Ludovic Roche
email: ludovic.roche@laas.fr

David Trémouilles

Emmanuel Marcault

Corinne Alonso

Study of GaN HEMTs Robustness to Application-Like, Software-Controlled Overshoots Emulating Different Gate Routings in Original 50 Ohms Environment

Keywords: GaN, Overshoot, Transient, Spike, Deported Control, 50 Ohm Environment, Robustness, characterization

In the field of transient tolerance tests, few studies have been conducted on gate voltage spikes of GaN HEMT components. A parametric generation of such overvoltage either implies precise and tedious hardware gate circuitry design, or require some simplification of the waveform making it less representative of actual and practical cases.

Such tests will be easier to conduct thanks to the original 50 Ohms environment setup proposed in this work. This original test bench improves usual measurement bandwidths. It also allows spatial isolation of a tested device with its driver and power circuits (e.g. over one meter), limiting their complex interactions.

As a first demonstration of the setup capability, the breakdown overvoltage of a p-GaN HEMT is determined. Several gate voltage overshoots as high as twice the DUT nominal rating are then demonstrated not to degrade the tested device. Moreover, allowing such overshoots may enable to significantly reduce switching losses.

I. INTRODUCTION

Wide BandGap (WBG) devices display better behavior than Si devices regarding power and volume efficiency for static converters. GaN High Electron Mobility Transistors (HEMTs) enable higher working frequencies and reduced switching losses. Thus, smaller passive components are needed as filters in power applications. However, fast switching tend to trigger parasitic oscillations in both gate and drain circuitry. These oscillations result from the complex interaction of the inductive and capacitive behaviors of the components, circuits, and parasitics. They can lead to energy losses and failures. Equivalent models are proposed in literature [START_REF] Kozak | An Analytical Model for Predicting Turn-ON Overshoot in Normally-OFF GaN HEMTs[END_REF], but identification of their parameters from actual PCBs is complex. Therefore, precisely predicting oscillations is much more difficult than modeling experimental ones. In particular, overvoltage studies require the generation of reproducible, precisely controlled and measurable OVSs. This entails either tedious PCB design, or the use of simpler waveforms, making them less representative of actual cases.

This work presents a generic characterization-setup design with full control of a power component state and its circuit parasitics. To do so, concepts from the radiofrequency transmission line model, and their application to move away a GaN HEMT from its command and power circuits are presented. A setup achieving high-frequency bandwidth is proposed along with its theoretical analysis. Several OVS robustness tests are conducted as a demonstration of the performances of the setup.

In section II, the origins and typical waveforms of gate OVerShoots (OVSs) are explored and compared to the existing literature of OVS robustness studies. The proposed setup and the underlying theoretical background is presented in section III. Experimental results are discussed in section IV.

II. GAN HEMTS GATE OVERVOLTAGES: ORIGINS, CHARACTERIZATION, CONSEQUENCES

Several phenomena can lead gate-source (or source sensing, if applicable) to hazardous voltage ringing and overshoots. Depending on the amplitude of the oscillations, the component is degraded at varying speeds [START_REF] Wang | Gate Lifetime of P-Gate GaN HEMT in Inductive Power Switching[END_REF]. An application-scale consequence is the false turn-off; upon switching on, the gate voltage oscillates lower than its threshold, causing undesired, partial switching [START_REF] Ishibashi | An analysis of false turn-on mechanism on high-frequency power devices[END_REF].

Firstly is discussed typical origins and waveforms of gate OVS. Then, in the second part is a review of existing gate OVS robustness characterization methods.

A. Origins and Consequences of Gate Overshoots

GaN transistors feature intrinsic fast switching speed and excellent radiation tolerance. However, while fast transient are beneficial regarding the decrease of switching losses, the parasitic RLC of the surrounding circuits are all the more excited by the high frequency content of the waveforms, creating oscillations jeopardizing the components.

Copper tracks alone provide enough inductance to create oscillations. A typical copper track section is presented in Fig. 1. According to [START_REF] Simons | Coplanar Waveguide Circuits, Components, and Systems[END_REF], this section creates a linear inductance L l = 0.4 nH mm -1 .

As an example, the effects of the length of the copper track between a typical driver and a p-GaN HEMT GS-065-004-1-L gate are simulated in Fig. 2; the longer the track, the higher its inductance and the higher the perturbation on the gate signal. Indeed, the track inductance resonates with the transistor gatecapacitance, creating oscillations that can be modeled by a 2 nd or 4 th order oscillator [START_REF] Kozak | An Analytical Model for Predicting Turn-ON Overshoot in Normally-OFF GaN HEMTs[END_REF]. This is the reason why it is common to reduce the length of copper tracks to achieve better signal [START_REF] Strydom | Using Enhancement Mode GaN-on-Silicon Power FETs (eGaN® FETs)[END_REF].

Furthermore, gate signal distortions may have many other origins like radiated coupling of adjacent tracks, especially power signal tracks. Perturbations from other parts of a circuit can also propagate through power supplies and tracks. These perturbations can in turn resonate with the presented equivalent LC circuitry.

B. Robustness Characterization

Since overshoots can occur in the most common converter designs, as shown in Part. II-A, it is essential to quantify the robustness of GaN HEMTs to those overshoots.

Several setups allow studying GaN gate robustness. DC studies are the most straightforward to conduct since LC elements does not interfere in this case. The gate DC Breakdown Voltage (BV) of GaN HEMTs is thus generally provided in all device datasheets. He et al. [START_REF] He | Frequencyand Temperature-Dependent Gate Reliability of Schottky-Type p -GaN Gate HEMTs[END_REF] present a study between stepped DC and pulsed test, which progressively steps up a DC stress applied at 100 kHz, with 50 % duty cycle, until failure.

A setup approaching the typical waveforms presented in Part. 1 was designed by Wang et al. [2] [7], to find that gate overshoot robustness can be fitted with both lognormal and Weibull distribution. This design lets an inductance be charged upon a controlled level, to then discharge it in the gate of a GaN transistor. The inductance and gate capacitance then produce ringing similar to the expected realistic waveform but around 0 V.

OVS robustness studies are less common than DC ones due to the strong impact of the circuitry on the gate signals.

To perform parametric testing, the waveforms applied must be precisely controlled, and there must be no parasitics due to, nor in the measurement path. Since only few millimeters of copper tracks suffice to create parasitics for fast transient signals, the resulting gate and measurement circuitry must be dense; its conception is therefore tedious. With GaN packages of different sizes and shapes, redesigning optimized test-PCBs can be time-consuming.

Moreover, while dense PCB design allows for the reduction of parasitic inductances, it makes impossible to apply an environmental stress to the device only, rising the need for robust and costly circuits for each device, and hampering any efforts to study GaN environmental reliability.

III. PROPOSED 50 OHM SETUP

A common solution for parasitic management at any length of conductors is controlled impedance. This concept is based on transmission lines equations, which is standard in the field of radio-frequency.

The proposed 50 Ohms circuitry is presented in the following parts. The transmission line theory completely encompasses the lumped impedance approach developed in Part. II-A, but is fairly different in its calculations and reasoning. Therefore, it is presented in the following parts independently of the lumped-model approach.

A. Transmission Lines: Selected Theoretical Elements

While lumped elements are a valid model for the propagation of electromagnetic signals in conductors of small length compared to the signal spatial wavelength, the tools provided by the transmission line model are valid for any length of cable.

A set of two conductors part of a current loop is defined by two main parameters in the transmission line model:

Near-end Circuitry Far-end Circuitry Z n-e Z f-e Transmission Line Z, T d V n-e (t)
I n-e (t)

V f-e (t)

I f-e (t)
Fig. 3: Parameters of a transmission line and its environment.

• its characteristic impedance Z;

• the travel time of an electromagnetic signal across the length of the conductor T d .

A circuit containing a transmission line can be described with the following set of parameters, schematized in Fig. 3):

• the transmission line, defined by its Z and T d ;

• the output impedance of the near-end side of the transmission line Z n-e ; • the input impedance of the far-end side of the transmission line Z f-e .

These elements allow calculating the near-end and far-end reflection coefficients:

R n-e (s) = Z n-e (s) -Z Z n-e (s) + Z ; R f-e = Z f-e (s) -Z Z f-e (s) + Z (1)
While Z is usually real, these coefficients can be complex depending on Z n-e and Z f-e . Note that a reflection coefficient is null only if the termination impedance is equal to the line impedance. With these parameters, the system can be analyzed as such:

• By replacing the transmission line with its characteristic impedance Z, the voltage across Z is calculated. This voltage is the incident voltage wave V i,n-e (t)H(t); • This voltage wave travels across the transmission line.

Upon arriving at the far-end, the voltage wave:

V i,f-e (t) = V i,n-e (t -T d )H T d (t) (2)
is reflected. The reflected wave is defined by:

V r1,f-e = V i,f-e R f-e (3)
• This reflected voltage wave then travels back to the nearend and reaches it with a delay:

V r1,n-e (t) = V r1,f-e (t -T d ) (4)
This wave is reflected as V r2,n-e (t), defined by:

V r2,n-e = V r1,n-e R n-e (5)
• Subsequent calculations of reflections can be made following this method. If set as in Fig. 3, the same relations apply for current calculations.

• At any time, the apparent voltage is defined by the sum of all the voltage waves defined previously:

           V tot,n-e (t) = V i,n-e (t)H(t) + ∞ k=0 V rk,n-e (t) V tot,f-e (t) = V i,f-e (t) + ∞ k=0 V rk,f-e (t) (6)
• The DC analysis of the currents and voltages of the circuitry are obtained with an infinite number of rebounds that converge to a stationary DC state; thus, they do not depend on T d . They can be obtained for any T d , especially T d = 0.

