Measurement of nitrous acid in Milan, Italy, by DOAS and diffusion denuders

Measurements of the atmospheric mixing ratio of nitrous acid have been carried out by means of the differential optical absorption system (DOAS) during a wintertime field study carried out in the urban area of Milan (Italy). Preliminary laboratory calibration of the DOAS was carried out by making use of a standard source of pure HONO which provided an interference-free UV absorption spectrum of HONO and recalculation of the absorption cross-section of this compound. Very high HONO mixing ratios, up to values exceeding 10 ppb, were measured during the campaign. DOAS results were compared with those obtained by using the annular denuder technique and good agreement was observed during the whole measurement period. On the basis of these measurements, a study on the relationship between the dynamic conditions of the lower boundary layer and the temporal trend of HONO mixing ratio can be carried out, in order to obtain indications about the sources of this species in urban atmospheres.