The hydroxyl radical ((•)OH) is one of the most powerful oxidizing agents, able to react unselectively and instantaneously with the surrounding chemicals, including organic pollutants and inhibitors. The (•)OH radicals are omnipresent in the environment (natural waters, atmosphere, interstellar space, etc.), including biological systems where (•)OH has an important role in immunity metabolism. We provide an extensive view on the role of hydroxyl radical in different environmental compartments and in laboratory systems, with the aim of drawing more attention to this emerging issue. Further research on processes related to the hydroxyl radical chemistry in the environmental compartments is highly demanded. A comprehensive understanding of the sources and sinks of (•)OH radicals including their implications in the natural waters and in the atmosphere is of crucial importance, including the way irradiated chromophoric dissolved organic matter in surface waters yields (•)OH through the H2O2-independent pathway, and the assessment of the relative importance of gas-phase vs aqueous-phase reactions of (•)OH with many atmospheric components. Moreover, considering the fact that people spend so much more time in dwellings than outside, the impact of the reactivity of indoor hydroxyl radicals on health and well-being is another emerging research topic of great concern.

The hydroxyl (OH) radical is the most important oxidant in the atmosphere since it controls its self-oxidizing capacity. The main sources of OH radicals are the photolysis of ozone and the photolysis of nitrous acid (HONO). Due to the attenuation of solar radiation in the indoor environment, the possibility of OH formation through photolytic pathways indoors has been ignored up to now. In the indoor air, the ozonolysis of alkenes has been suggested as an alternative route of OH formation. Models and indirect measurements performed up to now according to this hypothesis suggest concentrations of OH radicals on the order of 10 4 –10 5 molecules per cubic centimeter. Here, we present direct measurements of significant amounts of OH radicals of up to 1.8⋅10 6 molecules per cubic centimeter during an experimental campaign carried out in a school classroom in Marseille. This concentration is on the same order of magnitude of outdoor OH levels in the urban scenario. We also show that photolysis of HONO is an important source of OH radicals indoors under certain conditions (i.e., direct solar irradiation inside the room). Additionally, the OH concentrations were found to follow a linear dependence with the product J(HONO)⋅[HONO]. This was also supported by using a simple quasiphotostationary state model on the OH radical budget. These findings force a change in our understanding of indoor air quality because the reactivity linked to OH would involve formation of secondary species through chemical reactions that are potentially more hazardous than the primary pollutants in the indoor air.

The OH radical is the most important oxidant in both the tropospheric gas and aqueous phase. Its main sink processes in clouds appear to be reactions with organics but due to the lack of appropriate kinetic data current cloud chemistry models consider only reactions with C 1 and C 2 compounds. Therefore, in this study temperature dependent rate constants for the reactions of the OH radical with organic compounds (!C 2 ) were determined. These investigations were performed by competition kinetics (reference substance: SCN À ). Initially the experimental system was checked reinvestigating kinetic data for OH reactions with formate (R-1) and tert-butanol (R-2) available from literature. For the reactions (R-1) and (R-2) the following results were obtained: À1 for formate and tert-butanol, respectively. Temperature dependent rate constants for the reactions of OH with ethanol (k 3 (298 K)10 ¼ (36 AE 8) kJ mol À1 ; pyruvic acid k 11 (298 K) ¼ (1.2 AE 0.4) Â 10 8 M À1 s À1 ; A 11 ¼ (1.0 AE 0.1) Â 10 12 M À1 s À1 ; E A,11 ¼ (23 AE 4) kJ mol À1 ; pyruvate: k 12 (298 K) ¼ (7 AE 2) Â 10 8 M À1 s À1 ; A 12 ¼ (1.3 AE 0.1) Â 10 12 M À1 s À1 ;

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