Abstract
Studies on the wintertime photochemical air pollution, which was merely a hypothesis 10 years ago and now is taken for granted in the numerical studies, are reviewed. The phenomenon was first assumed in the analysis of severe NO2 pollution episodes in early winter, which was unable to explain by a conventional photostationary state concept. Subsequently, the existence of wintertime photochemical air pollution was confirmed by field studies. Severe NO2 pollution episodes caused by such mechanism was then reproduced by more detailed numerical study. On the basis of these studies, an urban aerosol model which built-in the NO3- formation scheme is under development. Although studies in European countries have generally neglected the photochemical reactions as a source of high winter NO2, some studies in the United States regarded it as a source of NO3- in urban aerosol pollution. It suggests that the wintertime photochemical air pollution could be a common phenomenon at metropolitan cites in the mid-latitude zone.
