# Measurements and Modelling of OH and Peroxy Radicals in an Indoor Environment Under Different Light Conditions and VOC Levels

Eve-Agnès Fiorentino, Hui Chen, Adrien Gandolfo, Victor Lannuque, Karine Sartelet, Henri Wortham

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## To cite this version:

Eve-Agnès Fiorentino, Hui Chen, Adrien Gandolfo, Victor Lannuque, Karine Sartelet, et al., Measurements and Modelling of OH and Peroxy Radicals in an Indoor Environment Under Different Light Conditions and VOC Levels. Atmospheric Environment, 2023, 292, 10.1016/j.atmosenv.2022.119398. insu-03839178

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Submitted on 15 Jul 2025

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# Measurements and modelling of OH and peroxy radicals in an indoor environment under different light conditions and VOC levels

Eve-Agnès Fiorentino<sup>a,*</sup>, Hui Chen<sup>b,1</sup>, Adrien Gandolfo<sup>a,2</sup>, Victor Lannuque<sup>c</sup>, Karine Sartelet<sup>c</sup> and Henri Wortham<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Indoor air

Radical chemistry

Photolysis

Air mixing

Light distribution

VOCs

## ABSTRACT

Indoor measurements of OH and sum of peroxy radicals $XO_2=HO_2+RO_2$ were conducted in a room using window glasses of different transparencies and applying different coatings to the walls. Average OH and $XO_2$ concentrations were found to vary in the range $(0.6-4) \times 10^5$ molecule cm$^{-3}$ and $(1-7) \times 10^7$ molecule cm$^{-3}$ respectively with anti-UV windows, and $(6-10) \times 10^5$ molecule cm$^{-3}$ and $(4-16) \times 10^7$ molecule cm$^{-3}$ respectively with borosilicate glasses. The OH and $XO_2$ concentrations were compared with simulation results obtained using the H$^2$I model, which accounts for the mixing between the sunlit and shaded volumes of the room. Taking into account the measurement uncertainty, the simulated OH concentrations agree with the observations on average while the simulated $XO_2$ concentrations tend to be overestimated. Based on the model results, ozonolysis of unsaturated VOCs and photolysis of HONO and of organic compounds were found to represent the main primary sources of OH and $XO_2$ radicals, the latter being more important in the sunlit volume. Despite the lower rate of the radical initiation in the shaded volume, the difference of radical concentrations in the shaded and sunlit volumes is mitigated by an interplay between the mixing time and the lifetime of $XO_2$ radicals, allowing efficient transport of $XO_2$ radicals into the shaded volume and acting there as a source of OH via radical propagation processes.

## 1. Introduction

Nowadays, the increasing need to improve the insulation of buildings is leading to growing concerns about exposure to indoor air pollution (Nazaroff, 2013). In addition to the pollutants brought from outdoors such as ozone, nitrogen oxides and particulate matter, numerous pollutant sources can be found in the indoor environment (Carslaw et al., 2017). These indoor sources include cleaning products and air fresheners, combustion-based activities such as cooking, heating and smoking, electronic equipment, furnishings, building materials, biocides, microbes, pets and humans (Abbatt and Wang, 2020). As air remains confined, pollutants and their oxidation products can reach very high concentrations (Geiss et al., 2011; Verriele et al., 2016) which potentially induces health effects (Sterling, 2018). Because the OH radical is recognized as the major oxidant of the outdoor atmosphere (e.g., Seinfeld and Pandis (2008)), its role in the transformation of indoor air composition deserves particular investigations.

Outdoors, the dominant primary source of OH radicals is photolysis, and particularly the $O_3$ photolysis,

which occurs at wavelengths shorter than ~ 320 nm (Young et al., 2019). Indoors, the OH radical concentration was at first considered insufficient to influence indoor chemistry. Indeed, the OH radical atmospheric life-time is too short to allow its transport from outdoors, and the actinic flux passing through the windows was regarded as too weak to produce OH by photolysis. Later, it was realized and confirmed by modelling that indoor sources may result in OH concentrations high enough to initiate indoor chemistry. Models predicted that the indoor OH concentrations are typically lower than the diurnal outdoor concentrations but can exceed the nocturnal ones (Weschler and Shields, 1996; Carslaw, 2007), with typical background concentrations of the order of $10^5$ molecule cm$^{-3}$, close to the detection limit of the current measuring instruments (Carslaw et al., 2017; Young et al., 2019).

The first major indoor primary source of OH radicals to be suggested was the ozonolysis of some unsaturated VOCs, namely alkenes and monoterpenes (Carslaw, 2007). But despite a lower actinic flux than outdoors, photolysis reactions can contribute to the OH and $XO_2$ ($HO_2+RO_2$) production too. Outdoor light below ~ 330 nm is usually strongly attenuated by windows, but some species like HONO and carbonyls can be photolysed at wavelengths greater than 330 nm. In addition to natural light, some artificial light sources like halogen, incandescent and fluorescent bulbs can

**ORCID(s):**

<footnote><sup>a</sup>Laboratoire de Chimie de l'Environnement (LCE), UMR7376 CNRS-Université d'Aix-Marseille, Marseille, 13331, France</footnote>

<footnote><sup>b</sup>Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), UMR6115 CNRS-Université d'Orléans, Orléans CEDEX 2, 45071, France</footnote>

<footnote><sup>c</sup>CEREA, Ecole des Ponts ParisTech, EdF R&D, Marne-la-Vallée, 77 455, France</footnote>

<footnote n="1">¹Present address: Key laboratory of organic compound pollution control engineering, School of Environmental and Chemical Engineering, Shanghai University, Shanghai, China</footnote>

<footnote n="2">²Present address: Centre for Research into Atmospheric Chemistry, School of Chemistry, University College Cork, Ireland</footnote>
emit short-wave radiation that can also initiate photochemistry (Young et al., 2019).

In terms of termination, reaction with NO₂ is a major sink for OH (Weschler and Shields, 1996; Carslaw, 2007). Furthermore, besides primary production and termination reactions, the OH concentration is governed by radical propagation reactions which convert OH into XO₂ in reactions with VOCs, and regenerate OH and HO₂ in reactions of XO₂ with NO. The relative importance of propagation, primary initiation and termination depends on the actinic flux and on the concentrations of the compounds present (Sarwar et al., 2002; Carslaw, 2007).

Because of the importance of photolytic processes in the OH chemistry, the variation of the actinic flux is expected to be one of the main factors determining the OH concentration distribution indoors (Won et al., 2019). In a classroom, Alvarez et al. (2013) found a correlation between the maximum measured OH concentrations and peaks of HONO photolysis occurring during direct sunlight periods. They also observed that the OH production by HONO photolysis was one order of magnitude larger than the OH production by d-limonene ozonolysis during these periods. The maximum OH concentration measured by Alvarez et al. (2013) reached values of up to 1.8x10⁶ molecule cm⁻³, which is of the same order of magnitude as outdoor daytime OH concentrations. Similarly large OH concentration was also encountered indoors during a desk cleaning experiment causing terpene emissions (Carslaw et al., 2017). During this experiment, HONO photolysis was a minor contributor, so light distribution was not a critical factor. The HO₂ concentration measured by Alvarez et al. (2013) was one order of magnitude lower than the ones reported by Carslaw (2007), which corresponds to quite different conditions and thus yield a different HONO influence. Despite the importance of OH and the development of numerical models, only a few direct measurements of OH have been performed, making comparisons difficult. HO₂ concentrations have been reported by Carslaw et al. (2017) and Farmer et al. (2019), but regarding XO₂, no published indoor measurements are currently available in the literature, to the authors' knowledge.

To improve the quality of indoor atmospheres, the use of photocatalytic paints has been suggested as a means to reduce VOCs and nitrogen oxides (NOx) concentrations. When irradiated by UV light (< 390 nm), the titanium dioxide (TiO₂) nanoparticles (NPs) contained in these paints are able to oxidize the pollutants that deposit on them. To assess the efficiency of this possible remediation technology, an in-situ campaign was conducted from 27 to 31 October 2016 in Martigues (France) by Gandolfo (2018). Five experiments were performed, during which OH, XO₂, HONO, NOx, O₃ and VOCs concentrations were measured. However, this remediation technology was still in its infancy,

and it is now well established that this first generation of photocatalytic paints was featured by strong VOC release (Salthammer and Fuhrmann, 2007; Auvinen and Wirtanen, 2008; Geiss et al., 2012; Gandolfo et al., 2018). It was suggested that the TiO₂ NPs contained by these paints could degrade the organic constituents of the paint binder, thus leading to pollutant emission, instead of pollutant elimination. To cope with this paradoxical effect, a new photocatalytic paint made of a mineral binder has been conceived. The total VOC emission measured for this paint is 43% lower than the VOC emission of the paint made of fully organic binder (Morin et al., 2019). Other improvements have been made through the development of safer-by-design TiO₂ NPs, coated with a bio-inspired ligand, or grafted onto cellulose nanocrystals (CNC) functionalizing the TiO₂ NPs (Rosset et al., 2021). These specifically developed TiO₂ NPs allow to decrease the degradation of the paint matrix and to reduce the amount of TiO₂ NPs incorporated in it while keeping the same photocatalytic efficiency.

Whatever these progresses, the photocatalytic paints deployed during the campaign performed by Gandolfo (2018) provide an innovative dataset that allows to study an indoor reactivity under high VOC conditions. Two types of windows were used, UV-blocking and UV-transparent, which allows to assess the contribution of important photochemical processes like the HONO photolysis. To interpret the radical observations acquired during this campaign, the H₂I (homogeneous heterogeneous indoor) model, an open source air-quality model developed to simulate compound concentrations in indoor environments (Fiorentino et al., 2021), is used. The OH and XO₂ sources and sinks are analyzed, and the influence of the HONO photolysis on the indoor OH concentration is assessed.

## 2. Methods

### 2.1. Experimental

The measurements were conducted in October 2016 in an office room of a new building situated in Martigues, France (43°24'59"N, 5°02'33"E), less than six months after its construction. To change the type of paint tested from one day to the other, a system of plasterboards was used. Three types of wall coating were tested: plasterboards covered by a 3.5% TiO₂ paint, plasterboards covered with the same paint free from nanoparticles (same organic matrix), denoted reference paint, and no plasterboards (bare walls). The floor was made of waxed concrete and the false ceiling was made of glass wool tiles. Two types of windows were used: an anti-UV glass that blocked 90% of the UV sunlight, and a borosilicate glass that allowed light transmission from 320 nm (UV-transparent). The ventilation rate coefficient kAER [h⁻¹] was determined by analysis of an inert gaseous tracer (CO₂) injected in the room once to thrice a day. A summary of the
<caption>**Table 1**</caption>

Parameters of the experiments: type of light, type of paint, ventilation rate coefficient $k_{AER}$, minimum and maximum temperature and humidity, average total VOC concentration in part per billion carbon, duration.

<table><thead><tr><th>Name</th><th>Date</th><th>UVs</th><th>Paint</th><th>k<sub>AER</sub><br>[h<sup>-1</sup>]</th><th>T<sub>min</sub><br>[°C]</th><th>T<sub>max</sub><br>[°C]</th><th>H<sub>min</sub><br>[%]</th><th>H<sub>max</sub><br>[%]</th><th>&lt;VOC&gt;<br>[ppbC]</th><th>Duration<br>[h]</th></tr></thead><tbody><tr><td>Experiment 1</td><td>27 October 2016</td><td>No</td><td>No paint board</td><td>0.25</td><td>22.7</td><td>26.7</td><td>42</td><td>44</td><td>770</td><td>8.7</td></tr><tr><td>Experiment 2</td><td>28 October 2016</td><td>No</td><td>0% TiO<sub>2</sub></td><td>0.29</td><td>24.3</td><td>27.8</td><td>39</td><td>45</td><td>1063</td><td>6.2</td></tr><tr><td>Experiment 3</td><td>29 October 2016</td><td>No</td><td>3.5% TiO<sub>2</sub></td><td>0.19</td><td>21.7</td><td>27.2</td><td>44</td><td>49</td><td>2911</td><td>7.9</td></tr><tr><td>Experiment 4</td><td>30 October 2016</td><td>Yes</td><td>3.5% TiO<sub>2</sub></td><td>0.46</td><td>23.2</td><td>29.6</td><td>39</td><td>43</td><td>2484</td><td>7.6</td></tr><tr><td>Experiment 5</td><td>31 October 2016</td><td>Yes</td><td>0% TiO<sub>2</sub></td><td>0.38</td><td>23.8</td><td>29.2</td><td>39</td><td>47</td><td>1015</td><td>7.0</td></tr></tbody></table>

parameters featuring the five performed experiments are presented in Tab. 1.

All the instrumentation, except a spectroradiometer and a CO₂-RH-Temperature gauge was installed in an adjacent room. The air was sampled from the room under study via interfaces arranged in the wall separating the rooms. Air mixing in the studied room was enforced by two fans installed in the room corners. Apart from these elements, the measurement's room was kept unfurnished. It was 3.2 m wide, 4.1 m long, and 2.5 m high, resulting in a ground surface of 13.1 m² and a room volume $V_{room} = 32.8 m^3$. The total surface of the room, including floor, ceiling, walls and window was 62.7 m², giving a surface to volume ratio of ~ 1.9 m⁻¹. The surface covered by plasterboards was 24 m². The window opening was of 1.44 m² (1.8 m long and 0.8 m high).

HONO was monitored using a LOPAP (LOng Path Absorption Photometer) commercialized by QUMA GmbH, at a sampling flow rate of about 1 slm from point A in Fig. 1a through a 1/8 inch 0.5 m long Teflon line. Point A was approximately 0.5 m above the floor. The overall uncertainty on the HONO concentration was 10%, for concentration values comprised between 4 and 3000 ppt. The HONO lower limit of detection was estimated at 3 ppt.

NOx, ozone and VOCs were monitored by chemiluminescence, spectrophotometry and with a PTR-ToF-MS using a time-of-flight mass spectrometer (PTR-ToF-MS 8000 instrument, Ionicon Analytik GmbH), respectively. The ozone analyzer was the O342M instrument marketed by the company Environnement SA (now ENVEA). NOx was monitored using the commercial device CLD 88p Eco Physics. Air for these three instruments was sampled from the centre of the measurement's room at 1.5 m height (point C in Fig. 1a) using a 1/4 inch 4 m long Teflon line. The sampling flow rate was of 2 slm. VOCs were also measured outdoors, in front of the window of the experimental room using a 4 m long 1/4 inch Teflon line. An automatic valve switching between both lines allowed to monitor VOCs alternatively for 10 minutes inside and 5 minutes outside, with a measurement frequency of one per minute. In terms of uncertainty, observing that the HONO and NO₂ levels were comparable, with a relative humidity of

the order of 40%, the global interference of HONO with the NOx measurement was estimated at 30%. The other possible interferences are considered as negligible. The NOx lower limit of detection was estimated at 330 ppt. The ozone analyzer can measure O₃ concentrations ranging from 0.4 ppb to 10 ppm. The VOCs detected by the PTR-ToF-MS were identified based on molecular fragmentation patterns (see Gandolfo et al. (2018) for similar work with laboratory measurements). The list of the VOCs detected and identified with the PTR-ToF-MS is provided in Tab. A.1 in Appendix, with their average concentrations for each experiment.

OH and sum of peroxy radicals ($XO_2=HO_2+RO_2$) concentrations were measured using a chemical ionization mass spectrometer (CIMS), based on the principle developed by Eisele and Tanner (1991) and described by Kukui et al. (2008, 2012), Michoud et al. (2012). Sampling was performed using a PTFE interface built into the room wall (point B in Fig. 1a). The sampling aperture was located 1 m away from the window-bearing wall, about 1 m above floor level, and was in the room's shaded area up to 14:30 (GMT). From this time and to the end of the day, the CIMS inlet was exposed to a direct sun radiation.

The overall estimated calibration accuracy ($2\sigma$) for OH was about 25%. $RO_2$ were detected by their conversion in the CIMS reactor to $HO_2$ in reaction with NO. By assuming a unitary yield of $HO_2$ for all $RO_2$, the estimated calibration uncertainty was about 30%. However, some $RO_2$ produce $HO_2$ with a yield lower than unity. In particular, the organic peroxy radicals that are converted into nitrates in the CIMS reactor are not detected. This introduces a bias in the measurement of the sum of $XO_2$ radicals which is systematically underestimated. The magnitude of this underestimation depends on the $XO_2$ composition, which is specific to the room. Based on the $XO_2$ composition derived from the chemical mechanism used in this paper (see section 2.2.1), this bias is estimated at 17% for the present campaign (see section 3.1).

The measurement's precision for 5 minutes of integration time during this campaign was about 25% (or $5 \times 10^5$ molecule cm⁻³, whichever is larger) for OH, and about 5% for $XO_2$. The lower limits of detection for OH and $XO_2$ radicals at S/N=2 and a 15 minutes
<figure><img src="image_3.png" /><figcaption>**Figure 1:** (a) Schematic representation of the experimental room. All the instruments were installed in an adjacent room. HONO was monitored from point A, OH and peroxy radicals were sampled via a PTFE interface built into the room wall (point B), NOx, ozone and VOCs were monitored from point C. The spectroradiometer is placed near the window and records the actinic flux in the office. (b) $J_{HONO}$ daily variation during Experiment 4 (borosilicate windows) in the sunlit and shaded areas. Note that the values are multiplied by 10 in the shaded area. (c) Actinic flux spectra of the direct and indirect lights. The spectra measured with anti-UV windows were recorded at 11:00 (GMT) on 27 and 29 October, respectively; the spectra measured with borosilicate windows were recorded at 11:32 (GMT) and 12:36 (GMT) respectively, on 30 October.</figcaption></figure>

integration time was $4 \times 10^5$ molecule cm$^{-3}$ and $1 \times 10^6$ molecule cm$^{-3}$, respectively.