B. Global Description and Functioning of the Setup

The proposed setup is presented in Fig. 4. Following are presented the parts of the setup: driver-side, power-side, measurements; and its advantages for efficient characterization.

1) Driver side: The gate of the DUT is interfaced with a 50 Ω interconnection, and a voltage probe. The far-end of the gate 50 Ω coaxial cable is connected to a 50 Ω-outputimpedance waveform generator, in order for the reflection coefficient (1) at this end to be null.

As illustrated in Fig. 5, for any incident voltage V i (t) sent through the line by the generator, one reflection V r (t) occurs on the DUT side and there is no further reflection on the generator, thanks to the null reflection coefficient on its side.

Applying the results of the previous section (III-A), the following parameters are defined:

• the cable is defined by its impedance Z c = 50 Ω and delay T c ; • the input impedance of the DUT gate is Z g , and the output impedance of the generator is Z gen = 50 Ω; • the reflection coefficient on the DUT side of the cable is therefore:

R gate = Z g -Z c Z g + Z c (7)
Then, the two voltage waves present at any time at the DUT gate are:

• the generator incident voltage V i , with a delay T c created by the cable:

V i,DUT (t) = V i (t -T c )H Tc (t) (8)
• and the reflected wave V r , defined by R gate and V i,DUT :

V r,DUT = R gate V i,DUT (9)
These waves are represented in Fig. 4. The apparent voltage at the DUT gate is thus:

V g (t) = V i,DUT (t) + V r,DUT (t) (10)
Then, by combining (9) and (10):

V g = V i,DUT (1 + R gate ) (11)
Hence, any desired gate voltage can be obtained by generating the appropriate V i defined by ( 11) and ( 7

Z gen = Z c Z c , T c DUT Z g V g I g V i V r 2V i
Fig. 5: Gate circuitry model. With a Thevenin model for the waveform generator, V i is generated with a 2V i voltage source.

V i,DUT = V g Z g + Z c 2Z g (12)
Since Z g is highly non-linear and, in some cases, depends on the power circuitry as well as the DUT intrinsic properties, (12) can be rewritten as:

V i,DUT = V g + I g Z c 2 (13)
Since V i,DUT is only a T g -shifted V i , V i is entirely defined by V g and I g . Moreover, with Z c being real, ( 14) is similar in time-domain:

V i,DUT (t) = V g (t) + I g (t)Z c 2 ( 14
)
The reproduction of an in-application measured (V g , I g ) is thus possible with the proposed setup. While V g is usually measurable on PCBs, I g can be obtained through simulation, knowing V g .

As displayed in Fig. 5, a Z c Thevenin voltage source providing 2V i generates the appropriate V i through the line.

In the following experiments, the theoretical V i waveforms obtained by this method are synthesized thanks to a homemade generator. This waveform generator is currently in patenting process. 2) Power side: In the same way as in Part. III-B1, the drain of the DUT is connected to a cable of controlled impedance. This cable is terminated by the drain voltage generator, which is constituted of a high voltage source, a high value resistor R, a decoupling capacitor and an adaptation 50 Ω resistor. The drain voltage generator is represented in Fig. 6. This generator acts as a Z C -adapted voltage source: the high resistor branch charges the capacitance and ensures the high-frequency stability of the voltage generator. Fast drain transients generated by the DUT are not reflected by the generator thanks to the capacitance and Z c resistor.

R >> Z c Z c Z c C Z c Z c Z c V V Equivalent setup :
The equivalent drain circuit is actually described by the same schematic as in Fig. 5, with Z DS in place of Z G . Eq. ( 14) can thus be rewritten as:

V i,DUT (t) = V DS (t) + Z c I DS (t) 2 (15)
Eq. 15 is equivalent to the circuitry in Fig. 7. Correspondence between Fig. 6 and Fig. 7 confirms that V = 2V i . The power side circuitry thus behaves as in Fig. 7. Hence, even tho the generator is deported, the drain-source voltage behaves as if commuting on an adjacent resistive charge.

3) Critical signal paths: In order to eliminate all parasitic, all copper tracks or cable must be of controlled impedance.

Z c DUT Z DS 2V i V DS
I DS Fig. 7: Power side circuitry equivalent according to Fig. 6. As represented in Fig. 5,

2V i = V .
However, some signal paths must be as short as possible to prevent impedance discontinuity and unwanted signal rebounds.

Those are marked with red crosses in Fig. 4.

• High value resistors, either for probing (R p ) or injecting small currents (R) must be as close to the main signal path as possible. If not, a signal travelling through the main line would be divided between its main track and this sub-path, with the sub-path termination not being adapted. Thus, rebounds would occur and be visible as parasitics on the main signal. • Adaptation resistors must be connected directly to the ground. The rebounds and parasitic are as dampened as the ground paths are short. • For voltage probing, the voltage probe must be as close as possible to the main line termination. If not, the delay δT between the probe and the line termination would be apparent in the measurement. In this later case:

V probe = V i,DUT (t + δT ) + V r,DUT (t -δT ) (16)
4) Experimental implementation and results: Several measurements are made to extract data using the proposed setup.

• The voltage probe allows monitoring V g . As displayed in Fig. 7 and demonstrated in the corresponding calculations, a cable Z c adapted at its termination behaves like a Z c resistor. Thus, the oscilloscope voltage is

V oscilloscope = 50 50 + R p V GS .
• The drain current probe is designed to keep the 50 Ω impedance and thus to avoid parasitic. Contrarily to the gate voltage probe that must be placed close to the gate, the position of the drain current probe on the line does not really matter. V i is DC, so the time-shifting described by (16) only shifts the measurement in time without distorting it. This time-shift can be measured independently and compensated in the oscilloscope settings. • The oscilloscope used for this series of experiment is a DSOS104A, with a 1 GHz bandwidth. 5) Advantages of the setup; Parasitics: By controlling the impedance of all interconnections, copper tracks inductances presented in part II-A are integrated in the calculations within the complete model of transmission lines. Therefore, there are no parasitics as long as the line impedance is controlled. Fig. 8: Typical experimental waveforms applied to the gate to find its spike breakdown (22 V OVS). The drain switching energies are of 3.3 µJ (1) and 0.6 µJ (2). However, electromagnetic coupling can still occur from the power side to the command side. This is prevented by only using well-defined, shielded sections, as in Fig. 1. These section have a well-defined impedance and the current loop is very small, due to the proximity of all conductors. 50 Ω cables are shielded and bring very little to no parasitics. Clean signal paths of controlled impedance are straightforward to route in this application due to the permitted long track length and small number of tracks. The only remaining source of parasitics is the interconnections or welds; these are however as small as the signal path through the welding is short, thus actually creating very little to no parasitics.

IV. ROBUSTNESS TESTING

Measurements were conducted with the proposed circuitry and a homemade gate voltage generator. Firstly, p-GaN HEMTs GS-065-004-1-L were submitted to a wide range of OVS, as displayed in Fig. 8, to find their approximate dynamic Breakdown Voltage BV dyn . Their apparent gate-source resistance was measured with an ohmmeter (Keysight U1241C) before and after each stress. The resistance was not measurable by the ohmmeter for an unbroken device (over 10 MΩ). The breakdown voltage was found to be around 22 V OVS, after which the measured resistance was around 7.5 MΩ.

After determining the approximate 1-pulse BV dyn , the gate of two p-GaN HEMTs GS-065-004-1-L were submitted to a series of three 15 V OVS displayed in Fig. 9. The superposition of the three pulses show the excellent replicability of the stress.

To monitor the gate and its possible degradation, two indicators are used: gate leakage and gate threshold measurements. The corresponding measurements are respectively presented in Fig. 10a and Fig. 10b.

No degradation was detected in Fig. 10, thus showing the robustness of GaN HEMTs to at least few, short gate voltage OVS. This result and the measured 1-pulse BV dyn are coherent with Wang et al. [START_REF] Wang | Gate Lifetime of P-Gate GaN HEMT in Inductive Power Switching[END_REF], although with different gate OVS waveforms. The switching energies are much lower with voltage OVS, as displayed in Fig. 8; this is related to the higher dV /dt with overshot waveforms, or the overshoot itself.

V. CONCLUSION

This work develops theoretical tools to apply any given voltage or current on a power device gate through ordinary 50 Ω cables. Similarly, the drain side circuit can be moved away (e.g. one meter).

As a demonstration, the spike breakdown voltage of a GaN gate was measured at four times the nominal gate voltage. A few stress waveforms as high as twice the DUT nominal rating have no measurable impact on the gate health.

This demonstration emphasizes the advantages of the setup: the interactions between the instrumentation circuitry and the DUT are suppressed, as well as parasitics.

Thus, this setup can be used for various testings, including long term gate reliability studies (e.g. repeated overshoots using realistic gate waveforms). Easier gate health monitoring is enabled, as well as gate signal optimization to achieve optimal switching in power converters.

Cable and tracks of any length can be used with the proposed setup. Especially, the use of long cables highly simplify environmental, thermal and wafer-level dynamic characterization.

Fig. 1 :Fig. 2 :

 12 Fig. 1: A typical copper track section on a FR4 power PCB. H = 1.5 mm, S = 0.15 mm, W = 0.2 mm, T = 35 µm. ϵ r = 4.5, ϵ r,ef f ective = 2.20. S and W are defined accordingly to EPC GaNs gate pads.

