Formaldehyde is a primary and secondary air pollutant. As an intermediate product of hydrocarbon photooxidn., it contributes to the formation of photochem. pollution in urban areas. In addn., chronic or acute exposure to HCHO (a possible carcinogen) constitutes an important health hazard. Differential optical absorption spectroscopy (DOAS) is one of the best-suited techniques for monitoring formaldehyde. Within the frame of a European Union project, a field measurement campaign (PIPAPO) was carried out from May to June, 1998, in the Milan, Italy, area. During PIPAPO, a large no. of air pollutants, including formaldehyde, were monitored at several sites from Milan to the Swiss Alps foothills. Located at Seregno (25 km north of downtown Milan), EPFL (the Ecole Polytechnique Federale de Lausanne) carried out formaldehyde measurements by DOAS and performed intercomparison DNPH (2,4-dinitrophenylhydrazine) samples, which were analyzed at JRC (the Joint Research Center). Despite having different spatial and time resoln., DOAS and DNPH measurements of formaldehyde compare fairly well. The formaldehyde concn. time series retrieved from DOAS were analyzed with respect to the meteorol. conditions, particularly regarding heterogeneous removal processes, and the concn. of other air pollutants assocd. with formaldehyde emission and prodn. Pollutant dynamics calcns., made with a 3-dimensional photochem. grid (Eulerian) model over a wide spatial domain, were compared to the measurements and provided a basis for the interpretation of the formaldehyde measurements performed during one of PIPAPO's intensive observation periods (IOP).