Matthieu Fricaudet
email: matthieu.fricaudet@centralesupelec.fr

Katarina Žiberna

Samir Salmanov

Jens Kreisel

Delong He

Brahim Dkhil

Tadej Rojac

Mojca Otoničar

Pierre-Eymeric Janolin

Andraž Bradeško

Multifunctional properties of polyvinylidene-fluoride-based materials: from energy harvesting to energy storage

Keywords: Energy harvesting, PVDF, piezoelectricity, pyroelectricity, triboelectricity, energy storage

Organic ferroelectrics are increasingly important due to their complementary properties to classical, inorganic ferroelectrics. Flexibility, chemical resistance, scalability, high breakdown fields, and biocompatibility are attractive for many applications like energy harvesting and storage. The most known energy harvesting methods are piezoelectric, pyroelectric, and triboelectric. Here, we apply the well-established material's figures of merit to five polyvinylidene-fluoride-based compositions ranging from ferroelectric to relaxor-like behavior to emphasize the importance of several key material parameters contributing to the maximal power output of energy harvesting devices. Afterward, we discuss the possibility of the same functional material storing the output energy for the development of scalable multifunctional devices.

Introduction

Ferroelectrics and their energy conversion ability are utterly crucial for various applications in modern electronics. The most common ferroelectric materials are inorganic oxides in single crystal or ceramic form 1 .

In recent years, the need to develop flexible electronics has emerged due to the increased popularity of body-worn devices in all aspects of our life [START_REF] Awolusi | Wearable Technology for Personalized Construction Safety Monitoring and Trending: Review of Applicable Devices[END_REF][START_REF] Taraldsen | Physical Activity Monitoring by Use of Accelerometer-Based Body-Worn Sensors in Older Adults: A Systematic Literature Review of Current Knowledge and Applications[END_REF] . This raised the need for materials that can be fabricated on flexible substrates, scalable, withstand wear and temperature variations, and are chemically resistant and biocompatible. Organic ferroelectrics, especially polyvinylidene-fluoride-(PVDF)-based materials, proved suitable for such applications [START_REF] Chen | PVDF-Based Ferroelectric Polymers in Modern Flexible Electronics[END_REF] . PVDF-based materials range from classical ferroelectric-like behavior to the relaxor-like, depending on the chemical composition or treatment [START_REF] Lovinger | Vinylidene Fluoride)[END_REF][START_REF] Furukawa | Structure and Properties of Ferroelectric Polymers[END_REF][START_REF] Yang | Novel Polymer Ferroelectric Behavior via Crystal Isomorphism and the Nanoconfinement Effect[END_REF] . This versatility uncovers opportunities to integrate these flexible materials to various applications, from electrocaloric cooling [START_REF] Meng | A Cascade Electrocaloric Cooling Device for Large Temperature Lift[END_REF] and neuromorphic computing [START_REF] Tian | A Robust Artificial Synapse Based on Organic Ferroelectric Polymer[END_REF] to different actuation [START_REF] Lheritier | Actuation Efficiency of Polyvinylidene Fluoride-Based Co-and Ter-Polymers[END_REF] , sensing [START_REF] Maity | Self-Powered Human-Health Monitoring through Aligned PVDF Nanofibers Interfaced Skin-Interactive Piezoelectric Sensor[END_REF] , and energy harvesting technologies [START_REF] Costa | Recent Progress on Piezoelectric, Pyroelectric, and Magnetoelectric Polymer-Based Energy-Harvesting Devices[END_REF] .

The leading energy harvesting technologies in organic ferroelectrics are designed to exploit piezoelectricity, pyroelectricity, and triboelectricity. Piezoelectric energy harvesting benefits from the piezoelectric response, generating charges under applied mechanical stress. The piezoelectric effect converts mechanical vibrations into useful electrical energy [START_REF] Yang | High-Performance Piezoelectric Energy Harvesters and Their Applications[END_REF] . The pyroelectric effect is, by definition, a change of the surface charge under varying temperatures [START_REF] Bhalla | Primary and Secondary Pyroelectricity[END_REF][START_REF] Bowen | Pyroelectric Materials and Devices for Energy Harvesting Applications[END_REF] . Latest studies show this effect can power simple electronic circuits by converting the energy from the finger touch and chemical reactions to electricity [START_REF] Zhao | Flexible Pyroelectric Device for Scavenging Thermal Energy from Chemical Process and as Self-Powered Temperature Monitor[END_REF] . For both these effects, organic ferroelectric benefit from their low dielectric constant, which allows them to compensate for their relatively smaller pyroelectric and piezoelectric constants compared to inorganics materials and gives them competitive harvesting capabilities. Recently, the triboelectric effect has shown great promise due to the relative simplicity and large power output demonstrated [START_REF] Fan | Flexible Triboelectric Generator[END_REF] . In principle, the triboelectric effect is the generation of surface charge upon contacting two surfaces and the corresponding electrical work (electrostatic induction) to compensate for the formed surface charges.

In the next stage of the harvesting process, the electrical energy must be efficiently manipulated by energy reclamation circuits and stored in a medium [START_REF] Liu | Active Piezoelectric Energy Harvesting: General Principle and Experimental Demonstration[END_REF][START_REF] Liang | Energy Flow in Piezoelectric Energy Harvesting Systems[END_REF] . Surprisingly, organic ferroelectrics are also prominent candidates for energy storage, especially some PVDF-based compositions [START_REF] Chu | A Dielectric Polymer with High Electric Energy Density and Fast Discharge Speed[END_REF][START_REF] Liu | Relaxor Ferroelectric Polymers: Insight into High Electrical Energy Storage Properties from a Molecular Perspective[END_REF] .

Until now, many research articles have been published on energy harvesting and energy storage with PVDF-based materials. However, there were no systematic investigations to uncover the good practices in the selection process for the optimal PVDF-based material within the poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) system for a given energy harvesting application [START_REF] Lheritier | Actuation Efficiency of Polyvinylidene Fluoride-Based Co-and Ter-Polymers[END_REF][START_REF] Jiang | VDF-Content-Guided Selection of Piezoelectric P(VDF-TrFE) Films in Sensing and Energy Harvesting Applications[END_REF] . Furthermore, the emerging idea is to combine several stages of energy harvesting within one material to make compact, scalable devices [START_REF] Bai | Energy Harvesting Research: The Road from Single Source to Multisource[END_REF] .

Here, we illustrate the multifunctionality of PVDF-based polymers with different amounts of trifluoroethylene (TrFE) and chlorofluoroethylene (CFE) co-and ter-polymers, respectively. We focus on the energy conversion aspect at room temperature. A benchmarking investigation is made to determine which compositions are optimal for specific energy harvesting methods. The thick films used in this study were prepared by the solution casting method and their electric, electromechanical, pyroelectric, and triboelectric properties are presented herein. After illustrating the energy generation aspect of these multifunctional materials, we show that they can also be used to store the harvested energy. Finally, the optimal composition as the best all-in-one material for efficient harvesting of energy from all possible sources and its efficient storage is pointed out and discussed.

Experimental section

The polymers in the form of powders were obtained from Piezotech-Arkema, France. The composition of powders and notations used in this paper are given in Table 1. These PVDFbased compositions are the most investigated and reported ones. The powders were dissolved in the dimethylformamide solution (99.8 %, Alfa Aesar, Germany) in the weight ratio of 20 % and mixed for 2h at ~60 °C. The deposition was done by solvent casting (Elcometer 4340, United Kingdom), with the speed of the blade (Elcometer 3580/7, United Kingdom) and its height adjusted to get uniform ~30 µm thick films. The temperature of the glass substrate was set to 55 °C. The films were dried at 55 °C for 1 hour, then removed from the substrate and finally annealed at 120 °C for 50 min. For electrical measurements 50-nm-Au electrodes were deposited on both sides by magnetron sputtering (MP300, PLASSYS, France). X-ray data collection was performed by a homemade diffractometer (radius = 500 mm) associated with a Rigaku RA-HF18 rotating anode generator (50 kV, 200mA). The wavelength was copper Kα radiation (0.1541 nm), and the step was 0.02°. The microstructures of the surfaces of the polymer films were investigated using a field-emission scanning electron microscope (SEM) Verios 4G HP (Thermo Fischer, Waltham, Massachusetts, USA), operated in secondary electron emission mode at 2 keV. To ensure sample surface conductivity and avoid charging during SEM imaging, the samples were coated with 5 nm of carbon using a Precision Etching and Coating System 628A (Gatan, Pleasanton, California, USA). Electrical measurements were performed by an impedance meter (Agilent 4294, CA, USA).