Fig. 4 :

 4 Fig. 4: Proposed 50 Ω setup. Each 50 Ω cable can be of arbitrary length, contrarily to the short critical paths.

Fig. 6 :

 6 Fig. 6: Drain voltage generator circuitry.

Fig. 9 :

 9 Fig.9: Experimental stress waveforms. The excellent repeatability and low parasitics level of the generated waveforms are in line with the developped theoretical elements.

II

  GSS before stress I GSS after stress (a) Gate leakage monitoring, before and after stress. DS before stress I DS after stress (b) Gate threshold monitoring. The B1505 compliance was set to 100 mA.

Fig. 10 :

 10 Fig. 10: Degradation monitoring on the gate. The chosen health monitoring parameters show no sign of degradation.

This work was supported by the LAAS-CNRS PROOF platform, partly financed by the Occitanie region.


Carmen Kalalian

Antoine Depoorter

Letizia Abis

Sébastien Perrier

Christian George
email: christian.george@ircelyon.univ-lyon1.fr

Indoor heterogeneous photochemistry of molds and their contribution to HONO formation

Keywords: Microbial surfaces, Aspergillus niger, photochemistry, VOC, nitrous acid (HONO), indoor air quality

To better understand the impact of molds on indoor air quality, we studied the photochemistry of microbial films made by Aspergillus niger species, a common indoor mold. Specifically, we investigated their implication in the conversion of adsorbed nitrate anions into gaseous nitrous acid (HONO) and nitrogen oxides (NOx), as well as the related VOC emissions under different indoor conditions, using a high-resolution proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) and a long path absorption photometer (LOPAP). The different mold preparations were characterized by the means of direct injection into an Orbitrap highresolution mass spectrometer with a heated electrospray ionization (ESI-Orbitrap-MS). The formation of a wide range of VOCs, having emission profiles sensitive to the types of films (either doped by potassium nitrate or not), cultivation time, UV-light irradiation, potassium nitrate concentration and relative humidity was observed. The formation of nitrous acid from these films was also determined and found to be dependent on light and relative humidity.

Finally, the reaction paths for the NOx and HONO production are proposed. This work helps to better understand the implication of microbial surfaces as a new indoor source for HONO emission.

Introduction

The health risk due to exposure to various types of indoor pollutants becomes greater as people spend exceedingly their time indoors. [START_REF] Park | Investigation of nitrous acid concentration in an indoor environment using an in-situ monitoring system[END_REF][START_REF] Gandolfo | Unexpectedly High Levels of Organic Compounds Released by Indoor Photocatalytic Paints[END_REF] Variety of pollutants might be emitted directly from indoor sources, from secondary chemistry initiated by oxidants and surface reactions or via infiltration of outdoor air. [START_REF] Zhou | Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home[END_REF][START_REF] Mendez | Identification of the major HOx radical pathways in an indoor air environment[END_REF] The high surface to volume ratio (1 to 4 m -1 ) [START_REF] Manuja | Total surface area in indoor environments[END_REF][START_REF] Wainman | Effects of surface type and relative humidity on the production and concentration of nitrous acid in a model indoor environment[END_REF][START_REF] Schwartz-Narbonne | Indoor lighting releases gas phase nitrogen oxides from indoor painted surfaces[END_REF][START_REF] Weschler | Indoor Chemistry[END_REF] in indoor environments points to the importance of heterogeneous chemistry through mechanisms including adsorption, oxidation reactions and acid-base chemistry. [START_REF] Schwartz-Narbonne | Water uptake by indoor surface films[END_REF][START_REF] Nazaroff | Indoor chemistry: research opportunities and challenges[END_REF] Indoor surfaces involving different types of building materials like gypsum board, paint, glass, wood, tile, laminate, carpet, and fabric as well as furniture, human activities, and microbial activity, can be important sources and sinks of reactive pollutants indoors. [START_REF] Weschler | Indoor Chemistry[END_REF] Volatile Organic Compounds (VOCs) are among these pollutants. VOCs can react with the indoor oxidants (like ozone (O3) and OH radicals), resulting in the formation of multifunctional compounds in the gas phase, and can contribute to the formation of secondary organic aerosol. [START_REF] Weschler | Indoor Chemistry[END_REF][START_REF] Farmer | Analytical challenges and opportunities for indoor air chemistry field studies[END_REF] VOCs are often detected indoors with higher concentrations than in the outdoor environment, [START_REF] Weschler | Indoor Chemistry[END_REF][START_REF] Brown | Concentrations of Volatile Organic Compounds in Indoor Air -A Review[END_REF][START_REF] Lee | Investigation of indoor air quality at residential homes in Hong Kong -Case study[END_REF] and their emissions depend on various environmental factors such as light, relative humidity and temperature. [START_REF] Schwartz-Narbonne | Water uptake by indoor surface films[END_REF][START_REF] Uhde | Impact of reaction products from building materials and furnishings on indoor air quality-A review of recent advances in indoor chemistry[END_REF] However, indoor VOCs concentrations are mainly controlled by air ventilation, human occupancy and activity, alongside their indoor chemical reactions with indoor oxidants. [START_REF] Schwartz-Narbonne | Water uptake by indoor surface films[END_REF][START_REF] Subramaniam | Comparative study on indoor fungi growth incorporated with different antifungal and wall finishings[END_REF] In addition, growing molds in damp dwellings can be an indoor source of spores, fragments and microbial VOCs (mVOCs). [START_REF] Polizzi | Influence of various growth parameters on fungal growth and volatile metabolite production by indoor molds[END_REF] The most common molds identified in indoor environments are Penicillium, Aspergillus and Cladosporium. [START_REF] Matysik | Determination of volatile metabolites originating from mould growth on wall paper and synthetic media[END_REF][START_REF] Reboux | Indoor mold concentration in Eastern France[END_REF] The growth of these fungi on different surfaces is accelerated by the adequate presence of nutrients, temperature, pH and relative humidity. [START_REF] Subramaniam | Comparative study on indoor fungi growth incorporated with different antifungal and wall finishings[END_REF] In an indoor environment with moisture and/or high humidity, fungal spores can stick to a wide variety of organic and inorganic materials, which is the first step of colonial development and reproduction. [START_REF] Priegnitz | The role of initial spore adhesion in pellet and biofilm formation in Aspergillus niger[END_REF] After their proliferation, they germinate and grow producing thousands of new spores using nutrients present on the different colonized substrates. [START_REF] Subramaniam | Comparative study on indoor fungi growth incorporated with different antifungal and wall finishings[END_REF] It was also demonstrated that the cell-to-cell interactions between the spores and/or hyphae are essential for the development of three-dimensional mycelial structures like biofilms. [START_REF] Priegnitz | The role of initial spore adhesion in pellet and biofilm formation in Aspergillus niger[END_REF] Moreover, as fungal cells might not be detected in the ambient air in the early stage of contamination, microbial VOCs (mVOCs) produced either by the microorganisms metabolism or by the VOCs processing, can then be used as tracers of fungal contamination within a dwelling. [START_REF] Korpi | Volatile Compounds Originating from Mixed Microbial cultures on building materials under various humidity conditions[END_REF] One should note that the indoor concentrations of mVOCs in sick buildings range from few ng/m 3 to 1 mg/m 3 . [START_REF] Korpi | Microbial volatile organic compounds[END_REF][START_REF] Lorenz | Practical experiences with MVOC as an indicator for microbial growth[END_REF][START_REF] Kalalian | Influence of indoor chemistry on the emission of mVOCs from Aspergillus niger molds[END_REF][START_REF] Velazquez | From one species to another: A review on the interaction between chemistry and microbiology in relation to cleaning in the built environment[END_REF] In addition to producing chemicals indoors, some microorganisms are able to degrade chemical contaminants, like phthalates, emitted from home vinyl flooring. [START_REF] Velazquez | From one species to another: A review on the interaction between chemistry and microbiology in relation to cleaning in the built environment[END_REF] Furthermore, previous studies have shown a positive correlation between the gases SO2, NO2, and CO and the airborne microorganisms. This correlation is probably due to the use of sulfate, nitrate, and carbonate (transformed from these gases) as the nutrients by the viable microbes. [START_REF] Xie | Characteristics of total airborne microbes at various air quality levels[END_REF] In a recent study by Guo et al., [START_REF] Guo | Effect of formaldehyde exposure on bacterial communities in simulating indoor environments[END_REF] the effect of different formaldehyde levels, a harmful indoor VOC emitted by various indoor surfaces, on indoor bacterial communities was established. This study has shown the interaction between formaldehyde levels and indoor bacteria and gave new insight for future studies on the interaction between indoor pollutants and microbial species. Furthermore, one should note that the indoor environment contains highly reactive molecules and radicals such as ozone (O3), nitrate (NO3) and hydroxyl radicals (OH), nitrogen oxides (NOX), sulfur dioxide (SO2). [START_REF] Uhde | Impact of reaction products from building materials and furnishings on indoor air quality-A review of recent advances in indoor chemistry[END_REF] Hydroxyl radicals are now considered as the most important oxidant indoors. Recent measurements have shown that their concentrations can reach 10 6 molecules cm -3 , similar to outdoor daytime levels. [START_REF] Mendez | Identification of the major HOx radical pathways in an indoor air environment[END_REF] OH radicals were thought to be mainly produced during the ozonolysis of unsaturated VOC. Nonetheless, recent studies showed that the photolysis of nitrous acid (HONO), at wavelengths lower than 400 nm, can also be a source of OH radicals, contributing to more than 50% of their emissions. [START_REF] Mendez | Identification of the major HOx radical pathways in an indoor air environment[END_REF][START_REF] Alvarez | Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid[END_REF][START_REF] Bartolomei | Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint[END_REF][START_REF] Alvarez | Light-induced nitrous acid (HONO) production from NO2heterogeneous reactions on household chemicals[END_REF][START_REF] George | Heterogeneous Photochemistry in the Atmosphere[END_REF] Meanwhile, HONO can result from different sources such as combustion processes, gas-phase reactions between OH and NO, or (photo)chemical heterogeneous reactions. [START_REF] Park | Investigation of nitrous acid concentration in an indoor environment using an in-situ monitoring system[END_REF][START_REF] Bartolomei | Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint[END_REF][START_REF] Ammar | Photoenhanced NO2 loss on simulated urban grime[END_REF][START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF][START_REF] Wang | Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter[END_REF][START_REF] Bhattarai | Emissions of atmospherically important nitrous acid (HONO) gas from northern grassland soil increases in the presence of nitrite (NO2-)[END_REF] The measured HONO concentrations indoors are typical around a few ppb, reaching higher values in poorly ventilated residences with gas-fired stoves. [START_REF] Bartolomei | Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint[END_REF][START_REF] Alvarez | Light-induced nitrous acid (HONO) production from NO2heterogeneous reactions on household chemicals[END_REF] It can also form carcinogenic nitrosamines via its reaction with secondary amines. [START_REF] Bartolomei | Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint[END_REF] Several studies have shown that the HONO formation can be enhanced by light (300 nm< λ <400 nm). [START_REF] Schwartz-Narbonne | Indoor lighting releases gas phase nitrogen oxides from indoor painted surfaces[END_REF][START_REF] Alvarez | Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid[END_REF][START_REF] Bartolomei | Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint[END_REF][START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF][START_REF] Morrison | Recent Advances in Indoor Chemistry[END_REF] Besides, nitrate anions can result from the oxidation of NOx into nitric acid (HNO3). Arata et al. performed direct measurements of N2O5 and NO3 indoors, where these species react to produce HNO3. [START_REF] Arata | Measurement of NO3 and N2 O5 in a Residential Kitchen[END_REF] Nitric acid is likely to deposit onto surfaces, where the presence of water layers could lead to its hydrolysis and thus to the production of surface nitrate. [START_REF] Schwartz-Narbonne | Indoor lighting releases gas phase nitrogen oxides from indoor painted surfaces[END_REF] Schwartz-Narbonne et al. characterized real indoor films (collected from the living room and kitchen) in terms of water uptake and ionic composition and found concentrations of nitrate anion ranging from 7.57 to 25.17 µg/cm 2 . [START_REF] Schwartz-Narbonne | Water uptake by indoor surface films[END_REF] Liu et al. [START_REF] Liu | Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO2Transformation on Glass Windows[END_REF] have also identified different anions (F -, Cl -, Br -, NO3 -, PO4 3-, SO4 2-) on glass plates collected from the kitchen with concentrations from 0.117 to 142.499 ppm.

