A systematic review of the source, formation mechanism, and environmental effects of HONO in the indoor air

Abstract

The vast majority of human life is spent in indoor environments, and indoor air quality is closely related to human health and well-being. Nitrous acid (HONO), as the main pollutant in indoor environment, has become an emerging indoor contaminant which is widely concerned at home and abroad. HONO is prone to produce hydroxyl free radicals (center dot OH) upon exposure to ultraviolet radiation, which leads to a series of photochemical reactions and ultimately indoor pollution, causing adverse effects on human health. HONO is ubiquitous in indoor environment, and its production process is dynamic and complex. In this work, the primary sources and mechanisms of indoor HONO were reviewed, including the homogeneous reactions, heterogeneous reactions, photolysis of surface nitrates, and other major reaction processes as well as detailed mechanisms leading to the formation of HONO were analyzed. The research progress related to NO2 heterogeneous light-induced reaction on indoor surfaces were summarized. Furthermore, the environmental implications of HONO on indoor air quality and health risk were emphasized, and some important research directions such as indoor multiphase chemical processes, emerging contaminants transformation, modeling and health risk assessment under complex environment were discussed. Indoor HONO serves as a bridge connecting indoor chemistry with the assessment of indoor air quality. Consequently, the comprehensive understanding of the primary sources and formation mechanisms of HONO will help to further improve the current major air quality models, ultimately optimizing the research of indoor chemistry and its impact on human health