Thi Khanh

Ly Nguyen

Sebastien Livi

Bluma G Soares

Sebastien Pruvost

Jannick Duchet-Rumeau

Jean-Francois Geŕard

Ionic liquids: A New Route for the Design of Epoxy Networks

Keywords: Ionic liquids, Epoxy networks, Morphology, Mechanical properties, Reactive additives

A new way to synthesize epoxy networks was reported in this work using phosphonium based ionic liquids combined with phosphinate, carboxylate, and phosphate counteranions. The influence of the chemical nature of the anions was investigated on the polymerization kinetics of epoxy systems as well as the thermal and mechanical properties of epoxy-IL networks. In all cases, ILs displayed a high reactivity toward epoxy prepolymer and led to the formation of poly epoxy networks with high epoxy group conversion, i.e., up to 90%. In addition, epoxy-IL networks have high hydrophobicity and an excellent thermal stability (above 350 °C) under N 2 with the glass transition temperatures (T g ) ranging from 90 to 140 °C depending on the chemical structure of ILs. For the first time, the mechanical properties of epoxy-IL networks were also evaluated in terms of flexural properties and fracture toughness (K Ic ).

■ INTRODUCTION

Nowadays, there is a growing demand to design and develop new high-performance materials that are sustainable and environmentally friendly. Recently, thanks to their unique properties such as excellent thermal stability, good ionic conductivity, low saturated vapor pressure, nonflammability, but also high versatility, ionic liquids are become excellent functional additives to polymer matrixes. 1,2 In fact, ILs are more and more used as processing aids of renewable resources, 3,4 as a solvent of polymerization, 5-8 lubricants, 9-11 or plasticizers [START_REF] Scott | Application of ionic liquids as plasticizers for poly(methyl methacrylate)[END_REF][START_REF] Scott | Application of ionic liquids as low-volatility plasticizers for PMMA[END_REF][START_REF] Rahman | Ionic liquids: New generation stable plasticizers for poly(vinyl chloride)[END_REF] in advanced polymer materials, compatibilizers of thermoplastic and biopolymer blends, [START_REF] Yousfi | Ionic liquids: A new way for the compatibilization of thermoplastic blends[END_REF][START_REF] Livi | Ionic liquids-lignin combination: an innovative way to improve mechanical behaviour and water vapour permeability of eco-designed biodegradable polymer blends[END_REF][START_REF] Lins | Phosphonium ionic liquids as new compatibilizing agents of biopolymer blends composed of poly(butylene-adipate-co-terephtalate)/poly(lactic acid) (PBAT/PLA)[END_REF] building blocks for designing nanostructured polymers, [START_REF] Livi | Ionic liquids: structuration agents in a fluorinated matrix[END_REF][START_REF] Livi | Structuration of ionic liquids in a poly(butylene-adipate-co-terephthalate) matrix: its influence on the water vapour permeability and mechanical properties[END_REF] or as surfactant agents in nanocomposites. [START_REF] Huddleston | Room temperature ionic liquids as novel media for "clean" liquid-liquid extraction[END_REF] Very recently, ionic liquids have been also investigated as reactive additives within epoxy networks to develop new polymer electrolytes, anticorrosive coatings, or for composite applications. [START_REF] Sidorov | The study of ionic liquids influence on curing of epoxide composites[END_REF][START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Mąka | High performance epoxy composites cured with ionic liquids[END_REF] According to the literature, two main ways have been studied to combine ILs and epoxy networks: (i) synthesis and development of new functionalized ILs in order to provide ionic conductivity to the polymer electrolytes based on an epoxy-amine network for energy applications. [START_REF] Matsumoto | Synthesis of ion conductive networked polymers based on an ionic liquid epoxide having a quaternary ammonium salt structure[END_REF][START_REF] Matsumoto | Synthesis of networked polymers with lithium counter cations from a difunctional epoxide containing poly(ethylene glycol) and an epoxide monomer carrying a lithium sulfonate salt moiety[END_REF][START_REF] Matsumoto | Preparation and properties of ionicliquid-containing poly(ethylene glycol)-based networked polymer films having lithium salt structures[END_REF][START_REF] Matsumoto | Design and synthesis of ionicconductive epoxy-based networked polymers[END_REF][START_REF] Mcdanel | Cross-linked ionic resins and gels from epoxide-functionalized imidazolium ionic liquid monomers[END_REF][START_REF] Mcdanel | Effect of monomer structure on curing behavior, CO 2 solubility, and gas permeability of ionic liquid-based epoxy-amine resins and ion-gels[END_REF] Thus, ILs bearing epoxide groups either on the cation or the counteranion were synthesized by Matsumoto et al. [START_REF] Matsumoto | Synthesis of ion conductive networked polymers based on an ionic liquid epoxide having a quaternary ammonium salt structure[END_REF][START_REF] Matsumoto | Synthesis of networked polymers with lithium counter cations from a difunctional epoxide containing poly(ethylene glycol) and an epoxide monomer carrying a lithium sulfonate salt moiety[END_REF][START_REF] Matsumoto | Preparation and properties of ionicliquid-containing poly(ethylene glycol)-based networked polymer films having lithium salt structures[END_REF] to design ionic conductive epoxy-amine networks. [START_REF] Matsumoto | Design and synthesis of ionicconductive epoxy-based networked polymers[END_REF] Other authors synthesized new bis epoxide ILs monomers to prepare polymeric ionic liquids (PILs) and PIL/IL ion-gels based on epoxy-amine networks revealing significant improvements in CO 2 permeability and selectivity. [START_REF] Mcdanel | Cross-linked ionic resins and gels from epoxide-functionalized imidazolium ionic liquid monomers[END_REF][START_REF] Mcdanel | Effect of monomer structure on curing behavior, CO 2 solubility, and gas permeability of ionic liquid-based epoxy-amine resins and ion-gels[END_REF] (ii) ILs can be also combined as additives or comonomers to conventional epoxy prepolymers or epoxy-amine reactive systems. [START_REF] Soares | Synthesis and characterization of epoxy/MCDEA networks modified with imidazolium-based ionic liquids[END_REF][START_REF] Soares | Preparation of epoxy/MCDEA networks modified with ionic liquids[END_REF][START_REF] Matsumoto | Confinement of ionic liquid by networked polymers based on multifunctional epoxy resins[END_REF][START_REF] Sanes | Effect of the addition of room temperature ionic liquid and ZnO nanoparticles on the wear and scratch resistance of epoxy resin[END_REF][START_REF] Saurín | Self-healing of abrasion damage in epoxy resin-ionic liquid nanocomposites[END_REF][START_REF] Kowalczyk | Ionic liquids as convenient latent hardeners of epoxy resins[END_REF][START_REF] Rahmathullah | Room temperature ionic liquids as thermally latent initiators for polymerization of epoxy resins[END_REF][START_REF] Maka | Epoxy resin crosslinked with conventional and deep eutectic ionic liquids[END_REF][START_REF] Maka | Epoxy resin/ionic liquid systems: The influence of imidazolium cation size and anion type on reactivity and thermomechanical properties[END_REF][START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] In fact, the incorporation of a small amount of ionic liquids (few wt %) can lead to increase reaction (curing) kinetics and to modify the resulting morphologies leading to improvements of the properties of the networks such as the thermomechanical behavior, ionic conductivity, and scratch resistance. [START_REF] Soares | Synthesis and characterization of epoxy/MCDEA networks modified with imidazolium-based ionic liquids[END_REF][START_REF] Soares | Preparation of epoxy/MCDEA networks modified with ionic liquids[END_REF][START_REF] Kowalczyk | Ionic liquids as convenient latent hardeners of epoxy resins[END_REF] For example, Sanes et al. have demonstrated an improvement of wear and scratch resistance of epoxy-amine networks by using only 1.5 wt % of imidazolium IL. [START_REF] Sanes | Effect of the addition of room temperature ionic liquid and ZnO nanoparticles on the wear and scratch resistance of epoxy resin[END_REF] On the other hand, Saurin et al. have shown that the use of 9 wt % of IL in an epoxyamine system led to a self-healing effect after abrasion damage under multiple scratch tests. [START_REF] Saurín | Self-healing of abrasion damage in epoxy resin-ionic liquid nanocomposites[END_REF] In recent years, ILs have been also reported as new alternatives to conventional amines used as hardeners (comonomers) of epoxy prepolymer. [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] In 2009, Rhamathullah et al. highlighted that imidazolium salt associated with a dicyanamide counteranion could be used as a latent curing agent. [START_REF] Rahmathullah | Room temperature ionic liquids as thermally latent initiators for polymerization of epoxy resins[END_REF] In addition, various authors have demonstrated that ILs based on imidazolium and pyridinium cations acted as initiators and curing agents of diglycidyl ether of bisphenol A prepolymer (DGEBA) through cationic polymerization to prepare epoxy-IL networks. [START_REF] Soares | Synthesis and characterization of epoxy/MCDEA networks modified with imidazolium-based ionic liquids[END_REF][START_REF] Soares | Preparation of epoxy/MCDEA networks modified with ionic liquids[END_REF][START_REF] Maka | Epoxy resin/ionic liquid systems: The influence of imidazolium cation size and anion type on reactivity and thermomechanical properties[END_REF] However, high temperatures are required for the processing of these networks. More recently, other authors as well as our research group have found that phosphonium ILs combined with different counteranions were also very good candidates to replace amines. [START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF][START_REF] Maka | Epoxy resin/phosphonium ionic liquid/carbon nanofiller systems: Chemorheology and properties[END_REF] In fact, Soares et al., Livi et al., and Nguyen et al. clearly demonstrated that the basic nature of the phosphinate and dicyanamide anions allowed the polymerization of the epoxy prepolymer via anionic polymerization. [START_REF] Mąka | High performance epoxy composites cured with ionic liquids[END_REF][START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF][START_REF] Maka | Epoxy resin/phosphonium ionic liquid/carbon nanofiller systems: Chemorheology and properties[END_REF] In addition, they have demonstrated that these new epoxy-IL networks have excellent thermomechanical properties, including thermal stability under nitrogen (>400 °C) and a glass transition temperature between 80 and 140 °C depending on the chemical nature of the anion. These authors have highlighted the occurrence of nanostructuration of the resulting networks due to the limited miscibility between an epoxy prepolymer, i.e., DGEBA, and IL. [START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF] Very recently, Maka et al. have also studied epoxy networks cured with imidazolium and phosphonium ILs combined with dicyanamide anion. [START_REF] Mąka | High performance epoxy composites cured with ionic liquids[END_REF] They have obtained epoxy-IL networks with high transparency (85%) combined with a glass transition temperature of 170 °C.