Indoor surfaces are typically covered by a complex mix of organic and inorganic compounds. [START_REF] Schwartz-Narbonne | Water uptake by indoor surface films[END_REF][START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF][START_REF] Weschler | Growth of Organic Films on Indoor Surfaces[END_REF][START_REF] Yang | Significant HONO formation by the photolysis of nitrates in the presence of humic acids[END_REF] In this work, we investigated the photochemistry of microbial films made by Aspergillus niger species, a common indoor mold, and their implication in the conversion of adsorbed nitrate anions into gaseous nitrous acid, nitrogen oxides, and associated VOC emissions, as a function of light irradiation, relative humidity and molds cultivation time. This will unravel the implication of microbial surfaces, ubiquitous in sick buildings, in the HONO formation, affecting the indoors oxidative capacity and will help better understand interactions between microbes and chemistry in human-dominated habitation.

Experimental section

2.1.Molds and film preparations

Molds were cultivated on a synthetic malt agar extract, recommended for the cultivation of indoor mold species. [START_REF] Sunesson | Identification of volatile metabolites from five fungal species cultivated on two media[END_REF] The protocols used for the culture medium preparation, as well as the species identification by molecular biology are described by Kalalian et al. [START_REF] Kalalian | Influence of indoor chemistry on the emission of mVOCs from Aspergillus niger molds[END_REF] The grown molds belonged to the Aspergillus sp. type Aspergillus niger.

The first step of the film preparation was to wash a grown molds' petri dish with Milli-Q water (18.2 MΩ cm), in order to create a solution, named "molds extract", containing a suspension of molds in water. Then, the second step consisted in filling a petri dish (40 mm in diameter) with either (i) a solution of molds extract or (ii) a solution of potassium nitrate KNO3 and (iii) a mixed solution of molds extract and potassium nitrate KNO3. The concentrations of the potassium nitrate KNO3 solutions were varied from 0.001 M to 0.5 M. The preparations were dried over a period of 12 hours in an oven at 323.2 ± 0.1 K. Once dried, the petri dishes were weighed again (the empty petri dishes were weighed beforehand) and used directly (within one day) in the planned experiments.

2.2.Experimental setup

Experiments were performed in an 80 cm 3 borosilicate reactor under a constant flow of dry/humid synthetic air, or synthetic N2, and irradiated from the top by a 340 nm LED lamp (ThorLabs, 60mW, nominal maximum irradiance of 2.22 W/m 2 ). The humidity in the reactor was varied by changing the mixing ratio of dry or humid carrier gas. The temperature (T = 25 ± 2°C) and the relative humidity in the reactor (RH = 0-85% ± 2%) were controlled by a humidity and temperature probe (Vaisala HUMICAP® HMP110).

In this study, two different experimental systems were used as shown in Figure S1 in the supporting materials (SM). Setup 1 was designed for VOC analysis, where the reactor was coupled to a high-resolution proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) and flushed by 120 mL/min of air (or N2). For this setup (setup 1), a measurement cycle consisted of measuring the VOC concentrations at the outlet of the reactor containing the film for 5 min, then the LED lamp was switched on for 15 min. In contrast, the second approach was designed to measure NOX and HONO formation. For setup 2, the reactor was coupled to a NOX analyzer and a Long Path Absorption Photometer (LOPAP) and flushed by 500 mL/min of synthetic air (or N2). The irradiation time was around 60 min.

Nonetheless, prior to each experiment, a blank control involving sterile petri dishes was performed under the same experimental conditions.

Analytical tools

UV-visible analysis

The UV-visible spectra of initial solutions, as well as extracted films of molds, potassium nitrate and mix film of molds and potassium nitrate, were determined in the range of 200-600 nm using a Cary 60 UV-Vis spectrophotometer from Agilent Technologies. Films were extracted with 2 mL of Milli-Q water and diluted from 2 to 20 times. Samples were then placed within a 3500 µL Macro Fluorescence Cuvette (Thorlabs) to perform the measurement. Three replicas were performed for each type of extracted film or solution.

Mass spectrometry analysis

It was demonstrated by Depoorter et al. [START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF] that a possible reaction may occur between nitrate anions and reactive organic compounds during the drying process of a wet film, resulting in the formation of nitro groups. Therefore, we characterized the potential formation of nitrogenated compounds in a mixed solution of molds extract and potassium nitrate using a direct infusion in an Orbitrap high-resolution mass spectrometer (HRMS, Q Exactive, Thermo Scientific, Bremen, Germany) with a heated electrospray ionization (ESI). These results were compared to different blanks using water, molds extract solution and potassium nitrate solution, respectively. It is noteworthy to mention that the solutions were filtered through a 0.2 μm Hydrophilic Polypropylene membrane (GHP,13 mm, Pall Corporation, USA).

Samples were injected into the MS through a steel capillary by a Hamilton syringe of 500 µL (SGE Analytical Science) at a flow rate of 10 μL/min and were analyzed using a spray voltage of -4 kV and +4 KV for negative and positive ionization mode (ESI-and ESI+) measurements, respectively. The temperature of the Orbitrap capillary was set at 250°C. The system was operated in both ESI+ and ESI-mode with a resolving mass power of 140 000 at m/z 200. A sodium acetate solution (2 mM in 50/50, v/v, ACN/water) was used for the mass calibration; this solution provides a series of negative and positive adduct ions in the scanning range of m/z 50-750. [START_REF] Wang | Chemical Characteristics of Organic Aerosols in Shanghai: A Study by Ultrahigh-Performance Liquid Chromatography Coupled With Orbitrap Mass Spectrometry[END_REF][START_REF] Wang | Chemical Characteristics and Brown Carbon Chromophores of Atmospheric Organic Aerosols Over the Yangtze River Channel: A Cruise Campaign[END_REF] The calibration method consisted in filling the syringe with a 2 mM solution of sodium acetate and setting the syringe pump flow rate to 20 µL/min. Once the calibration started, few minutes wait are needed for a stable system with a stable TIC (deviation <10%). The accuracy of the calibration is then evaluated (lower than 0.5 ppm). This procedure is made for both modes (positive and negative). Signals in the range of m/z from 50 to 750 were then recorded and processed for formula calculations.

Data were processed and evaluated using Xcalibur 2.2 (Thermo, USA). The subsequent non-target screening approach was conducted using MZmine 2.53 software. [START_REF] Wang | Chemical Characteristics of Organic Aerosols in Shanghai: A Study by Ultrahigh-Performance Liquid Chromatography Coupled With Orbitrap Mass Spectrometry[END_REF][START_REF] Wang | Chemical Characteristics and Brown Carbon Chromophores of Atmospheric Organic Aerosols Over the Yangtze River Channel: A Cruise Campaign[END_REF] This software provides the main functionality for MS data processing: raw data import, mass detection, chromatogram building, deisotoping, alignment, identification, and duplicate peak filtering.