A parallel RC circuit was used to calculate the dielectric permittivity of the samples in the temperature range from room temperature to 140 °C. The samples were poled at 1 MV/cm (Keithley 248, CLE, USA) for 10 min at room temperature. The direct piezoelectric response was measured on the Berlincourt-type piezometer (PM10, Piezotest, Singapore) under 20 Hz sinusoidal force excitation. Polarization loops were measured using a piezoelectric testing equipment (TF-2000, Aixacct, Germany) at a frequency of 20 Hz and a magnitude of electric field of 1 MV/cm. Pyroelectric response was measured on an Aixacct TF-2000 system with a pyroelectric module. The heating rate was 5 K/min, and the temperature variation amplitude was 20 K. A Fast Fourier transform (FFT) filter was used to reduce the noise level.

The triboelectric effect was measured in the contact-separation mode. One Au electrode of 2 cm diameter was sputtered on the sample face, while the other electrical contact was placed on a contacting aluminum plate. The excitation frequency was 15 Hz, the displacement was generated with a linear electromagnetic motor, and the waveform was sinusoidal. The force was measured with a force sensor. The generated triboelectric voltage was measured by a multimeter (Keithley 2700, CLE, USA) and an oscilloscope (Agilent DSO-X 2012A, CA, USA) in a series with a 100 MΩ resistor (to not affect the signal with the measurements input impedances).

Results and Discussion

Structural and microstructural properties

The structural properties of the samples were analyzed to ensure the fabrication and crystallization process was properly executed. Figure 1a shows the room temperature XRD pattern of the polymers. The PVDF-based polymers show only one clear peak in the 2 range from 16° to 21°. The slightly asymmetric Bragg peak corresponds to the (110) reflections of the all-trans (TTTT) molecular ferroelectric conformation (β-phase), i.e., all the dipoles in each (-CH 2 -CF 2 -)-like monomer are aligned along the same direction that is quasiperpendicular to the polymer chain. No extra peak related to the non-polar (TGTG') α-phase could be evidenced. Note that Raman and IR spectroscopy measurements (not seen here) confirmed such results. The asymmetry of the left side of the peak comes from the presence of the (200) reflections [START_REF] Lovinger | Crystalline Forms in a Copolymer of Vinylidene Fluoride and Trifluoroethylene (52/48 Mol %)[END_REF] . With the increased amount of TrFE, the peaks shift to lower angles, and with the addition of CFE the shift is even more pronounced. This diffraction data were used to calculate the interchain spacing, which shows that the spacing increases with the addition of TrFE and CFE (Figure 1b). This is due to the partial substitution of the hydrogen (Van der Walls radius of r=1.20 Å) by the larger fluorine (r=1.47 Å), and even larger chlorine (r=1.75 Å) atoms in CFE, leading to the larger unit cell.

3.2.Electrical response

The structural properties are reflected in the electrical response, as shown in Fig. 3. At first, the permittivity of the fabricated compositions was measured as a function of temperature.

The peak in the permittivity in ferroelectric materials corresponds to the Curie temperature (T C ), the transition between the polar and non-polar phases. Meanwhile, in relaxor-like materials, the dielectric peak is not associated with a phase transition but instead shows a permittivity that is frequency-dependent and spreads over a broad temperature range.

Among the analyzed compositions, 80/20 had the highest T C , above the melting point (at ~150°C), while the relative permittivity amplitude at the peak maximum is the lowest. The highest dielectric permittivity at room temperature was measured in CFE.

Obtained results in Fig. 3b show that in compositions with the amount of TrFE < 30%, the dielectric peak position is fixed within 1 °C around the same temperature, whatever the frequency of measurements. This observation is typical of a classical ferroelectric behavior with a fixed Curie transition temperature. In compositions 55/45 and CFE there is a frequency dispersion in the peak temperature position suggesting a relaxor-like behavior. However, the frequency dispersion does not follow a straight line as it should according to the Vogel-Fulcher (VF) relationship, which was observed in 65/35 gamma-irradiated samples [START_REF] Casar | Electrical and Thermal Properties of Vinylidene Fluoride-Trifluoroethylene-Based Polymer System with Coexisting Ferroelectric and Relaxor States[END_REF] . This irradiation is known to induce a relaxor state by forming irradiation-induced polar defects which have a similar chain-interspacing increase as adding CFE or TrFE to the PVDF [START_REF] Yang | Novel Polymer Ferroelectric Behavior via Crystal Isomorphism and the Nanoconfinement Effect[END_REF] . It is worth mentioning that the presence of the amorphous phase and/or its interface with the crystallites (Fig. 2) can also contribute to the dielectric frequency behavior altering the total dielectric response.

The transition from ferroelectric, which is the pristine PVDF and the compositions up to 70/30, to relaxor-like, which are the high-TrFE and CFE systems, is related to the increase in the interchain distance. Simulations in ref. [START_REF] Yang | Novel Polymer Ferroelectric Behavior via Crystal Isomorphism and the Nanoconfinement Effect[END_REF] show that with increasing interchain distance, the correlation length of the dipoles is decreasing, causing the dipoles to rotate more easily in the more spacious regions and even to be more susceptible to thermal fluctuations, resulting in the formation of polar clusters or nanodomains. These polar nanodomains are at the origin of frequency dispersion of the maximum dielectric permittivity [START_REF] Cross | Relaxor Ferroelectrics[END_REF] . Pramanick et al. [START_REF] Pramanick | Origin of Dielectric Relaxor Behavior in PVDF-Based Copolymer and Terpolymer Films[END_REF] , further suggest that the relaxor behavior in terpolymers is also largely influenced by entanglement and complex interplay between the crystalline and amorphous phases.

The investigated compositions exhibit a peak in dielectric response from room temperature to 140 °C. This behavior not only defines the working region of these materials but is also expected to dramatically influence the functional response of these materials, which we will elaborate on in the next paragraphs.

Piezoelectric energy harvesting

In this section we highlighted the structural, microstructural and dielectric features of each composition, we focus on the functional response and the functional peculiarities that the In terms of maximizing the piezoelectric response, piezoelectric coefficients are not the only essential term [START_REF] Roscow | Modified Energy Harvesting Figures of Merit for Stress-and Strain-Driven Piezoelectric Systems[END_REF] . The piezoelectric energy harvesting figure of merit (FOM piezo ) is a ratio between the square of the piezoelectric response (d 33 ) over the relative permittivity (ε 33 ). The figure of merit for the the off-resonance piezoelectric response FOM piezo [START_REF] Priya | Criterion for Material Selection in Design of Bulk Piezoelectric Energy Harvesters[END_REF] is:

(

The 70/30 composition has the highest FOM piezo despite the highest d 33 measured in 55/45 (Fig. 4b), since the permittivity, due to the proximity of the dielectric anomaly (see Fig. 1b), is high in 55/45. It implies that the proximity of phase transitions/anomalies, where the losses are maximized, is not the optimal operating region for energy harvesters. As both d 33 and ε 33 depend on the temperature, FOM piezo vs. T plots would precisely define the optimal working region of a specific material. PVDF away from the onset of the Curie transition [START_REF] Burkard | Reversible Pyroelectricity and Inverse Piezoelectricity in Polyvinylidene Fluoride[END_REF][START_REF] Sagar | Effect of BaZrO3 Nanoparticles on Pyroelectric Properties of Polyvinylidene Fluoride (PVDF)[END_REF] . Not only p is needed to benchmark the compositions in terms of maximal energy output. Indeed, Sebald et al. [START_REF] Sebald | Electrocaloric and Pyroelectric Properties of 0.75Pb(Mg 1/3Nb2/3)O3-0.25PbTiO3 Single Crystals[END_REF] defined the figure of merit (FOM pyro ) for integrating materials into pyroelectric energy harvesters as:

(

) 2
where ε' r is the relative permittivity, and p is the pyroelectric constant. The FOM pyro for the studied compositions are given in Fig. 5b. 70/30 is again the dominant material due to its high p and low permittivity relative to the other compositions.

It should be stressed that the figure of merit used here does not include the heat transport properties, which are also of key importance because they define the rate at which the temperature can be varied. It follows that this rate defines the amount of energy that can be generated with pyroelectric energy harvesting [START_REF] Pandya | New Approach to Waste-Heat Energy Harvesting: Pyroelectric Energy Conversion[END_REF] . Highly textured composites with anisotropic thermal transport properties should be designed and fabricated to maximize further the response of the pyroelectric harvesting materials [START_REF] Yan | Performance Enhancements in Poly(Vinylidene Fluoride)-Based Piezoelectric Nanogenerators for Efficient Energy Harvesting[END_REF][START_REF] Anand | Effect of Sodium Niobate (NaNbO3) Nanorods on β-Phase Enhancement in Polyvinylidene Fluoride (PVDF) Polymer[END_REF] .

