A DOAS study on the origin of nitrous acid at urban and non-urban sites

Measurements have been performed to study the origin of the tropospheric nighttime concentrations of HNO2. The experiments were carried out at three locations with different levels of pollution by using a commercial differential optical absorption spectroscopy (DOAS) and a point sampling analyser. The DOAS observations allowed study of the concentrations 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 aerosols S along the beam path. NO was measured by a chemiluminescent monitor. A HNO2 formation is clearly indicated by the nighttime increase of the HNO2/NOx ratio. Correlation studies suggest the participation of the aerosol surface in the heterogeneous formation in the less polluted areas. In Milan the measured HNO2 concentrations are considered to be influenced by the nighttime boundary layer and the heterogeneous formation on permanent surfaces like buildings.