The detailed processing steps and settings can be found in Wang et al. [START_REF] Wang | Chemical Characteristics and Brown Carbon Chromophores of Atmospheric Organic Aerosols Over the Yangtze River Channel: A Cruise Campaign[END_REF] and in Table S1.

Molecular formulas tentatively identified were expressed as CcHhOoNnSs, where c, h, o, n, and s correspond to the numbers of carbon, hydrogen, oxygen, nitrogen, and sulfur atoms in the molecular formula, respectively. Molecular formulas were assigned to detected signals satisfying the following constraints: 1-50 for 12 C, 1-50 for 1 H, 0-40 for [START_REF] Polizzi | Influence of various growth parameters on fungal growth and volatile metabolite production by indoor molds[END_REF] O, 0-10 for 14 N, 0-3 for 32 S, 0-1 for 13 C, 0-1 for [START_REF] Reboux | Indoor mold concentration in Eastern France[END_REF] O, and 0-1 for 34 S. The mass tolerance was 2 ppm for ESIand 3 ppm for ESI+. Several rules were also considered:

H/C ratio, 0.3 ≤ H/C ≤ 3; NOPS/C ratios, N/C ≤ 2, O/C ≤ 2, S/C ≤ 2.
In the positive mode, 0-2 of [START_REF] Kalalian | Influence of indoor chemistry on the emission of mVOCs from Aspergillus niger molds[END_REF] Na was also included in the formula calculation because of the high tendency of Na to form adducts with polar organic molecules. [START_REF] Lin | Elemental composition of HULIS in the Pearl River Delta Region, China: Results inferred from positive and negative electrospray high resolution mass spectrometric data[END_REF] The ring and double bond equivalence (𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅) value was calculated as follow:

𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 2×𝐶𝐶+2+𝑁𝑁-𝐻𝐻 2 (Eq. 1)
The 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 values considered in this study were in the range of 0-25. However, 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 may not accurately indicate the level of unsaturation for compounds with heteroatoms (e.g., O, N, and S). For this reason, the aromaticity equivalent (𝑋𝑋 𝑐𝑐 ) was also calculated. A threshold values for 𝑋𝑋 𝑐𝑐 were proposed in the literature, where 𝑋𝑋 𝑐𝑐 ≥ 2.5 and 𝑋𝑋 𝑐𝑐 ≥ 2.7143 are the unambiguous minimum criterion for the presence of aromatic and condensed aromatic structures in a molecule, respectively. [START_REF] Yassine | Structural characterization of organic aerosol using Fourier transform ion cyclotron resonance mass spectrometry: aromaticity equivalent approach[END_REF] 𝑋𝑋 𝑐𝑐 for compounds containing carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus can be calculated as follow:

𝑋𝑋 𝑐𝑐 = 3×(𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅-(𝑝𝑝×𝑂𝑂+𝑞𝑞×𝑆𝑆))-2 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅-(𝑝𝑝×𝑂𝑂+𝑞𝑞×𝑆𝑆) (Eq. 2)
where 𝑝𝑝 and 𝑞𝑞 correspond to the fraction of oxygen and sulfur atoms involved in π-bond structures of a compound, respectively. 𝑝𝑝 = 𝑞𝑞 = 1 for aldehydes, ketones, nitroso and cyanate compounds, 𝑝𝑝 = 𝑞𝑞 = 0.5 for carboxylic acid, ester and nitro compounds, and 𝑝𝑝 = 𝑞𝑞 = 0 for alcohols, ethers and peroxides. [START_REF] Yassine | Structural characterization of organic aerosol using Fourier transform ion cyclotron resonance mass spectrometry: aromaticity equivalent approach[END_REF] Aligned peak lists from MZmine workflow and xcalibur were further processed with three different databases Swiss-Prot, [START_REF]Swiss-Prot database[END_REF] metabolomics workbench, [START_REF]metabolomics workbench[END_REF] and the protein data bank [START_REF] Rscb | protein data bank[END_REF] to ensure the most relevant tentatively identified formulas. Only peaks with intensities at least 10-100 times higher than the mentioned blanks were considered to check the possible formation of nitrogenated compounds.

HONO and NOX measurements

A long path absorption photometer (LOPAP, QUMA) was used to measure the concentration of gaseous nitrous acid. For such measurements, HONO is sampled in a glass stripping coil by a fast-chemical reaction and converted into an azo dye which is photometrically detected in long path absorption inside a special Teflon tubing (Teflon AF2400). During the experiments, the integrated time resolution was 7 minutes, and the detection limit was less than 10 pptv under our operating conditions (inlet gas flow set to 500 mL/min and liquid flow of the reagents set to 20 mL/min). The sampling unit was maintained at 293 ± 0.02 K, using a Thermo Scientific ARCTIC Refrigerated/Heated Bath (SC100 A10 -230V). In addition, one should note that the instrument was calibrated each week. The full description of this device is detailed elsewhere. [START_REF] Heland | A new instrument to measure gaseous nitrous acid (HONO) in the atmosphere[END_REF][START_REF] Villena | Development of a new Long Path Absorption Photometer (LOPAP) instrument for the sensitive detection of NO2 in the atmosphere[END_REF] Furthermore, a chemiluminescence NOX analyzer (CLD 88 CY analyzer, Eco Physics) was used to measure the NO and NO2 concentrations, with a detection limit of 50 ppt.

Prior to a series of measurements, the NOx analyzer was calibrated. the calibration consisted of doing a zero point of the instrument using a nitrogen/air flow and then doing a span point with a known NO concentration. Then, to check the linearity of the instrument in the working range, calibration curves for NO (~1 ppm in N2, Linde) and NO2 (850 ppb in N2, Linde) in the range 0-100 ppb were established.

VOC analysis

VOCs released by the films were continuously analyzed using a high-resolution proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS 8000, Ionicon Analytik). The ionization was carried out in the H3O + mode. This instrument allows the estimation of the VOC concentrations without an external calibration [START_REF] Cappellin | On quantitative determination of volatile organic compound concentrations using proton transfer reaction time-of-flight mass spectrometry[END_REF] and its full description can be found elsewhere. [START_REF] Cappellin | On quantitative determination of volatile organic compound concentrations using proton transfer reaction time-of-flight mass spectrometry[END_REF][START_REF] De Gouw | Validation of proton transfer reaction-mass spectrometry (PTR-MS) measurements of gas-phase organic compounds in the atmosphere during the New England Air Quality Study (NEAQS) in 2002[END_REF] For all the experiments, the instrument settings were chosen as follows: drift tube voltage at 500 V, drift tube temperature at 353 K with a drift pressure of 2.2 mbar. The E/N ratio was 125 Td where 1 Td = 10 -17 V cm [START_REF] Gandolfo | Unexpectedly High Levels of Organic Compounds Released by Indoor Photocatalytic Paints[END_REF] . Furthermore, in order to reduce the adsorption and the products loss, the reactor's outlet and the PTR-TOF-MS sample inlet tubing were constantly heated at 333 K. All the spectra were recorded with a time resolution of 1 s and a mass resolution of 4500 m/Δm. A calibration gas standard containing 14 VOCs with a concentration of 100 ± 10 ppb in nitrogen (TO-14A Aromatic Mix, Restek Corporation, Bellefonte, USA) was used to control the instrument performance in terms of mass resolution, mass accuracy, sensitivity, and relative mass-dependent transmission efficiency. TofViewer v1.4.3 software was used for raw spectral data analysis, mass calibration, peak fitting, and area extraction. The mass calibration was performed using internal calibration based on two references ions with an exactly known mass (i) hydronium ion isotope (H3 [START_REF] Reboux | Indoor mold concentration in Eastern France[END_REF] O + m/z 21.022) and (ii) protonated acetone (C3H7O + m/z 59.049). [START_REF] Lindinger | On-line monitoring of volatile organic compounds at pptv levels by means of Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) Medical applications, food control and environmental research[END_REF][START_REF] Cappellin | On data analysis in PTR-TOF-MS: From raw spectra to data mining[END_REF] The used protonation reaction rate constant was 2×10 -9 cm³ s -1 for all the detected masses. Conjointly, the VOC concentrations were calculated according to the equation given by Cappellin et al., [START_REF] Cappellin | On quantitative determination of volatile organic compound concentrations using proton transfer reaction time-of-flight mass spectrometry[END_REF] where the uncertainties on the absolute concentrations can be estimated to be about 30-40 % arising from systematic errors. Furthermore, as the variation of relative humidity in the sampled air can affect the H3O + and H3O + (H2O) ions signals and thus the H3O + to H3O + (H2O) ratio, the VOC ion signals were normalized to a standard reagent ion signal of 10 6 counts s -1 (cps) according to the procedure given by de Gouw et al. [START_REF] De Gouw | Validation of proton transfer reaction-mass spectrometry (PTR-MS) measurements of gas-phase organic compounds in the atmosphere during the New England Air Quality Study (NEAQS) in 2002[END_REF] The collected experimental dataset contains around 160 variables corresponding to the number of peaks detected in PTR-TOF-MS measurements. In order to evaluate the effect of the different tested conditions of relative humidity, cultivation time, type of films and potassium nitrate concentrations on the VOC emission profiles, the principal component analysis (PCA, package FactormineR, Ade4 Version 1.0.153-©2009-2017 R Studio) was performed on the datasets using R software (Version 1.0.153 -©2009-2017 RStudio). The PCA allows a statistical representation of all data without any preconception as to the type of emitted compounds and without bias. PCA can generate optimum and similar graphical representations of the scatterplot representing the data matrix. Each PCA factor is a linear combination of variables representing the maximum variance in the scatterplot. As the factors have an orthogonal relationship, only independent sources of variance are taken into account. [START_REF] Moularat | Detection of fungal development in a closed environment through the identification of specific VOC: Demonstration of a specific VOC fingerprint for fungal development[END_REF]

Reagents

Potassium nitrate (99%), sodium acetate (≥99 %), and lactic acid (90%) were purchased from Sigma Aldrich. Malt agar extract was provided by Biokar diagnostics. The air (>99.9%) and dinitrogen N2 (>99.9992%) were either synthetic or obtained from Air Products. NO (~1 ppm in N2) and NO2 (850 ppb in N2) were provides by Linde.