Finally, the light actinic flux in the experimental room was recorded using a commercial Metcon spectroradiometer in front of the window, close to the CIMS inlet. This instrument allows to measure the actinic flux in the range 290-660 nm and the photolysis frequencies of NO₂, HONO, O₃ (O¹D), HCHO and NO₃, with a time resolution of one minute. Fig. 1b presents diurnal profiles of the HONO photolysis frequencies that were measured with borosilicate windows in the sunlit and shaded areas, while Fig. 1c shows the actinic flux spectra that were recorded with the two types of glass. The borosilicate windows allow radiations from 320 nm to pass through, while the anti-UV ones allow radiations from 395 nm only. In both spectra, the maximum wavelength transmitted is 660 nm. In both cases, the magnitude of the actinic flux is about one order of magnitude larger in the direct light than in the shaded area.

## 2.2. Simulations

### 2.2.1. Presentation of the model

The H²I model is a 0D chemistry model developed for indoor environments. It incorporates the elements necessary to simulate indoor concentrations, including ventilation, intake of outdoor pollutants, indoor emissions and deposition. The chemical mechanism corresponds to the Regional Atmospheric Chemistry Mechanism (RACM2, Goliff et al. (2013)), modified by Lannuque et al. (2021) for the oxidation of toluene and xylene. In this scheme, the chemical species are grouped in a reduced number of compounds, depending on their reactivity properties. The error on the evolutions of the OH and HO₂ radicals concentrations has been assessed by comparison with deterministic chemical mechanisms generated by the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) (Aumont et al., 2005) and the Master Chemical Mechanism (MCM; v3.3.1) (Jenkin et al., 2003; Bloss et al., 2005). This error amounts to 11% for OH and
4% for HO₂ during the oxidation of toluene, against 6% for OH and 7% HO₂ during the oxidation of xylene, in conditions of high VOC concentrations.

One of the specificities of the H²I model is to consider two parts in the room, one irradiated by direct light, and another one irradiated indirectly by diffuse and reflected light. It is thus a two-box model, representing two volumes communicating via a mixing term.

The volumes of each of the two parts of the room were determined using the approach described in Tlili et al. (prep). First, the position of the sun relative to the window was calculated for each hour of the day. From these information, an extrapolation of the sunbeams from the window was performed with the Autodesk Revit 2018 software. The directly irradiated surface area and room volume, denoted S<sub>box</sub><sup>L</sup> and V<sub>box</sub><sup>L</sup> respectively, were then calculated by vertical and horizontal projections of the indoor solar flux using Autodesk Autocad 2016. The diurnal evolutions of S<sub>box</sub><sup>L</sup> and V<sub>box</sub><sup>L</sup> are presented in Fig. A.1 in Appendix. Their maximum values are reached between 11:10 and 11:30 (GMT), and amount to 6.5 m² and 6.6 m³ respectively. The sunlit area thus represents a small fraction of the total room, only 20% at the maximum.

The mixing term is implemented similarly to Furtaw Jr. et al. (1996). This term is proportional to the air velocity in the room, denoted u<sub>inf</sub>, to the concentration difference between the two volumes and to the area of the surface separating the volumes, denoted S<sub>gas</sub>, which was determined using the same methods as the ones used for S<sub>box</sub><sup>L</sup> and V<sub>box</sub><sup>L</sup>. The air velocity controls the air mixing, which is recognized as an important component of indoor air quality modelling (Lakey et al., 2021). The value of u<sub>inf</sub> was calibrated by adjusting the simulated O<sub>3</sub>, NO, NO<sub>2</sub> and HONO concentrations to the observed ones, and is set to 0.24 m.s<sup>-1</sup> (see Fiorentino et al. (2021)).

The air exchange with the outdoor is implemented using the ventilation rate coefficient k<sub>AER</sub> determined experimentally. The measurements obtained for a given day were almost constant, thus allowing to perform the simulations using a daily average.

A deposition loss term is incorporated in the model for NO<sub>3</sub>, HNO<sub>3</sub>, HNO<sub>4</sub>, H<sub>2</sub>O<sub>2</sub>, HO<sub>2</sub> and the RO<sub>2</sub> listed in Tab. A.2 in Appendix. No OH deposition is implemented, considering that OH is too reactive to be affected by deposition at the room scale. The deposition term is computed using the approach suggested by Mendez et al. (2017b) (see Fiorentino et al. (2021) for details). We consider a unity uptake coefficient, so that deposition is only controlled by transport. For each compound, the diffusion coefficient is computed using the critical temperature and volume of a representative molecule.

### 2.2.2. Calculation of the photolysis rates

The evolution of the photolysis rate coefficients J<sub>i</sub> [s<sup>-1</sup>] with daytime in the sunlit and shaded volumes is parameterized as a function of the solar zenith angle θ using the photolysis frequencies J<sub>i</sub><sup>ref</sup> derived from the spectroradiometer actinic flux measurements for i=HONO, NO<sub>2</sub>, HCHO, H<sub>2</sub>O<sub>2</sub> and NO<sub>3</sub> at some reference time t<sub>ref</sub> corresponding to zenith angle θ<sub>ref</sub>:

$$J_i(\theta) = J_i^{\text{ref}} \exp\left(-\frac{(\theta - \theta_{\text{ref}})}{B}\right), \quad (1)$$

where B=10.7 (see Fiorentino et al. (2021) for details). Light was monitored at one point for each volume, shaded and sunlit, which amounts to considering that the actinic flux is homogeneous throughout each volume. This assumption is correct in the direct light, but in the shade, the intensity of diffuse light decreases exponentially with distance from the window (Zhou et al., 2021). Due to the fact that radical formation by photolysis is minor in the shade, we consider the effect of this approximation as being negligible.

Using anti-UVs windows, the actinic flux in the direct and indirect light was measured at t<sub>ref</sub>=11:00 (GMT) during Experiments 1 and 3, respectively. The light intensity computed in the UV domain (λ < 400 nm) is 0.55 W.m<sup>-2</sup> in the direct light, and 0.0096 W.m<sup>-2</sup> in the indirect light. Using borosilicate windows, the actinic flux in the direct and indirect light was measured during Experiment 4, at t<sub>ref</sub>=11:32 (GMT) and t<sub>ref</sub>=12:36 (GMT), respectively. The intensity computed in the UV domain is 27.42 W.m<sup>-2</sup> in the direct light, and 0.76 W.m<sup>-2</sup> in the indirect light. Note that the weather was fine during the first four experiments, making the solar irradiation comparable during these days. Cloudy periods of limited duration occurred during the fifth experiment, which may have caused variations of intensity on this day during which light was not measured.

The J<sub>i</sub> values of the RACM2 compounds corresponding to the species whose photolysis frequencies were not measured are computed using Eq. (1), with J<sub>i</sub><sup>ref</sup> calculated using cross section and quantum yield values from the RACM2 chemical mechanism implemented in the Polyphemus air-quality modelling platform (Kim et al., 2009) (see Sander et al. (2006) for reference).

### 2.2.3. Model constraints

The concentrations of the organic and inorganic compounds that were measured are assigned to their experimental values every 10 s, by linear interpolation of the measured values. The measured compounds are NO, NO<sub>2</sub>, HONO, O<sub>3</sub> and the VOCs listed in Tab.A.3 in Appendix, with their RACM2 definitions and median concentrations for each experiment. The observed concentration profiles of the species used to constrain the model are presented in Appendix (Figs. A.2-A.3) for the inorganic compounds NO, NO<sub>2</sub>, HONO, O<sub>3</sub> and
some representative VOCs. The C₁₀H₁₆ monoterpenes detected by the PTR-ToF-MS are represented by the compound LIM (d-limonene and other cyclic dieneterpenes), considering that d-limonene was reported as the most frequently detected terpene indoors (Weschler and Shields, 1996).

During Experiment 1, an artificial NO₂ injection of about 56 ppb was performed at 13:30 (GMT), and in the course of Experiment 5, unexpected welding work occurred in the building at approximately 13:20 (GMT), which acted as a NOx injection. Some of the VOC concentrations were affected by the latter (see formaldehyde concentration in Fig. A.3 for instance).

### 2.2.4. Initialization of the concentrations

The compounds which were not measured are initialized by running a spin-up simulation, in the course of which the air exchange acts exclusively as a sink for the unconstrained species, aside from OH and HO₂. The outdoor OH and HO₂ concentrations are set to realistic values of 10⁶ molecule cm⁻³ and 10⁸ molecule cm⁻³, respectively. The rest of the unconstrained species outdoor concentrations are set to zero.

During the spin-up simulation, the indoor and outdoor concentrations of the measured species, the photolysis rates, the box volumes, the temperature and the humidity are fixed to their values at the start time. The spin-up simulation is stopped when the OH radical concentration has reached a steady state, i.e. when the relative difference between OH concentrations of two iterations is less than 10⁻⁸%.

## 3. Results

### 3.1. Overview of the VOC conditions

The paints coating the wall panels constituted a major source of VOCs. These paints had been characterized experimentally prior to the indoor campaign (Gandolfo et al., 2018). For the photocatalytic paint, high emission rates had been found for formaldehyde and acetaldehyde. An important number of saturated and unsaturated carboxylic acids and carbonyls had been observed, as well as alkenes and unsaturated cyclic compounds. Even in the dark, the photocatalytic paint had shown higher formaldehyde, acetaldehyde, formic acid, acetic acid and acrylic acid emission rates than the paint made of the same binder but free from TiO₂ nanoparticles (reference paint).

Among the VOCs identified during the campaign under study, five of them were subject to guide values proposed by the French Indoor Air Quality Observatory (OQAI), namely formaldehyde, acetaldehyde, acrolein, benzene and naphtalene. The concentrations of these compounds were higher during the experiments performed with photocatalytic paints. Exceedances of the OQAI guide values were observed for formaldehyde, acetaldehyde and acrolein in these experiments.

Going into details, it is noteworthy that during the experiments performed with reference paint, the formaldehyde concentration was close to the OQAI guide value of 40 ppb, which can be attributed to the building itself, owing to the freshness of its construction. Using photocatalytic paints, the formaldehyde concentration surpassed the guide value by up to 200% (Experiment 3) and 265% (Experiment 4). The acetaldehyde and acrolein concentrations measured with reference paint were significant, likely for the same reason as formaldehyde, but remained below their guide values of 90 ppb and 3 ppb, respectively. These concentrations exceeded the guide values by up to 15% for acetaldehyde and 100% for acrolein in the presence of photocatalytic paint.

These observations are in agreement with the increased emission rates observed in laboratory experiments using photocatalytic paints, apart from the naphtalene which had not been not detected in the photoreactor. The monoterpane and sesquiterpane species are eligible to the same finding, considering that they had not been detected in laboratory experiments, and that their concentrations were multiplied by two during this campaign, in the presence of photocatalytic paint. In general, the species that were found at high concentrations, mainly acids, aldehydes and alcohols, followed the same trend as formaldehyde and acetaldehyde. The VOCs concentrations measured with and without UVs were comparable, which can indicate that the photocatalyst was activated even by visible light (> 400 nm).

More details about the VOC data analysis are provided by Gandolfo et al. (2020).

### 3.2. Radicals concentrations

Measured OH and XO₂ concentration profiles are presented in Fig. 2. In Experiments 1 and 2 performed with reference paint and anti-UV windows, the observed XO₂ and OH concentrations were about 4 × 10⁷ molecule cm⁻³ and less than 7 × 10⁵ molecule cm⁻³, respectively. Experiments 3 and 4 were performed with photocatalytic paints, which generated a large release of VOCs (see Fig. A.3 in Appendix). Experiments 4 and 5 were performed with borosilicate windows, which allowed the occurrence of photolytic processes. The combination of both factors results in a doubling of the XO₂ concentration in Experiment 4. The weighted mean OH and XO₂ concentrations and weighted standard errors of the weighted means are presented in Tab. 2. The weights are inversely proportional to the square of the measurement uncertainties.

The simulated OH, HO₂ and RO₂ concentrations are compared to the OH and XO₂ measurements in Figs. 2a and 2b-2c, respectively. The list of the RACM2 compounds forming the RO₂ radicals in this study is provided in Tab. 3. The error bars correspond to the measurement precision without accounting for the
Measurements and modelling of OH and XO₂ radicals in an indoor environment

<figure><img src="image_4.png" /><figcaption>**Figure 2:** Comparison between simulated (lines) and observed (black dots) radical concentrations: (a) OH radicals in the sunlit (red line) and shaded (yellow line) volumes, (b) XO₂ radicals in the sunlit volume, (c) XO₂ radicals in the shaded volume. A detailed description of this figure can be found in section 3.2.</figcaption></figure>
<caption>**Table 2**</caption>

Weighted averages $\bar{x}_{wtd}$ [molecule cm$^{-3}$] and weighted standard errors ($s_{\bar{x}}$)$_{wtd}$ of the weighted averages of the measured OH and XO₂ concentrations. The weights are inversely proportional to the square of the measurement uncertainties.

<table><thead><tr><th rowspan="2"></th><th colspan="2">27 October</th><th colspan="2">28 October</th><th colspan="2">29 October</th><th colspan="2">30 October</th><th colspan="2">31 October</th></tr><tr><th>x&#773;<sub>wtd</sub></th><th>(s<sub>x&#773;</sub>)<sub>wtd</sub></th><th>x&#773;<sub>wtd</sub></th><th>(s<sub>x&#773;</sub>)<sub>wtd</sub></th><th>x&#773;<sub>wtd</sub></th><th>(s<sub>x&#773;</sub>)<sub>wtd</sub></th><th>x&#773;<sub>wtd</sub></th><th>(s<sub>x&#773;</sub>)<sub>wtd</sub></th><th>x&#773;<sub>wtd</sub></th><th>(s<sub>x&#773;</sub>)<sub>wtd</sub></th></tr></thead><tbody><tr><td>[OH]<sup>exp</sup> (×10<sup>5</sup>)</td><td>0.62</td><td>0.20</td><td>1.61</td><td>0.27</td><td>3.97</td><td>0.28</td><td>9.79</td><td>0.47</td><td>5.55</td><td>0.51</td></tr><tr><td>[XO<sub>2</sub>]<sup>exp</sup> (×10<sup>7</sup>)</td><td>1.49</td><td>0.077</td><td>1.59</td><td>0.089</td><td>7.09</td><td>0.26</td><td>15.6</td><td>0.88</td><td>4.47</td><td>0.26</td></tr></tbody></table>

calibration uncertainty. The dashed vertical line at 14:30 (GMT) denotes the light transition on the CIMS instrument: the radical species were measured in the shade before this marker, and in the direct sunlight from this marker until the end of the day.

To assess the difference between two simulated profiles, or between a simulated profile and measurements, two metrics are used: the Mean Normalized Bias Difference (MNBD)

$$ \text{MNBD} = \frac{1}{N} \sum_{i}^{N} \left( \frac{C_{i}^{k} - C_{i}^{j}}{C_{i}^{j}} \right) \cdot 100\%, \quad (2) $$

and Mean Normalized Gross Difference (MNGD)

$$ \text{MNGD} = \frac{1}{N} \sum_{i}^{N} \left( \frac{|C_{i}^{k} - C_{i}^{j}|}{C_{i}^{j}} \right) \cdot 100\%, \quad (3) $$

where $C_{i}^{j}$ is the concentration profile considered as reference and $C_{i}^{k}$ is the evaluated one. N is the number of values constituting the reference profile. Tab. 4 summarizes the statistics obtained for the OH and XO₂ concentrations presented in Fig. 2. To take into account the uncertainty of the measured data in this analysis, the MNBD and MNGD computed between measured and simulated profiles are presented with the deviation $\sigma_{\text{MNBD}}$ derived as

$$ \sigma_{\text{MNBD}} = \frac{1}{N} \sqrt{\sum_{i}^{N} \left( \frac{C_{i}^{k}}{C_{i}^{j}} \right)^{2} \left( \left( \frac{E_{C_{i}^{k}}}{C_{i}^{k}} \right)^{2} + \left( \frac{E_{C_{i}^{j}}}{C_{i}^{j}} \right)^{2} \right)} \quad (4) $$

where $E_{C_{i}^{j}}$ is the measurement uncertainty at time i and $E_{C_{i}^{k}}$ the simulation uncertainty. Given the error values estimated by Lannuque et al. (2021) (see section 2.2.1), we consider a simulation uncertainty $E_{C_{i}^{k}}$ of 10% for OH and 20% for XO₂.

