Sunlight driven processes in the indoor air are highly probable to occur given the heterogeneity of the available light absorbing compounds. A number of compounds absorb photons in the wavelength range of interest and light-induced processes leading to a change in the secondary organic components are highly probable. Various photochemical reactions occur on the indoor surfaces where radicals and photosensitizers can affect the rates and the degradation pathways resulting in ageing of the available organic compounds.
Here, I discuss some recent work which further explores these photochemical processes, hitherto little-considered atmospheric mechanism: hydroxyl radicals (OH) and VOCs formation via photosensitized reactions. With examples of reactions driven by these mechanisms, it is shown that solar radiation may play an important role in indoor atmospheric chemistry, especially in the production of oxidants. Such radical-generating processes will give rise both to increased OH production and to new aerosol formation, via condensation of the oxidized products.
OH radicals, despite their low (10⁶ cm^-3) daytime concentrations, are the most important oxidant species in indoor atmosphere, controlling the concentration of organic pollutants. Understanding what controls the OH concentrations is crucial for accurately modeling the atmosphere’s oxidation capacity. By enhancing the production of OH radicals which initiate the degradation reactions of atmospheric pollutants, these photosensitized reactions could have a significant impact on atmospheric processes.
High OH levels, which were in line with typical outdoor concentrations, were measured for the first time in a high school in France. It was demonstrated that photolysis of nitrous acid (HONO) is the most important source of these highly reactive OH radicals. This set of innovatory first direct measurements of indoor OH radicals was followed with also avant-garde measurements of the solar actinic fluxes which can penetrate indoors and photolysis frequencies of key indoor species. The first time-resolved measurements of oxidant species in an indoor environment in China has confirmed these high OH levels demonstrating the importance of OH radical as the main driving force of the indoor oxidation capacity. The results obtained are of enormous repercussions and need to be studied in the next years much more profoundly due to their still unexplored implications. It also points out that this complex chemistry occurring indoors will become even more important as the development of the modern society imposes us to be confined more and more indoors.