Sjrdin and Ferm (1985) have studied atmospheric concentrations of nitrous acid during a 5 month period in the city of Gothenburg in Sweden. Samples of HNO2 were collected in 12-h intervals with the sodium carbonate denuder technique described in detail by Ferm and Sjrdin (1985) and simultaneous measurements of NO and NO2 (by chemiluminescence) were observed. These authors conclude that HNO2 is decomposed during daytime (HNO2 + hv ~ NO + OH.) and the correlation coefficient calculations indicate that the reaction NO + NO2 + H20 -~ 2HNO2 is a major dark source of nitrous acid in urban air.