The scores presented in Tab. 4 do not take into account the error caused by the estimation of the concentrations used as input. The influence of the measurement's uncertainty on the model's results can be assessed by performing sensitivity tests. Let $C_{i}^{\text{meas}}$ be the concentration of species i constrained to the observed data. The varied concentration $C_{i}^{\text{test}}$ used as input is defined as

$$ C_{i}^{\text{test}}(t) = C_{i}^{\text{meas}}(t_{0}) + \alpha(C_{i}^{\text{meas}}(t) - C_{i}^{\text{meas}}(t_{0})) \quad (5) $$

<caption>**Table 3**</caption>

List of the RACM2 compounds included in the RO₂ group.

<table><thead><tr><th>Species</th><th>Definition</th></tr></thead><tbody><tr><td>ACO<sub>3</sub></td><td>Acetyl peroxy radicals</td></tr><tr><td>ACTP</td><td>Peroxy radicals formed from ACT</td></tr><tr><td>ADDC</td><td>Aromatic-HO adduct from CSL</td></tr><tr><td>APIP</td><td>Peroxy radicals formed from API</td></tr><tr><td>BAL1</td><td>Peroxy radicals formed from BALD</td></tr><tr><td>BAL2</td><td>Peroxy radicals formed from BALD</td></tr><tr><td>BALP</td><td>Peroxy radicals formed from BALD</td></tr><tr><td>BENP</td><td>Peroxy radicals formed from BEN</td></tr><tr><td>ETEP</td><td>Peroxy radicals formed from ETE (Ethene)</td></tr><tr><td>ETHP</td><td>Peroxy radicals formed from ETH (Ethane)</td></tr><tr><td>HC3P</td><td>Peroxy radicals formed from HC3</td></tr><tr><td>HC5P</td><td>Peroxy radicals formed from HC5</td></tr><tr><td>HC8P</td><td>Peroxy radicals formed from HC8</td></tr><tr><td>ISOP</td><td>Peroxy radicals formed from ISO+HO</td></tr><tr><td>KETP</td><td>Peroxy radicals formed from KET</td></tr><tr><td>LIMP</td><td>Peroxy radicals formed from LIM</td></tr><tr><td>MACP</td><td>Peroxy radicals formed from MACR+HO</td></tr><tr><td>MCP</td><td>Peroxy radical formed from MACR+HO which does not form MPAN</td></tr><tr><td>MEKP</td><td>Peroxy radicals formed from MEK (Methyl ethyl ketone)</td></tr><tr><td>MO2</td><td>Methyl peroxy radical</td></tr><tr><td>MVKP</td><td>Peroxy radicals formed from MVK (Methyl vinyl ketone)</td></tr><tr><td>OLIP</td><td>Peroxy radicals formed from OLI</td></tr><tr><td>OLNN</td><td>NO<sub>3</sub>-alkene adduct reacting to form carbonitrates</td></tr><tr><td>OLTP</td><td>Peroxy radicals formed from OLT</td></tr><tr><td>ORAP</td><td>Peroxy radical formed from ORA2+ HO reaction</td></tr><tr><td>PER1</td><td>Peroxy intermediate formed from TOL</td></tr><tr><td>PER2</td><td>Peroxy intermediate formed from TOL</td></tr><tr><td>RCO<sub>3</sub></td><td>Higher saturated acyl peroxy radicals</td></tr><tr><td>TLP1</td><td>Peroxy radicals formed from TOL</td></tr><tr><td>TOLP</td><td>Peroxy radicals formed from TOL</td></tr><tr><td>TR2</td><td>Peroxy radicals formed from TOL</td></tr><tr><td>UALP</td><td>Peroxy radicals formed from UALD</td></tr><tr><td>XY2</td><td>Peroxy radicals formed from XYL</td></tr><tr><td>XYL1</td><td>Peroxy radicals formed from XYL</td></tr><tr><td>XYLP</td><td>Peroxy radicals formed from XYL</td></tr><tr><td>XYO2</td><td>Peroxy radicals formed from XYO</td></tr><tr><td>XYOP</td><td>Peroxy radicals formed from XYO</td></tr></tbody></table>

where $t_{0}$ is the start time of the simulation and $\alpha$ is a multiplicative factor. Four cases are tested: (a) the average HONO concentration is increased by 179% ($\alpha$=8), (b) the average NO concentration is decreased by 54% ($\alpha$=1.5), (c) the average NO concentration
<caption>Table 4: Assessment of the simulated OH and XO₂ concentrations vs. measurements. The measured concentrations are denoted by “exp”. The simulated concentrations are denoted by “L” in the sunlit volume, “S” in the shaded volume. By “red”, we denote the reduced simulated XO₂ concentrations, i.e. the XO₂ concentrations as if they were detected by the CIMS. The difference between two curves is quantified by computing the MNBD and MNCD (see Eqs. 2-3): (a) [OH]L vs [OH]S (first row), [XO₂]L vs [XO₂]S (second row), (b) [OH]L vs [OH]exp (first row), [OH]S vs [OH]exp (second row), (c) [XO₂]L vs [XO₂]S (first row), [XO₂]S,red vs [XO₂]exp (second row). The σ<sub>MNBD</sub> values (see Eq. 4) provide and error estimate of these metrics. (d) [XO₂]L,red vs [XO₂]exp (first row), [XO₂]S,red vs [XO₂]exp (second row).</caption>

Measurements and modelling of OH and XO₂ radicals in an indoor environment

<table><thead><tr><th></th><th colspan="3">27 October</th><th colspan="3">28 October</th><th colspan="3">29 October</th><th colspan="3">30 October</th><th colspan="3">31 October</th></tr><tr><th></th><th>MNBD</th><th>MNGD</th><th>σ<sub>MNBD</sub></th><th>MNBD</th><th>MNGD</th><th>σ<sub>MNBD</sub></th><th>MNBD</th><th>MNGD</th><th>σ<sub>MNBD</sub></th><th>MNBD</th><th>MNGD</th><th>σ<sub>MNBD</sub></th><th>MNBD</th><th>MNGD</th><th>σ<sub>MNBD</sub></th></tr></thead><tbody><tr><td>a. [OH]</td><td>2.83</td><td>2.83</td><td>-</td><td>3.52</td><td>3.52</td><td>-</td><td>1.19</td><td>1.19</td><td>-</td><td>51.83</td><td>51.83</td><td>-</td><td>108.37</td><td>108.37</td><td>-</td></tr><tr><td>[XO₂]</td><td>1.85</td><td>1.85</td><td>-</td><td>2.37</td><td>2.37</td><td>-</td><td>0.87</td><td>0.87</td><td>-</td><td>9.43</td><td>9.43</td><td>-</td><td>21.93</td><td>21.93</td><td>-</td></tr><tr><td>b. [OH]</td><td>302.24</td><td>352.02</td><td>1023.33</td><td>-331.55</td><td>-242.47</td><td>-</td><td>874.38</td><td>-75.15</td><td>59.66</td><td>0.84</td><td>-67.63</td><td>73.13</td><td>0.40</td><td>76.29</td><td>97.01</td><td>6.15</td></tr><tr><td>[XO₂]</td><td>290.66</td><td>341.61</td><td>998.47</td><td>-326.84</td><td>-236.74</td><td>-</td><td>858.19</td><td>-75.46</td><td>59.94</td><td>0.84</td><td>-76.72</td><td>81.27</td><td>0.36</td><td>-19.89</td><td>51.79</td><td>3.06</td></tr><tr><td>c. [OH]</td><td>13.44</td><td>13.44</td><td>-</td><td>14.85</td><td>14.85</td><td>-</td><td>16.47</td><td>16.47</td><td>-</td><td>16.24</td><td>16.24</td><td>-</td><td>17.17</td><td>17.17</td><td>-</td></tr><tr><td>d. [OH]</td><td>193.80</td><td>193.80</td><td>0.09</td><td>10.59</td><td>65.64</td><td>0.04</td><td>70.77</td><td>86.77</td><td>0.06</td><td>64.69</td><td>71.32</td><td>0.05</td><td>158.58</td><td>158.58</td><td>0.08</td></tr><tr><td>[XO₂]</td><td>189.25</td><td>189.25</td><td>0.09</td><td>9.27</td><td>65.49</td><td>0.04</td><td>69.74</td><td>86.40</td><td>0.06</td><td>55.77</td><td>67.23</td><td>0.05</td><td>122.34</td><td>124.45</td><td>0.07</td></tr></tbody></table>

is increased by 85% ($\alpha$ = 0.25), (d) the average VOC concentration is decreased by 63% ($\alpha$ = 0.25). The tests are performed using the data of Experiment 4 (see Fig. A.4 in Appendix). The influence of the input variations on the OH and HO₂ concentrations is estimated by computing the MNBD between the base case (OH and HO₂ concentrations simulated with observed data) and the test cases in the shaded and sunlit volumes (Tab. 5). The resulting differences are significant, which illustrates that the measurement's uncertainties contribute to the model uncertainty for a large part, in addition the uncertainty of the chemical scheme.

In the experiments performed with anti-UV windows, the OH concentrations simulated in the light and shade are very similar (see Tab. 4a, 3.5% of difference at most). Because of the window properties, the photochemistry was restricted to the NO₃ photo-lysis ($J \sim 10^{-1} s^{-1}$), and to a lesser extent, to the NO₂, MGLY (methylglyoxal) and DCB (unsaturated dicarbonyls) photolyses ($J \sim 10^{-4} s^{-1}$), which produce HO₂ and ACO₃ (Acetyl peroxy radicals), but not OH radicals. The absence of photo-induced production of OH radicals with anti-UV windows leads to similar simulated OH concentrations in the sunlit and shaded volumes. Conversely, in the experiments performed with borosilicate windows, the difference between the OH concentrations simulated in the light and in the shade can reach a factor of two.

Regarding XO₂ radicals, the concentrations in the sunlit and shaded volumes are very close in the absence of UV light (less than 2.5% of discrepancy), whereas a gap, significantly lower than for OH, appears in the experiments performed with borosilicate windows, of 9.43% and 21.93%, respectively.

In terms of agreement between simulations and measurements, the MNBD computed for the OH profiles is of the order of 300% during the two first experiments, against 50-75% for the three others. The OH concentrations during the two first experiments were close to the limit of detection and are associated to significant uncertainty (see σ<sub>MNBD</sub> values in Tab. 4). The comparison between simulations and measurements in less reliable in these experiments. Graphically, the simulated OH concentration profiles are within the measurement uncertainty, except in Experiment 4.

As explained in section 2.1, the CIMS instrument detects the peroxy radicals based on their reactions with NO, by assuming a unitary yield of HO₂ for all RO₂. The bias introduced by this assumption can be quantified by comparing the sum of the simulated RO₂ to the sum of the simulated RO₂ weighted by their RACM2 yield in HO₂, which ranges between 0 and 1. Let $\alpha_i$ be the yield of HO₂ for particular RO₂ with NO. Fig. 2 presents the “reduced” XO₂ (light blue) and RO₂ (light orange) concentrations given by the sum of the $\alpha_i C_i$, i.e. the radicals concentrations as if they were
<caption>**Table 5**</caption>

Sensitivity tests: variations in OH and HO₂ concentrations estimated by computing the MNBD in the sunlit (MNBD<sup>L</sup>) and shaded (MNBD<sup>S</sup>) volumes with respect to the base case. Case (b): increased HONO. Case (c): decreased NO. Case (d): increased NO. Case (e): decreased VOC.

<table><thead><tr><th rowspan="2"></th><th colspan="2">case (b)</th><th colspan="2">case (c)</th><th colspan="2">case (d)</th><th colspan="2">case (d)</th></tr><tr><th>MNBD<sup>L</sup></th><th>MNBD<sup>S</sup></th><th>MNBD<sup>L</sup></th><th>MNBD<sup>S</sup></th><th>MNBD<sup>L</sup></th><th>MNBD<sup>S</sup></th><th>MNBD<sup>L</sup></th><th>MNBD<sup>S</sup></th></tr></thead><tbody><tr><td>[OH]</td><td>46</td><td>21</td><td>-44</td><td>-55</td><td>33</td><td>40</td><td>132</td><td>101</td></tr><tr><td>[HO₂]</td><td>23</td><td>22</td><td>34</td><td>37</td><td>-27</td><td>-28</td><td>-42</td><td>-42</td></tr></tbody></table>

detected by the CIMS. The MNBD calculated between “reduced” and “raw” simulated XO₂ does not exceed ~17% (Tab.4c). It should be noted, however, that the yields αᵢ are average values calibrated for the RACM2 model species, and may not completely capture the measurement bias of the CIMS instrument.

Based on this reduced simulated XO₂ concentration, the agreement between measurements and simulations amounts to ~10%, 70% and 60% for Experiment 2, 3 and 4, respectively (Tab.4d). The difference increases in the last experiment, and up to a factor of 3 in the first experiment. The variations of the XO₂ concentrations are well reproduced, but the concentration values are generally overestimated. We underline that this discrepancy is an aggregation of the discrepancies existing between simulated and detected XO₂, for HO₂ and all the RO₂ listed in Tab. 3.

### 3.3. Analysis of radicals sources and sinks

This section focuses on the sources and sinks that arise from reactivity, without considering the transport by forced convection. The OH and XO₂ production rates as a function of time for Experiments 3 and 4 are presented in Figs. 3-4. The average production and consumption rates in molecule cm⁻³ s⁻¹ for each experiment are presented in Tab. A.4 in Appendix. The percentages of contribution of each source and sink are presented in Tab. A.5 in Appendix. In addition, given the very high VOC concentrations measured, a detailed analysis of the radical consumption and production by VOCs in percentage of contribution is provided in Tabs. 6-8. Tab. 6 presents the OH consumption by VOCs in the shaded volume only, considering that the differences with the sunlit volume are negligible (< 0.33%). Tabs. 7-8 present the HO₂ and RO₂ production by VOCs in the sunlit and shaded volumes, by distinguishing the primary production reactions and the propagation reactions.

In all experiments, the contribution from NO₃ reactions is minor because the NO₃ concentrations are very low. The NO₃ radical is formed when both O₃ and NO₂ concentrations are high (Young et al., 2019). In the present study, the measured O₃ concentrations are low (see Fig. A.2), which results in NO₃ concentrations of the order of 10⁴ molecule cm⁻³.

<caption>**Table 6**</caption>

Average rates of OH consumption by VOCs in the shaded volume, in percentages of contribution.

<table><thead><tr><th>Species</th><th>Exp 1</th><th>Exp 2</th><th>Exp 3</th><th>Exp 4</th><th>Exp 5</th></tr></thead><tbody><tr><td>OLT</td><td>18.95</td><td>18.68</td><td>22.88</td><td>22.38</td><td>15.33</td></tr><tr><td>OLI</td><td>9.22</td><td>9.46</td><td>12.18</td><td>8.12</td><td>9.29</td></tr><tr><td>XYL</td><td>16.06</td><td>13.52</td><td>5.21</td><td>6.47</td><td>12.37</td></tr><tr><td>ISO</td><td>1.00</td><td>1.24</td><td>2.13</td><td>2.24</td><td>1.06</td></tr><tr><td>LIM</td><td>13.48</td><td>10.38</td><td>5.58</td><td>6.10</td><td>9.12</td></tr><tr><td>HCHO</td><td>12.46</td><td>10.21</td><td>7.73</td><td>10.68</td><td>8.43</td></tr><tr><td>ACD</td><td>7.66</td><td>10.64</td><td>10.69</td><td>13.12</td><td>16.47</td></tr><tr><td>ALD</td><td>11.96</td><td>15.24</td><td>23.33</td><td>21.83</td><td>16.84</td></tr><tr><td>MACR</td><td>2.15</td><td>2.34</td><td>1.88</td><td>2.14</td><td>3.01</td></tr><tr><td>ROH</td><td>1.89</td><td>3.38</td><td>4.46</td><td>2.77</td><td>2.37</td></tr><tr><td>ORA2</td><td>1.09</td><td>1.25</td><td>1.07</td><td>1.04</td><td>1.17</td></tr><tr><td>Others</td><td>4.08</td><td>3.67</td><td>2.87</td><td>3.10</td><td>4.54</td></tr></tbody></table>

#### 3.3.1. Primary production

In the shade, or using anti-UV windows, the HONO photolysis is negligible and VOC ozonolysis is the major primary source of OH radicals. Nevertheless, it represents less than 7% of the total OH production. In the two last experiments, the HONO photolysis can occur in the direct light, which makes the OH primary production increase up to 30% and 42% of the total OH production in this volume. The strength of this OH radical source is determined by J<sub>HONO</sub>, shown in Fig. 4, and by the HONO gas-phase concentration, which was observed to increase during the day.

The HO₂ primary production is ensured by VOC+O₃ reactions and VOC photolysis, but due to the low O₃ concentration (less than 5 ppb as shown in Fig. A.2), ozonolysis represents only 1-2% of the total HO₂ production. The primary sources of HO₂ by ozonolysis are alkenes (OLT and OLI), unsaturated dicarbonyls (DCB2 and DCB1), LIM and, for the last experiment, EPX (epoxide formed in TOL, XYL and XYO reactions). In the direct light, the photolysis of methylglyoxal (MGLY) and unsaturated dicarbonyls is efficient, even without UV light. It can constitute a significant HO₂ primary source. With borosilicate windows, the photolysis of HCHO reinforces the HO₂ primary production, especially in Experiment 4, where the photocatalytic paint generates a massive release of formaldehyde (see section 3.1). In total, the HO₂
<figure><img src="image_5.png" /><figcaption>**Figure 3:** OH, HO₂ and RO₂ budgets in the shaded and sunlit volumes computed from the concentrations measured on 29 October.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>(a) OH budget in the light</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(b) OH budget in the shade</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(c) HO₂ budget in the light</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(d) HO₂ budget in the shade</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(e) RO₂ budget in the light</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(f) RO₂ budget in the shade</figcaption></figure>

**Figure 4:** OH, HO₂ and RO₂ budgets in the shaded and sunlit volumes computed from the concentrations measured on 30 October.
<caption>**Table 7**</caption>

Average rates of HO₂ and RO₂ production by VOCs in the shaded volume, either as primary source or by propagation, in percentages of contribution.

