Observations of HNO2 in the polluted winter atmosphere: possible heterogeneous production on aerosols

Measurements of HNO2 and NO2 were obtained in Christchurch, New Zealand, during the winter of 1997, using differential optical absorption spectroscopy (DOAS). HNO2 concentrations ranged from below 50 ppt to 2.9 ppb and were found to be correlated with those of NO2. The highest HNO2 values occurred during the night when general pollution levels, particularly those of suspended particulate matter, were also high. The aerosol surface density in the light path was estimated from the light attenuation measured by the spectroscopic system, and a strong correlation between HNO2/NO2 and the aerosol surface density was observed. This correlation suggests that significant heterogeneous chemical production of HNO2 may occur through reactions of NO2 on aerosol surfaces. This hypothesis is further supported by a detailed analysis of selected pollution episodes where the HNO2/NO2 ratio was highly correlated with short-term changes of the aerosol density during episodes with consistently high NO2 concentrations. The observed HNO2 concentrations are also consistent with recent studies of the oxidation of NO2 to HNO2 on aerosol surfaces. The evidence for heterogeneous production of HNO2 on aerosol surfaces is limited, however, by the lack of data on inversion layer height which dominates trace gas concentrations in the boundary layer.