An INdoor air Detailed Chemical Model was developed to investigate the impact of ozone reactions with indoor surfaces (including occupants), on indoor air chemistry in simulated apartments subject to ambient air pollution. The results are consistent with experimental studies showing that approximately 80% of ozone indoors is lost through deposition to surfaces. The human body removes ozone most effectively from indoor air per square meter of surface, but the most significant surfaces for C 6 -C 10 aldehyde formation are soft furniture and painted walls owing to their large internal surfaces.Mixing ratios of between 8 and 11 ppb of C 6 -C 10 aldehydes are predicted to form in apartments in various locations in summer, the highest values are when ozone concentrations are enhanced outdoors. The most important aldehyde formed indoors is predicted to be nonanal (5-7 ppb), driven by oxidation-derived emissions from painted walls. In addition, ozone-derived emissions from human skin were estimated for a small bedroom at nighttime with concentrations of nonanal, decanal, and 4-oxopentanal predicted to be 0.5, 0.7, and 0.7 ppb, respectively. A detailed chemical analysis shows that ozone-derived surface aldehyde emissions from materials and people change chemical processing indoors, through enhanced formation of nitrated organic compounds and decreased levels of oxidants. K E Y W O R D SC 6 -C 10 aldehydes, indoor air quality, nitrated organic species, ozone deposition, skin emissions, surface chemistry

People are an important source of pollution indoors, through activities such as cleaning, and also from “natural” emissions from breath and skin. This paper investigates natural emissions in high‐occupancy environments. Model simulations are performed for a school classroom during a typical summer in a polluted urban area. The results show that classroom occupants have a significant impact on indoor ozone, which increases from ~9 to ~20 ppb when the pupils leave for lunch and decreases to ~14 ppb when they return. The concentrations of 4‐OPA, formic acid, and acetic acid formed as oxidation products following skin emissions attained maximum concentrations of 0.8, 0.5, and 0.1 ppb, respectively, when pupils were present, increasing from near‐zero concentrations in their absence. For acetone, methanol, and ethanol from breath emissions, maximum concentrations were ~22.3, 6.6, and 21.5 ppb, respectively, compared to 7.4, 2.1, and 16.9 ppb in their absence. A rate of production analysis showed that occupancy reduced oxidant concentrations, while enhancing formation of nitrated organic compounds, owing to the chemistry that follows from increased aldehyde production. Occupancy also changes the peroxy radical composition, with those formed through isoprene oxidation becoming relatively more important, which also has consequences for subsequent oxidant concentrations.

We report on the development of a modelling consortium for chemistry in indoor environments that connects models over a range of spatial and temporal scales, from molecular to room scales and from sub-nanosecond to days, respectively.

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