<table><thead><tr><th rowspan="2"></th><th colspan="2">Exp1</th><th colspan="2">Exp2</th><th colspan="2">Exp3</th><th colspan="2">Exp4</th><th colspan="2">Exp5</th></tr><tr><th>source</th><th>propag</th><th>source</th><th>propag</th><th>source</th><th>propag</th><th>source</th><th>propag</th><th>source</th><th>propag</th></tr></thead><tbody><tr><td colspan="11"><strong>VOCs forming HO<sub>2</sub></strong></td></tr><tr><td>HCHO</td><td>0.00</td><td>57.58</td><td>0.00</td><td>52.50</td><td>0.00</td><td>48.91</td><td>3.08</td><td>58.54</td><td>1.29</td><td>46.44</td></tr><tr><td>XYL</td><td>0.00</td><td>13.13</td><td>0.00</td><td>12.31</td><td>0.00</td><td>5.84</td><td>0.00</td><td>6.27</td><td>0.00</td><td>12.06</td></tr><tr><td>MGLY</td><td>0.83</td><td>0.00</td><td>0.18</td><td>0.00</td><td>0.19</td><td>0.00</td><td>0.48</td><td>0.00</td><td>0.97</td><td>0.00</td></tr><tr><td>DCB1</td><td>1.32</td><td>0.36</td><td>0.61</td><td>0.23</td><td>0.43</td><td>0.17</td><td>0.82</td><td>0.19</td><td>2.40</td><td>0.83</td></tr><tr><td>DCB2</td><td>1.62</td><td>0.45</td><td>0.78</td><td>0.29</td><td>0.54</td><td>0.22</td><td>1.03</td><td>0.24</td><td>2.92</td><td>1.01</td></tr><tr><td>EPX</td><td>0.93</td><td>1.50</td><td>0.60</td><td>0.90</td><td>0.41</td><td>0.69</td><td>0.60</td><td>0.81</td><td>2.25</td><td>3.47</td></tr><tr><td>MOH</td><td>0.00</td><td>3.47</td><td>0.00</td><td>3.66</td><td>0.00</td><td>2.53</td><td>0.00</td><td>2.10</td><td>0.00</td><td>4.18</td></tr><tr><td>ROH</td><td>0.00</td><td>8.75</td><td>0.00</td><td>17.40</td><td>0.00</td><td>28.26</td><td>0.00</td><td>15.17</td><td>0.00</td><td>13.05</td></tr><tr><td>ORA2</td><td>0.00</td><td>1.82</td><td>0.00</td><td>1.60</td><td>0.00</td><td>1.21</td><td>0.00</td><td>1.32</td><td>0.00</td><td>0.96</td></tr><tr><td>OLT</td><td>2.03</td><td>0.00</td><td>2.52</td><td>0.00</td><td>3.63</td><td>0.00</td><td>3.45</td><td>0.00</td><td>1.89</td><td>0.00</td></tr><tr><td>OLI</td><td>2.30</td><td>0.00</td><td>2.93</td><td>0.00</td><td>4.47</td><td>0.00</td><td>2.82</td><td>0.00</td><td>2.68</td><td>0.00</td></tr><tr><td>LIM</td><td>1.76</td><td>0.00</td><td>1.67</td><td>0.00</td><td>1.06</td><td>0.00</td><td>1.06</td><td>0.00</td><td>1.29</td><td>0.00</td></tr><tr><td>UALD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.05</td><td>0.00</td><td>0.02</td><td>0.00</td></tr><tr><td>ACD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.03</td><td>0.00</td><td>0.02</td><td>0.00</td></tr><tr><td>ALD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.30</td><td>0.00</td><td>0.13</td><td>0.00</td></tr><tr><td>BALD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.13</td><td>0.00</td><td>0.02</td><td>0.00</td></tr><tr><td>Others</td><td>0.40</td><td>1.76</td><td>0.28</td><td>1.53</td><td>0.27</td><td>1.17</td><td>0.36</td><td>1.14</td><td>0.94</td><td>1.18</td></tr><tr><td><strong>Total</strong></td><td>11.19</td><td>88.82</td><td>9.57</td><td>90.42</td><td>11.00</td><td>89.00</td><td>14.21</td><td>85.78</td><td>16.82</td><td>83.18</td></tr><tr><td colspan="11"><strong>VOCs forming RO<sub>2</sub></strong></td></tr><tr><td>OLT</td><td>0.24</td><td>16.26</td><td>0.32</td><td>19.40</td><td>0.33</td><td>20.95</td><td>0.30</td><td>17.33</td><td>0.19</td><td>13.71</td></tr><tr><td>OLI</td><td>3.17</td><td>7.91</td><td>4.40</td><td>9.82</td><td>4.80</td><td>11.15</td><td>2.96</td><td>6.29</td><td>3.23</td><td>8.31</td></tr><tr><td>LIM</td><td>1.89</td><td>11.57</td><td>1.95</td><td>10.78</td><td>0.89</td><td>5.11</td><td>0.87</td><td>4.73</td><td>1.21</td><td>8.15</td></tr><tr><td>ISO</td><td>0.01</td><td>0.86</td><td>0.01</td><td>1.29</td><td>0.02</td><td>1.95</td><td>0.02</td><td>1.74</td><td>0.01</td><td>0.94</td></tr><tr><td>ACD</td><td>0.00</td><td>6.57</td><td>0.00</td><td>11.05</td><td>0.00</td><td>9.78</td><td>0.00</td><td>10.16</td><td>0.00</td><td>14.73</td></tr><tr><td>UALD</td><td>0.00</td><td>0.44</td><td>0.00</td><td>0.67</td><td>0.00</td><td>0.67</td><td>0.00</td><td>0.69</td><td>0.00</td><td>0.41</td></tr><tr><td>MGLY</td><td>0.15</td><td>0.07</td><td>0.04</td><td>0.04</td><td>0.03</td><td>0.02</td><td>0.07</td><td>0.03</td><td>0.16</td><td>0.12</td></tr><tr><td>MACR</td><td>0.00</td><td>1.89</td><td>0.00</td><td>2.45</td><td>0.00</td><td>1.72</td><td>0.00</td><td>1.67</td><td>0.00</td><td>2.71</td></tr><tr><td>ORA2</td><td>0.00</td><td>0.93</td><td>0.00</td><td>1.30</td><td>0.00</td><td>0.98</td><td>0.00</td><td>0.81</td><td>0.00</td><td>1.05</td></tr><tr><td>ALD</td><td>0.00</td><td>10.26</td><td>0.00</td><td>15.83</td><td>0.00</td><td>21.36</td><td>0.04</td><td>16.91</td><td>0.02</td><td>15.06</td></tr><tr><td>XYL</td><td>0.00</td><td>11.34</td><td>0.00</td><td>11.56</td><td>0.00</td><td>3.93</td><td>0.00</td><td>4.12</td><td>0.00</td><td>9.11</td></tr><tr><td>PAN</td><td>10.17</td><td>0.00</td><td>3.06</td><td>0.00</td><td>5.10</td><td>0.00</td><td>11.60</td><td>0.00</td><td>9.10</td><td>0.00</td></tr><tr><td>MPAN</td><td>1.60</td><td>0.00</td><td>0.41</td><td>0.00</td><td>0.54</td><td>0.00</td><td>1.10</td><td>0.00</td><td>0.98</td><td>0.00</td></tr><tr><td>PPN</td><td>13.55</td><td>0.00</td><td>4.34</td><td>0.00</td><td>9.70</td><td>0.00</td><td>17.76</td><td>0.00</td><td>9.36</td><td>0.00</td></tr><tr><td>Others</td><td>0.20</td><td>0.90</td><td>0.13</td><td>1.17</td><td>0.06</td><td>0.90</td><td>0.10</td><td>0.69</td><td>0.37</td><td>1.07</td></tr><tr><td><strong>Total</strong></td><td>30.98</td><td>69.00</td><td>14.66</td><td>85.36</td><td>21.47</td><td>78.52</td><td>34.82</td><td>65.17</td><td>24.63</td><td>75.37</td></tr></tbody></table>

production by VOC photolysis represents 11% to 19% of the total HO₂ production.

The RO₂ primary production is comprised of VOC+O₃ reactions and VOC photolysis and represents 4-9% of the total RO₂ production. The main VOC group that generates RO₂ by ozonolysis is OLI, followed by LIM. The distribution of the VOC contributions is comparable in both volumes, indicating that the influence of photolytic reactions on the RO₂ chemistry is minor.

### 3.3.2. Termination

The main OH termination route is NO₂, aside from Experiment 2 where NO and NO₂ weigh comparably.

HO₂ radicals are eliminated by reacting with NO, HO₂, RO₂ and by deposition. There are two types of

reaction with NO, a propagation one which leads to the formation of OH radicals, and a termination one, which leads to the formation of HNO₃. It should be mentioned that the latter is still under discussion (see e.g., Atkinson et al. (2004), Burkholder et al. (2020)). In these experiments, it represents 0.5% of the total HO₂ consumption. Depending on the considered experiment, the main HO₂ sink can be deposition, NO or RO₂. There is no clear main HO₂ sink, except in Experiment 5, where deposition is particularly efficient. However, we recall that deposition is only controlled by transport in this paper, which maximizes its effect.

The RO₂ radicals can be terminated by reaction with HO₂ and by deposition. Both routes represent a small fraction of the total RO₂ loss.
<caption>**Table 8**</caption>

Average rates of HO₂ and RO₂ production by VOCs in the sunlit volume, either as primary source or by propagation, in percentages of contribution.

<table><thead><tr><th rowspan="2"></th><th colspan="2">Exp1</th><th colspan="2">Exp2</th><th colspan="2">Exp3</th><th colspan="2">Exp4</th><th colspan="2">Exp5</th></tr><tr><th>source</th><th>propag</th><th>source</th><th>propag</th><th>source</th><th>propag</th><th>source</th><th>propag</th><th>source</th><th>propag</th></tr></thead><tbody><tr><td colspan="11"><strong>VOCs forming HO<sub>2</sub></strong></td></tr><tr><td>HCHO</td><td>0.00</td><td>37.06</td><td>0.00</td><td>45.91</td><td>0.00</td><td>43.51</td><td>34.13</td><td>32.14</td><td>14.38</td><td>33.65</td></tr><tr><td>XYL</td><td>0.00</td><td>8.45</td><td>0.00</td><td>10.76</td><td>0.00</td><td>5.19</td><td>0.00</td><td>3.40</td><td>0.00</td><td>8.34</td></tr><tr><td>MGLY</td><td>12.60</td><td>0.00</td><td>3.85</td><td>0.00</td><td>3.98</td><td>0.00</td><td>3.03</td><td>0.00</td><td>6.07</td><td>0.00</td></tr><tr><td>DCB1</td><td>11.32</td><td>0.23</td><td>4.44</td><td>0.20</td><td>3.53</td><td>0.15</td><td>2.94</td><td>0.10</td><td>6.52</td><td>0.53</td></tr><tr><td>DCB2</td><td>13.93</td><td>0.29</td><td>5.70</td><td>0.26</td><td>4.46</td><td>0.19</td><td>3.74</td><td>0.13</td><td>8.07</td><td>0.65</td></tr><tr><td>EPX</td><td>0.58</td><td>0.96</td><td>0.52</td><td>0.78</td><td>0.37</td><td>0.61</td><td>0.23</td><td>0.43</td><td>0.85</td><td>2.22</td></tr><tr><td>MOH</td><td>0.00</td><td>2.23</td><td>0.00</td><td>3.20</td><td>0.00</td><td>2.25</td><td>0.00</td><td>1.16</td><td>0.00</td><td>3.05</td></tr><tr><td>ROH</td><td>0.00</td><td>5.63</td><td>0.00</td><td>15.21</td><td>0.00</td><td>25.15</td><td>0.00</td><td>8.26</td><td>0.00</td><td>9.15</td></tr><tr><td>ORA2</td><td>0.00</td><td>1.17</td><td>0.00</td><td>1.40</td><td>0.00</td><td>1.08</td><td>0.00</td><td>0.72</td><td>0.00</td><td>0.70</td></tr><tr><td>OLT</td><td>1.27</td><td>0.00</td><td>2.17</td><td>0.00</td><td>3.20</td><td>0.00</td><td>1.30</td><td>0.00</td><td>0.71</td><td>0.00</td></tr><tr><td>OLI</td><td>1.44</td><td>0.00</td><td>2.52</td><td>0.00</td><td>3.95</td><td>0.00</td><td>1.06</td><td>0.00</td><td>1.01</td><td>0.00</td></tr><tr><td>LIM</td><td>1.10</td><td>0.00</td><td>1.43</td><td>0.00</td><td>0.93</td><td>0.00</td><td>0.40</td><td>0.00</td><td>0.49</td><td>0.00</td></tr><tr><td>UALD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.54</td><td>0.00</td><td>0.15</td><td>0.00</td></tr><tr><td>ACD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.40</td><td>0.00</td><td>0.27</td><td>0.00</td></tr><tr><td>ALD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>3.54</td><td>0.00</td><td>1.47</td><td>0.00</td></tr><tr><td>BALD</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>1.42</td><td>0.00</td><td>0.23</td><td>0.00</td></tr><tr><td>Others</td><td>0.60</td><td>1.13</td><td>0.31</td><td>1.34</td><td>0.40</td><td>1.04</td><td>0.33</td><td>0.62</td><td>0.68</td><td>0.81</td></tr><tr><td><strong>Total</strong></td><td>42.84</td><td>57.15</td><td>20.94</td><td>79.06</td><td>20.82</td><td>79.17</td><td>53.06</td><td>46.96</td><td>40.90</td><td>59.10</td></tr><tr><td colspan="11"><strong>VOCs forming RO<sub>2</sub></strong></td></tr><tr><td>OLT</td><td>0.22</td><td>15.67</td><td>0.31</td><td>19.22</td><td>0.32</td><td>20.82</td><td>0.23</td><td>18.96</td><td>0.11</td><td>15.34</td></tr><tr><td>OLI</td><td>2.98</td><td>7.62</td><td>4.29</td><td>9.73</td><td>4.74</td><td>11.09</td><td>2.25</td><td>6.88</td><td>1.91</td><td>9.00</td></tr><tr><td>LIM</td><td>1.78</td><td>11.15</td><td>1.90</td><td>10.68</td><td>0.87</td><td>5.08</td><td>0.66</td><td>5.25</td><td>0.72</td><td>9.17</td></tr><tr><td>ISO</td><td>0.01</td><td>0.83</td><td>0.01</td><td>1.27</td><td>0.02</td><td>1.94</td><td>0.01</td><td>1.89</td><td>0.00</td><td>1.03</td></tr><tr><td>ACD</td><td>0.00</td><td>6.33</td><td>0.00</td><td>10.94</td><td>0.00</td><td>9.72</td><td>0.11</td><td>11.16</td><td>0.07</td><td>16.32</td></tr><tr><td>UALD</td><td>0.00</td><td>0.42</td><td>0.00</td><td>0.66</td><td>0.00</td><td>0.66</td><td>0.10</td><td>0.75</td><td>0.03</td><td>0.46</td></tr><tr><td>MGLY</td><td>3.50</td><td>0.06</td><td>0.88</td><td>0.04</td><td>0.64</td><td>0.02</td><td>0.87</td><td>0.03</td><td>1.55</td><td>0.12</td></tr><tr><td>MACR</td><td>0.00</td><td>1.82</td><td>0.00</td><td>2.43</td><td>0.00</td><td>1.71</td><td>0.00</td><td>1.82</td><td>0.00</td><td>3.00</td></tr><tr><td>ORA2</td><td>0.00</td><td>0.90</td><td>0.00</td><td>1.29</td><td>0.00</td><td>0.97</td><td>0.00</td><td>0.89</td><td>0.00</td><td>1.17</td></tr><tr><td>ALD</td><td>0.00</td><td>9.88</td><td>0.00</td><td>15.68</td><td>0.00</td><td>21.21</td><td>1.01</td><td>18.40</td><td>0.38</td><td>16.27</td></tr><tr><td>XYL</td><td>0.00</td><td>10.92</td><td>0.00</td><td>11.45</td><td>0.00</td><td>3.90</td><td>0.00</td><td>4.52</td><td>0.00</td><td>9.88</td></tr><tr><td>PAN</td><td>9.55</td><td>0.00</td><td>2.98</td><td>0.00</td><td>5.03</td><td>0.00</td><td>8.79</td><td>0.00</td><td>5.37</td><td>0.00</td></tr><tr><td>MPAN</td><td>1.50</td><td>0.00</td><td>0.39</td><td>0.00</td><td>0.53</td><td>0.00</td><td>0.84</td><td>0.00</td><td>0.58</td><td>0.00</td></tr><tr><td>PPN</td><td>12.71</td><td>0.00</td><td>4.22</td><td>0.00</td><td>9.56</td><td>0.00</td><td>13.45</td><td>0.00</td><td>5.53</td><td>0.00</td></tr><tr><td>Others</td><td>1.27</td><td>0.87</td><td>0.47</td><td>1.16</td><td>0.25</td><td>0.90</td><td>0.40</td><td>0.76</td><td>0.80</td><td>1.21</td></tr><tr><td><strong>Total</strong></td><td>33.52</td><td>66.47</td><td>15.45</td><td>84.55</td><td>21.96</td><td>78.02</td><td>28.72</td><td>71.31</td><td>17.05</td><td>82.97</td></tr></tbody></table>

### 3.3.3. Transfers with the HNO₄ and PANs reservoir species

As a rule, the reaction HO₂+NO₂ → HNO₄ cannot be strictly considered as a termination reaction for HO₂. It leads to the formation of HNO₄ which can regenerate HO₂ following the reaction HNO₄ → HO₂+NO₂, thus acting as a temporary reservoir. The difference between consumption by NO₂ and production from HNO₄ is used to determine the influence of HNO₄ on HO₂. If this difference is positive, HNO₄ rather acts as a sink than a source of HO₂. As shown by the values presented in Tab. A.5, the contribution of HNO₄ to the HO₂ initiation or termination is non-significant, indicating that the HO₂, NO₂ and HNO₄ concentrations are close to an equilibrium. Albeit small, this difference is generally negative and a little more

pronounced in the direct light. In this study, the HNO₄ concentration is of the order of 10⁻³ ppb.

