inorganic aerosols were comparable to the measurements at an urban and a rural station. Sensitivity studies with reduced anthropogenic NOx and volatile organic carbon (VOC) emissions showed that SOC behaves in the same way as ozone, i.e., decreases with reduced VOC emissions and increases with reduced NOx emissions in the plume where ozone production is predicted to be VOC sensitive. Sensitivity of secondary aerosol formation to NH3 and NOx emissions was studied by reducing these emissions. Varying NH3 emissions led to an almost linear change in secondary aerosol mass at sites with low NH3. At ammonia-rich sites, on the other hand, availability of nitrate became important for the further formation of secondary aerosols. Monoterpene emissions were predicted to contribute about 25% of the secondary organic aerosols produced in the northern part of the model domain, which is mostly a forested area.