The results also imply that working near the transition temperature is not necessarily optimal due to the FOM pyro , which is related to the ratio between the pyroelectric coefficient and the dielectric permittivity, as both drastically increase near the transition.

Triboelectric energy harvesting

A relatively novel method of energy harvesting from the environment is triboelectricity.

The triboelectric effect has been known for centuries, but only recently it was shown that its uses in harvesting applications are interesting [START_REF] Fan | Flexible Triboelectric Generator[END_REF] . The underlying mechanism is not wellunderstood; in general, the effect occurs in two steps: contact electrification followed by electrostatic induction [START_REF] Wang | On the Origin of Contact-Electrification[END_REF]37 . Contact electrification happens when two surfaces/materials come in close contact and exchange electrons due to different work functions. When the contact is separated, the surface charges are compensated by the electrostatic induction phenomenon.

The charge redistribution during the compensation is responsible for the electric output work.

The force applied to the samples and the triboelectric voltages (U tribo ) generated by the samples is shown in Fig. 6a.

The peak voltages ranged from -36 V for 55/45 to ~0 V for CFE. One voltage peak appears just after the contact, as shown in Fig. 6a (see force and voltage peaks). The contact time is denoted by a dashed line and numbered 1. The separation time is again denoted by a dashed line and numbered 2, where another voltage peak is observed right after. Some more minor voltage variations were observed, most probably due to the non-equal distribution of the forces due to the non-perfect alignment of the sample and the aluminum side. It is also interesting to note that the voltage converged to a final value after a certain amount of time (cycles). The possible explanation for this is gradual migration of the defect ions [START_REF] Mackey | Layer Confinement Effect on Charge Migration in Polycarbonate/ Poly(Vinylidene Fluorid-Co-Hexafluoropropylene) Multilayered Films[END_REF] or surface poling of the material itself due to the electrostatic forces created.

Defining the figure of merit for triboelectric materials is difficult because the phenomenon is related to the electronic structure of the dielectric and metal in contact. Nonetheless, some figures of merit have been proposed 37,[START_REF] Peng | Optimization Principles and the Figure of Merit for Triboelectric Generators[END_REF] . The figure of merit that best describes triboelectric energy harvesting is 39 :

(3)

σ is the generated surface charges of the surface under investigation and ε 0 is the dielectric permittivity of vacuum.

However, since σ is challenging to determine, we used the root mean square of the voltage signal (U OC-RMS ) as the figure of merit. This factor is proportional to the power output since all the samples are connected to the same resistance (R of 100 MΩ in Fig. 6b) value.

Interestingly, the values of the power (seen in Fig. 6c) extracted by the triboelectric experiment to some degree correlate with the FOM piezo (see Fig. 4b). This implies that similar mechanisms drive the triboelectric effect in (relaxor-)ferroelectric compositions. We assume effects are related to the degree of (spontaneous) polarization, as already shown in some articles [START_REF] Bai | Dipole-Moment-Induced Effect on Contact Electrification for Triboelectric Nanogenerators[END_REF] , and to the dielectric permittivity of the materials. It is also possible that during the triboelectric cycling, the surface region was self-poled by the charged surface and the migration of point defects, which means that a part of the contribution to the generated voltage was from the piezoelectric effect. The oscillation of the voltage during the contact phase (between 1 and 2 in Fig. 6c), which seems to directly correlate with the oscillation of pressure would support the idea of self-poled regions and piezoelectric response since the generated voltage is negative when under pressure which is expected from the piezoelectric response with a negative d 33 . It is therefore not excluded that here some piezoelectric effect also contributes to the total triboelectric response, as well as other mechanisms such as electrostriction and flexoelectricity.

Energy storage

Energy generated by the three energy harvesting methods, piezoelectric, pyroelectric, and triboelectric must now be effectively stored. It was considered whether the same compositions that generated the energy could be used to store the energy until it is supplied to the consumer. PVDF-based materials were recently shown to possess high storage capability [START_REF] Liu | Relaxor Ferroelectric Polymers: Insight into High Electrical Energy Storage Properties from a Molecular Perspective[END_REF] .

The energy storage properties were calculated from unipolar loops shown in Fig. 7a. The polarization values at the maximal field are the largest in the CFE composition and the smallest in the 80/20. The latter reflects the short-and long-range order, respectively. While the dipoles in 80/20 were permanently oriented with poling, as for classical ferroelectrics, CFE possesses randomly oriented dipoles aligned only under the applied electric field [START_REF] Jin | Decoding the Fingerprint of Ferroelectric Loops: Comprehension of the Material Properties and Structures[END_REF] . The energy storage at depolarization was evaluated from the polarization values. In contrast to the trends of the harvesting methods, the energy storage is minimal for 80/20 and increases with increasing the TrFE amount and is the highest in CFE with 1.52 J/cm 3 (see Fig. 7b) Although the stored power for 55/45 is higher than for the other copolymers, its efficiency is much lower, possibly due to the relaxor-to-ferroelectric phase transitions.

The values are in general comparable to some well-known energy storage materials such as antiferroelectric NaNbO 3 -based materials [START_REF] Fulanović | NaNbO3-Based Antiferroelectric Multilayer Ceramic Capacitors for Energy Storage Applications[END_REF] , owing mainly to the larger breakdown field of PVDF-based materials.

Summary and conclusions

Within this study, we try to promote the agenda that showing merely the conversion coefficients does not provide sufficient information in the selection process for materials for energy harvesting. The so-called figures of merit are more informative and give a more straightforward answer because they are proportional to the energy output.

A radar plot was utilized (Fig. 8a) to summarize this study findings and to find the best allaround composition. The radar plot was used to plot the FOM piezo , FOM pyro , U RMS-OC , and For each composition, the area defined by these four parameters was then evaluated and normalized again to the highest area obtained amongst the compositions. In our case, this was for 70/30. Fig. 8b indicates the best all-around material (70/30, area is by definition 100%) and how much the followers are behind the optimal composition. It can be seen from Fig. 8b, that 70/30 is leading when considering all four sections. The 70/30 is the best all-around material, leading in all three harvesting categories. However, its long-range ferroelectric order with remanent polarization prevents high energy storage capabilities since the polarization change with the application of electric field is relatively low compared to relaxor CFE. It seems that 70/30 is an optimal choice in terms of material responsivity, which is tuned by varying the interchain distance of the material. The CFE composition has superior energy storage capability, while its relaxor-like nature prevents it from having high piezo-, pyro-, tribo-electric responses.

Figure 1 .

 1 Figure 1. Structural analysis of the PVDF-based compositions. a) XRD patterns and b)

Figure 2

 2 Figure2shows surface morphologies of the films with randomly oriented and intertwined

Figure 2 .

 2 Figure 2. Microstructural images of the surface of 80/20, 75/25, 70/30, 55/45 and CFE

Figure 3 .

 3 Figure 3. The dielectric response of five PVDF-based materials. a) the real part of the

  different compositions exhibit. The direct piezoelectric effect is one of the key parameters that define the energy output of the energy harvesters. The values of direct effective longitudinal piezoelectric coefficient (d 33 ) measured on the compositions are given in Fig. 4a. The highest d 33 value was observed in 55/45 (-21 pC/N), while no response was measured in the CFE. In the ferroelectric compositions 80/20, 75/25, and 70/30, d 33 increases with the content of TrFE until reaching a peak value at 55/45. The microscopic mechanism behind the increased d 33 at this content of TrFE is interconnected and three-fold. First, the permittivity anomaly is close to room temperature (see Fig. 3a), which signals the high mobility of domain walls and dipoles. Second, the decreased interchain distance (see Fig. 1a&b) additionally eases the switching. Third, the vicinity of the morphotropic phase boundary (MPB, located at ~50% TrFE) provides a flattened energy landscape favorable for enhanced responses.

Figure 4 . 3 . 4 .

 434 Figure 4. The dielectric and piezoelectric performance of the PVDF-based materials at room

Figure 5 .

 5 Figure 5. Pyroelectric effect of the P(VDF-TrFE). a) the pyroelectric current density

Figure 6 .

 6 Figure 6. Triboelectric energy harvesting at room temperature. a) force applied to the samples

Figure 7 .

 7 Figure 7. Energy storage and efficiency calculated at room temperature. a) unipolar

W

  S ·ζ normalized to the highest value obtained in the specific section (FOM piezo : 70/30, FOM pyro : 70/30, U RMS-OC : 70/30, W S ·ζ: CFE).

Figure 8 .

 8 Figure 8. The final benchmarking of the samples. a) a radar plot of the four normalized

Table 1 .