Similarly to the HNO₄/HO₂ equilibrium, the decomposition of PANs (peroxyacetyl nitrates) into RO₂+NO₂ cannot be strictly considered as a primary source of RO₂, and RO₂+NO₂ → PANs as a termination path for RO₂ in general. Precisely, RO₂+NO₂ → PANs refers to the reactions ACO₃+NO₂ → PAN (Peroxyacetyl nitrate and higher saturated PANs), RCO₃+NO₂ → PPN (Peroxypropionyl nitrate), MACP+NO₂ → MPAN (Peroxymethacryloylnitrate and other higher peroxyacylnitrates from isoprene oxidation) and BAL2+NO₂ → ONIT (Organic nitrate), while PANs → RO₂+NO₂ accounts for the decomposition of PAN, PPN and MPAN. In these experiments, the PAN and PPN concentrations are of the order of 10⁻¹ ppb, while
the MPAN concentration is of the order of $10^{-2}$ ppb. The difference between consumption by NO₂ and production by PANs is positive, which means that PANs rather act as sinks for RO₂ in these experiments. The contribution from the RO₂+NO₂↔PANs turnover represents 1% to 9% of the RO₂ loss, and is again more important in the light than in the shade. It is clearly the main termination path, whatever the volume considered.

### 3.3.4. Propagation reactions

In these experiments, formation and consumption fluxes are dominated by propagation reactions, for the three radical types considered. To ease the visualization, a schema of the radicals chemistry occurring with borosilicate windows is presented in Fig. 5, with the average radical fluxes computed for Experiment 4, in both volumes.

The recycling reaction NO+HO₂ is the major source of OH radicals. This reaction constitutes up to 99% of the total HO₂ consumption.

The OH consumption by VOCs represents more than 97% of the total OH consumption for each experiment. The main VOCs that consume OH are OLT (terminal alkenes) and ALD (C3 and higher aldehydes) followed by ACD (acetaldehyde), XYL (xylene and less reactive aromatics), HCHO, OLI (internal alkenes) and LIM (d-limonene and other cyclic diene-terpenes).

The HO₂ production by recycling reactions arises from RO₂+NO, VOC+OH, RO₂+RO₂ reactions and from the decomposition of intermediate aromatics. Intermediate aromatics are peroxy radicals formed by the reaction of OH with xylene, toluene and less reactive aromatics. In particular, these reactions lead to the formation of the RACM2 compounds XY2, XYO2 and TR2, which dissociate themselves to form XYLP, XYOP, TOLP and other radicals (see definitions of radicals in Tab. 3). In turn, XYLP, XYOP and TOLP dissociate to form additional radicals. In these experiments, the dissociation of intermediate aromatics represents 4% of the total HO₂ production, while the production from RO₂+RO₂ reactions is close to negligible. The HO₂ production caused by VOC+OH reactions represents 14% to 20% of the total HO₂ production. In all experiments, the main VOCs that convert OH to HO₂ are HCHO (close to 50% of the total), followed by XYL and alcohols (ROH and MOH). Above all, the major part of the HO₂ production comes from the RO₂+NO reactions.

Propagation reactions leading to RO₂ are VOC+OH, RO₂+HO₂, RO₂+RO₂, intermediate aromatics decomposition and RO₂+NO reactions. Whereas the VOC+OH reactions represent 14-20% of the total HO₂ production, VOC+OH reactions cause more than 60% of the total RO₂ production, against 20-28% for the RO₂+NO reactions. The VOCs that propagate OH radicals to form RO₂ are OLT, ALD, ACD, OLI,

<caption>**Table 9**</caption>

Average rates of HO₂ and RO₂ production by RO₂ in the shaded volume, in percentages of contribution.

<table><thead><tr><th></th><th>Exp 1</th><th>Exp 2</th><th>Exp 3</th><th>Exp 4</th><th>Exp 5</th></tr></thead><tbody><tr><td colspan="6"><strong>RO₂ forming HO₂</strong></td></tr><tr><td>PER2</td><td>4.69</td><td>3.91</td><td>1.43</td><td>1.96</td><td>3.46</td></tr><tr><td>XY2</td><td>5.02</td><td>4.17</td><td>1.53</td><td>2.02</td><td>3.42</td></tr><tr><td>MO2</td><td>17.22</td><td>20.02</td><td>20.00</td><td>21.15</td><td>26.40</td></tr><tr><td>ETHP</td><td>20.08</td><td>23.27</td><td>30.08</td><td>27.53</td><td>24.54</td></tr><tr><td>OLTP</td><td>19.35</td><td>19.05</td><td>22.06</td><td>23.59</td><td>15.44</td></tr><tr><td>OLIP</td><td>10.05</td><td>10.27</td><td>12.52</td><td>9.14</td><td>9.90</td></tr><tr><td>ISOP</td><td>1.17</td><td>1.43</td><td>2.34</td><td>2.70</td><td>1.18</td></tr><tr><td>MCP</td><td>0.62</td><td>0.66</td><td>0.50</td><td>0.63</td><td>0.83</td></tr><tr><td>UALP</td><td>0.46</td><td>0.58</td><td>0.62</td><td>0.79</td><td>0.40</td></tr><tr><td>LIMP</td><td>17.73</td><td>13.59</td><td>6.91</td><td>8.29</td><td>11.82</td></tr><tr><td>KETP</td><td>1.82</td><td>1.75</td><td>0.96</td><td>1.10</td><td>1.49</td></tr><tr><td>Others</td><td>1.80</td><td>1.30</td><td>1.05</td><td>1.10</td><td>1.11</td></tr><tr><td colspan="6"><strong>RO₂ forming RO₂</strong></td></tr><tr><td>KETP</td><td>1.13</td><td>1.02</td><td>0.57</td><td>0.65</td><td>0.80</td></tr><tr><td>MACP</td><td>1.14</td><td>1.07</td><td>0.86</td><td>0.85</td><td>1.20</td></tr><tr><td>BALP</td><td>0.52</td><td>0.87</td><td>1.10</td><td>0.95</td><td>0.21</td></tr><tr><td>BAL1</td><td>0.52</td><td>0.87</td><td>1.10</td><td>0.95</td><td>0.22</td></tr><tr><td>RCO₃</td><td>38.96</td><td>42.53</td><td>57.50</td><td>51.44</td><td>40.55</td></tr><tr><td>ACO₃</td><td>27.16</td><td>29.96</td><td>29.72</td><td>33.28</td><td>39.12</td></tr><tr><td>XY2</td><td>24.14</td><td>18.79</td><td>7.04</td><td>9.25</td><td>14.08</td></tr><tr><td>XYLP</td><td>5.25</td><td>4.08</td><td>1.53</td><td>2.01</td><td>3.06</td></tr><tr><td>Others</td><td>1.18</td><td>0.81</td><td>0.58</td><td>0.61</td><td>0.75</td></tr></tbody></table>

LIM and XYL. The contribution of RO₂+HO₂ and RO₂+RO₂ reactions is negligible, while the decomposition of intermediate aromatics represents up to 9% of the total RO₂ production.

In terms of loss, the RO₂ radicals can be consumed by propagation through NO, RO₂, and dissociation. Similarly to HO₂, the propagation reaction with NO constitutes the major part of the RO₂ consumption, more than 80% of the total RO₂ loss in all cases. The dissociation of intermediate aromatics into other RO₂ can represent up to 13% of the RO₂ loss and seem less important in the experiments performed with photocatalytic paints.

### 3.4. Estimation of the radical lifetimes

The production of HO₂ and RO₂ radicals by RO₂ + NO reactions or the decomposition of intermediate aromatics is presented in Tab. 9, to identify the most active RO₂. Results are presented for the shaded volume only, since the intensity and distribution of this production is comparable in both volumes (3.6% of difference at most). The termination of RO₂ radicals by HO₂+RO₂ and RO₂+RO₂ reactions appears in Tab. A.5 as a minor sink, and is thus not investigated in the present section.

The main RO₂ producing HO₂ are MO2, ETHP, OLTP, OLIP and LIMP. MO₂ and ETHP originate from a variety of reactions, whereas OLTP, OLIP and LIMP are produced by OLT, OLI and LIM only.
<figure><img src="image_12.png" /><figcaption>Figure 5: Schema of the ROx chemistry in the shaded in sunlit volumes. The rates values are the ones computed for Experiment 4 and are expressed in 10⁶ molecule cm⁻³ s⁻¹.</figcaption></figure>

The main RO₂ producing RO₂ are ACO₃, RCO₃ and XY2. The radical XY2 is formed by XYL. Like MO2 and ETHP, ACO₃ and RCO₃ are formed by a variety of reactions.

The median lifetimes computed for OH and for the main RO₂ identified are summarized in Tab. 10. The lifetimes as a function of time are presented in Appendix (Fig. A.5).

The median OH radical's lifetime ranges between 3 and 15 ms in these experiments. This time can be compared to the mixing time $t_{mix}$, estimated as

$$t_{\text{mix}} = L/u_{\text{inf}} \qquad (6)$$

where L is the room characteristic length. By considering $L = (V_{\text{room}})^{1/3} = 3.2\,\text{m}$, Eq. (6) gives $t_{mix} = 13.3\,\text{s}$. As a result, the OH radical has no time
<table><caption>**Table 10**
Median lifetimes [s] of OH and a selection of RO₂.</caption><thead><tr><th></th><th>Exp1</th><th>Exp2</th><th>Exp3</th><th>Exp4</th><th>Exp5</th></tr></thead><tbody><tr><td>OH</td><td>0.0146</td><td>0.0108</td><td>0.0035</td><td>0.0039</td><td>0.0104</td></tr><tr><td>ACO<sub>3</sub></td><td>0.656</td><td>0.417</td><td>0.682</td><td>0.753</td><td>0.553</td></tr><tr><td>RCO<sub>3</sub></td><td>0.656</td><td>0.417</td><td>0.682</td><td>0.754</td><td>0.553</td></tr><tr><td>ETHP</td><td>3.14</td><td>1.18</td><td>2.36</td><td>4.29</td><td>2.07</td></tr><tr><td>MO2</td><td>3.62</td><td>1.35</td><td>2.72</td><td>4.85</td><td>2.38</td></tr><tr><td>LIMP</td><td>6.90</td><td>2.58</td><td>5.18</td><td>9.11</td><td>4.52</td></tr><tr><td>OLT</td><td>6.91</td><td>2.58</td><td>5.18</td><td>9.15</td><td>4.52</td></tr><tr><td>OLI</td><td>6.91</td><td>2.58</td><td>5.18</td><td>9.16</td><td>4.52</td></tr></tbody></table>

to be transported and homogenized at the scale of the room, which creates the difference between the simulated concentrations in the shaded and the sunlit volumes during the morning, when the OH photolytic production is the most efficient. At the time when the OH sampling point becomes illuminated, the difference between the simulated OH concentrations in the shaded and sunlit volumes becomes small, in agreement with measurements which do not show distinct change of OH concentration by passing from shadow to illuminated conditions.

Regarding RO₂, the median life time is ~ 0.6 s for ACO₃ and RCO₃, ~ 2.8 s for ETHP and MO₂, and ~ 5.7 s for LIMP, OLTP and OLIP. These values are close to the mixing time tₘix, which explains why the model predicts very small difference for RO₂ in the sunlit and shaded volumes, in agreement with measurements.

## 4. Discussion

The use of different windows during the campaign made it possible to compare the influence of photolytic processes on radical concentrations with and without UV light. When using anti-UV windows, the photolysis of MGLY and unsaturated dicarbonyls is found to be a non negligible source of HO₂. The other terms are comparable in both volumes since ozone and alkenes are supposed to be homogeneously distributed in the room. With borosilicate windows, the OH and HO₂ productions increase in the area irradiated by direct light, due to the photolysis of HONO, and also HCHO which is emitted in large quantity when using photo-catalytic paints. However, the difference of OH and XO₂ radical concentrations in the shaded and sunlit volumes is mitigated by a fast, relative to the XO₂ lifetime, mixing of XO₂ over the entire room volume, acting in the shadow volume as the major source of OH via radical propagation processes. The OH radicals are essentially produced by propagation of HO₂ radicals, which are mainly formed by NO+RO₂ reactions. RO₂ are themselves formed by propagation of OH radicals through the reactions VOC+OH (see Fig. 5). Thanks to the air mixing, the RO₂ produced by VOC oxidation in the direct light can travel from light to shade and

regenerate OH radicals in the whole room by radical propagation. As a result, even if the OH primary production mainly occurs in the volume irradiated by direct light, the important VOC level observed in these experiments attenuates the OH concentration difference between light and shade.

We now compare the results presented in this paper to the study performed by Alvarez et al. (2013), where a strong increase of measured OH concentration was observed when the OH sampling aperture was illuminated by direct light. Conversely, no significant increase in OH concentration is observed in the present work when moving from shade to sun. Despite these different behaviours, the maximum OH concentrations observed in this work with borosilicate windows are close to OH concentrations observed by Alvarez et al. (2013) in presence of direct light.

The inorganic compounds and OH radicals concentrations measured in both studies are summarized in Tab. 11. It should be mentioned that similarly to the 27 October 2016 experiment, an artificial NO₂ injection was performed on the 22 July 2011, thus explaining the high NO₂ concentration that day. We can observe that the inorganic compound concentrations reported in this study are lower than the ones of Alvarez et al. (2013), even in the absence of NO₂ injection. The difference in NOₓ and O₃ concentrations between both studies can derive from the location of the experiments (outlying suburb vs. city centre).

Second, the VOC concentration level reported in this study is much higher. Using the mean concentrations of the eight VOCs detected on the 21 July 2011 and reported by Mendez et al. (2017b), the VOC concentration is 80 ppbC for Alvarez et al. (2013). In the present paper, the VOC concentration amounts to 2484 ppbC and 1015 ppbC for Experiments 4 and 5, respectively (see Tab. 1). In Alvarez et al. (2013), the very low VOC concentrations decrease the RO₂ production, thus the OH production by propagation from RO₂ to HO₂, and then from HO₂ to OH. In the present paper, using borosilicate windows, the OH production in the direct light is increased by HONO photolysis, and so does the XO₂ production by VOC photolysis and VOC+OH reactions.

To assess the relative influence of the HONO photolysis on the radical initiation in the total room volume, the HONO photolysis rate normalized to volume, r<sub>HONO</sub>, defined as

$$r_{\text{HONO}} = \frac{J_{\text{HONO}}^{L} V^{L} + J_{\text{HONO}}^{S} V^{S}}{V^{L} + V^{S}} [\text{HONO}] \quad (7)$$

is compared to the ozonolysis rate in Fig. 6, for the two experiments performed with borosilicate windows. We can observe that the HONO photolysis acts as a ROₓ (OH+HO₂+RO₂) source comparable with ozonolysis during the morning. However, it becomes less significant in the afternoon due to the increasing VOCs
<caption>**Table 11**</caption>

Comparison between the concentrations measured by Alvarez et al. (2013) and the ones reported in this paper. The inorganic concentrations of Alvarez et al. (2013) are converted from the values reported in ppb by Mendez et al. (2017a), assuming a temperature of 26°C. The averages presented in this table are simple averages (no weighting by uncertainty).