In this work, different phosphonium ionic liquids combined with phosphinate, carboxylate, and phosphate counteranions have been used as new reactive components of epoxy systems. Thus, the influence of the chemical nature of the anion (phosphinate versus carboxylate) and the cation (phosphonium functionalized with short versus long alkyl chains combined with the same phosphate counteranion) was investigated on the polymerization kinetics of epoxy prepolymer, thermal (ATG, DSC), and thermomechanical properties (DMS) as well as on the morphology and surface properties of epoxy-IL networks. In addition, for the first time to our knowledge, a comparative study on the mechanical behavior (flexural properties and fracture toughness) of these new epoxy networks versus conventional epoxy-amine systems was studied.

■ EXPERIMENTAL SECTION

Materials. Diglycidyl ether of bisphenol A (DGEBA) based epoxy prepolymer (DER 332, Dow Chemical) with epoxide equivalent weight of 175 g•mol -1 was considered. Two conventional diamine comonomers, i.e., an aromatic diamine, 4,4′-methylene bis(3-chloro-2,6-diethylaniline-MCDEA (Lonzacure, AHEW = 95 g•mol -1 ) and an aliphatic diamine, polyoxy-propylene-diamine-Jeffamine D230 (Hunstmann, AHEW = 60 g•mol -1 ) were used as the reference curing agents. Four types of phosphonium ionic liquids were kindly supplied by Cytec, Inc. All the chemical structures are presented in Figure 1. The ionic liquids denoted IL-EPC (trihexyl(tetradecyl)phosphonium 2-ethylhexanoate), IL-EHP (trihexyl(tetradecyl)phosphonium bis(2-ethylhexyl)phosphate), and IL-TMP (trihexyl-(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl) phosphinate) have the same cation combined with carboxylate, phosphate, and phosphinate counteranions, respectively. IL-DEP (tributyl(ethyl)phosphonium diethylphosphate) possesses the same phosphate anion as IL-EHP, but combined with a phosphonium cation, functionalized with shorter alkyl groups.

Preparation of Samples. DGEBA and curing agents (ionic liquid or diamine) were mixed with a suitable ratio under stirring at room temperature for 30 min. The mixture was then degassed in an ultrasonic bath for 15 min, was poured into silicone molds, and was cured under different conditions to confirm the formation of epoxyionic liquid networks. The ratio between DGEBA and hardeners as well as the curing procedure are given in Table 1.

The amount of ionic liquids depending on the chemical structure of ILs was chosen based on previous studies. [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] In fact, our previous works on the use of ionic liquids combined with phosphinate and phosphate counteranions as an initiator of the epoxy polymerization allowed us to identify the optimal concentrations, i.e., 10 phr (phosphinate) and 30 phr (phosphate), to obtain solid samples with better properties and without exudation phenomenon. [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] In addition, 34 (a/e = 1) 2 h@80 + 3 h@125 + 1 h@200 IL-TMP 10 (a/e = 0.03) b 2 h@80 + 3 h@120 + 1 h@200 IL-EPC 10 (a/e = 0.02) 2 h@80 + 3 h@120 + 1 h@200 IL-EHP 30 (a/e = 0.06) 2 h@80 + 3 h@180 + 3 h@200 IL-DEP 30 (a/e = 0.14) 2 h@80 + 3 h@160 + 3 h@200 a a/e is the amino hydrogen to epoxy ratio. b In the case of IL, a/e is the IL anion to epoxy ratio considering that each IL anion reacts to one epoxy group.

knowing that IL-EPC and IL-EHP have the same reactivity as ILs denoted IL-TMP and IL-DEP respectively, the same amount of ILs was used. The curing protocol was also tuned for each epoxy-ILs system in order to complete the curing reaction and obtain the fully cured networks. Characterization of Epoxy-IL Mixtures and Networks. Differential scanning calorimetry measurements (DSC) of epoxy-IL reactive mixtures and resulting networks were carried out by using Q20 (TA Instruments) from 20 to 250 °C at a rate of 10 K•min -1 under nitrogen flow of 50 mL•min -1 . The weight of samples was between 9 and 10 mg in all cases.

Fourier transform infrared absorption spectra (FTIR) were recorded using a Thermo Scientific Nicolet iS10 spectrometer in a transmission mode (32 scans, resolution 4 cm -1 ) from 4000 to 500 cm -1 to follow the curing reaction. The conversion of epoxide group was calculated from the area of absorption peaks at 1184 cm -1 (corresponding to the C-O bonds) and 914 cm -1 (epoxy function) at a given reaction time, t, using the following equation:

22 χ = - × A A A % ( ) 100% t o o (1)
where A o and A t are the A 914 /A 1184 area ratio for the reactive system at t = 0 and at a reaction time t, respectively.