 1 The chemical composition and the notation used (ref. datasheet).

	Commercial	VDF (wt %)	TrFE (wt %)	CFE (wt %)	Notation
	name
	FC20	79.6	20.4	/	80/20
	FC25	75.1	24.9	/	75/25
	FC30	70	30	/	70/30
	FC45	54.5	45.5	/	55/45
	FS-RS	64.6	26.2	9.2	CFE

Acknowledgments

The authors acknowledge the "Investissements d'Avenir" program (ANR-10-LABX-0035, through the flagship NanoVibes), PHC Slovenian-French Proteus mobility grant (BI-FR/21-22-PROTEUS-004), Slovenian Research Agency (program P2-0105), ARRS (Slovenian research agency) J2-2508, and FNR-Luxembourg INTERmobility grant (INTER/Mobility/19/13992074).


Chaoyang Xue

Gisèle Krysztofiak

Yangang Ren

Min Cai

Patrick Mercier

Frédéric Le Fur

Corinne Robin

Benoit Grosselin

Véronique Daële

Max R Mcgillen

Yujing Mu

Valéry Catoire
email: valery.catoire@cnrs-orleans.fr

Abdelwahid Mellouki
email: mellouki@cnrs-orleans.fr

A study on wildfire impacts on

Keywords: Wildfire, Air Quality, Emission Factor, Emission Inventory, VOLTAIRE Supersite

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The frequency of wildfire events as well as the extent of the burned areas shows an increasing trend globally due to climate change [START_REF] Artés | A global wildfire dataset for the analysis of fire regimes and fire behaviour[END_REF][START_REF] Barbero | Climate change presents increased potential for very large fires in the contiguous United States[END_REF][START_REF] Liu | Trends in global wildfire potential in a changing climate[END_REF][START_REF] Randerson | Global burned area and biomass burning emissions from small fires[END_REF]. During the combustion of biomass, numerous pollutants, including particles and gases, are emitted into the atmosphere, which could have potential impacts on air quality and global climate [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF][START_REF] Jaffe | Ozone production from wildfires: A critical review[END_REF][START_REF] Jost | Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations[END_REF][START_REF] Randerson | Global burned area and biomass burning emissions from small fires[END_REF]. On a regional scale, the emitted pollutants could exacerbate air pollution downwind of the wildfire. For instance, [START_REF] David | Could the exception become the rule? "Uncontrollable" air pollution events in the U.S. due to wildland fires[END_REF] found that wildfire events in the US significantly enhanced the nationwide particulate matter with aerodynamic diameters smaller than 2.5 or 10 µm (PM 2.5 and PM 10 ). Considering that the number and severity of wildfire events are increasing, they suggested that wildfire impacts should be considered in the national ambient air quality standards and implementations. Besides, the emitted pollutants also affect ground-level O 3 [START_REF] Jaffe | Ozone production from wildfires: A critical review[END_REF]. As a secondary pollutant, O 3 could be consumed or accumulated within a biomass burning (wildfire) plume varying from different aging stages. In a very fresh plume, O 3 is typically consumed regarding its rapid reaction with the emitted NO. It is also possible that in the very early stages of plume formation, highly reactive organic molecules may serve to suppress ozone concentrations. As the plume is diluted and transported, O 3 can be produced through chemical reactions involving volatile organic compounds (VOCs) and NO x (NO + NO 2 ) under sunlight irradiation [START_REF] Granier | The Impact of Biomass Burning on the Global Budget of Ozone and Ozone Precursors[END_REF][START_REF] Jaffe | Ozone production from wildfires: A critical review[END_REF]. For example, [START_REF] Wang | Chemical Tomography in a Fresh Wildland Fire Plume: A Large Eddy Simulation (LES) Study[END_REF] observed 10-40 ppbv O 3 enhancement at 30 km downwind of a wildfire in the US. In addition, when the plumes (typically VOC-rich) mix into urban plumes (typically NO x -rich) substantial enhancement in O 3 formation is observed, which deteriorates urban air quality [START_REF] Xu | Ozone chemistry in western[END_REF].

Carbon monoxide (CO) is considered to be the second-largest primary carbon compound (after carbon dioxide, CO 2 ) emitted from the wildfire processes, which accounts for nearly half of its global emissions [START_REF] Zheng | Global atmospheric carbon monoxide budget 2000-2017 inferred from multi-species atmospheric inversions[END_REF]. CO is typically treated as a tracer for incomplete combustion, including biomass burning. The tropospheric lifetime of CO against OH with a global mean of 1.5×10 6 molecules cm -3 [START_REF] Seinfeld | Atmospheric Chemistry and Physics: From Air Pollution to Climate Change[END_REF] is about 5 weeks, implying that its concentration in the wildfire plume is dominated by the downwind dilution during transport over one week or so. Hence, the Emission factor (EF (X)=∆X/∆CO, where X represents the target species) of O 3 obtained in the wildfire plume was usually used for estimating the plume age [START_REF] Jaffe | Ozone production from wildfires: A critical review[END_REF]. Generally, its values were reported to be negative in a fresh wildfire plume and turned positive and increased progressively with the plume age [START_REF] Jaffe | Ozone production from wildfires: A critical review[END_REF].

In addition to the long-lived greenhouse gases (GHGs, e.g., CO 2 , CH 4 , N 2 O), non-methane hydrocarbons (NMHCs) could also be emitted from wildfires. Due to their high reactivity (with OH, O 3 , or NO 3 ), NMHCs play important role in the in-plume photochemistry to produce secondary pollutants like particulate matter (PM), O 3 , and other photo-oxidants. Furthermore, numerous NMHCs are toxic and the increase in their ambient concentration levels constitutes a risk to public health. To quantify their emissions from wildfires, their emission factors can be calculated [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF][START_REF] Jost | Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations[END_REF].

On a global scale, wildfires are an important GHG source [START_REF] Friedlingstein | Global Carbon Budget[END_REF]Saunois et al., 2020). Saunois et al. (2020) have estimated 17 Tg CH 4 yr -1 emissions from wildfire emissions from 2008 to 2017, which represents 5% of the total anthropogenic CH 4 emissions. In addition to GHGs emissions, depending on their size and location, wildfire plumes can be transported to the upper troposphere-lower stratosphere (UTLS) region, and hence, impact the global climate and stratospheric ozone layer [START_REF] Baray | Influence of stratosphere-to-troposphere exchange and long-range transport of biomass burning plumes[END_REF][START_REF] Hirsch | Record-breaking aerosol levels explained by smoke injection into the stratosphere[END_REF][START_REF] Hooghiem | Wildfire smoke in the lower stratosphere identified by in situ CO observations[END_REF][START_REF] Kloss | Australian Fires 2019-2020: Tropospheric and Stratospheric Pollution Throughout the Whole Fire Season[END_REF][START_REF] Kloss | Transport of the 2017 canadian wildfire plume to the tropics via the asian monsoon circulation[END_REF][START_REF] Yu | Black carbon lofts wildfire smoke high into the stratosphere to form a persistent plume[END_REF]. For example, [START_REF] Hirsch | Record-breaking aerosol levels explained by smoke injection into the stratosphere[END_REF] reported a record-breaking aerosol level in the stratosphere over the southern hemisphere caused by the intensive Australian bushfires. This wildfire smoke could have a lifetime of days to months, which allows the smoke to be uniformly mixed over most of the southern hemisphere, leading to a significant cooling effect of 1.0 ± 0.6 W m -2 . Satellite observations can be used for estimating the burned area and emissions of wildfires. However, scaling this to small fires remains challenging [START_REF] Ramo | African burned area and fire carbon emissions are strongly impacted by small fires undetected by coarse resolution satellite data[END_REF]Saunois et al., 2020). For instance, [START_REF] Ramo | African burned area and fire carbon emissions are strongly impacted by small fires undetected by coarse resolution satellite data[END_REF] found that the Moderate Resolution Imaging Spectroradiometer (MODIS) images underestimated the burned area of small fires (<100 ha) by 94% in sub-Saharan Africa. Hence, the regional impact on air quality and/or the global impact on climate from both small and large fires should be studied [START_REF] Ramo | African burned area and fire carbon emissions are strongly impacted by small fires undetected by coarse resolution satellite data[END_REF], in particular in those regions where few fire events were recorded. Herein, in the middle of September 2020, a wildfire occurred in southern Orléans city where few wildfire events have occurred in the past.

To the best of our knowledge, no study has been conducted to quantify the impacts of wildfires on the air quality in this region. Based on atmospheric measurements along with trajectory model simulations, the impact of this wildfire on the air quality in suburban Orléans was quantified.

Emissions of this wildfire were evaluated and compared with the fire emission inventories.

Atmospheric implications are presented after analysis and discussion of the field measurements and model simulations.