<table><thead><tr><th rowspan="2">Study</th><th rowspan="2">UVs</th><th>O<sub>3</sub></th><th>NO</th><th>NO<sub>2</sub></th><th>HONO</th><th colspan="2">OH (10<sup>5</sup>)</th></tr><tr><th>[ppb]<br>mean</th><th>[ppb]<br>mean</th><th>[ppb]<br>mean</th><th>[ppb]<br>mean</th><th>[molecule cm<sup>-3</sup>]<br>mean</th><th>[molecule cm<sup>-3</sup>]<br>max</th></tr></thead><tbody><tr><td>Alvarez et al. (2013), 21 July 2011</td><td>Yes</td><td>5.8</td><td>6.9</td><td>27.4</td><td>5.2</td><td>7.0</td><td>13.0</td></tr><tr><td>Alvarez et al. (2013), 22 July 2011</td><td>Yes</td><td>9.3</td><td>5.8</td><td>40.8</td><td>6.6</td><td>6.3</td><td>14.0</td></tr><tr><td>Alvarez et al. (2013), 27 July 2011</td><td>Yes</td><td>2.4</td><td>8.0</td><td>28.0</td><td>5.2</td><td>4.4</td><td>11.0</td></tr><tr><td>This paper, 27 October 2016</td><td>No</td><td>0.7</td><td>1.5</td><td>7.1</td><td>2.3</td><td>0.5</td><td>3.6</td></tr><tr><td>This paper, 28 October 2016</td><td>No</td><td>0.8</td><td>3.9</td><td>2.3</td><td>2.9</td><td>1.8</td><td>6.4</td></tr><tr><td>This paper, 29 October 2016</td><td>No</td><td>0.9</td><td>2.3</td><td>2.6</td><td>1.9</td><td>4.1</td><td>8.7</td></tr><tr><td>This paper, 30 October 2016</td><td>Yes</td><td>1.5</td><td>1.4</td><td>3.7</td><td>3.0</td><td>10.1</td><td>15.7</td></tr><tr><td>This paper, 31 October 2016</td><td>Yes</td><td>2.6</td><td>2.6</td><td>3.3</td><td>4.0</td><td>6.1</td><td>13.6</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Figure 6:** Comparison of the ozonolysis rate (blue line) and the HONO photolysis rate averaged over the room volume (orange line), in the experiments performed with UV-transparent windows.</figcaption></figure>

and O₃ concentrations, and due to decreasing *J*<sub>HONO</sub> and illuminated volume (see Fig. A.1 in Appendix). Experimentally, the OH concentration is found to decrease at the end of the experiments performed with borosilicate windows, in accordance with the decrease of *J*<sub>HONO</sub>.

The effect of XO₂ transport from sunlit to shaded volume followed by OH regeneration makes it possible to consider the HONO photolysis and other photolytic processes active in the sunlit volume as an affective OH initiation process in the total room volume. The discussed above effect of transport reducing the influence of the HONO photolysis on the difference of OH concentrations in the shadow and illuminated volumes is emphasized by the mixing rate, and may account for the different OH dynamics observed in the two studies. Despite similar OH production and loss rates in both experiments, no artificially enhanced mixing was apparently used in the experiments of Alvarez et al. (2013). Besides, significantly increased OH concentration in absence of direct light was well observed by Alvarez et al. (2013) during short periods

of ventilation performed with accelerated mixing using electrical fans.

## 5. Conclusions

OH and XO₂ measurements were performed in an indoor environment. Depending on the light conditions and the VOC levels, the average observed OH concentration was found to vary in the range (0.6-4)×10⁵ molecule cm⁻³ with anti-UV windows, and (6-10)×10⁵ molecule cm⁻³ with borosilicate windows. The XO₂ concentration was found to vary in the range (1-7)×10⁷ molecule cm⁻³ with anti-UV windows, and (4-16)×10⁷ molecule cm⁻³ with borosilicate windows. Taking into account the measurement uncertainty, the simulated OH concentrations agree in general with the observations, while simulated XO₂ concentrations are overestimated. It is likely that this overestimation would be reduced if some species which have not been measured, but are found to be important actors of the radical chemistry, were constrained to measurements. For future prospects, HNO₄ PAN, PPN, MGLY and DCB measurements should be of help to reduce modelled XO₂ uncertainty.
According to the radical budget analysis the domi-
nant OH production and loss processes are through the
radical propagation mechanism, with OH formation in
reaction of HO₂ with NO and OH loss in reactions with
VOCs, mostly aldehydes, alkenes, aromatic compounds
and terpenes. The primary source of OH in the sunlit
volume is represented mainly by HONO photolysis, up
to about 40% of total OH production. Ozonolysis of
unsaturated VOCs is the major OH initiation mecha-
nism in the shaded volume or in the room with anti-
UV windows accounting for about 3-6% of the total OH
production.

Similarly to OH, the dominant production and loss of XO₂ radicals proceed by radical propagation mechanism via XO₂+NO and OH+VOCs reactions. The main RO₂ primary source is represented by the ozonolysis of unsaturated VOCs with contribution of VOCs photolysis in the sunlit volume, with both sources contributing on average less than 10% of the total XO₂ production. Besides such XO₂ termination processes as peroxy radical self-reactions and deposition, the major termination pathway for RO₂ has appeared to be the reaction with NO₂ forming organic nitrates. The reaction of HO₂ with NO₂ was found to be in equilibrium with HNO₄ thermal decomposition and was not contributing to HO₂ termination or generation.

The particular conditions of these experiments,
namely the large VOC amount emitted by the paints,
which fosters the production of XO₂ radicals, combined
with an efficient air mixing, leads to decrease the
influence of the HONO photolysis on the OH radical
concentration spatial heterogeneity. Despite the lower
rate of the radical initiation in the shaded volume, the
difference of radical concentrations in the shaded and
the sunlit volumes is mitigated by an interplay between
the mixing time and the lifetime of XO₂ radicals,
allowing efficient transport of XO₂ radicals into the
shaded volume and acting there as a source of OH
via radical propagation processes. In such conditions,
the increase in OH concentration level, a major at-
mospheric oxidant, does not remain confined in the
illuminated volume and can affect the whole room.

Acknowledgements

The authors thank Alexandre Kukui for provid-
ing the radicals measurements setup and enlighten-
ing advice and guidance in analysis. This work is
a contribution to the Labex Serenade (no. ANR-11-
LABX-0064) funded by the "Investissements d'Avenir"
French Government programme of the French National
Research Agency (ANR) through the A MIDEX project
(no. ANR11-IDEX-0001-02).

Authors' contributions

EAF performed the simulations, formal analysis and
scientific visualization, analyzed the simulation results

and wrote the draft. HC acquired the radicals data. AG
acquired the VOCs, NOx, HONO, O₃ and light data.
VL provided the updated chemical scheme. KS and HW
reviewed the draft. HW acquired the funding.

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A. Appendix

<figure><img src="image_14.png" /><figcaption>**Figure A.1:** Directly irradiated room volume ($V_{box}^{L}$) and surface area ($S_{box}^{L}$) as a function of day hour. The dots denote the values that were calculated by Tlili et al. (prep).</figcaption></figure>
<caption>**Table A.1**</caption>

List of the VOCs identified and quantified with the PTR-ToF-MS with their average concentrations in ppb for each experiment.

<table><thead><tr><th>Compounds</th><th>Formula</th><th>Experiment 1</th><th>Experiment 2</th><th>Experiment 3</th><th>Experiment 4</th><th>Experiment 5</th></tr></thead><tbody><tr><td>Acetaldehyde</td><td>C<sub>2</sub>H<sub>4</sub>O</td><td>13.56</td><td>25.62</td><td>77.90</td><td>81.57</td><td>38.98</td></tr><tr><td>Acetone/Propanal</td><td>C<sub>3</sub>H<sub>6</sub>O</td><td>9.34</td><td>13.83</td><td>26.07</td><td>25.84</td><td>21.35</td></tr><tr><td>Benzaldehyde</td><td>C<sub>7</sub>H<sub>6</sub>O</td><td>0.41</td><td>0.98</td><td>4.02</td><td>2.75</td><td>0.50</td></tr><tr><td>Butanone</td><td>C<sub>4</sub>H<sub>8</sub>O</td><td>1.74</td><td>4.23</td><td>10.07</td><td>8.02</td><td>6.62</td></tr><tr><td>Formaldehyde</td><td>CH<sub>2</sub>O</td><td>38.99</td><td>44.54</td><td>101.20</td><td>119.51</td><td>38.08</td></tr><tr><td>Heptanal</td><td>C<sub>7</sub>H<sub>14</sub>O</td><td>4.30</td><td>6.29</td><td>20.09</td><td>17.62</td><td>6.64</td></tr><tr><td>Hexanal</td><td>C<sub>6</sub>H<sub>12</sub>O</td><td>6.47</td><td>12.60</td><td>78.36</td><td>60.44</td><td>12.65</td></tr><tr><td>Octanal</td><td>C<sub>8</sub>H<sub>16</sub>O</td><td>1.32</td><td>1.73</td><td>4.74</td><td>4.53</td><td>1.34</td></tr><tr><td>Pentanal</td><td>C<sub>5</sub>H<sub>10</sub>O</td><td>2.11</td><td>3.47</td><td>18.17</td><td>17.17</td><td>3.48</td></tr><tr><td>Pentenal</td><td>C<sub>5</sub>H<sub>8</sub>O</td><td>0.39</td><td>0.66</td><td>2.27</td><td>2.49</td><td>0.50</td></tr><tr><td>Pentanedione/C5 acid</td><td>C<sub>5</sub>H<sub>8</sub>O<sub>2</sub></td><td>0.14</td><td>0.33</td><td>1.33</td><td>1.22</td><td>0.58</td></tr><tr><td>Vinyl acetate</td><td>C<sub>4</sub>H<sub>6</sub>O<sub>2</sub></td><td>1.29</td><td>2.42</td><td>8.51</td><td>7.97</td><td>1.70</td></tr><tr><td>Acetic acid</td><td>C<sub>2</sub>H<sub>4</sub>O<sub>2</sub></td><td>27.35</td><td>43.16</td><td>99.29</td><td>97.47</td><td>42.14</td></tr><tr><td>Acrylic acid</td><td>C<sub>3</sub>H<sub>4</sub>O<sub>2</sub></td><td>4.64</td><td>5.10</td><td>23.90</td><td>32.61</td><td>5.83</td></tr><tr><td>Butyric acid</td><td>C<sub>4</sub>H<sub>8</sub>O<sub>2</sub></td><td>0.32</td><td>1.26</td><td>5.35</td><td>5.00</td><td>1.76</td></tr><tr><td>Formic acid</td><td>CH<sub>2</sub>O<sub>2</sub></td><td>23.02</td><td>24.66</td><td>46.32</td><td>51.01</td><td>15.46</td></tr><tr><td>Glycolic acid</td><td>C<sub>2</sub>H<sub>4</sub>O<sub>3</sub></td><td>0.27</td><td>0.44</td><td>1.03</td><td>0.87</td><td>0.35</td></tr><tr><td>Propanoic acid</td><td>C<sub>3</sub>H<sub>6</sub>O<sub>2</sub></td><td>13.64</td><td>21.06</td><td>65.58</td><td>43.54</td><td>17.51</td></tr><tr><td>Butadiene</td><td>C<sub>4</sub>H<sub>6</sub></td><td>0.34</td><td>0.78</td><td>4.12</td><td>3.29</td><td>1.00</td></tr><tr><td>Butene</td><td>C<sub>4</sub>H<sub>8</sub></td><td>1.65</td><td>3.90</td><td>16.31</td><td>8.76</td><td>2.74</td></tr><tr><td>C5 alkenes</td><td>C<sub>5</sub>H<sub>10</sub></td><td>2.55</td><td>3.21</td><td>12.12</td><td>6.65</td><td>2.77</td></tr><tr><td>C6 alkenes</td><td>C<sub>6</sub>H<sub>12</sub></td><td>1.29</td><td>1.56</td><td>5.93</td><td>3.03</td><td>1.49</td></tr><tr><td>Dodecene</td><td>C<sub>12</sub>H<sub>24</sub></td><td>0.10</td><td>0.13</td><td>1.72</td><td>0.45</td><td>0.08</td></tr><tr><td>Isoprene</td><td>C<sub>5</sub>H<sub>8</sub></td><td>0.27</td><td>0.44</td><td>2.28</td><td>2.13</td><td>0.38</td></tr><tr><td>Propene</td><td>C<sub>3</sub>H<sub>6</sub></td><td>8.04</td><td>9.62</td><td>28.31</td><td>17.75</td><td>7.18</td></tr><tr><td>Butanol</td><td>C<sub>4</sub>H<sub>10</sub>O</td><td>9.90</td><td>23.42</td><td>97.83</td><td>52.58</td><td>16.43</td></tr><tr><td>Methanol</td><td>CH<sub>4</sub>O</td><td>22.37</td><td>28.52</td><td>49.66</td><td>40.29</td><td>29.97</td></tr><tr><td>Benzene</td><td>C<sub>6</sub>H<sub>6</sub></td><td>0.79</td><td>1.07</td><td>1.64</td><td>2.01</td><td>1.16</td></tr><tr><td>Cymene</td><td>C<sub>10</sub>H<sub>14</sub></td><td>1.62</td><td>1.73</td><td>2.27</td><td>2.73</td><td>1.80</td></tr><tr><td>Phenol</td><td>C<sub>6</sub>H<sub>6</sub>O</td><td>0.14</td><td>0.17</td><td>0.20</td><td>0.24</td><td>0.03</td></tr><tr><td>Styrene</td><td>C<sub>8</sub>H<sub>8</sub></td><td>0.33</td><td>0.39</td><td>0.90</td><td>0.64</td><td>0.35</td></tr><tr><td>Methylstyrene</td><td>C<sub>9</sub>H<sub>10</sub></td><td>0.15</td><td>0.17</td><td>0.32</td><td>0.32</td><td>0.15</td></tr><tr><td>Naphtalene</td><td>C<sub>10</sub>H<sub>8</sub></td><td>0.05</td><td>0.08</td><td>0.20</td><td>0.22</td><td>0.05</td></tr><tr><td>TMB</td><td>C<sub>9</sub>H<sub>12</sub></td><td>9.06</td><td>10.03</td><td>11.91</td><td>13.88</td><td>8.47</td></tr><tr><td>Toluene</td><td>C<sub>7</sub>H<sub>8</sub></td><td>2.63</td><td>1.81</td><td>2.52</td><td>2.94</td><td>1.88</td></tr><tr><td>Xylene/Ethylbenzene</td><td>C<sub>8</sub>H<sub>10</sub></td><td>11.89</td><td>12.85</td><td>15.26</td><td>17.10</td><td>11.98</td></tr><tr><td>Acrolein</td><td>C<sub>3</sub>H<sub>4</sub>O</td><td>1.54</td><td>2.10</td><td>4.78</td><td>5.01</td><td>2.55</td></tr><tr><td>MVK/Methacrolein</td><td>C<sub>4</sub>H<sub>6</sub>O</td><td>0.44</td><td>0.83</td><td>2.30</td><td>2.17</td><td>1.31</td></tr><tr><td>Monoterpenes</td><td>C<sub>10</sub>H<sub>16</sub></td><td>2.15</td><td>2.26</td><td>3.54</td><td>3.51</td><td>1.95</td></tr><tr><td>Cymenene</td><td>C<sub>10</sub>H<sub>12</sub></td><td>0.06</td><td>0.08</td><td>0.24</td><td>0.19</td><td>0.07</td></tr><tr><td>Phenyl acetaldehyde</td><td>C<sub>8</sub>H<sub>8</sub>O</td><td>0.32</td><td>0.33</td><td>0.52</td><td>0.69</td><td>0.01</td></tr><tr><td>Cresol</td><td>C<sub>7</sub>H<sub>8</sub>O</td><td>0.06</td><td>0.09</td><td>0.24</td><td>0.21</td><td>0.00</td></tr><tr><td>Sesquiterpenes</td><td>C<sub>15</sub>H<sub>24</sub></td><td>0.17</td><td>0.28</td><td>0.54</td><td>0.52</td><td>0.14</td></tr></tbody></table>
**Table A.2**

List of the RACM2 compounds that can deposit on surfaces, with their molar mass *M* [g.mol⁻¹], the representative species considered to compute the diffusion coefficient, the critical temperature *T*<sub>c</sub> [K] and critical molar volume *V*<sub>c</sub> [cm³.mol⁻¹] of the representative species, and the diffusion coefficient *D* [m².s⁻¹] at T=21.7°C, computed following the procedure described in (Fiorentino et al., 2021). No reference means that *T*<sub>c</sub> and *V*<sub>c</sub> are calculated using the method of Joback and Reid (1987).

