Abstract
Measurements have been performed to study the origin of the tropospheric nighttime concentrations of HNO 2 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 HNO 2, NO 2, SO 2 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 HNO 2 formation is clearly indicated by the nighttime increase of the HNO 2/NO x ratio. Correlation studies suggest the participation of the aerosol surface in the heterogeneous formation in the less polluted areas. In Milan the measured HNO 2 concentrations are considered to be influenced by the nighttime boundary layer and the heterogeneous formation on permanent surfaces like buildings.
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