During the Pianura Padana Produzione di Ozono (PIPAPO) field campaign (May--June 1998), aerosol measurements were performed at an urban site and a semirural site to determine the particulate matter chemical composition in the Milan area. Aerosol samples were collected on filters for subsequent chemical analysis using virtual impactors. Possible sampling artifacts for semivolatile particulate species were addressed. Our NH4NO3 measurements were successfully compared with artifact free wet denuder-wet aerosol collector sampler data. Positive sampling artifacts for organic species were corrected using the back-to-back filter technique, allowing us to assess the lower limit for particulate organic carbon (POC) concentrations. Aerosol size distributions were measured on-line with differential mobility analyzers (DMA). The variations in the submicron aerosol mass concentration estimated from chemical analyses compared well with the variations in the submicron particle volume calculated from number size distributions. At both sites, 70% of the PM10 mass was found in the submicron fraction. The mean submicron aerosol mass concentrations were 28 ¿g m-3 and 20 ¿g m-3 at the urban site and the semirural site, respectively. The correlations between NH4+ and NO3- + 2SO4= indicate that strong acids were fully neutralized at both sites. At the urban site the submicron concentrations of NH4NO3, (NH4)2SO4, and POC were 8.1 ¿g m-3, 6.3 ¿g m-3, and 6.4 ¿gC m-3, respectively. At the semi-rural site, these concentrations were 4.9 ¿g m-3, 4.0 ¿g m-3, and 5.6 ¿gC m-3, respectively. These results show that sulfate and nitrate contribute about the same amount to submicron aerosol mass concentration, and that particulate organic matter (POM) could be the major component of the aerosol submicron mass in the Milan region, especially in semirural areas. The correlation between normalized concentrations of POM and O3 observed at the semirural site suggests that POM results at least partly from photochemical oxidation of volatile organic compounds (VOC). It is likely that reducing anthropogenic VOC emissions would lead to lower POC and total submicron aerosol concentrations in semirural areas around Milan.