Methods

The Sologne Forest Fire Event

At 15:44 (local time, UTC+2) on 14 th September 2020, firemen in Orléans received the information about the fire in the Sologne forest (47.3726°N, 2.12987° E, Figure 1). Its spread was controlled but it was still active at 2:31 on 15 th September (information obtained from Firefighters Loire-et-Cher, https://sdis41.fr/, last access: 17 January 2022). On 22 nd September, the fire was considered permanently extinguished. In total, about 36.3 hectares of forests (vegetation above ground: coniferous trees, mainly Laricio pines; surface vegetation: heather, moors, etc.) were burned during this event.

Figure 1

Measurement Site and Instrumentation

Measurements used in this study were conducted at the VOLTAIRE supersite. It is located on the campus of the Centre National de Recherche Scientifique (CNRS, 47.8377º N, 1.9445º E) which is about 10 km south from Orléans city center and about 120 km southwest from Paris (Figure 1).

Details about this station can be found in previous studies based on measurements at this site [START_REF] Cai | Ambient BTEX concentrations during the COVID-19 lockdown in a peri-urban environment[END_REF][START_REF] Jiang | Seasonal and diurnal variations of BTEX compounds in the semi-urban environment of Orleans, France[END_REF]. , NH 4 + , Na + , K + , etc.). Black carbon (BC) was measured by a black carbon monitor (Magee Scientific Aethalometer ® Model AE33), which also includes the aethalometer model to quantify the contributions of wood-burning (BCwd) and fossil fuel (BCff) [START_REF] Herich | A 2.5 year's source apportionment study of black carbon from wood burning and fossil fuel combustion at urban and rural sites in Switzerland[END_REF].

At

Models

The Lagrangian transport and diffusion model, FLEXible PARTicle (FLEXPART) v10.4, was used to calculate back trajectories of air masses reaching our measurement site [START_REF] Brocchi | Intercontinental transport of biomass burning pollutants over the Mediterranean Basin during the summer 2014 ChArMEx-GLAM airborne campaign[END_REF][START_REF] Pisso | The Lagrangian particle dispersion model FLEXPART version 10.4[END_REF] [START_REF] Fountoukis | ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+-Ca2+-Mg2+-NH4+-Na+-SO42--NO3--Cl--H2O aerosols[END_REF]. The model was run based on the AIM data (particle-and gas-phase), temperature, and relative humidity.

Inventories

Data from three fire emission inventories were used in this study. 3 Results and Discussion

General Overview of the Measurements

All the observations in the whole of September 2020 are presented and overviewed here to represent the typical meteorological and air quality conditions, while detailed analysis was mostly focused on the wildfire plume and its comparison with measurements at the same hours on other days of September.

Meteorological Conditions

During the measurement period, it was mostly sunny and cloud-free except for a limited number of days (5 th -6 th Sept. and 28 th Sept. -1 st Oct.) when it was rainy or cloudy as reflected by the measured photosynthetically active radiation (PAR, Figure 2). The air temperature was typically around 20 ºC except for the last week (25 th Sept. -1 st Oct.) when the temperature decreased to around 10 ºC caused by a cold front. The campaign-averaged temperature is 18.0 ± 6.0 ºC, respectively (Table 1). Throughout the campaign, air masses observed at this site originated from various directions (but few from South/Southeast/East), as reflected by the wind rose plot (Figure S1A) and the trajectory frequencies obtained from the HYSPLIT model (Figure S1B). Wind speed was generally less than 2 m s -1 except for the periods of 17 th and from 23 rd to 28 th Sept.

when the wind speed reached 4 m s -1 . The average wind speed was 1.3 ± 1.0 m s -1 during the campaign.

In particular, the highest temperature of 35.4 ºC and the lowest RH of 21.8% during the campaign occurred at 16:45 on 14 th September, which is close to the occurrence of the forest wildfire. Daily average T of 22.6 ± 8.8 ºC and RH of 53 ± 22% on this day were the highest and lowest ones during the campaign, respectively. The wind direction was dominated by the south (the direction of the forest fire location) on the following day after the occurrence of the forest wildfire, which blew the emitted pollutants to the north of the fire, including the VOLTAIRE supersite.

Figure 2: Meteorological parameters measured at the VOLTAIRE supersite. RH: relative humidity; P: pressure; WS: wind speed; WD: wind direction; T: temperature.

Pollutants

The VOLTAIRE supersite is located in a relatively clean area with the average PM 2.5 (6.7 µg m - 3 ), PM 10 (12.8 µg m -3 ), NO 2 (7.4 µg m -3 ), and O 3 (63.6 µg m -3 ) being lower than the World Health Organization (WHO) air quality guideline values (see WHO website: https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health, last access: 7 March 2022) of 15 µg m -3 (24 h mean) for PM 2.5 , 45 µg m -3 (24 h mean) for PM 10 , 25 µg m -3 (24 h mean) for NO 2 , and 100 µg m -3 (8 h daily maximum) for O 3 . All the pollutants exhibited much lower values during the period from 23 rd to 28 th Sept., as a result of fast dilution caused by high wind speeds (Figure 2).

Regarding the mass distribution of different particle sizes, average concentrations of PM 1 , PM 1- 2.5 , PM 2.5-4 , and PM 4-10 were 5.0 ± 3.9, 1.7 ± 0.8, 1.6 ± 0.8, and 4.5 ± 3.7 µg m -3 (Table 1), with their contributions of 39%, 13%, 13%, and 35% to PM 10 , respectively. BC was generally lower than 2 µg m -3 , with an average of 0.6 ± 0.4 µg m -3 . Significant enhancements in PM and BC were found in the wildfire plume observed on the morning of 15 th Sept. and more analysis is presented in Section 3.2. Ambient ions in the particle or gas phase are presented in Figure 4. In terms of the measured concentrations, sodium and ammonium were the most important cations in the particle phase, while nitrate and sulfate were the most important anions. On average, concentrations of sodium, ammonium, nitrate, and sulfate were 0.8±0.3, 1.0±0.4, 0.5±0.5, and 1.1±0.5 µg m -3 (Table 1),

with their mass contributions of 19.9%, 24.8%, 13.0%, and 25.9% to the total particle-phase ions (Figure S2), respectively. In contrast, the measured organics (acetate and formate), chloride, nitrite, calcium, magnesium, and potassium showed much lower levels, with a sum of their contributions of 16.4%. In the gas phase, the dominant ions were ammonium (33.2%), acetate (28.6%), formate (13.6%), nitrite (8.6%), and sulfate (6.1%) while contributions of other ions were all less than 5% (Figure S2).

Observations of the wildfire Plume

Figure S3 shows the concentrations of CO, NO 2 , CH 4 , and N 2 O measured by SPIRIT at the supersite. Stable measurements started at 9:27 on 15 th September and the first measured CO concentration was 837 ppbv, which is much higher than the background level of around 100 ppbv. Later, the highest CO concentration of 1050 ppbv was observed at 09:38. High concentrations of NO 2 (9.3 ppbv), N 2 O (334 ppbv), and CH 4 (2203 ppbv) were also observed within 10 mins around the CO peak. Meanwhile, the levels of other measured pollutants (e.g., PM, BC, sulfate) showed a simultaneous increase (Figure 3 and Figure 4). For example, the PM 10 level of 75.9 µg m -3 was the highest one in September and more than 6 times higher than the campaign average value of 12.7 µg m -3 , revealing the significant impact of this forest fire on the air quality in this region. In addition, a view from the satellite (Figure S4) also observed the wildfire and the regional spread of its emissions.

Emission Factors (EF)

As the forest fire occurred around 15:35 on 14 th September and the wildfire plume was observed at 9:38 (local time, the occurrence of the CO peak) on 15 th September, the plume age should be <18 h (from the trajectory a plume age <12 h could be obtained and mostly during night-time).