Surface energy of epoxy networks was determined from the sessile drop method using a Dataphysics goniometer. Water and diiodomethane were used as probe liquids for contact angle measurements on compacted MCDEA pellets and resulting epoxy networks. Nondispersive (polar and H-bond) and dispersive components of surface energy were determined according to the Owens-Wendt theory. [START_REF] Owens | Estimation of the surface free energy of polymers[END_REF] Surface tensions of phosphonium ionic liquids (denoted IL-TMP, IL-EPC, IL-DEP, and IL-EHP) and Jeffamine D230 were measured from a pendant drop method. [START_REF] Stauffer | The measurement of surface tension by the pendant drop technique[END_REF] Thermogravimetric analyses (TGA) of resulting epoxy networks were performed using a Q500 thermogravimetric analyzer (TA Instruments) from 30 to 600 °C for a heating rate of 20 K•min -1 under nitrogen atmosphere.

Dynamic mechanical spectroscopy (DMS) was carried out on a Rheometrics Solid Analyzer RSAII at 0.01% tensile strain and a frequency of 1 Hz. The heating rate was 3 K•min -1 for a temperature range from 30 to 200 °C.

Flexural Tests. Three points bending tests were conducted under ISO 178:2001(F) standard with the samples geometries of 75 × 10 × 4 mm. Five samples for each system were analyzed using a MTS tensile instrument with a 1 kN load cell and a fixed span length of 60 mm at a speed of 2 mm•min -1 at room temperature. Flexural strength at break was calculated using the following equation:

= FL bh flexural strength 3 2 2 (2)
where F (N) is the load at break, L (m) the span length, and b (m) and h (m) the width and the thickness of specimens, respectively. The flexural modulus was calculated from the force deflection curve using the following equation:

= E L bh F d 4 f 3 3 (3)
where F (N) and d (m) is the load and the deflection at break, respectively. (L 3 /(4bh 3 )) is the correction factor for the slope of the load-deflection curve.

Fracture toughness (K Ic ) of epoxy-IL samples was determined using compact tension specimen figuration according to ISO 13586 Standard (Figure S1, Supporting Information). The initial portion of V notch was machined with a milling cutter and the starter crack was introduced at the root of the notch by tapping a fine razor blade. The ratio of crack length to width (a/w) was to be maintained between 0.2 and 0.8. The K Ic specimens were then tested in tension mode using a MTS tensile instrument with a 1 kN load cell at a speed of 10 mm• min -1 . Then fracture toughness was calculated using the following equation:

= K f a w F h w ( / ) Ic Q (4)
where F Q (N) is the load at the initiation of crack, a (m) is the crack length, w (m) is the width of the specimen, h (m) is the thickness of the specimen, and f(a/w) is the geometry constant (eq 5):

= + - + - + - f a w a a a a a a ( / ) (2 )
(1 ) (0.886 4.64 13.32 14.72

5.6 ) 3/2 2 3 4 (5)
Transmission electron microscopy (TEM) was carried out at the Technical Center of Microstructures (University of Lyon) using a Phillips CM 120 microscope operating at 80 kV to characterize the structuration of ILs in the epoxy networks. The 60 nm-thick ultrathin sections of samples were obtained using ultramicrotome equipped with a diamond knife and were then set on copper grids. The size of ionic domains was estimated using ImageJ Software (U.S National Institutes of Health).

Scanning electron microscopy (SEM) on a Phillips XL20 microscope with a tension of acceleration of 25 kV was used to characterize the fracture surface of K Ic specimens. The samples were cleaned using ethanol [START_REF] Yang | Preparation and mechanical properties of modified epoxy resins with flexible diamines[END_REF] and then coated with gold.

■ RESULTS AND DISCUSSION

Reactivity of Epoxy-Amine and Epoxy-IL Systems. The reaction kinetics of the epoxy-curing agents reactive systems were investigated using DSC from the reaction enthalpy. Thus, the position of exothermic enthalpy peak is illustrated in Figure 2.

Concerning epoxy combined with conventional diamines, exothermic peaks occurred at temperatures of 140 °C (with Jeffamine D230) and 269 °C (with MCDEA). This difference is explained by the aromatic nature of the MCDEA, which requires higher curing temperatures compared to aliphatic Jeffamine D230. [START_REF] De Nograro | Effects of chemical structure of hardener on curing evolution and on the dynamic mechanical behavior of epoxy resins[END_REF] In all cases, phosphonium ILs combined with phosphinate, carboxylate, and phosphate counteranions induce reaction enthalpy highlighting the ability of ILs to act as initiators and hardeners of DGEBA epoxy prepolymer. Thus, 10 phr of IL-EPC (carboxylate) and IL-TMP (phosphinate) led to exothermic peaks at 100 and 140 °C, respectively. This result confirms our previous work on the use of IL-TMP as a hardener but demonstrates a better reactivity of IL-EPC. [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] In contrast, ILs combined with phosphate counteranions denoted IL-DEP and IL-EHP led to higher temperatures, i.e., close to 285 and 344 °C, confirming a lower reactivity of these anions. These observations were reported by Nguyen et al. between IL-DEP and IL-TMP. [START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] Indeed, they have demonstrated that the reactivity of the ionic liquids is controlled by their basicity. [START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] Thus, ILs may be ranked according to their basicity in the following order: IL-EPC > IL-TMP > IL-DEP > IL-EHP, which corresponds to their reactivity. Other authors have shown that the Lewis base such as imidazoles initiates a self-perpetuating anionic polymerization verified by NMR that the imidazole participates in the formation of the network by covalent bonds, confirming the role of imidazole as a curing agent. [START_REF] Wu | Influence of post-curing and temperature effects on bulk density, glass transition and stress-strain behaviour of imidazolecured epoxy network[END_REF][START_REF] Barton | Preparation and characterization of imidazole?metal complexes and evaluation of cured epoxy networks[END_REF] These results confirm the mechanisms that we have proposed in previous works where the phosphinate, phosphate, and dicyanamide counteranions bind covalently to the network and initiate the polymerization. [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] However, compared to epoxy-amine networks where a stoichiometric composition is required to form and to obtain the optimum properties, the use of a small amount of ionic liquids is necessary to prepare epoxy-IL networks.

In conclusion, ILs are excellent alternatives to conventional amines as lower amounts are required for reaction: 10 phr for IL-TMP and IL-EPC, 30 phr for IL-DEP and IL-EHP compared to 34 phr for Jeffamine D230 and 54 phr for MCDEA. In addition, the reactivity of the phosphonium ILs plays a key role on the kinetics of polymerization, i.e., on the formation of the epoxy network as well as on its final properties.

FTIR Analysis. As the Jeffamine D230 and MCDEA amines have been widely studied as epoxy hardeners in the literature, [START_REF] Hong | DSC and FTIR analysis of the curing behaviors of epoxy/DICY/solvent open systems[END_REF][START_REF] Gonzaĺez | Applications of FTIR on epoxy resins-identification, monitoring the curing process, phase separation and water uptake[END_REF][START_REF] Zhao | Curing kinetics, mechanism and chemorheological behavior of methanol etherified amino/novolac epoxy systems[END_REF] only the influence of ILs discussed in this Section. To highlight the influence of the chemical nature of the ionic liquid and to investigate precisely the reaction paths, FTIR analysis was used to follow the reaction between epoxy prepolymer (DGEBA) and ILs. Thus, FTIR spectra of each system were recorded at different reaction times during curing. The epoxy group conversion of different epoxy-IL systems calculated according to eq 1 is presented in Figure 3 as a function of curing time.