Results and discussion

3.1.Film characterization

The UV analyses were performed on three types of films (molds, KNO3 and mix of molds and KNO3) extracted with milli-Q water and were compared to their initial corresponding solutions.

Both solutions and extracted films showed similar trends (Figure S2 in the SM). For the KNO3 extracted films, a band centered at 300 nm was observed consistent with the literature. [START_REF] Sager | Spectral absorbance of some aqueous solutions in the range 10degrees to 40-degrees-C[END_REF] For the molds extracted films, the UV spectrum shows a band with three large peaks: at 313 nm, 285 nm and 247 nm, which could be a combination of different compounds exhibiting various functional groups and conjugations. As for the mixed extracted film of molds and potassium nitrate, a band centered at 292 nm was observed. It was however not possible to assign a functional group for this band. In fact, the presence of organic compounds (molds extract in this study) modifies the molecular structure of adsorbed nitrate anions, and leads to a red shift in the actinic region, which can increase the light absorbed by nitrate anions. [START_REF] Ye | Matrix effect on surface-catalyzed photolysis of nitric acid[END_REF] Moreover, to check if the mixing time of the molds extract and potassium nitrate affects its UV spectrum, the UV spectrum of a mixed solution was recorded at different times after mixing i.e., 3 and 12 hours of mixing (same time used for the film preparation) and the results are presented in Figure S3 in the SM. These results show that the mixing time did not affect the UV spectrum of the mixed solution of molds extract and potassium nitrate. Spectral analysis of irradiated films of molds, KNO3 and mix of molds and KNO3 by the LED at 340 nm revealed a change in the shape of the bands i.e., mainly a decrease in intensity suggesting a possible degradation of the films upon irradiation (Figure S4 in the SM).

The MS analysis allowed the identification of 7 molecular formulas only in ESI+ that are exclusively present in the mix solution of molds extract and potassium nitrate. Table 1 summarizes the detected masses in ESI+ with the tentatively identified formulas, 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅, 𝑋𝑋 𝑐𝑐 , potential origin and the ratios H/C, O/C, N/C, S/C, N/O. Moreover, O/N ratios were determined for each of the tentative compounds, in the range from 0.5 to 7. O/N ratios larger than 3 points toward the presence of a nitro (-NO2) function to the proposed formulas. However, formulas with O/N ≤ 1 could contain a reduced nitrogen functions, such as amines and amides. [START_REF] Wang | Chemical Characteristics and Brown Carbon Chromophores of Atmospheric Organic Aerosols Over the Yangtze River Channel: A Cruise Campaign[END_REF][START_REF] Lin | Elemental composition of HULIS in the Pearl River Delta Region, China: Results inferred from positive and negative electrospray high resolution mass spectrometric data[END_REF] These compounds have high H/C ratios compared to O/C ones. Formulas with 1 ≤ O/N ≤ 3 could be a combination of different functional groups like nitramines (N-NO2). Therefore, the detected molecules are mainly amino acids, nucleic acid, peptide and dipeptide fragments, membrane fractions and metabolites. Most of the tentatively identified formulas possesses a nitro group (-NO2), possibly related to a nitration reaction during the preparation of the mix solution of molds extract and potassium nitrate.

As demonstrated in previous studies, protein nitration can occur in the gas phase in the presence of nitrogen dioxide (NO2) and ozone (O3). [START_REF] Franze | Protein nitration by polluted air[END_REF][START_REF] Meusel | Light-induced protein nitration and degradation with HONO emission[END_REF] However, as proposed by Ghiani et al., [START_REF] Ghiani | Nitration of pollen aeroallergens by nitrate ion in conditions simulating the liquid water phase of atmospheric particles[END_REF] the protein nitration can also take place in the aqueous phase in the presence of nitrate ions (NO3 -) with concentrations and pH levels comparable to that found in the aerosol liquid water phase both in the dark and under UV-A irradiation. The nitration of aromatic compounds, such as phenols, can occur both in the gas phase in the presence of OH and NO2 or NO3 and NO2, and by multiphase various reactions involving nitrogen dioxide, N2O5 and/or ClNO2, NO3, nitrate or nitrite when irradiated. Nitration is also induced by the presence of dissolved Fe(III), nitrate, semiconductor oxides. [START_REF] Vione | Nitration and hydroxylation of benzene in the presence of nitrite/nitrous acid in aqueous solution[END_REF] Additionally, HNO2 may react in the dark with hydrogen peroxide to form peroxynitrous acid (HOONO), allowing consequently the oxidation and nitration of aromatic compounds. [START_REF] Vione | Nitration and hydroxylation of benzene in the presence of nitrite/nitrous acid in aqueous solution[END_REF] On the other hand, it is well known that Aspergillus sp can use a broad range of nitrogen sources, such as ammonium, nitrate, amino acids (e.g., histidine or proline), or complex substrates such as collagen and elastin. The nitrate assimilation by fungi leads to the complete reduction of nitrate forming ammonium, which is finally incorporated in the amino acids glutamate and glutamine acting as nitrogen donors. [START_REF] Krappmann | Nitrogen metabolism of Aspergillus and its role in pathogenicity[END_REF] Moreover, fungi can also synthesize NO from nitrate through the action of nitrate reductase. [START_REF] Cánovas | Nitric oxide in fungi: is there NO light at the end of the tunnel?[END_REF] In the presence of superoxide (O2 -), peroxynitrite (ONO2 -) can be formed and cause protein modification, lipid oxidation, and DNA damage. [START_REF] Mclean | Peroxynitrite toxicity in Escherichia coli K12 elicits expression of oxidative stress responses and protein nitration and nitrosylation[END_REF][START_REF] Warris | Oxidative responses and fungal infection biology[END_REF] It was further demonstrated that peroxynitrite can nitrate and oxidize tyrosine residues in membrane proteins and can cause as well lipid oxidation and nitration. [START_REF] Mclean | Peroxynitrite toxicity in Escherichia coli K12 elicits expression of oxidative stress responses and protein nitration and nitrosylation[END_REF][START_REF] Yang | Quantification of nitrotyrosine in nitrated proteins[END_REF][START_REF] Szabó | Peroxynitrite: Biochemistry, pathophysiology and development of therapeutics[END_REF] For all the reasons cited above, we hypothesized that a combination of biotic and abiotic factors might have contributed to the formation of the nitro compounds that were observed in our study. NI: not identified 318

VOC emission from films

The VOC emissions from molds films, either mixed or not with KNO3, were investigated for different molds cultivation time, under different relative humidities, under dark and irradiation conditions, under N2 and air streams, and for different concentrations of KNO3 (0.001-0.5M).

In this section, only the results for the effect of KNO3 on VOC emission profiles are presented.

More details about the impact of the other cited conditions are available in the supporting materials.

To visualize the impact of the type of film on the VOC emission profiles, a PCA was performed and is presented in Figure 1. PCA_1 described 38.8 % of the total dataset variability (28.8% for the first dimension and 10% for the second one). According to Figure 1, each film shows a very differentiated VOC emission profile (ellipses grouping the different conditions (i.e., film type) are well separated). It is also noticeable that all the differences between the three film types are almost driven by compounds emitted by molds' films doped with potassium nitrate. Furthermore, in order to determine the effect of the potassium nitrate concentration on the VOC emission profiles, another PCA (PCA_2) was performed on the dataset (Figure 2-a Nonetheless, in the absence or presence of UV-light, the concentration of some VOCs increased with the increase of potassium nitrate concentrations as shown in Figure 2-b. Among those VOCs: succinimide (C4H5NO2) and butene (C4H8), where their relative abundances increase from 0% for [KNO3] = 0-0.001M to 6.1% and 20.5% for [KNO3] = 0.5M, respectively.

The evolution of some VOC concentrations in response to the different potassium nitrate concentrations in the films reflects the possible interaction between the nitrate solution and the molds extract during the film preparation. As for the VOCs that are both sensitive to UV-light irradiation and potassium nitrate concentration, they might be released in the gas phase in response to the photodegradation of nitrogenated compounds that can be formed during the film preparation (Figure 2-b andTable S2).