<table><thead><tr><th>Compounds</th><th><i>M</i></th><th>Representative species</th><th><i>T</i><sub>c</sub></th><th><i>V</i><sub>c</sub></th><th>Reference</th><th><i>D</i> (10<sup>-5</sup>)</th></tr></thead><tbody><tr><td>ACO<sub>3</sub></td><td>75</td><td>Methyl peroxide</td><td>499.60</td><td>130</td><td>-</td><td>1.18</td></tr><tr><td>ACTP</td><td>89</td><td>Acetone</td><td>508.10</td><td>213</td><td>Ambrose et al. (1974b)</td><td>0.924</td></tr><tr><td>BAL1</td><td>121</td><td>Benzaldehyde</td><td>695.00</td><td>340</td><td>Ambrose et al. (1975)</td><td>0.682</td></tr><tr><td>BAL2</td><td>105</td><td>Benzaldehyde</td><td>695.00</td><td>340</td><td>Ambrose et al. (1975)</td><td>0.692</td></tr><tr><td>BALP</td><td>137</td><td>Benzaldehyde</td><td>695.00</td><td>340</td><td>Ambrose et al. (1975)</td><td>0.674</td></tr><tr><td>BENP</td><td>127</td><td>Benzene</td><td>562.05</td><td>256</td><td>Tsonopoulos and Ambrose (1995)</td><td>0.804</td></tr><tr><td>ETHP</td><td>61</td><td>Ethane</td><td>305.30</td><td>147</td><td>Ambrose and Tsonopoulos (1995)</td><td>1.24</td></tr><tr><td>ISOP</td><td>117</td><td>Isoprene</td><td>480.20</td><td>280</td><td>-</td><td>0.793</td></tr><tr><td>KETP</td><td>117</td><td>2-Pentanone</td><td>561.08</td><td>320</td><td>Ambrose et al. (1974a)</td><td>0.727</td></tr><tr><td>LIMP</td><td>185</td><td>Limonene</td><td>657.16</td><td>500</td><td>-</td><td>0.544</td></tr><tr><td>MACP</td><td>101</td><td>Methacrolein</td><td>516.54</td><td>260</td><td>-</td><td>0.827</td></tr><tr><td>MO2</td><td>47</td><td>Methyl peroxide</td><td>499.60</td><td>130</td><td>-</td><td>1.28</td></tr><tr><td>OLIP</td><td>117</td><td>Pentene</td><td>464.80</td><td>298</td><td>Tsonopoulos and Ambrose (1996)</td><td>0.772</td></tr><tr><td>OLTP</td><td>91</td><td>Propene</td><td>364.90</td><td>185</td><td>Tsonopoulos and Ambrose (1996)</td><td>1.03</td></tr><tr><td>ORAP</td><td>75</td><td>Acetic acid</td><td>590.70</td><td>171</td><td>D'Souza and Teja (1987)</td><td>1.02</td></tr><tr><td>PER1</td><td>141</td><td>Toluene</td><td>822.28</td><td>490</td><td>-</td><td>0.544</td></tr><tr><td>PER2</td><td>157</td><td>Toluene</td><td>822.28</td><td>490</td><td>-</td><td>0.539</td></tr><tr><td>RCO<sub>3</sub></td><td>90</td><td>Methyl peroxide</td><td>499.60</td><td>130</td><td>-</td><td>1.15</td></tr><tr><td>TLP1</td><td>91</td><td>Toluene</td><td>822.28</td><td>490</td><td>-</td><td>0.569</td></tr><tr><td>UALP</td><td>133</td><td>Pentenal</td><td>548.86</td><td>310</td><td>-</td><td>0.732</td></tr><tr><td>XYLP</td><td>155</td><td>m-Xylene</td><td>617.00</td><td>375</td><td>Tsonopoulos and Ambrose (1995)</td><td>0.646</td></tr><tr><td>NO<sub>3</sub></td><td>62</td><td>NO<sub>3</sub></td><td>534.15</td><td>140</td><td>-</td><td>1.17</td></tr><tr><td>HNO<sub>3</sub></td><td>63</td><td>HNO<sub>3</sub></td><td>648.46</td><td>140</td><td>-</td><td>1.13</td></tr><tr><td>HNO<sub>4</sub></td><td>79</td><td>HNO<sub>4</sub></td><td>669.82</td><td>155</td><td>-</td><td>1.05</td></tr><tr><td>HO<sub>2</sub></td><td>33</td><td>H<sub>2</sub>O<sub>2</sub></td><td>728.00</td><td>70</td><td>Nikitin et al. (1995)</td><td>1.67</td></tr><tr><td>H<sub>2</sub>O<sub>2</sub></td><td>34</td><td>H<sub>2</sub>O<sub>2</sub></td><td>728.00</td><td>70</td><td>Nikitin et al. (1995)</td><td>1.66</td></tr></tbody></table>
**Table A.3**

List of the RACM2 organic and inorganic compounds constrained to measurements, with their median concentrations in ppb for each experiment.

<table><thead><tr><th>Species</th><th>Definition</th><th>Exp 1</th><th>Exp 2</th><th>Exp 3</th><th>Exp 4</th><th>Exp 5</th></tr></thead><tbody><tr><td>ACD</td><td>Acetaldehyde</td><td>13.91</td><td>27.21</td><td>83.25</td><td>87.73</td><td>41.64</td></tr><tr><td>ACT</td><td>Acetone</td><td>7.34</td><td>11.65</td><td>22.00</td><td>22.34</td><td>18.08</td></tr><tr><td>ALD</td><td>C3 and higher aldehydes</td><td>16.99</td><td>28.43</td><td>135.00</td><td>113.83</td><td>31.87</td></tr><tr><td>BALD</td><td>Benzaldehyde and other aromatic aldehydes</td><td>0.43</td><td>1.03</td><td>4.20</td><td>2.95</td><td>0.00</td></tr><tr><td>BEN</td><td>Benzene</td><td>0.81</td><td>1.08</td><td>1.80</td><td>1.99</td><td>1.19</td></tr><tr><td>CSL</td><td>Cresol and other hydroxy substituted aromatics</td><td>0.07</td><td>0.10</td><td>0.27</td><td>0.23</td><td>0.00</td></tr><tr><td>HCHO</td><td>Formaldehyde</td><td>39.41</td><td>46.87</td><td>106.51</td><td>124.59</td><td>38.11</td></tr><tr><td>ISO</td><td>Isoprene</td><td>0.28</td><td>0.46</td><td>2.39</td><td>2.28</td><td>0.43</td></tr><tr><td>KET</td><td>Ketones</td><td>1.89</td><td>4.87</td><td>11.84</td><td>10.41</td><td>7.35</td></tr><tr><td>LIM</td><td>d-limonene and other cyclic diene-terpenes</td><td>2.27</td><td>2.49</td><td>4.19</td><td>3.98</td><td>2.11</td></tr><tr><td>MACR</td><td>Methacrolein</td><td>2.01</td><td>3.07</td><td>7.67</td><td>7.74</td><td>4.02</td></tr><tr><td>MOH</td><td>Methanol</td><td>22.59</td><td>30.03</td><td>50.27</td><td>42.77</td><td>30.86</td></tr><tr><td>OLI</td><td>Internal alkenes</td><td>3.69</td><td>4.96</td><td>20.09</td><td>12.40</td><td>4.67</td></tr><tr><td>OLT</td><td>Terminal alkenes</td><td>16.99</td><td>23.19</td><td>86.75</td><td>74.45</td><td>19.22</td></tr><tr><td>ORA1</td><td>Formic acid</td><td>23.52</td><td>25.82</td><td>50.50</td><td>55.67</td><td>15.38</td></tr><tr><td>ORA2</td><td>Acetic acid and higher acids</td><td>42.23</td><td>68.48</td><td>180.90</td><td>158.41</td><td>66.74</td></tr><tr><td>PHEN</td><td>Phenol</td><td>0.14</td><td>0.17</td><td>0.21</td><td>0.25</td><td>0.00</td></tr><tr><td>ROH</td><td>C3 and higher alcohols</td><td>10.52</td><td>24.20</td><td>104.12</td><td>58.93</td><td>19.05</td></tr><tr><td>UALD</td><td>Unsaturated aldehydes</td><td>0.41</td><td>0.68</td><td>2.44</td><td>2.72</td><td>0.50</td></tr><tr><td>TOL</td><td>Toluene and less reactive aromatics</td><td>3.32</td><td>2.55</td><td>4.05</td><td>3.96</td><td>2.48</td></tr><tr><td>XYL</td><td>Xylene and less reactive aromatics</td><td>24.12</td><td>26.58</td><td>32.03</td><td>36.86</td><td>25.87</td></tr><tr><td>NO</td><td>Nitric oxide</td><td>1.34</td><td>3.98</td><td>1.97</td><td>1.09</td><td>2.23</td></tr><tr><td>NO<sub>2</sub></td><td>Nitrogen dioxide</td><td>3.73</td><td>2.21</td><td>2.72</td><td>3.85</td><td>3.21</td></tr><tr><td>HONO</td><td>Nitrous acid</td><td>2.02</td><td>2.88</td><td>1.89</td><td>2.99</td><td>4.19</td></tr><tr><td>O<sub>3</sub></td><td>Ozone</td><td>0.70</td><td>0.50</td><td>0.70</td><td>1.30</td><td>3.00</td></tr></tbody></table>
<figure><img src="image_15.png" /><figcaption>**Figure A.2:** Observed concentration profiles of the four inorganic species that were monitored in the five experiments. During the 1st and 5th experiments, two events occured : for Experiment 1, a voluntary NO₂ injection performed at 13:30 and, for Experiment 5, a NOx injection caused by welding work that occurred in the building at approximately 13:20.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure A.3:** Measured concentration profiles for five representative VOCs, namely HCHO (formaldehyde), ACD (acetaldehyde), ALD (C3 and higher aldehydes), LIM (d-limonene and other cyclic diene-terpenes), OLT and OLI (terminal and internal alkenes).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure A.4:** Sensitivity tests performed with the data of Experiment 4. On the top row, the light curves denote the observed data, used for reference, while the dark curves denote the varied data. The OH (blue) and HO₂ (yellow) concentrations simulated using the observed data are shown in the first column (case a), in the sunlit (dark lines) and shaded (light lines) volumes. The OH and HO₂ concentrations shown in the subsequent columns are simulated by varying the HONO (b), NO (c,d) and VOC concentrations (e). For practical reasons, HCHO is the only VOC presented in case (e). The OH and HO₂ concentrations are expressed in molecule.cm⁻³ while the HONO, NO and HCHO concentrations are expressed in ppb.</figcaption></figure>
<caption>Table A.4</caption>

OH, HO₂ and RO₂ budgets: average production and consumption rates in molecule cm⁻³ s⁻¹. Primary production reactions are identified in orange, propagation reactions are identified in green, termination reaction are identified in blue.

<table><thead><tr><th rowspan="2"></th><th colspan="5">Shaded volume</th><th colspan="5">Sunlit volume</th></tr><tr><th>Exp1</th><th>Exp2</th><th>Exp3</th><th>Exp4</th><th>Exp5</th><th>Exp1</th><th>Exp2</th><th>Exp3</th><th>Exp4</th><th>Exp5</th></tr></thead><tbody><tr><td colspan="11"><strong>OH sources</strong></td></tr><tr><td>HONO + hν</td><td>4.05E+00</td><td>6.15E+00</td><td>3.17E+00</td><td>5.22E+05</td><td>7.88E+05</td><td>1.37E+03</td><td>2.08E+03</td><td>1.07E+03</td><td>1.56E+07</td><td>2.36E+07</td></tr><tr><td>VOCs + O<sub>3</sub></td><td>3.35E+05</td><td>5.21E+05</td><td>1.83E+06</td><td>2.25E+06</td><td>1.75E+06</td><td>3.35E+05</td><td>5.22E+05</td><td>1.83E+06</td><td>2.26E+06</td><td>1.75E+06</td></tr><tr><td>NO + HO<sub>2</sub></td><td>4.70E+06</td><td>7.20E+06</td><td>2.65E+07</td><td>3.84E+07</td><td>2.99E+07</td><td>4.84E+06</td><td>7.33E+06</td><td>2.67E+07</td><td>4.17E+07</td><td>3.57E+07</td></tr><tr><td>others</td><td>1.18E+03</td><td>8.29E+02</td><td>9.78E+03</td><td>4.51E+04</td><td>1.48E+04</td><td>1.24E+03</td><td>8.43E+02</td><td>9.88E+03</td><td>5.35E+04</td><td>2.19E+04</td></tr><tr><td colspan="11"><strong>OH sinks</strong></td></tr><tr><td>VOCs</td><td>4.89E+06</td><td>7.59E+06</td><td>2.82E+07</td><td>4.09E+07</td><td>3.18E+07</td><td>5.02E+06</td><td>7.72E+06</td><td>2.84E+07</td><td>5.92E+07</td><td>5.94E+07</td></tr><tr><td>NO</td><td>1.89E+04</td><td>4.92E+04</td><td>4.00E+04</td><td>4.25E+04</td><td>1.89E+05</td><td>1.95E+04</td><td>5.01E+04</td><td>4.04E+04</td><td>6.89E+04</td><td>3.61E+05</td></tr><tr><td>NO<sub>2</sub></td><td>9.22E+04</td><td>4.44E+04</td><td>5.60E+04</td><td>1.46E+05</td><td>2.57E+05</td><td>9.56E+04</td><td>4.52E+04</td><td>5.67E+04</td><td>2.23E+05</td><td>5.34E+05</td></tr><tr><td>HONO</td><td>2.45E+04</td><td>3.63E+04</td><td>2.97E+04</td><td>7.52E+04</td><td>2.08E+05</td><td>2.52E+04</td><td>3.69E+04</td><td>2.99E+04</td><td>1.10E+05</td><td>3.97E+05</td></tr><tr><td>others</td><td>8.06E+03</td><td>8.65E+03</td><td>1.19E+04</td><td>2.41E+04</td><td>4.43E+04</td><td>8.29E+03</td><td>8.80E+03</td><td>1.20E+04</td><td>3.59E+04</td><td>8.53E+04</td></tr><tr><td colspan="11"><strong>HO<sub>2</sub> sources</strong></td></tr><tr><td>VOCs + hν</td><td>2.60E+04</td><td>9.36E+03</td><td>2.40E+04</td><td>1.52E+05</td><td>1.92E+05</td><td>6.31E+05</td><td>2.28E+05</td><td>5.84E+05</td><td>3.08E+06</td><td>3.37E+06</td></tr><tr><td>HCHO + hν</td><td>0.00E+00</td><td>0.00E+00</td><td>0.00E+00</td><td>2.30E+05</td><td>7.48E+04</td><td>0.00E+00</td><td>0.00E+00</td><td>0.00E+00</td><td>6.79E+06</td><td>2.21E+06</td></tr><tr><td>VOCs + O<sub>3</sub></td><td>9.23E+04</td><td>1.32E+05</td><td>4.65E+05</td><td>6.79E+05</td><td>7.05E+05</td><td>9.25E+04</td><td>1.32E+05</td><td>4.66E+05</td><td>6.82E+05</td><td>7.06E+05</td></tr><tr><td>HNO<sub>4</sub></td><td>2.05E+06</td><td>7.18E+05</td><td>4.14E+06</td><td>1.59E+07</td><td>5.59E+06</td><td>2.06E+06</td><td>7.20E+05</td><td>4.15E+06</td><td>1.60E+07</td><td>5.69E+06</td></tr><tr><td>VOCs + OH</td><td>3.30E+05</td><td>5.59E+05</td><td>1.78E+06</td><td>2.03E+06</td><td>2.12E+06</td><td>3.39E+05</td><td>5.68E+05</td><td>1.80E+06</td><td>2.94E+06</td><td>3.90E+06</td></tr><tr><td>HCHO + OH</td><td>6.09E+05</td><td>7.74E+05</td><td>2.18E+06</td><td>4.37E+06</td><td>2.68E+06</td><td>6.26E+05</td><td>7.88E+05</td><td>2.19E+06</td><td>6.39E+06</td><td>5.17E+06</td></tr><tr><td>RO<sub>2</sub> + RO<sub>2</sub></td><td>1.21E+03</td><td>5.50E+02</td><td>2.23E+04</td><td>1.15E+05</td><td>2.83E+04</td><td>1.33E+03</td><td>5.72E+02</td><td>2.25E+04</td><td>1.32E+05</td><td>4.15E+04</td></tr><tr><td>Interm.aro.diss.</td><td>1.94E+05</td><td>2.50E+05</td><td>3.61E+05</td><td>6.49E+05</td><td>9.63E+05</td><td>1.99E+05</td><td>2.55E+05</td><td>3.64E+05</td><td>9.38E+05</td><td>1.77E+06</td></tr><tr><td>RO<sub>2</sub> + NO</td><td>3.49E+06</td><td>5.53E+06</td><td>2.21E+07</td><td>3.02E+07</td><td>2.61E+07</td><td>3.52E+06</td><td>5.58E+06</td><td>2.22E+07</td><td>3.20E+07</td><td>3.05E+07</td></tr><tr><td>others</td><td>7.88E+03</td><td>8.50E+03</td><td>1.10E+04</td><td>2.07E+04</td><td>4.16E+04</td><td>8.10E+03</td><td>8.64E+03</td><td>1.11E+04</td><td>3.08E+04</td><td>7.94E+04</td></tr><tr><td colspan="11"><strong>HO<sub>2</sub> sinks</strong></td></tr><tr><td>HO<sub>2</sub> + NO (1)</td><td>4.70E+06</td><td>7.20E+06</td><td>2.65E+07</td><td>3.84E+07</td><td>2.99E+07</td><td>4.84E+06</td><td>7.33E+06</td><td>2.67E+07</td><td>4.17E+07</td><td>3.57E+07</td></tr><tr><td>HO<sub>2</sub> + NO (2)</td><td>2.50E+04</td><td>3.75E+04</td><td>1.39E+05</td><td>1.99E+05</td><td>1.55E+05</td><td>2.57E+04</td><td>3.81E+04</td><td>1.41E+05</td><td>2.16E+05</td><td>1.85E+05</td></tr><tr><td>NO<sub>2</sub></td><td>2.08E+06</td><td>7.34E+05</td><td>4.24E+06</td><td>1.60E+07</td><td>5.55E+06</td><td>2.16E+06</td><td>7.45E+05</td><td>4.28E+06</td><td>1.72E+07</td><td>6.56E+06</td></tr><tr><td>HO<sub>2</sub></td><td>6.60E+03</td><td>2.10E+03</td><td>6.46E+04</td><td>3.27E+05</td><td>5.12E+04</td><td>6.93E+03</td><td>2.15E+03</td><td>6.52E+04</td><td>3.70E+05</td><td>6.84E+04</td></tr><tr><td>RO<sub>2</sub></td><td>1.15E+04</td><td>3.49E+03</td><td>1.14E+05</td><td>5.61E+05</td><td>9.47E+04</td><td>1.19E+04</td><td>3.56E+03</td><td>1.15E+05</td><td>6.26E+05</td><td>1.27E+05</td></tr><tr><td>Deposition</td><td>4.79E+04</td><td>2.47E+04</td><td>1.52E+05</td><td>3.86E+05</td><td>4.19E+06</td><td>2.66E+04</td><td>1.63E+04</td><td>1.03E+05</td><td>2.41E+05</td><td>2.71E+06</td></tr><tr><td>Others</td><td>1.38E+03</td><td>8.93E+02</td><td>5.67E+03</td><td>2.78E+04</td><td>1.56E+04</td><td>1.44E+03</td><td>9.10E+02</td><td>5.74E+03</td><td>3.16E+04</td><td>2.11E+04</td></tr><tr><td colspan="11"><strong>RO<sub>2</sub> sources</strong></td></tr><tr><td>VOCs + hν</td><td>1.16E+04</td><td>3.82E+03</td><td>1.08E+04</td><td>7.70E+04</td><td>8.62E+04</td><td>2.82E+05</td><td>9.28E+04</td><td>2.63E+05</td><td>1.69E+06</td><td>1.58E+06</td></tr><tr><td>VOCs + O<sub>3</sub></td><td>3.11E+05</td><td>4.97E+05</td><td>1.87E+06</td><td>2.23E+06</td><td>1.76E+06</td><td>3.12E+05</td><td>4.97E+05</td><td>1.87E+06</td><td>2.24E+06</td><td>1.77E+06</td></tr><tr><td>PANs</td><td>1.44E+06</td><td>5.70E+05</td><td>4.72E+06</td><td>1.61E+07</td><td>6.92E+06</td><td>1.44E+06</td><td>5.70E+05</td><td>4.72E+06</td><td>1.61E+07</td><td>6.92E+06</td></tr><tr><td>VOCs + OH</td><td>3.93E+06</td><td>6.23E+06</td><td>2.42E+07</td><td>3.44E+07</td><td>2.68E+07</td><td>4.04E+06</td><td>6.34E+06</td><td>2.44E+07</td><td>4.97E+07</td><td>5.00E+07</td></tr><tr><td>RO<sub>2</sub> + HO<sub>2</sub></td><td>3.89E+02</td><td>1.55E+02</td><td>6.10E+03</td><td>2.93E+04</td><td>5.33E+03</td><td>4.27E+02</td><td>1.61E+02</td><td>6.18E+03</td><td>3.49E+04</td><td>8.30E+03</td></tr><tr><td>RO<sub>2</sub> + RO<sub>2</sub></td><td>3.34E+02</td><td>1.45E+02</td><td>5.98E+03</td><td>3.05E+04</td><td>6.38E+03</td><td>3.85E+02</td><td>1.52E+02</td><td>6.08E+03</td><td>3.71E+04</td><td>1.08E+04</td></tr><tr><td>Interm.aro.diss.</td><td>5.31E+05</td><td>6.88E+05</td><td>9.90E+05</td><td>1.78E+06</td><td>2.65E+06</td><td>5.45E+05</td><td>7.00E+05</td><td>9.98E+05</td><td>2.57E+06</td><td>4.86E+06</td></tr><tr><td>RO<sub>2</sub> + NO</td><td>1.23E+06</td><td>2.26E+06</td><td>1.03E+07</td><td>1.37E+07</td><td>1.25E+07</td><td>1.30E+06</td><td>2.33E+06</td><td>1.04E+07</td><td>1.58E+07</td><td>1.72E+07</td></tr><tr><td colspan="11"><strong>RO<sub>2</sub> sinks</strong></td></tr><tr><td>NO</td><td>5.10E+06</td><td>8.41E+06</td><td>3.48E+07</td><td>4.72E+07</td><td>4.12E+07</td><td>5.20E+06</td><td>8.53E+06</td><td>3.51E+07</td><td>5.14E+07</td><td>5.08E+07</td></tr><tr><td>RO<sub>2</sub></td><td>2.20E+03</td><td>9.70E+02</td><td>3.85E+04</td><td>2.02E+05</td><td>4.68E+04</td><td>2.44E+03</td><td>1.01E+03</td><td>3.90E+04</td><td>2.34E+05</td><td>7.04E+04</td></tr><tr><td>Interm.aro.diss.</td><td>8.17E+05</td><td>1.05E+06</td><td>1.52E+06</td><td>2.73E+06</td><td>4.06E+06</td><td>8.38E+05</td><td>1.07E+06</td><td>1.53E+06</td><td>3.95E+06</td><td>7.46E+06</td></tr><tr><td>NO<sub>2</sub></td><td>1.52E+06</td><td>7.82E+05</td><td>5.41E+06</td><td>1.90E+07</td><td>7.63E+06</td><td>1.63E+06</td><td>8.02E+05</td><td>5.48E+06</td><td>2.16E+07</td><td>1.06E+07</td></tr><tr><td>HO<sub>2</sub></td><td>1.15E+04</td><td>3.49E+03</td><td>1.14E+05</td><td>5.61E+05</td><td>9.47E+04</td><td>1.19E+04</td><td>3.56E+03</td><td>1.15E+05</td><td>6.26E+05</td><td>1.27E+05</td></tr><tr><td>Deposition</td><td>2.53E+04</td><td>1.63E+04</td><td>1.17E+05</td><td>2.55E+05</td><td>1.15E+05</td><td>2.53E+04</td><td>1.63E+04</td><td>1.17E+05</td><td>2.55E+05</td><td>1.15E+05</td></tr></tbody></table>
<caption>Table A.5</caption>