As expected, FTIR spectra of epoxy-IL mixtures during the reaction show a decrease of absorption peak at 914 cm -1 corresponding to epoxide group justifying the opening of epoxide ring during curing process. [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF][START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] At the end of each curing protocol, this absorption peak was nearly disappeared suggesting a total conversion. In fact, a constant increase in the conversion of the epoxy groups is observed in Figure 3 to achieve at 6 h a conversion of about 90% for all the epoxy-IL blends. However, for the same curing time (1 h at 80 °C), epoxy systems based on IL-EPC and IL-TMP (10 phr) have exhibited a higher conversion of epoxy groups (40% and 50%) and are in the solid state compared to epoxy networks with 30 phr of IL-DEP and IL-EHP (30% and 10%). In addition, the curing conditions of epoxy-IL networks are different depending of the chemical nature of the ILs: 3 h at 180 and 160 °C as well as a postcuring at 200 °C during 3 h are required for IL-EHP and IL-DEP, respectively. In contrast, 3 h at 120 °C and 1 h at 200 °C are necessary for IL-TMP and IL-EPC. These results still show the best reactivity of epoxy systems containing IL-TMP and IL-EPC as all the reactive systems were cured at 80 °C for the two first hours (confirming the DSC analysis). This analysis also demonstrates that after curing schedule completion, the epoxy conversion is higher than 90%.

Morphology of the Resulting Epoxy Networks. To reveal a possible structuration of ILs in epoxy networks, transmission electronic microscopy (TEM) is the suitable tool. TEM micrographs of epoxy cured with IL-TMP, IL-EPC, IL-DEP, and IL-EHP are presented in Figure 4.

In the case of epoxy systems containing IL-TMP (10 phr), IL-DEP (30 phr), and IL-EHP (30 phr), no phase separation is observed, which could be attributed to (i) good miscibility between the ionic liquid and the epoxy prepolymer along the reaction as well as (ii) a slight difference in electronic densities between phases which prevents their identification. [START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF][START_REF] Nguyen | Ionic liquids as reactive additives for the preparation and modification of epoxy networks[END_REF] Nevertheless, in contrast, IL-EPC led to a (nano)phase separation characterized by the appearance of small IL inclusions due to the limited miscibility between the phosphonium salt and the growing epoxy polymer during curing (Figure 4,left). This phenomenon is commonly encountered in the modified reactive systems (thermosets) using elastomers, thermoplastics, or block copolymers. [START_REF] Pascault | Epoxy Polymers: New Materials and Innovations[END_REF][START_REF] Hillmyer | Self-assembly and polymerization of epoxy resin-amphiphilic block copolymer nanocomposites[END_REF][START_REF] Lipic | Nanostructured thermosets from self-assembled amphiphilic block copolymer/epoxy resin mixtures[END_REF][START_REF] Meng | Nanostructured thermosetting Blends of Epoxy Resin and Amphiphilic Poly(ε-caprolactone)-block-polybutadiene-block-poly(ε-caprolactone) triblock copolymer[END_REF][START_REF] Meng | Formation of ordered nanostructures in epoxy thermosets: A mechanism of reaction-induced microphase separation[END_REF] ILs have a similar behavior to ionomers that are polymers in which a relatively small number (up to 15%) of the monomer units consist of ionic moieties. [START_REF] Utracki | Multiphase polymers: Blends and ionomers[END_REF][START_REF] Hara | Mechanical Properties of lonomers[END_REF] The morphology of such ionomers consists of small dispersed ionomer particles imbedded in the polymer matrix when the ionomer constitutes the minor phase. It is well-known that the polar ionic groups of the ionomer tend to form aggregates, commonly referred to as multiplets and clusters. Multiplets are considered to consist of a small number of interacting ion pairs; whereas clusters are thought to arise from overlapping regions of restricted mobility surrounding multiplets. [START_REF] Eisenberg | A new multiplet-cluster model for the morphology of random ionomers[END_REF] With a similar behavior to ionomers, ILs open a new route in the preparation of nanostructured thermosets.

Surface Properties of Epoxy Networks. Surface energy of epoxy networks were determined by a sessile drop method. Then, the polar and dispersive components of the epoxy networks were calculated by using the Owens-Wendt method from the contact angles with water (Figure 5), and diiodomethane and are summarized in Table 2.

The incorporation of ILs, whatever their chemical nature (IL-TMP, IL-EPC, IL-EHP, and IL-DEP) as hardeners in the epoxy prepolymer (DGEBA), induces a more significant hydrophobic behavior compared to conventional epoxy-amine networks. These results are explained by the hydrophobic nature of ionic liquids that is characterized by surface energy values from 29 to 33 mJ•m -259,60 (Table S1, Supporting Information). Slight differences were observed depending on the chemical nature of 1 for curing schedules that differ for each reactive system). the ionic liquids. Indeed, among the ionic liquids IL-EPC, IL-EHP, and IL-TMP having the same phosphonium cation, the use of IL-EPC (with COO -group) leads to the formation of a slightly less hydrophobic epoxy network. On the opposite, IL-EHP and IL-TMP generate a reduction in the polar component of epoxy systems due to the presence of long alkyl chains on the phosphinate (1 mJ•m -2 ) and phosphate (0.12 mJ•m -2 ) counteranions. Thus, the chemical nature of the ILs also plays a key role in the hydrophobic behavior of thermosets.

In conclusion, the use of ionic liquids as reactive additives of epoxy matrix could open new perspectives in the development of coatings having a hydrophobic surface.

Thermal Behavior of Epoxy Networks. The influence of the conventional amines and phosphonium ILs on the thermal stability of epoxy networks was investigated by thermogravimetric analysis (TGA). The evolution of the weight loss as a function of the temperature of all the epoxy networks is presented in Figure 6. In all cases, the TGA curves highlight an excellent thermal behavior of the epoxy-amine and epoxy-IL networks, i.e., degradation temperature at maximum of DTGA curve of over 350 °C. In the case of epoxy cured with Jeffamine D230 and 4,4′-methylene-bis 3-chloro-2,6-diethylaniline (MCDEA), thermal degradation temperatures of 375 and 418 °C were obtained. According to the literature, this higher degradation temperature (418 °C) for the epoxy-MCDEA system is explained by the presence of aromatic groups. [START_REF]AminesAdvances in Research and Application[END_REF] Concerning the epoxy networks based on 10 phr of phosphonium ILs, i.e., IL-TMP and IL-EPC, the degradation temperatures determined at the maximum of the DTGA peaks are included between 450 and 460 °C. Although, the use of ionic liquids combined with phosphate counteranions, denoted IL-EHP and IL-DEP (30 phr), leads to lower degradation temperatures of 390 and 370 °C, respectively. The slight improvement of IL-EHP thermal stability of is due to the functionalization of IL by long alkyl chains on the cation but also on the counteranion (Figure 6). Thus, two types of behavior are observed: epoxy cured with IL-EHP and IL-DEP have a similar thermal behavior to an epoxy-Jeffamine network whereas the use of IL-EPC and IL-TMP as hardeners leads to a better thermal stability (+40 °C) compared to an epoxy-MCDEA network. This difference in thermal behavior between the epoxy-IL systems is attributed to their chemical nature but also to the amount of ionic liquid introduced (10 phr for IL-EPC and IL-TMP; 30 phr for IL-DEP and IL-EHP). Indeed, the use of a larger amount of ionic liquids denoted IL-DEP and IL-EHP generates more free ionic liquids in epoxy systems acting as plasticizers. [START_REF] Livi | Nanostructured thermosets from ionic liquid building block-epoxy prepolymer mixtures[END_REF] Thus, this phenomenon may explain the better thermal stability of epoxy-IL networks based on IL-TMP and IL-EPC.