Besides, a significant O 3 decrease by ca. 13 ppbv was observed in this wildfire plume indicating the plume did not undergo much photochemistry that could lead to O 3 production. For instance, [START_REF] Jost | Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations[END_REF] found a significant O 3 enhancement in a wildfire plume after 2 h photochemistry. Hence, the wildfire plume observed here should be relatively fresh (also see the trajectory in Section 3.3). However, previous field and laboratory studies have reported the impact of near-source transformation on the properties of wildfire plumes, which may influence the EF values [START_REF] Collier | Regional Influence of Aerosol Emissions from Wildfires Driven by Combustion Efficiency: Insights from the BBOP Campaign[END_REF][START_REF] Hodshire | Aging Effects on Biomass Burning Aerosol Mass and Composition: A Critical Review of Field and Laboratory Studies[END_REF][START_REF] Li | Evolution of Organic Aerosol From Wood Smoke Influenced by Burning Phase and Solar Radiation[END_REF]. The derived EF values are focused on gas species, for which the atmospheric oxidization process may cause major uncertainties. Hence, we employed a method to constrain the uncertainties, which are generally caused by the chemical loss mainly through OH oxidization. Take toluene as an example, its reaction rate with OH is 5.6×10 -12 cm 3 molecule -1 s -1 at 298 K [START_REF] Mellouki | Evaluated kinetic and photochemical data for atmospheric chemistry: volume VIIIgas-phase reactions of organic species with four, or more, carbon atoms ( ≥ C<sub>4</sub&gt[END_REF]. Assuming a relatively high OH concentration of 5×10 5 molecule cm -3 , a negligible amount of CO but 11% of toluene will be lost through OH oxidization in 12 hours, leading to an underestimation by <11% in EF(toluene). Similarly, other EF uncertainties caused by the OH oxidization process can be estimated, which is summarized in the updated Table 2. Please note that we used a higher OH concentration to constrain the upper limit of the uncertainties. The molar enhancement ratio of compound X compared to CO was defined as EF(X)=∆X/∆CO [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF][START_REF] Jost | Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations[END_REF], which was used here to estimate the wildfire emissions. To avoid the uncertainty caused by the selection of background levels, we used the slope of the regression line between species X and CO as the EF (X) during the pollution case [START_REF] Jost | Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations[END_REF]. Table 2 shows the derived EFs and the correlations for numerous species. As mentioned before, a decreased O 3 resulted in a negative EF(O 3 ) of -0.05 µg m -3 ppbv -1 (equivalent to -0.026 ppbv ppbv -1 ) was obtained here, indicating a young wildfire plume. Most correlation coefficients are high, implying that the increase in species X and CO are caused by the same source, the wildfire plume. Poor correlations between some species (e.g., xylenes, benzene) and CO are also found, which might be caused by the complexity of the emissions as well as the ambient dilution process. In particular, significant CH 4 emission reflected by a high CH 4 peak of 2203 ppbv and a positive EF(CH 4 ) of 0.23 ppbv ppbv -1 was found in this wildfire plume, implying the burning of the herbaceous layer [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF][START_REF] Jost | Trace gas chemistry in a young biomass burning plume over Namibia: Observations and model simulations[END_REF]. This may also explain the higher EF(CH 4 ) and EF(N 2 O) obtained here than those from the averages of temperate forest fires (0.08 and 0.001 ppbv ppbv -1 , respectively) [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF]. Other EFs are lower than those from temperate fires [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF], indicating the potential impact of dilution and highlighting the complexity of the wildfire emissions and their impacts on ambient pollutant levels. Although this fire was small (36.3 ha burned area) compared with the fires summarized in [START_REF] Andreae | Emission of trace gases and aerosols from biomass burning -An updated assessment[END_REF], it has been suggested that the relatively high EFs for greenhouse gases (N 2 O and CH 4 ) from small fires may be of importance to the global greenhouse gas budget [START_REF] Randerson | Global burned area and biomass burning emissions from small fires[END_REF].

Impacts on BC and Particle Size Distribution

We explored the possible impact of this forest fire, even though a small one, on the measured BC levels and its sources. Figure 5 shows the average concentration of measured BC wb (BC derived from wood burning) and BC ff (BC derived from fossil fuel) in the wildfire plume or non-wildfire plumes at the same hours (9:00-11:00). BC ff was the major source for the measured total BC at this site and about one order of magnitude higher than BC wd in non-wildfire plumes. Considering that the measurements covered rush hours, the source of BC in non-wildfire plumes was attributed to fossil fuel combustion. The corresponding BC wb /BC ff ratio of 0.08 was in the range of 0.02 -0.11 reported in other sites in Europe during summertime [START_REF] Herich | A 2.5 year's source apportionment study of black carbon from wood burning and fossil fuel combustion at urban and rural sites in Switzerland[END_REF].

However, BC wd increased significantly in the wildfire plume (0.3±0.19 µg m -3 , non-wildfire plume: 0.06±0.05 µg m -3 ) while BC ff kept a similar level (0.95±0.38 µg m -3 ) to that in non-wildfire plumes (0.77±0.57 µg m -3 ), leading to an increasing BC wb /BC ff ratio of 0.32, which is in the range of values (0.3 -0.5) impacted by wildfire emissions [START_REF] Helin | Characteristics and source apportionment of black carbon in the Helsinki metropolitan area, Finland[END_REF][START_REF] Herich | A 2.5 year's source apportionment study of black carbon from wood burning and fossil fuel combustion at urban and rural sites in Switzerland[END_REF][START_REF] Kalogridis | Assessment of wood burning versus fossil fuel contribution to wintertime black carbon and carbon monoxide concentrations in Athens, Greece[END_REF][START_REF] Yttri | Quantifying black carbon from biomass burning by means of levoglucosan -A one-year time series at the Arctic observatory Zeppelin[END_REF]. This indicates the potential impact of this wildfire on regional BC levels. Figure 5B and C show PM mass concentrations at different sizes (PM 1 , PM 1-2.5 , PM 2.5-4 , and PM 4-10 ) and their relative contributions in the wildfire or non-wildfire plumes during the same hours (9:00 -11:00). Average PM 2.5 (or PM 10 ) in the non-wildfire plumes during the same hours of the day was 7.5 (or 14.7) µg m -3 , which is much lower than the WHO air quality guideline PM 2.5 (or PM 10 ) value of 15 (or 45) µg m -3 (24 h mean). However, in the wildfire plume, the average PM 2.5 (or PM 10 ) increased to 29.6 (or 42.6) µg m -3 , which exceeds (or becomes close to)

the WHO air quality guideline PM 2.5 (or PM 10 ) value, indicating a potential public health risk caused by wildfire emissions (PM, BC, VOCs, etc.). Moreover, wildfire emissions can cause changes in particulate size distribution. For instance, mass concentrations of PM 1 , PM 1-2.5 , PM 2.5- 4 , and PM 4-10 in the non-wildfire plumes were 5.5, 2.1, 2.0, and 5.2 µg m -3 , with relative contributions of 37%, 14%, 13%, and 36%, respectively. All their mass concentrations showed a significant increase (from 5.5 to 25.0, 2.1 to 4.6, 2.0 to 3.9, and 5.2 to 9.1 µg m -3 for PM 1 , PM 1- 2.5 , PM 2.5-4 , and PM 4-10 , respectively), while their contributions changed remarkably within the biomass burning plume, particularly PM 1 and PM 4-10 whose contributions increased to 59% or decreased to 21%, respectively. As reported in previous studies, wildfire can emit various sizes of particles. However, particles with a smaller diameter (i.e., submicron particles) hold a relatively longer atmospheric lifetime against deposition so that they may undergo a regional transportation process [START_REF] Gallagher | Measurements and parameterizations of small aerosol deposition velocities to grassland, arable crops, and forest: Influence of surface roughness length on deposition[END_REF].

Impacts on Ambient Ions in the Particle-and Gas-Phase

Figure 6A displays the relative mass contribution of each particle-phase cation or anion to their total masses. During the non-wildfire period, anions were dominated by SO 4 2-and NO 3 -, with contributions of 49% and 40%, respectively. Within the young wildfire plume, SO 4 2-was enhanced from 1.1 to 2.3 µg m -3 while NO 3 -reduced from 0.9 to 0.2 µg m -3 , resulting in a dominant role of SO 4 2-(89%) in the total particle anion mass. NH 4 + and Na + were the two most important cations in the particle phase with contributions of 45% and 34% in the non-wildfire plumes, respectively, which is similar to other European locations [START_REF] Tang | Pan-European rural monitoring network shows dominance of NH 3 gas and NH 4 NO 3 aerosol in inorganic atmospheric pollution load[END_REF]. NH 4 + was slightly enhanced from 1.2 to 1.4 µg m -3 while Na + slightly decreased from 0.9 to 0.7 µg m -3 in the wildfire plume compared to the non-wildfire plumes, leading to a larger NH 4 + contribution of 51%.

Figure 6B shows the relative mass contribution of each gas-phase cation or anion to their total masses. The gas-phase anions are typically in their acid forms, i.e., the measured gas-phase HCOO -, CH 3 COO -and NO 2 -were in the form of HCOOH, CH 3 COOH, and HONO in the atmosphere, respectively. A remarkable increase in organic acids (CH 3 COOH and HCOOH) was found within the wildfire plume. Their concentrations increased from 2.0 to 5.7 or from 1.2 to 3.7 µg m -3 , respectively, suggesting strong emissions of organic acids from the wildfire [START_REF] Burling | Laboratory measurements of trace gas emissions from biomass burning of fuel types from the southeastern and southwestern United States[END_REF][START_REF] Chaliyakunnel | A Large Underestimate of Formic Acid from Tropical Fires: Constraints from Space-Borne Measurements[END_REF][START_REF] R'honi | Exceptional emissions of NH 3 and HCOOH in the 2010 Russian wildfires[END_REF]. Interestingly, significant gas-phase SO 4 2-(i.e., H 2 SO 4 ) enhancement was also found within this wildfire plume as its concentration increased from 0.8 µg m -3 to 1.7 µg m -3 , which may have an impact on sulfate aerosol formation and new particle formation.