OH, HO₂ and RO₂ budgets: production and consumption rates averaged over total experiment duration, in percentages of contribution. Primary production reactions are shown in orange, propagation reactions are shown in green, termination reaction are shown in blue. Shown in yellow, the values for HO₂ + NO₂ ⇌ HNO₄ and RO₂ + NO₂ ⇌ PANs indicate the HO₂/RO₂ productions or consumptions which result from the difference between the dissociation and formation of the HNO₄/PANs reservoir species (see flux values in Tab. A.4). VOCs* refers to VOCs other than HCHO.

<table><thead><tr><th rowspan="2"></th><th colspan="5">Shaded volume</th><th colspan="5">Sunlit volume</th></tr><tr><th>Exp1</th><th>Exp2</th><th>Exp3</th><th>Exp4</th><th>Exp5</th><th>Exp1</th><th>Exp2</th><th>Exp3</th><th>Exp4</th><th>Exp5</th></tr></thead><tbody><tr><td colspan="11"><strong>OH sources</strong></td></tr><tr><td>HONO + hν</td><td>0.00</td><td>0.00</td><td>0.00</td><td>1.27</td><td>2.43</td><td>0.03</td><td>0.03</td><td>0.00</td><td>26.22</td><td>38.67</td></tr><tr><td>VOCs + O₃</td><td>6.65</td><td>6.75</td><td>6.45</td><td>5.47</td><td>5.38</td><td>6.48</td><td>6.64</td><td>6.40</td><td>3.79</td><td>2.87</td></tr><tr><td>NO + HO₂</td><td>93.33</td><td>93.24</td><td>93.51</td><td>93.15</td><td>92.15</td><td>93.47</td><td>93.32</td><td>93.56</td><td>69.91</td><td>58.43</td></tr><tr><td>others</td><td>0.02</td><td>0.01</td><td>0.03</td><td>0.11</td><td>0.05</td><td>0.02</td><td>0.01</td><td>0.03</td><td>0.09</td><td>0.04</td></tr><tr><td colspan="11"><strong>OH sinks</strong></td></tr><tr><td>VOCs</td><td>97.15</td><td>98.21</td><td>99.51</td><td>99.30</td><td>97.85</td><td>97.13</td><td>98.21</td><td>99.51</td><td>99.26</td><td>97.74</td></tr><tr><td>NO</td><td>0.38</td><td>0.64</td><td>0.14</td><td>0.10</td><td>0.58</td><td>0.38</td><td>0.64</td><td>0.14</td><td>0.12</td><td>0.59</td></tr><tr><td>NO₂</td><td>1.83</td><td>0.58</td><td>0.20</td><td>0.35</td><td>0.79</td><td>1.85</td><td>0.57</td><td>0.20</td><td>0.37</td><td>0.88</td></tr><tr><td>HONO</td><td>0.49</td><td>0.47</td><td>0.10</td><td>0.18</td><td>0.64</td><td>0.49</td><td>0.47</td><td>0.10</td><td>0.18</td><td>0.65</td></tr><tr><td>others</td><td>0.16</td><td>0.11</td><td>0.04</td><td>0.06</td><td>0.14</td><td>0.16</td><td>0.11</td><td>0.04</td><td>0.06</td><td>0.14</td></tr><tr><td colspan="11"><strong>HO₂ sources</strong></td></tr><tr><td>VOCs* + hν</td><td>0.55</td><td>0.13</td><td>0.09</td><td>0.40</td><td>0.58</td><td>11.66</td><td>3.01</td><td>2.11</td><td>5.82</td><td>7.06</td></tr><tr><td>HCHO + hν</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.60</td><td>0.23</td><td>0.00</td><td>0.00</td><td>0.00</td><td>12.82</td><td>4.63</td></tr><tr><td>VOCs + O₃</td><td>1.94</td><td>1.82</td><td>1.73</td><td>1.77</td><td>2.14</td><td>1.71</td><td>1.75</td><td>1.68</td><td>1.29</td><td>1.48</td></tr><tr><td>HNO₄ ⇌ HO₂+NO₂</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.11</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>VOCs* + OH</td><td>6.95</td><td>7.69</td><td>6.61</td><td>5.29</td><td>6.43</td><td>6.26</td><td>7.52</td><td>6.51</td><td>5.55</td><td>8.18</td></tr><tr><td>HCHO + OH</td><td>12.82</td><td>10.66</td><td>8.07</td><td>11.37</td><td>8.15</td><td>11.56</td><td>10.42</td><td>7.94</td><td>12.07</td><td>10.83</td></tr><tr><td>RO₂ + RO₂</td><td>0.03</td><td>0.01</td><td>0.08</td><td>0.30</td><td>0.09</td><td>0.02</td><td>0.01</td><td>0.08</td><td>0.25</td><td>0.09</td></tr><tr><td>Interm. aro. diss.</td><td>4.08</td><td>3.45</td><td>1.34</td><td>1.69</td><td>2.92</td><td>3.67</td><td>3.37</td><td>1.32</td><td>1.77</td><td>3.72</td></tr><tr><td>RO₂ + NO</td><td>73.47</td><td>76.14</td><td>82.04</td><td>78.54</td><td>79.22</td><td>64.97</td><td>73.82</td><td>80.32</td><td>60.38</td><td>63.86</td></tr><tr><td>others</td><td>0.17</td><td>0.12</td><td>0.04</td><td>0.05</td><td>0.13</td><td>0.15</td><td>0.11</td><td>0.04</td><td>0.06</td><td>0.17</td></tr><tr><td colspan="11"><strong>HO₂ sinks</strong></td></tr><tr><td>HO₂ + NO → NO₂ + OH</td><td>97.33</td><td>98.85</td><td>97.87</td><td>95.98</td><td>86.90</td><td>96.49</td><td>98.83</td><td>97.93</td><td>94.06</td><td>89.94</td></tr><tr><td>HO₂+NO₂ ⇌ HNO₄</td><td>0.76</td><td>0.21</td><td>0.37</td><td>0.27</td><td>-</td><td>2.06</td><td>0.34</td><td>0.49</td><td>2.60</td><td>2.21</td></tr><tr><td>HO₂</td><td>0.14</td><td>0.03</td><td>0.24</td><td>0.82</td><td>0.15</td><td>0.14</td><td>0.03</td><td>0.24</td><td>0.83</td><td>0.17</td></tr><tr><td>RO₂</td><td>0.24</td><td>0.05</td><td>0.42</td><td>1.40</td><td>0.27</td><td>0.24</td><td>0.05</td><td>0.42</td><td>1.41</td><td>0.32</td></tr><tr><td>HO₂ + NO → HNO₃</td><td>0.52</td><td>0.51</td><td>0.52</td><td>0.50</td><td>0.45</td><td>0.51</td><td>0.51</td><td>0.52</td><td>0.49</td><td>0.47</td></tr><tr><td>Deposition</td><td>0.99</td><td>0.34</td><td>0.56</td><td>0.97</td><td>12.18</td><td>0.53</td><td>0.22</td><td>0.38</td><td>0.54</td><td>6.84</td></tr><tr><td>Others</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.07</td><td>0.05</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.07</td><td>0.05</td></tr><tr><td colspan="11"><strong>RO₂ sources</strong></td></tr><tr><td>VOCs + hν</td><td>0.19</td><td>0.04</td><td>0.03</td><td>0.15</td><td>0.20</td><td>4.35</td><td>0.93</td><td>0.69</td><td>2.35</td><td>2.10</td></tr><tr><td>VOCs + O₃</td><td>5.18</td><td>5.13</td><td>5.02</td><td>4.27</td><td>4.03</td><td>4.81</td><td>4.99</td><td>4.95</td><td>3.11</td><td>2.34</td></tr><tr><td>VOCs + OH</td><td>65.38</td><td>64.36</td><td>64.74</td><td>65.88</td><td>61.23</td><td>62.30</td><td>63.64</td><td>64.24</td><td>68.93</td><td>66.29</td></tr><tr><td>RO₂ + HO₂</td><td>0.01</td><td>0.00</td><td>0.02</td><td>0.06</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.02</td><td>0.05</td><td>0.01</td></tr><tr><td>RO₂ + RO₂</td><td>0.01</td><td>0.00</td><td>0.02</td><td>0.06</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.02</td><td>0.05</td><td>0.01</td></tr><tr><td>Interm. aro. diss.</td><td>8.84</td><td>7.11</td><td>2.65</td><td>3.41</td><td>6.04</td><td>8.42</td><td>7.02</td><td>2.63</td><td>3.57</td><td>6.45</td></tr><tr><td>RO₂ + NO</td><td>20.40</td><td>23.36</td><td>27.52</td><td>26.18</td><td>28.47</td><td>20.11</td><td>23.41</td><td>27.46</td><td>21.94</td><td>22.80</td></tr><tr><td colspan="11"><strong>RO₂ sinks</strong></td></tr><tr><td>NO</td><td>84.48</td><td>86.72</td><td>93.37</td><td>87.64</td><td>89.12</td><td>83.04</td><td>86.55</td><td>93.18</td><td>83.00</td><td>81.60</td></tr><tr><td>RO₂</td><td>0.04</td><td>0.01</td><td>0.10</td><td>0.38</td><td>0.10</td><td>0.04</td><td>0.01</td><td>0.10</td><td>0.38</td><td>0.11</td></tr><tr><td>Interm. aro. diss.</td><td>13.53</td><td>10.87</td><td>4.07</td><td>5.07</td><td>8.77</td><td>13.38</td><td>10.87</td><td>4.07</td><td>6.38</td><td>11.97</td></tr><tr><td>RO₂+NO₂ ⇌ PANs</td><td>1.34</td><td>2.19</td><td>1.84</td><td>5.40</td><td>1.55</td><td>2.94</td><td>2.36</td><td>2.03</td><td>8.83</td><td>5.93</td></tr><tr><td>HO₂</td><td>0.19</td><td>0.04</td><td>0.31</td><td>1.04</td><td>0.20</td><td>0.19</td><td>0.04</td><td>0.31</td><td>1.01</td><td>0.20</td></tr><tr><td>Deposition</td><td>0.42</td><td>0.17</td><td>0.31</td><td>0.47</td><td>0.25</td><td>0.40</td><td>0.17</td><td>0.31</td><td>0.41</td><td>0.18</td></tr></tbody></table>
<figure><img src="image_18.png" /><figcaption>**Figure A.5:** Daily evolution of the lifetimes computed for OH and main RO₂ identified in Tab. 9.</figcaption></figure>
Highlights

Measurements and modelling of OH and peroxy radicals in an indoor environment
under different light conditions and VOC levels

Eve-Agnès Fiorentino, Hui Chen, Adrien Gandolfo, Victor Lannuque, Karine Sartelet, Henri Wortham

* OH and of sum of peroxy radical measurements were performed in an indoor environment.

* Radical measurements were examined with a 0D chemistry model accounting for mixing.

* Main radical primary sources were ozonolysis in the shade and photolysis in the light.

* OH spatial heterogeneity is mitigated by transport of peroxy radicals over the room.