In conclusion, phosphonium ionic liquids are an excellent alternative as curing agents to conventional amines. In fact, the use of these new curing agents leads to very thermally stable epoxy networks and the presence of phosphorus compound could be an asset to the fire properties of these systems. Moreover, lower amounts of ionic liquids are required (10 phr for IL-EPC and IL-TMP and 30 phr for IL-EHP and IL-DEP) compared to stoichiometric ratios of 34 phr and 54 phr for diamines, i.e., Jeffamine D230 and MCDEA.

Dynamic Mechanical Properties of Epoxy Networks. The influence of the chemical nature of the curing agents (diamines versus ILs) on the dynamical mechanical properties of epoxy networks was investigated by DMS. The temperature of α-relaxation related to the glass transition T α from the maximum of tan δ as well as the storage modulus, E′, in the rubbery state (E′ R ) are listed in Table 3 and the DMS spectra are given in Figure S2 of the Supporting Information. In addition, cross-link density, ν e , (mol•m -3 ) of epoxy networks (assuming homogeneous networks) was also estimated using the elasticity theory: [START_REF] Maka | Epoxy resin/phosphonium ionic liquid/carbon nanofiller systems: Chemorheology and properties[END_REF][START_REF] Lemay | Structure and ultimate properties of epoxy resins[END_REF][START_REF] Wan | A comparative study of epoxy resin cured with a linear diamine and a branched polyamine[END_REF]

ν = ′ E RT 3 e R R (5)
where E′ R (Pa) is storage modulus determined in the rubbery state, i.e., at T R = T α + 30 (K) and R is ideal gas constant (R = 8.314 J•mol -1 •K -1 ).

For all epoxy systems, DMS spectra display only one relaxation peak suggesting the homogeneity of epoxy networks. However, in the case of IL-EPC, the phase separation observed by TEM micrographs cannot be evidenced with an additional relaxation peak in DMS spectrum.

The glass transition temperatures (considering T α value) and cross-link densities of the epoxy-amine systems (MCDEA and D230) are in agreement with the literature. [START_REF] Soares | Synthesis and characterization of epoxy/MCDEA networks modified with imidazolium-based ionic liquids[END_REF][START_REF] Shan | Effect of network structure of epoxy DGEBA-poly(oxypropylene)diamines on tensile behavior[END_REF][START_REF] Cook | Dynamic mechanical thermal analysis of thermally stable and thermally reactive network polymers[END_REF][START_REF] Soares | Preparation of epoxy/Jeffamine networks modified with phosphonium based ionic liquids[END_REF] The same ranges of cross-link density (1400-1600 mol•m -3 ) are obtained for the two amines. As expected, MCDEA cured epoxy networks have higher T α values (160 °C) compared to Jeffamine D230 cured epoxy networks (100 °C). This difference of T α is mainly due to the more rigid and more hindered structure of MCDEA based on aromatic rings preventing chain mobility and increasing the glass transition temperature. [START_REF] Garcia | Influence of chemical structure of hardener on mechanical and adhesive properties of epoxy polymers[END_REF] In the cases of epoxy-IL networks, two different behaviors have been observed. In fact, epoxy networks cured by phosphate ILs (IL-DEP and IL-EHP) display almost the same glass transition region as Jeffamine D230, i.e., 100-110 °C, whereas epoxy networks cured by IL-TMP and IL-EPC are close to epoxy-MCDEA networks with T α values above 140 °C. Moreover, the cross-link density of epoxy networks filled with IL-EPC (11 121 mol•m -3 ) and IL-TMP (9412 mol•m -3 ) are significantly higher compared to epoxy-MCDEA networks and epoxy networks based on phosphate-ILs. In the case of phosphate counteranions based ILs, i.e., IL-EHP and IL-DEP, the long alkyl chains of IL-EHP increase the flexibility of epoxy-IL networks leading to lower T α values compared to IL-DEP based epoxy network one.

Overall, the chemical nature of the ionic liquid controls the formation and the final properties of epoxy networks. Thus, IL-EPC and IL-TMP having the high value of T α is attributed to the very high cross-linking densities. [START_REF] Barton | Preparation and characterization of imidazole?metal complexes and evaluation of cured epoxy networks[END_REF] Mechanical Behavior of Epoxy Networks. Mechanical properties such as the flexural properties and the fracture toughness, K Ic , of epoxy networks were investigated. As the mechanical tests require thick samples, the low reactivity of IL-DEP did not allow to process the samples. As a consequence, only the epoxy-amine and epoxy-IL networks containing IL-TMP, IL-EPC, and IL-DEP were studied.

Fracture Toughness (K Ic ). The values of the critical stress intensity factor K Ic were measured for epoxy-amine and epoxy-IL networks (Table 4).

In the case of epoxy networks cured with Jeffamine D230 and MCDEA, the values of the fracture toughness are similar to the ones reported in the literature. In fact, many authors reported K Ic values of about of 0.6 and 1.24 MPa•m 1/2 for epoxy-MCDEA and epoxy-Jeffamine networks, respectively. [START_REF] Plangsangmas | Determination of fracture toughness of epoxy using fractography[END_REF][START_REF] Pluart | Epoxy/montmorillonite nanocomposites: influence of organophilic treatment on reactivity, morphology and fracture properties[END_REF][START_REF] Girard-Reydet | Use of block copolymers to control the morphologies and properties of thermoplastic/thermoset blends[END_REF][START_REF] Rebizant | Chemistry and mechanical properties of epoxy-based thermosets reinforced by reactive and nonreactive SBMX block copolymers[END_REF] This difference in mechanical behavior between the two diamine hardeners provides from their different chemical structure. Indeed, the aromatic amine, i.e., MCDEA, that has a rigid structure and a high steric hindrance leads to the formation of a more brittle epoxy network whereas the aliphatic amine, i.e., Jeffamine D230, composed of long ether chains induces a more flexible epoxy system confirming its higher value of the fracture toughness.

The fracture toughnesses of the epoxy-IL networks containing IL-EPC and IL-TMP are in the same range whereas the epoxy system based on IL-DEP has a higher K Ic value of 0.64 MPa•m 1/2 . This difference in fracture toughness between epoxy-IL networks is related to their cross-linking densities. In fact, the epoxy-IL networks having the highest cross-linking densities (IL-EPC and IL-TMP) are more brittle than the epoxy-MCDEA network. In fact, Chang and Brittain have shown a general trend in which an increase in the cross-link density has led to a decrease in fracture toughness. [START_REF] Chang | Studies of epoxy resin systems: Part D: Fracture toughness of an epoxy resin: A study of the effect of crosslinking and sub-Tg aging[END_REF] Other authors have also reported the same phenomenon on epoxyamine networks in the literature. [START_REF] Won | Internal antiplasticization in copolymer and terpolyrner networks based on diepoxides, diamines and monoamines[END_REF][START_REF] Amdouni | Epoxy networks based on Dicyandiamide: Effect of the cure cycle on viscoelastic and mechanical properties[END_REF][START_REF] Levita | Crosslink density and fracture toughness of epoxy resins[END_REF][START_REF] Van Der Sanden | Deformation and toughness of polymeric systems: 3. Influence of crosslink density[END_REF][START_REF] Min | The dependence of fracture properties on cure temperature in a DGEBA/DDS epoxy system[END_REF][START_REF] Urbaczewski-Espuche | Influence of chain flexibility and crosslink density on mechanical properties of epoxy/ amine networks[END_REF] However, the relationship between the cross-link density of a network and the fracture toughness remains ambiguous knowing as several other parameters are also involved such as chain flexibility, free volume, and intermolecular packing.