No apparent enhancement of HONO (gas-phase NO 2 -) was observed in the wildfire plume, contrary to previous studies (e.g., [START_REF] Nie | Influence of biomass burning plumes on HONO chemistry in eastern China[END_REF][START_REF] Peng | HONO Emissions from Western U.S. Wildfires Provide Dominant Radical Source in Fresh Wildfire Smoke[END_REF][START_REF] Theys | Global nitrous acid emissions and levels of regional oxidants enhanced by wildfires[END_REF], which presented significant HONO emissions from biomass burning process. Although this does not preclude the possibility of HONO emissions from the fire since HONO concentrations will be + (in the form of NH 3 ) predominated, with a concentration of 3.1 and 3.5 µg m -3 in the nonwildfire and wildfire plumes, respectively, which is comparable to other European cities [START_REF] Tang | Pan-European rural monitoring network shows dominance of NH 3 gas and NH 4 NO 3 aerosol in inorganic atmospheric pollution load[END_REF]. Figure 6: Relative contributions (%) of each cation/anion in the particle-phase (A) and gas-phase (B) masses. Percentages less than 5% were not shown. In the gas-phase, concentrations of the only cation source, NH 3 , are 3.1 and 3.5 µg m -3 during non-wildfire and wildfire periods, respectively.

Emission Estimation and Comparison with GFAS

FLEXPART Simulations

Figure 7 illustrates the 12-h back trajectory (starting from CO peak at 9:38 local time) obtained from the FLEXPART simulation. It was found that the air mass at the VOLTAIRE supersite mainly originated from the south and southwest, which is consistent with the wind measurements at this station on 14 th and 15 th September (Figure 2). Some clusters originated from the area above the wildfire (Figure 7). Hence, the large increase in pollutants, such as CO from ~100 ppbv to 1050 ppbv, is attributed to the transportation of the wildfire plume. The satellite also captured the widespread plumes (Figure S4), indicating the impacts of wildfire emissions on regional air quality. Besides, quantification of emissions from this wildfire is of significance for understanding the role and impacts of small wildfires on regional air quality and global climate.

Emission Estimation vs. GFAS vs. GFED vs. FINN

We estimated carbon emissions from this fire event and compared them with current biomass burning inventories. A standard equation, developed by [START_REF] Seiler | Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning[END_REF], was popularly used for calculating the total carbon emission from a forest fire (C total ). , (Eq-1)

where A, B, ƒ c , and β represent the total burned area (36.3 ha), biomass density (t ha -1 ), the fraction of carbon of the biomass, and the fraction of carbon consumed during the burning process, respectively. Considering the composition differences, such as organic mat on the forest floor and in peatlands, components on the ground layer and above the ground layer are estimated separately. [START_REF] French | Variability in the emission of carbon-based trace gases from wildfire in the Alaskan boreal forest[END_REF] furtherly modified the equation to:

, (Eq-2)

where B a , ƒ ca , β a , C g , and β g denote the average biomass density of the above vegetation (t ha -1 ), the carbon fraction of the aboveground biomass with an assumed value of 0.50, the fraction of the aboveground biomass consumed during the fire, the carbon density (15 t-C ha -1 from French et al. ( 2002)) of the organic mat exposed to the fire, and the fraction of the organic mat consumed during the fire, respectively. B a was set as 50 t ha -1 obtained from the European Space Agency BIOMASS mission [START_REF] Quegan | The European Space Agency BIOMASS mission: Measuring forest above-ground biomass from space[END_REF]. 50-year average values of β a (0.33) and β g (0.25) for forest fires obtained from [START_REF] French | Variability in the emission of carbon-based trace gases from wildfire in the Alaskan boreal forest[END_REF] were adopted in this calculation. Then we obtained the C total of 435.6 t for this forest fire. This leads to an average carbon emission of 12 t-C ha -1 , which is comparable to previous studies of 5 -40 t-C ha -1 [START_REF] French | Variability in the emission of carbon-based trace gases from wildfire in the Alaskan boreal forest[END_REF][START_REF] Randerson | Global burned area and biomass burning emissions from small fires[END_REF].

Emission factors of CO 2 , CO, and CH 4 during flaming (EF f , kg gas (g total carbon) -1 ) and smoldering (EF s , kg gas (g total carbon) -1 ) periods were taken from [START_REF] French | Variability in the emission of carbon-based trace gases from wildfire in the Alaskan boreal forest[END_REF] in which they calculated the average of values reported by several previous studies (Table S1). For emission factors of BC and N 2 O, the derived Emission factors in Table 2 were used. As shown in 2.4 (6.6), 0.017 (0.048), and 0.019 (0.054) t (unit: ton), assuming the fire was always at the flaming (or smoldering) stage, respectively. Note that the actual emission should be between those two numbers as a wildfire event typically undergoes both flaming and smoldering stages.

For the GFAS inventory, emissions were only detected on 14 th September, with a daily average of grid CO emission of 1.5×10 -9 kg m -2 s -1 . Previous studies reported underestimation of small fire emissions possibly due to the uncertainties in burned area estimation [START_REF] Ramo | African burned area and fire carbon emissions are strongly impacted by small fires undetected by coarse resolution satellite data[END_REF]Saunois et al., 2020). In the present study, we found a similar underestimation by the GFAS inventory. In contrast, significant overestimation in the burned area, and total carbon, CO 2 , CO, and CH 4 emissions were found in the GFED and the FINN inventories. Regarding the significant overestimation in the burned area, in general, the reason is that current inventories use a coarse spatial resolution, such as 0.25×0.25 for GFED. For the FINN inventory, the burned area was classified according to land cover. For instance, in our case, woody savanna/shrublands, temperate forest, and temperate evergreen forest were identified in the FINN inventory, then the burned area can be overestimated when summing up all the burned areas. As far as we know, a new version of the FINN inventory is forthcoming (https://www2.acom.ucar.edu/modeling/finn-fire-inventory-ncar, last access: 11 July 2022), which should improve the performance in estimating small fire emissions. Hence, improvement of the inventory performance in small fire emissions is needed to quantify emissions of small fires and their environmental impacts. Differences between different inventories also suggest the necessary inter-comparison between inventories.

Conclusion and Implication

In the middle of September 2020, a forest wildfire occurred in the Sologne forest in Souesmes (Loir-et-Cher, Sologne, France) and its plume spread to 200 km in the following day, as observed by the MODIS satellite. At the VOLTAIRE supersite in suburban Orléans (about 50 km northwest of the wildfire location), instruments for various trace gases and particle compositions detected young biomass burning plumes from this wildfire. The data obtained enabled us to quantify the emissions from that fire and evaluate the possible impact on air quality in suburban Orléans.

Average concentrations of PM 2.5 and PM 10 in September were 6.7 and 12.8 µg m -3 , respectively, which are close to or lower than the updated WHO air quality guideline values of 5 and 15 µg m - 3 , respectively. However, PM 2.5 and PM 10 levels in the periods that were affected by wildfire emissions increased substantially (29.6 and 42.6 µg m -3 , respectively) and exceeded the WHO air quality guideline values (Figure 3). Additionally, levels of a series of VOCs including BTEX were also enhanced by the wildfire event.

Besides, the impacts of the wildfire on the size, composition, and properties of aerosol were quantified. Within the wildfire plume, an increase in BC was observed and the observed BC wb /BC ff ratio of 0.32 was in the range of values (0.3 -0.5) impacted by wildfire emissions.

Aerosol pH was significantly reduced in the plume as a result of the increase in sulfate. Besides, Emission factors compared to CO were calculated for various gaseous pollutants and compared with previous studies. Relatively higher EF ratios for GHGs (CH 4 and N 2 O) were found within the plume from this wildfire, indicative of the possibly important role of small fires in GHGs burden.

Furthermore, emissions of this wildfire were estimated and compared with GFAS emission inventory. We found that the GFAS emission inventory significantly underestimated the emission from this small fire, which agrees with other studies [START_REF] Ramo | African burned area and fire carbon emissions are strongly impacted by small fires undetected by coarse resolution satellite data[END_REF][START_REF] Wang | Chemical Tomography in a Fresh Wildland Fire Plume: A Large Eddy Simulation (LES) Study[END_REF]. This study suggested that the detection and prediction of small fire emissions still need further assessment and improvement.