3.3.HONO and NOx emission

The temporal evolution of HONO formation under an air stream, from the three sets of film samples (irradiated films of molds, KNO3 and mold films doped with KNO3) and compared to a reactor blank with a relative humidity of 85±2% is presented in Figure 3. The uncertainties on the concentrations were twice the standard deviation. When the LED lamp was switched ON, an enhancement of the HONO concentration was observed. The HONO concentration increases to a maximum value of 900 ppt and then stabilizes at a value of 700 ppt. It is noteworthy mentioning that the formation of HONO from irradiated mold films doped with KNO3 was also observed under a nitrogen stream with concentrations in the same order as that under air stream. However, individually molds and nitrate films showed a negligible formation of HONO compared to a blank experiment with an empty reactor. In fact, the wavelengths of the LED lamp used in this study ranged between 335 and 380 nm. Thus, at these wavelengths, the photochemistry of potassium nitrate cannot be activated efficiently because due to a low absorption beyond 335 nm. [START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF] Figure 3: Formation of HONO (ppt) in response to the irradiation of molds films, KNO3 films and molds films doped with 0.04M of KNO3 and compared to a reactor blank. RH=85±2%, error is 2σ, yellow area corresponds to light irradiation. Additionally, the concentration of nitrate in the film was also varied from 0.001M to 0.5M. As the KNO3 concentration increases, a slight increase of the HONO concentration was observed in Figure 4-a. However, for higher concentration values of KNO3, the formation of HONO decreased. Likewise, the proportion of nitrate and molds in the films was varied. These results show a maximum HONO formation for a stoichiometric mix of molds and nitrate in the initial solution (Figure 4-b) and a decrease of the HONO concentration when KNO3 was in excess.

The decrease of the HONO concentration for high KNO3 concentration can be explained by the fact that the gaseous products formed during the photolysis of the film may hardly escape from the nitrate crystal that is saturating the surface. A similar behavior was previously observed from mixed films of furfural and KNO3 [START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF] as well as from films of humic acids and KNO3 [START_REF] Yang | Significant HONO formation by the photolysis of nitrates in the presence of humic acids[END_REF] where a decrease of the HONO formation was also witnessed for high potassium nitrate concentrations. tested (RH= 0-85 ±2%), and the results are shown in Figure 5. The maximum tested relative humidity was below the deliquescence point of potassium nitrate (>92%). [START_REF] Freney | Deliquescence and efflorescence of potassium salts relevant to biomass-burning aerosol particles[END_REF] Each point corresponds to the average of 3-4 replicas and the error bars correspond to one standard deviation. It was found that with increasing relative humidity, both NO2 and HONO concentrations increase. However, NO formation seemed to be less sensitive to the variation of relative humidity. Pure KNO3 films and mold films were unable to produce HONO and NOX.

These results show that an increased amount of HONO is generated by the photolysis of nitrates when mixed with organic matter (molds extract) with increasing relative humidity. The observed HONO formation during irradiation might also result, in part, from the photodegradation of nitro compounds (mainly nitrated proteins, amino acids or metabolites), formed during the film preparation process.

Reaction mechanism

Based on the results obtained in this study and that of the available literature, different routes explaining the photochemical formation of NO, NO2 and HONO are proposed.

The mass spectrometry analysis of the mixed solution of molds extract and potassium nitrate showed the formation of some nitro and nitrogenated compounds during the film preparation.

Depoorter et al. [START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF] have also shown a possible nitration reaction during the preparation of furfural and potassium nitrate films. The authors have proposed a mechanism for the photochemical degradation of 2-nitrofuran (a proxy for nitro compounds formed during the preparation of furfural films doped with KNO3). The photodegradation of 2-nitrofuran results in the (i) formation of NO and C4H3O2, (ii) formation of NO2 and C4H4O, and (iii) formation of HONO following the reduction of NO2 on surfaces containing reducing functionalities. This photodegradation can occur under both nitrogen and air streams, where the authors have probed the possible formation of O2 -on the surface which can increase the oxidative capacity of the surfaces leading to an evolution of the concentrations of nitrogen containing compounds at the gas-surface interface. Moreover, Wang et al. [START_REF] Wang | Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter[END_REF] reported that reactions between NOx and HO2/O2 - in the aerosol phase are a significant pathway by which HONO production is enhanced in the presence of dissolved aliphatic organic matter. This chemistry can be applied to many aqueous environments (e.g., cloud droplets, aqueous aerosols, and surface water) and may represent a significant source of gaseous HONO in areas where surface waters or aqueous microlayers contain a high concentration of nitrate and dissolved aliphatic organic matter.

On the other hand, the presence of molds extract as a source of organic compounds on the surface can modify the molecular structure of nitrate anions leading to a red shift into the actinic region and an enhancement of the light absorbed by nitrate anions. [START_REF] Ye | Matrix effect on surface-catalyzed photolysis of nitric acid[END_REF] Depending on the surface pH, nitrate can be transformed into nitric acid. In fact, among the VOC released from molds film doped with KNO3, some acids were detected that can promote surface acidity. The photolysis of adsorbed nitric acid/ nitrate (HNO3/nitrate(ads)) can directly produce HONO and NO2 via reactions R1-R3 or indirectly through reactions R4 and R5. [START_REF] Ye | Matrix effect on surface-catalyzed photolysis of nitric acid[END_REF][START_REF] Zhou | Nitric acid photolysis on surfaces in low-NOx environments: Significant atmospheric implications[END_REF][START_REF] Finlayson-Pitts | The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism[END_REF][START_REF] Ma | Review of heterogeneous photochemical reactions of NOy on aerosol -A possible daytime source of nitrous acid (HONO) in the atmosphere[END_REF] The produced NO2 from reaction (R2) and HONO from reactions (R3, R4 and R5) are released from the surface to the gas phase following reactions (R6) and (R7). The Org(ads) corresponds to molds extract in this study.

𝐻𝐻𝐻𝐻𝑂𝑂 3 /𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 𝑎𝑎𝑎𝑎𝑎𝑎 ℎ𝑣𝑣 � � [𝐻𝐻𝐻𝐻𝑂𝑂 3 /𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛] (𝑎𝑎𝑎𝑎𝑎𝑎) * (R1) ℎ𝑣𝑣 � � 𝐻𝐻𝑂𝑂 2(𝑎𝑎𝑎𝑎𝑎𝑎) + 𝑂𝑂𝐻𝐻 (𝑎𝑎𝑎𝑎𝑎𝑎) (R2) ℎ𝑣𝑣 � � 𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂 (𝑎𝑎𝑎𝑎𝑎𝑎) + 𝑂𝑂( 𝑃𝑃 3 ) (𝑎𝑎𝑎𝑎𝑎𝑎) (R3) 𝐻𝐻𝑂𝑂 2(𝑎𝑎𝑎𝑎𝑎𝑎) +𝐻𝐻 2 𝑂𝑂 (𝑎𝑎𝑎𝑎𝑎𝑎) ℎ𝑣𝑣 � � 𝑂𝑂𝐻𝐻 (𝑎𝑎𝑎𝑎𝑎𝑎) + 𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂 (𝑎𝑎𝑎𝑎𝑎𝑎) (R4) 𝐻𝐻𝑂𝑂 2(𝑎𝑎𝑎𝑎𝑎𝑎) +𝑂𝑂𝑛𝑛𝑂𝑂 (𝑎𝑎𝑎𝑎𝑎𝑎) ℎ𝑣𝑣 � � 𝑂𝑂𝑛𝑛𝑂𝑂 -𝑜𝑜𝑜𝑜𝑛𝑛𝑜𝑜𝑛𝑛𝑜𝑜𝑛𝑛𝑜𝑜 (𝑎𝑎𝑎𝑎𝑎𝑎) + 𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂 (𝑎𝑎𝑎𝑎𝑎𝑎) (R5) 𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂 (𝑎𝑎𝑎𝑎𝑎𝑎) → 𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂 (𝑔𝑔) (R6) 𝐻𝐻𝑂𝑂 2(𝑎𝑎𝑎𝑎𝑎𝑎) → 𝐻𝐻𝑂𝑂 2(𝑔𝑔) (R7)
Nevertheless, Aspergillus niger species contain Aspergillin that is classified as melanin or a humic acid-containing chromophore groups. [START_REF] Ray | Studies on the Biosynthesis of Aspergillin byAspergillus niger[END_REF] Yang et al. [START_REF] Yang | Significant HONO formation by the photolysis of nitrates in the presence of humic acids[END_REF] and Stemmler et al. [START_REF] Stemmler | Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid[END_REF] suggested that the presence of chromophoric centers, like aromatic carbonyls in humic acids, could act as photosensitizers in the photolysis of nitrates or in the conversion of adsorbed NO2 into HONO.

Additionally, during the photolysis of nitrate and humic acids, aromatic nitro compounds are expected to be formed by the photochemical oxidation of nitrates. Correspondingly, it has been reported that the photolysis of nitro-aromatic compounds may lead to the HONO formation. [START_REF] Yang | Significant HONO formation by the photolysis of nitrates in the presence of humic acids[END_REF] Furthermore, the photolysis of species containing a nitrogen atom in humic acids may release gaseous products such as HONO and NO2. [START_REF] Yang | Significant HONO formation by the photolysis of nitrates in the presence of humic acids[END_REF] And it was previously demonstrated that the photolysis of ortho-nitrophenols (as is the case for 3-nitrotyrosine) can generate nitrous acid (HONO). [START_REF] Meusel | Light-induced protein nitration and degradation with HONO emission[END_REF] Thus the photodegradation of nitro compounds can occur as follow:

𝑂𝑂𝑛𝑛𝑂𝑂 -𝐻𝐻𝑂𝑂 2 ℎ𝑣𝑣 � � 𝑂𝑂𝑛𝑛𝑂𝑂 + 𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂 (𝑔𝑔) (R8)
Hence, the obtained results from this study and the available literature suggest that the HONO formation might result from a photodegradation of nitro-compounds and nitrogencontaining compounds formed in the film, and/or from the photolysis of nitrate/nitric acid in the presence of molds extract as organic compounds on the surfaces.