SEM micrographs of fractured surface after crack initiation and catastrophic failure are presented in Figure 7.

Two types of fracture behaviors are evidenced depending on the nature of curing agents used. Indeed, the fracture surfaces of brittle networks, i.e., IL-EPC, IL-TMP, and MCDEA, are very smooth, which can be denoted as mirror-like fracture surfaces (Figures 7a,b,c). In contrast, epoxy cured with Jeffamine D230 and also IL-DEP show a very rough fracture surface with ridges, river marks, and tortuous cracks (Figure 7e,f). According to the literature, such features are the signature a higher energy for the crack propagation, i.e., a higher fracture toughness. [START_REF] Yang | Preparation and mechanical properties of modified epoxy resins with flexible diamines[END_REF][START_REF] Nohales | Morphology, flexural, and thermal properties of sepiolite modified epoxy resins with different curing agents[END_REF] Flexural Properties. The flexural modulus and the flexural strength of epoxy-amine and epoxy-IL networks are summarized in Table 3. The load-deflection traces are also given in Figure S3 of the Supporting Information.

Overall, the obtained values of the flexural modulus and flexural strength for the epoxy cured with Jeffamine D230 and MCDEA are close to the values reported in the literature. [START_REF] Yang | Preparation and mechanical properties of modified epoxy resins with flexible diamines[END_REF][START_REF] Nohales | Morphology, flexural, and thermal properties of sepiolite modified epoxy resins with different curing agents[END_REF][START_REF] Giannotti | Morphology and fracture properties relationship of epoxy-diamine systems simultaneously modified with polysulfone and poly(ether imide)[END_REF][START_REF] Qi | Investigation of the mechanical properties of DGEBA-based epoxy resin with nanoclay additives[END_REF][START_REF] Garcia | Mechanical properties of epoxy networks based on DGEBA and aliphatic amines[END_REF][START_REF] Amaral | Impact of aliphatic amine comonomers on DGEBA epoxy network properties[END_REF] Thus, the addition of the flexible amine (Jeffamine D230) leads to a ductile behavior with a high flexural modulus of 2.7 GPa whereas the use of the aromatic amine (MCDEA) displays a brittle behavior with a lower flexural modulus (2.30 GPa).

Then, the addition of ILs as reactive additives leads to the formation of stiff epoxy networks. Indeed, a similar behavior to the conventional system using MCDEA is obtained for epoxy cured with IL-EPC (2.4 GPa) and IL-DEP (2.2 GPa). Only the network containing IL-TMP displays a very different behavior with a lower value of flexural modulus (1.6 GPa).

In conclusion, these first results on the mechanical performances such as the flexural properties and the fracture toughness of epoxy-IL networks have shown similar results to an epoxy prepolymer cured with MCDEA. Thus, it becomes possible to replace the conventional diamine hardeners with ionic liquids. According to the numerous cation/anion combinations, this new route for the preparation of epoxy networks looks very promising.

■ CONCLUSION

In conclusion, different phosphonium ionic liquids were studied as new curing agents for epoxy prepolymer and compared to epoxy cured with aliphatic or aromatic diamines (Jeffamine D230, MCDEA). Thus, we demonstrated that a small amount of ILs (from 10 to 30 phr) could lead to the formation of epoxy-IL networks with improved final properties. In fact, the use of ionic liquids as reactive additive leads to poly epoxy networks with excellent thermal properties (>350 °C), glass transition temperatures between 90 and 140 °C as well as highly hydrophobic networks. Furthermore, in terms of mechanical properties, the values of the fracture toughness and flexural moduli are similar to epoxy-MCDEA networks. In summary, ILs open a new route for the design of new epoxy networks. Nevertheless, further studies are required to demonstrate the real relevancy of ionic liquids as comonomers in thermosetting materials.

■ ASSOCIATED CONTENT

* S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00953.

Configuration for fracture toughness specimens, DMS spectra (storage modulus and tan δ) and force-deflection curves of epoxy networks, and table of materials surface tension (DGEBA, amines, and ionic liquids) (PDF).

■ AUTHOR INFORMATION

Corresponding Author *S. Livi. E-mail: sebastien.livi@insa-lyon.fr.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support from Ministere de l'Enseignement Supeŕieur et de la Recherche (MESR).

■ REFERENCES

Figure 1 .

 1 Figure 1. Chemical structures of DGEBA, Jeffamine D230, MCDEA, and phosphonium ionic liquids.

Figure 2 .

 2 Figure 2. Reaction enthalpy peak temperature (DSC, 10 K•min -1 ) of different epoxy reactive systems.

Figure 3 .

 3 Figure 3. Epoxide group conversion as a function of curing time calculated from FTIR spectra of different epoxy-ILs systems during cure (see Table1for curing schedules that differ for each reactive system).

Figure 4 .

 4 Figure 4. TEM micrographs of IL-EPC (left), IL-TMP (middle), and IL-DEP (right) networks.

Figure 5 .

 5 Figure 5. Contact angle with water on surfaces of epoxy networks cured with different types of ILs and different IL amounts.

Figure 6 .

 6 Figure 6. Weight loss as a function of temperature (TGA, DTGA) of epoxy networks: MCDEA ( ■ ), Jeffamine D230 (○), IL-TMP (Δ), IL-EPC ( ⧫ ), IL-DEP (☆), and IL-EHP (◀) (heating ramp 20 K•min -1 , N 2 atmosphere).

  Flexural modulus and flexural strength from flexural tests.

Figure 7 .

 7 Figure 7. Fractured surfaces of epoxy networks cured with IL-EPC (a), IL-TMP (b), and MCDEA (c) at magnification of ×500 (up) and MCDEA (d), IL-DEP (e) and D230 (f) at magnification of ×50 (down).

( 1 )

 1 Lu, J.; Yan, F.; Texter, J. Advanced applications of ionic liquids in polymer science. Prog. Polym. Sci. 2009, 34 (5), 431-448.(2) Livi, S.; Duchet-Rumeau, J.; Geŕard, J.-F.; Pham, T. N. Polymers and ionic liquids: A successful wedding. Macromol. Chem. Phys. 2015, 216 (4), 359-368. (3) Wang, H.; Gurau, G.; Rogers, R. D. Ionic liquid processing of cellulose. Chem. Soc. Rev. 2012, 41 (4), 1519. (4) Isik, M.; Sardon, H.; Mecerreyes, D. Ionic liquids and cellulose: Dissolution, chemical modification and preparation of new cellulosic materials. Int. J. Mol. Sci. 2014, 15 (7), 11922-11940. (5) Vygodskii, Y. S.; Lozinskaya, E. I.; Shaplov, A. S.; Lyssenko, K. A.; Antipin, M. Y.; Urman, Y. G. Implementation of ionic liquids as activating media for polycondensation processes. Polymer 2004, 45 (15), 5031-5045. (6) Chiappe, C.; Pieraccini, D. Ionic liquids: solvent properties and organic reactivity. J. Phys. Org. Chem. 2005, 18 (4), 275-297. (7) Kubisa, P. Ionic liquids as solvents for polymerization processesProgress and challenges. Prog. Polym. Sci. 2009, 34 (12), 1333-1347. (8) Chesnokov, S. A.; Zakharina, M. Y.; Shaplov, A. S.; Lozinskaya, E. I.; Malyshkina, I. A.; Abakumov, G. A.; Vidal, F.; Vygodskii, Y. S. Photopolymerization of poly(ethylene glycol) dimethacrylates: The influence of ionic liquids on the formulation and the properties of the resultant polymer materials. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (11), 2388-2409. (9) Sanes, J.; Carrioń, F. J.; Bermudez, M. D.; Martínez-Nicolaś, G. Ionic liquids as lubricants of polystyrene and polyamide 6-steel contacts. Preparation and properties of new polymer-ionic liquid dispersions. Tribol. Lett. 2006, 21 (2), 121-133.

