A DOAS STUDY ON THE ORIGIN OF NITROUS-ACID AT URBAN AND NONURBAN SITES

Measurements have been performed to study the origin of the tropospher ic nighttime concentrations of HNO2. The experiments were carried out at three locations with different levels of pollution by using a comme rcial differential optical absorption spectroscopy (DOAS) and a point sampling analyser. The DOAS observations allowed study of the concentr ations of HNO2, NO2, SO2 and O-3. Light intensities given by the DOAS system were used to estimate rough values for the total surface of aer osols S along the beam path. NO was measured by a chemiluminescent mon itor. A HNO2 formation is clearly indicated by the nighttime increase of the HNO2/NOx ratio. Correlation studies suggest the participation o f the aerosol surface in the heterogeneous formation in the less pollu ted areas. In Milan the measured HNO2 concentrations are considered to be influenced by the nighttime boundary layer and the heterogeneous f ormation on permanent surfaces like buildings.