The year 2020 was a warm year in Europe, with the autumn having the highest recorded temperatures since 1950 (see ECWMF news at https://climate.copernicus.eu/esotc/2020/temperature, last access: 17 January 2022). Higher temperatures are expected to lead to wildfire events in wildfire hotspot regions as well as those regions where few wildfires were recorded before (e.g., Orléans) [START_REF] Liu | Trends in global wildfire potential in a changing climate[END_REF][START_REF] Randerson | Global burned area and biomass burning emissions from small fires[END_REF], which may be exacerbated by climate change. This not only reduces regional air quality near those wildfires but also increases global greenhouse gas emissions. We suggest that besides the major wildfire regions, attention should also be paid to the regions with few wildfires recorded so far, in order to better understand their environmental effects and how their distribution may change over time.

  Figure 1. (A): Locations of Paris and Orléans (obtained from https://www.arcgis.com/, last access: 25 February 2022); (B): Locations of the forest fire (red star) and the VOLTAIRE

Figure 4 :

 4 Figure 4: Gas-(upper panel) and particle-phase compositions of PM 10 (lower panel) measured at the VOLTAIRE supersite. The green and red vertical lines indicate the occurrence of the forest fire in Souesmes and the observation of the wildfire plume at the VOLTAIRE supersite, respectively.

Figure 5 :

 5 Figure 5: Levels of (A) BC wb and BC ff , and (B) PM at different sizes, and (C) their relative mass contributions of different PM sizes to PM 10 in the wildfire or non-wildfire plumes during the same hours (9:00 -11:00) considering that the detection of the plume covered rushing hours.

Figure 7 :

 7 Figure 7: FLEXPART back trajectory with clusters. The black and the yellow stars represent the locations of the VOLTAIRE supersite and the wildfire.

Table 1 :

 1 Figure 3: Time series of pollutants (PM, O 3 , NO, NO 2 and BC, in µg m -3 ) concentrations at the VOLTAIRE supersite (data from Lig'Air, https://www.ligair.fr/, last access: 7 March 2022). The green and red vertical lines indicate the occurrence of the forest fire in Souesmes and the observation of the wildfire plume at the VOLTAIRE supersite. Statistic description of the observations at the VOLTAIRE supersite during the whole of September 2020. Except for the meteorological parameters, all the pollutants listed in this tableare in µg m -3 . For AIM data, p and g represent particle-or gas-phases, respectively.

	Parameters	Average	Range	Parameters	Average	Range
	T (ºC)	18.0±6.0	6.6-35.4	Sulfate (p)	1.1±0.5	≤2.9
	PAR (µmol m -2	17.7±24.2	0-91.2	Ammonium (p) 1.0±0.4	0.3-2.2
	s -1 )
	WS (m s -1 )	1.3±1.0	0-5.9	Calcium (p)	0.3 ± 0.3	≤2.6
	RH (%)	70.6±21.0	21.8-100	Magnesium (p) 0.04±0.03	≤0.19
	P (mBar)	1018±5.9	1001-1030	Potassium (p)	0.2±0.1	≤0.8
	BC	0.6±0.4	≤3.0	Sodium (p)	0.8±0.3	0.6-2.1
	NO	0.8±1.9	≤29.7	Acetate (g)	2.4±1.3	0.7-7.4
	NO 2	7.4±6.1	0.9-44.6	Formate (g)	1.1±0.9	0.1-4.1
	O 3	63.6±26.5	2.4-146.4	Chloride (g)	0.19±0.14	0.05-0.90
	PM 1	5.0±3.9	0.6-42.1	Nitrate (g)	0.4±0.2	≤1.1
	PM 1-2.5	1.7±0.8	0.4-9.0	Nitrite (g)	0.7±0.6	0.1-4.4
	PM 2.5-4	1.6±0.8	0.3-7.8	Sulfate (g)	0.5±0.8	≤4.9
	PM 4-10	4.5±3.7	0.1-22.9	Ammonium (g) 2.7±1.4	0.1-4.4
	Acetate (p)	(8.7±2.8)×10 -4	≤0.002	Calcium (g)	0.04±0.02	0.01-0.16
	Formate (p)	(23±8.4)×10 -3	≤0.04	Magnesium (g)	(1.8±1.4)×10 -3 ≤0.02
	Chloride (p)	0.13±0.21	≤1.2	Potassium (g)	0.12±0.07	≤0.36
	Nitrate (p)	0.5±0.5	≤3.0	Sodium (g)	0.08±0.06	0.03-0.31
	Nitrite (p)	0.02±0.01	≤0.06

Table 2 :

 2 Emission factors (EF) inferred from the wildfire plume. Units: ppbv ppbv -1 for Efs of

	NO 2 , CH 4 , and N 2 O (measured by SPIRIT); µg m -3 ppbv -1 for all the others. BTEX = Benzene +
	Toluene + Ethylbenzene + m/p/o -Xylenes. Comparisons with temperate forest fire summarized
	in Andreae (2019) were also shown. Note that the original values in Andreae (2019) are given in
	gram species per kilogram dry matter burned and they are converted to EFs for comparison.
	Atmospheric OH rate constants were taken from Mellouki et al. (2021) or Atkinson and Arey
	(2003).
	X	EF (X)	Uncertainty %	R 2	Andreae (2019)
	O 3	-4.9E-2		0.70
	NO	8.8E-4		0.47	2.5E-2
	CH 4	2.3E-1		0.79	8.2E-2
	N 2 O	1.7E-2		0.84	1.4E-3
	BC	2.8E-4		0.70	1.2E-2
	123-TMB	7.8E-6	51	0.99
	124-TMB	1.7E-5	50	0.80
	135-TMB	2.7E-6	71	0.86
	BTEX	1.2E-4		0.62	2.5E-3
	Ethylbenzene	1.6E-5	14	1.00	9.7E-5
	Toluene	5.2E-5	11	0.90	7.5E-4
	α-Pinene	1.3E-5	68	0.69
	n-Octane	8.7E-5	16	0.88
	n-Propylbenzene	7.5E-6	12	0.67
	p-Ethytoluene	3.5E-6	22	0.92
	m-Diethylbenzene	6.8E-7		0.45
	m-Ethyltoluene	6.2E-6	33	0.95
	methyl ethyl ketone	2.5E-4	35	0.35
	n-Decane	3.5E-6	21	1.00

Table 3

 3

	, the calculated emissions of CO 2 , CO, CH 4 , BC, and N 2 O were 1370 (1128), 82.8 (200),

Table 3 :

 3 The total CO emission based on the GFAS inventory was ca. 11 t. Similarly, the GFAS-derived total carbon, CO 2 , CH 4 , N 2 O, and BC emissions from Emission factors during flaming (EF f , g gas (kg total carbon) -1 ) and smoldering (EF s , kg gas (g total carbon) -1 ) periods, and the total released mass of each gas/particle during the two periods (M f and M s , in t).

	this wildfire were about 161.4, 1.0, 0.01, 0.05 t, respectively. All the GFAS-derived emissions
	except for BC and N 2 O were much lower than the estimated ones, either M f or M s .
	GFED and FINN inventories captured the wildfire on both 14 th and 15 th . However, the burned
	area in the FINN inventory is largely overestimated, either for fire counted by MODIS (175.5 ha)
	or MODIS+VIIRS (270.6 ha). This leads to larger emissions such as total carbon of 2210.7
	(1455.6) t, CO 2 of 7283.8 (4791.0) t, CO of 493.3 (328.0) t, etc. in FINN with fire counted by
	MODIS+VIIRS (or by MODIS only). Likewise, for the GFED inventory, similar emission levels

a : obtained from Sapeurs-Pompiers Loire-et-Cher; s : smoldering period; f : flaming period. Note that differences between FINN (MODIS) and FINN (MODIS+VIIRS) were possibly caused by the fact that the fire was detected twice on 15 th Sept. in FINN (MODIS+VIIRS), which can be obtained from Table

S2

; b : sum of carbon in the form of CO 2 , CO, CH 4 , and BC; c : using the average of the twice detection.

Acknowledgments

We thank Ministère de l'Agriculture et de l'Alimentation for providing information (including the photo) about the wildfire.

Funding

This study was supported by the VOLTAIRE project (ANR-10-LABX-100-01) funded by the ANR and the PIVOTS project provided by the Region Centre -Val de Loire (ARD 2020 program and CPER 2015 -2020).

Data Availability

All the data presented in this study are available upon request from the corresponding authors.

Author Contribution

Competing Interests

The authors declare no competing financial interest.