Table 1 .

 1 Curing Conditions of Epoxy-Amine and Epoxy-IL Systems

	epoxy	curing agent	concentration (phr)	curing protocol
	DGEBA	MCDEA	54 (a/e = 1) a	1 h@150 + 2 h@180 + 1 h@200
		D230

Table 2 .

 2 Contact Angles and Surface Energy of Epoxy Networks Measured by Sessile Drop Method

	MCDEA	81	50	4.5	29.8	34.3
	D230	72	50	9.2	27.5	36.7
	IL-TMP	91	44	1.0	37.3	38.3
	IL-EPC	89	62	3.0	24.4	27.4
	IL-DEP	102	73	1.1	21.2	22.3
	IL-EHP	103	69	0.3	23.1	23.4

curing agent

Θ water (deg)

Θ CH 2 I 2 (deg) γ nondispersive (mJ•m -2 ) γ dispersive (mJ•m -2 ) γ total (mJ•m -2 )

Table 3 .

 3 Thermomechanical Behavior of Epoxy Networks Cured by Different Curing Agents Relaxation Temperature from DMS at 1 Hz. b Rubbery state storage modulus from DMS, determined at T α + 30 (K) c Cross-link density calculated from DMS

	curing agent	T α a (°C)	E′ R b (MPa)	ν e c (mol•m -3 )
	MCDEA	160	16	1385
	D230	100	16	1591
	IL-TMP	140	104	9412
	IL-EPC	144	124	11 121
	IL-DEP	117	16	1527
	IL-EHP	98	14	1399

a α:

Table 4 .

 4 Mechanical Behavior (Determined at Room Temperature) of Epoxy Networks Cured by Different Curing Agents

	curing agent	E f a (GPa)	σ R b (MPa)	K Ic (MPa•m 1/2 )
	MCDEA	2.30	77.1	0.56
	D230	2.71	81.5	1.05
	IL-TMP	1.61	24.6	0.39
	IL-EPC	2.39	49.5	0.37
	IL-DEP	2.22	68.8	0.64
	IL-EHP


Xinke Wang

Dandan Li

Pierre-Marie Flaud

Haiyan Li

Sebastien Perrier

Eric Villenave

Sebastien Dusanter

Alexandre Tomas

Emilie Perraudin
email: emilie.perraudin@u-bordeaux.fr

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

Matthieu Riva
email: matthieu.riva@ircelyon.univ-lyon1.fr

Atmospheric nitrous acid measurement in the French Landes forest

Keywords: nitrous acid, Landes forest, atmospheric photochemistry, micrometeorological process, decoupled layer, soil emission

Nitrous acid (HONO) is a major hydroxyl radical (OH) source, which plays a central role in atmospheric chemistry. In this study, continuous measurements of HONO and other related parameters were conducted from 5 th to 25 th July 2018 in the French Landes forest. The mean daytime and nighttime HONO concentrations were 60 ppt and 139 ppt, respectively, whilst the mean daily OH production rate from HONO photolysis below the forest canopy was estimated to be 7.6 × 10 4 molecules cm -3 s -1 , much less than those from ozone (O3) photolysis in the presence of water vapor (3.1 × 10 5 molecules cm -3 s -1 ) and alkene ozonolysis (4.4 × 10 6 molecules cm -3 s -1 ).

Interestingly, the HONO levels were daily peaking between 18:00 to 24:00, that cannot be explained by the known chemical formation pathways. Analysis of collocated measurements suggested that the accumulation of HONO during the evening might be explained by micrometeorological processes. Hence creation of a stable decoupled layer retaining the HONO emission from the soil was associated with the formation of a convective decoupled layer.

Increasing relative humidity within this period enhanced HONO gaseous deposition leading to its fast depletion.

Introduction

Nitrous acid (HONO) plays an important role in atmospheric photochemistry due to its fast photolysis yielding hydroxyl radicals (OH) and nitric oxide (NO), both of which contribute to the ground-level ozone (O3) and aerosol production. [START_REF] Alicke | Impact of Nitrous Acid Photolysis on the Total Hydroxyl Radical Budget during the Limitation of Oxidant Production/Pianura Padana Produzione Di Ozono Study in Milan[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] Harris | Observations of Nitrous Acid in the Los Angeles Atmosphere and Implications for Predictions of Ozone-Precursor Relationships[END_REF][START_REF] Xue | Oxidative Capacity and Radical Chemistry in the Polluted Atmosphere of Hong Kong and Pearl River Delta Region: Analysis of a Severe Photochemical Smog Episode[END_REF] Many field and modeling studies have shown that HONO photolysis can contribute to 25-56% of OH radical production in the daytime, [START_REF] Elshorbany | Oxidation Capacity of the City Air of Santiago, Chile[END_REF][START_REF] Kleffmann | Daytime Formation of Nitrous Acid: A Major Source of OH Radicals in a Forest[END_REF][START_REF] Michoud | Radical Budget Analysis in a Suburban European Site during the MEGAPOLI Summer Field Campaign[END_REF] even reaching up to 80% in the early morning. [START_REF] Acker | Nitrous Acid in the Urban Area of Rome[END_REF] Despite the significance of HONO to atmospheric chemistry, a molecular-level understanding of its sources remains unclear. [START_REF] Kleffmann | Daytime Formation of Nitrous Acid: A Major Source of OH Radicals in a Forest[END_REF][START_REF] Lee | Detailed Budget Analysis of HONO in Central London Reveals a Missing Daytime Source[END_REF] During the past decades, a large number of ambient HONO measurements have been conducted at various sites including rural, suburban, urban, forest, mountain, ocean and even in polar regions, showing that HONO concentration ranges from several ppt to more than 10 ppb. 6,10- 18 Anthropogenic direct emissions such as vehicle exhaust with emission ratios of HONO/NOx ranging from 0.3% to 1.6% and natural sources including soil emission are regarded as important HONO sources for urban/suburban, and rural/forest regions, respectively. [19][20][21] The study of Su et al. 19 showed that HONO can be released from nitrite degradation present in the soil and might strongly increase globally due to the enhanced fertilizer use and soil acidification. However, soil biological activity has been recently suggested as the most important HONO source from soil emission. 22 This process can release HONO in greater quantities than from the acid-base and Henry's law equilibria of the aqueous phase in soil, but its production is influenced by the soil temperature and water content as a result of their influence on microbial activity.