Implications for indoor air quality

In order to determine the concentration of HONO resulting from irradiated indoor surfaces contaminated with molds, the indoor concentration of HONO was estimated following the equation given by Willem et al. [START_REF] Willem | Ventilation Control of Volatile Organic Compounds in New U . S . Homes : Results of a Controlled Field Study in Nine Residential Units[END_REF] :

𝑜𝑜[𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂] 𝑜𝑜𝑛𝑛 = 𝑅𝑅 𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 × 𝑆𝑆 𝑉𝑉 -𝐻𝐻 ℎ [𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂] -𝐽𝐽 𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 [𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂] (Eq. 3)
where 𝑅𝑅 𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 is the emission rate of HONO in µg m -2 h -1 , 𝑆𝑆 is the surface area of the room contaminated by molds (m 2 ), 𝑉𝑉 is the room volume (m 3 ), 𝐻𝐻 ℎ is the air exchange rate (h -1 ) and 𝐽𝐽 𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 is the photolysis rate of indoor HONO. It is noteworthy mentioning that the loss on surfaces was already taken into account in the measured HONO concentration.

As the HONO outdoor levels are usually lower than indoor levels, it was assumed that there is no influence of outdoor concentration of HONO on the indoor concentration for this simulation. [START_REF] Lee | Nitrous acid, nitrogen dioxide, and ozone concentrations in residential environments[END_REF] The steady-state HONO concentration can be then simplified as:

[𝐻𝐻𝑂𝑂𝐻𝐻𝑂𝑂] = 𝑅𝑅 𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 × 𝑆𝑆 𝑉𝑉 × (𝐽𝐽 𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 + 𝐻𝐻 ℎ ) (Eq. 4)

For indoor relevant relative humidity (RH = 45%), with an air exchange rate of 0.5 h -1 73 and a photolysis rate of HONO equal to 0.25 h -1 3,27 and considering a standard living room (S= 20 m 2 , 𝑉𝑉 =50 m 3 ) with a mold damaged surface of 0.25 m 2 and a nitrate concentration of 0.04M, the resulting HONO concentration from these surfaces is of the order of 60 ppt.

Moreover, the UV fraction of sunlight penetrating in indoor spaces ranges from 330 up to 400 nm depending on the location, time of the year, room orientation as well as the size and type of the glass window. [START_REF] Gandolfo | Spectrally resolved actinic flux and photolysis frequencies of key species within an indoor environment[END_REF] Additionally, Liu et al. [START_REF] Liu | Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO2Transformation on Glass Windows[END_REF] have shown high HONO yields during the irradiation period of the glass windows in the kitchen. Hence, the wavelength of the LED lamp used in this study corresponds to the lowest possible wavelength encountered indoor. Likewise, the corresponding photon flux is of 3.8×10 12 photons cm -2 s -1 , which is lower than the direct indoor UV actinic flux (with windows closed) measured by Gandolfo et al. (i.e., 10 14 photons cm -2 s -1 ) but significantly higher than the one in the shadows (i.e., 10 10 photons cm -2 s -1 ).

Nevertheless, our experimental conditions can be somehow representative of realistic indoor conditions during sunny periods. Thus, for future studies, it will be interesting to evaluate the Finally, mold growth is especially found in damp dwellings with high relative humidity (>70%). In fact, sick buildings infested with visible molds, present a wide contaminated surface area compared to the one used in our calculation and have usually a low ventilation rate. For example, if the mold damaged surface is 25 m 2 , and for the same nitrate concentration of 0.04M, the resulting HONO concentration in the gas phase will be around 6 ppb. Hence, indoor surfaces covered with molds can be a non-negligible source of HONO mainly in sick buildings exhibiting large, contaminated areas, alongside high relative humidity, and low ventilation rates. Typical indoor levels are of the order of few ppbs. [START_REF] Bartolomei | Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint[END_REF][START_REF] Alvarez | Light-induced nitrous acid (HONO) production from NO2heterogeneous reactions on household chemicals[END_REF][START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF] These levels could significantly be affected by surface chemistry as it was previously demonstrated for various indoor surfaces [START_REF] Alvarez | Light-induced nitrous acid (HONO) production from NO2heterogeneous reactions on household chemicals[END_REF][START_REF] Depoorter | Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid[END_REF][START_REF] Wang | Surface reservoirs dominate dynamic gas-surface partitioning of many indoor air constituents[END_REF] and for microbial indoor surface in the present work.

Conclusions

In this study, we show that Aspergillus niger, a common indoor mold, can react on surfaces and produce a complex mixture of adsorbed products that initiate photochemistry indoors.

Figure 1 :

 1 Figure 1 :VOC emission profiles at RH = 85% for different types of films: molds, KNO3 and molds + KNO3. The molds were incubated for 10 days and [KNO3] = 0.04M. The m/z of the 50 compounds that are most explaining the variance in the two first components are presented on the graph. The percentages of the eigenvalues of the two first components are shown on each axis.

1 and 2 ,

 12 ) and the relative abundance of each VOC detected from molds films doped with different concentrations of potassium nitrate were determined and compared in Figure 2-b. PCA_2 described 22 % of the total dataset variability (17.5% for the first dimension and 4.5% for the second one). The ellipses grouping the different conditions (KNO3 concentrations) are separated with an overlap between the film with higher potassium nitrate concentration (0.04M, 0.1M and 0.5M). It is also noticeable that 42 of the 50 compounds are driven by the films with higher potassium nitrate concentration, explaining the variance between the different conditions. Likewise, Figure 2-b shows variability in the VOC emission profiles, which is in line with the results obtained by PCA_1 and PCA_2 analyses. Hence, according to Figures the presence of potassium nitrate in the films affects their composition and thus their VOC emission profiles.

Figure 2 :

 2 Figure 2: (a) VOC emission profiles for mold films doped with different KNO3 concentrations. The m/z of the 50 compounds that are most explaining the variance in the two first components are presented on the graph. The percentages of the eigenvalues of the two first components are shown on each axis (b) Relative abundance of the most emitted VOCs from these films. RH=85% and molds were incubated for 14 days.

Figure 4 :

 4 Figure 4: Variation of [HONO] (ppt/cm 2 ) as (a) function of [KNO3] (M) used in the film preparation and (b) for different volume ratio KNO3/molds. The reactor blank is subtracted and error bars correspond to 1σ.

Figure 5 :

 5 Figure 5: Variation of [HONO] and [NOX] in ppt/cm 2 as a function of the relative humidity from mold films doped with 0.04M of KNO3, error bars correspond to 1σ.

  influence of different type of indoor light sources with different intensities on the HONO yields from different surfaces and mainly those covered by microbial species. This will help improving the model used to accurately predict the HONO concentrations values under different light intensities.

  These microbial species lead to the formation of VOCs and enhances HONO production under various indoor conditions including light irradiation, relative humidity and KNO3 concentration. As microbial species are often found in areas with high relative humidity, the resulting HONO concentrations become significant. Hence, this work allows the recognition of microbials species (Aspergillus niger) as a new indoor HONO source. This will definitely shed a new light on the role of molds grown in sick dwellings in affecting the indoor air chemistry and quality.

Table 1 :

 1 Potential identities and sources for the identified compounds from a mixed solution of molds extract and potassium nitrate in ESI+ mass 316 spectra. 317

	Detected Masses (Da)	Neutral mass (Da)	Possible Formula	Possible identity	Potential origin	H/C	O/C	N/C S/C	O/N	RDBE	Xc (p=q=1)	Xc (p=q=0.5)
	219.14902	218.14902	C13H18N2O	5-Methoxydimethyltrypta mine	Tryptamine derivatives	1.4	0.1	0.2	0.0	0.5	6.0	2.60	2.64
					Alkaloid -
	223.18029	222.18029	C13H22N2O	Nigragillin	metabolites from	1.7	0.1	0.2	0.0	0.5	4.0	2.33	2.43
					the A.niger
				N-nitro-L-cysteinyl-
	246.01611	223.01611	C5H9N3O5S	OH) glycine (NO2-Cys-Gly-	Peptide L-cys-gly	1.8	1.0	0.6	0.2	1.7	3.0	3.67	0.00
			C13H5NO3	Possible nitro	NI	0.4	0.2	0.1	0.0	3.0	12.0	2.78	2.81
				1-ethynyl-7-
				nitrodibenzofuran /	Miscellenious
				3-nitro-15-	compound a.niger /
			C14H7NO3	oxatetracyclo[10.2.1.05	n-	0.5	0.2	0.1	0.0	3.0	12.0	2.78	2.81
				,14.08,13]pentadeca-	myristoyltransferas
		237.03182		heptaene 1(14),2,4,6,8(13),9,11-	e
	260.03182		C5H8N4O7	4,5-dimethoxy-1,3-dinitroimidazolidin-2-trinitrobutanamide one / N-methyl-4,4,4-	Imidazolinone /Amino acids or Ala-Gly glutamine or beta-dipeptides:	1.6	1.4	0.8	0.0	1.8	4.0	3.67	1.00
				3-nitro-4-pyrimidin-2-
		259.03182	C11H6N4O2S	ylsulfanylbenzonitrile / 3,6-bis(2-furanyl)-[1,2,4]triazolo[3,4-	Nucleic acid derivatives	0.5	0.2	0.4	0.1	0.5	11.0	2.75	2.79
				b][1,3,4]thiadiazole
			C13H5NO4	Possible nitro	NI	0.4	0.3	0.1	0.0	4.0	12.0	2.75	2.80
	262.01109	239.01109	C4H6N4O8	Tetranitrobutane	Protein hydrolase/toxin n-	1.5	2.0	1.0	0.0	2.0	4.0	3.50	0.00
					butane

Acknowledgments

This work was supported by the Alfred P. Sloan Foundation under its Chemistry of Indoor Environments program.