In addition to primary emissions, the atmospheric secondary reactions including gas phase reaction of NO with OH radicals [START_REF] Lee | Detailed Budget Analysis of HONO in Central London Reveals a Missing Daytime Source[END_REF]23 and heterogeneous reactions of NO2 on various surfaces such as ground, leaf, etc., are also suggested to significantly contribute to HONO formation. [START_REF] Finlayson-Pitts | The Heterogeneous Hydrolysis of NO2 in Laboratory Systems and in Outdoor and Indoor Atmospheres: An Integrated Mechanism[END_REF] In recent years, many laboratory studies have shown that photolysis of nitric acid (HNO3) and nitrate ions (NO3¯) can be an important daytime source of HONO. [24][START_REF] Zhou | Nitric Acid Photolysis on Surfaces in Low-NOx Environments: Significant Atmospheric Implications[END_REF][START_REF] Scharko | Release of Nitrous Acid and Nitrogen Dioxide from Nitrate Photolysis in Acidic Aqueous Solutions[END_REF] In addition, HONO production from aqueous nitrate photolysis can be enhanced by dissolved aliphatic organic matter due to the reaction of O2ˉ with NOx producing nitrite (NO2ˉ). [START_REF] Wang | Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter[END_REF] Moreover, the photosensitized conversion of NO2 on surfaces was also proved to be an efficient HONO formation pathway, [START_REF] George | Photoenhanced Uptake of Gaseous NO2 on Solid Organic Compounds: A Photochemical Source of HONO? Faraday Discuss[END_REF][START_REF] Stemmler | Photosensitized Reduction of Nitrogen Dioxide on Humic Acid as a Source of Nitrous Acid[END_REF] whilst the ozonolysis and photolysis of organic nitrogen substances, such as neonicotinoid nitenpyram ozonolysis [START_REF] Wang | Unexpected Formation of Oxygen-Free Products and Nitrous Acid from the Ozonolysis of the Neonicotinoid Nitenpyram[END_REF] and ortho-nitrophenols [START_REF] Bejan | The Photolysis of Ortho-Nitrophenols: A New Gas Phase Source of HONO[END_REF] and nitrotyrosine photolysis, [START_REF] Meusel | Light-Induced Protein Nitration and Degradation with HONO Emission[END_REF] are also suggested to be responsible for the formation of HONO. These findings are helpful to fill the gaps between the field observations and numerical models. However, the sources and fate of HONO in ambient conditions remain still unclear. [START_REF] Lee | Detailed Budget Analysis of HONO in Central London Reveals a Missing Daytime Source[END_REF] In addition to these direct emission and formation pathways, the local micrometeorological processes can also significantly influence the primary pollutant concentrations and should not be neglected. As an example, it has been recently reported that the formation of a shallow decoupled layer below the canopy, can lead to a significant increase of monoterpenes and monoterpene oxidation products near ground level in the Finnish boreal forest [START_REF] Zha | Vertical Characterization of Highly Oxygenated Molecules (HOMs) below and above a Boreal Forest Canopy[END_REF] and the French Landes forest. [START_REF] Bsaibes | Variability of Hydroxyl Radical (OH) Reactivity in the Landes Maritime Pine Forest: Results from the LANDEX Campaign 2017[END_REF] This work aims to contribute to unravel biological, chemical, physical and micrometeorological processes on HONO sources and sinks in the context of a forested site characterized by high levels of biogenic volatile organic compounds (BVOCs) and weakly influenced by anthropogenic emissions (i.e., low NOx air masses). Measurements of atmospheric HONO as well as meteorological parameters, trace gases, and volatile organic compounds (VOCs) were performed in a maritime pine forest located in the French Landes (southwestern part of France), previously reported as a site highlighting high monoterpene concentrations. [START_REF] Kammer | Observation of Nighttime New Particle Formation over the French Landes Forest[END_REF][START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF] Very surprisingly, a daily high HONO peak ranging from 18:00 to midnight was observed. The contribution of HONO photolysis to the primary OH production was evaluated and compared to those of O3 photolysis and reaction of the O( 1 D) product with water vapor as well as alkene ozonolysis. Finally, a HONO budget including soil emission, chemical formation and loss pathways as well as local micrometeorological processes was estimated and analyzed for the summer of 2018.

Experimental section

Site description. The measurements presented in this study were performed as part of the CERVOLAND (Characterization of Emissions and Reactivity of Volatile Organic Compounds in the Landes forest) campaign and conducted from 5 th to 25 th July 2018. The measurement site was located next to the European Integrated Carbon Observation System (ICOS) station at Bilos in the Landes forest (44 °29 ′ 39.69 ″ N, 0 °57 ′ 21.75 ″ W), which is around 40 km far from the nearest urban area of the Bordeaux metropole and 25 km from the North Atlantic Oceanic. More descriptions about the site were given in previous studies. [START_REF] Kammer | Observation of Nighttime New Particle Formation over the French Landes Forest[END_REF][START_REF] Mermet | Atmospheric Reactivity of Biogenic Volatile Organic Compounds in a Maritime Pine Forest during the LANDEX Episode 1 Field Campaign[END_REF][START_REF] Kammer | Biogenic Volatile Organic Compounds (BVOCs) Reactivity Related to New Particle Formation (NPF) over the Landes Forest[END_REF] Briefly, this field site is mainly covered by maritime pine trees (Pinus pinaster Aiton) and weakly influenced by anthropogenic pollution. It is characterized by very high levels of biogenic VOCs (up to 20 ppb for the total monoterpene concentration) [START_REF] Mermet | Optimization of a Gas Chromatographic Unit for Measuring Biogenic Volatile Organic Compounds in Ambient Air[END_REF][START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF] and the soil is a sandy acidic hydromorphic podzol with a discontinuous layer of iron hard pan at a 75 cm depth. [START_REF] Bsaibes | Variability of Hydroxyl Radical (OH) Reactivity in the Landes Maritime Pine Forest: Results from the LANDEX Campaign 2017[END_REF] Measurement techniques. Atmospheric HONO was measured by a commercial long path absorption photometer instrument (QUMA, Model LOPAP-03). A detailed description of the LOPAP instrument can be found elsewhere. [START_REF] Wang | Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter[END_REF][START_REF] Heland | A New Instrument to Measure Gaseous Nitrous Acid (HONO) in the Atmosphere[END_REF] In brief, the sampling stripping coil (at 4 m above the ground level (a.g.l.)) was kept at a constant temperature of 15℃ and consists of two channels: channel 1 uptakes HONO and a small fraction of interfering species, while the interfering species are quantified by channel 2. [START_REF] Kleffmann | Technical Note: Quantification of Interferences of Wet Chemical HONO LOPAP Measurements under Simulated Polar Conditions[END_REF] The ambient air sampling flow rate and peristaltic pump velocity were set to 1.1 L min -1 and 10 revolutions per minute, with a HONO collection efficiency of 99.9% and an uncertainty of 10%. The LOPAP was calibrated by a known concentration nitrite (NO2¯, Titrisol, 1000 mg L -1 NO2ˉ in water) solution every 4 days, whilst zero measurements were conducted every 8 hours by injecting pure N2. Detection limit of the LOPAP was estimated to be 0.3 ppt (i.e., 2 times noise of zero measurements). Then, HONO concentration was obtained by subtracting the calibrated signal of channel 2 from channel 1 with a time resolution of 30 s.

In addition, the Vocus proton-transfer-reaction mass spectrometry (PTR-MS), with a high time response and sensitivity (detection limit < 1 ppt), was used to measure VOCs during this field campaign. More descriptions about the setup and the results of Vocus PTR-MS were reported elsewhere. [START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF][START_REF] Li | Atmospheric Organic Vapors in Two European Pine Forests Measured by a Vocus PTR-TOF: Insights into Monoterpene and Sesquiterpene Oxidation Processes[END_REF] Additionally, the photolysis frequencies of NO2 and O3 represented by JNO2 and JO3 were quantified by a spectroradiometer (Meteorologie Consult GmbH 6007) in a clearing. The photolysis frequency of HONO (JHONO) was obtained by multiplying JNO2 by the ratio of calculated JHONO and JNO2 based on the NCAR Tropospheric Ultraviolet and Visible (TUV) model. [START_REF] Li | Characteristics and Sources of Nitrous Acid in an Urban Atmosphere of Northern China: Results from 1-Yr Continuous Observations[END_REF][START_REF] Cui | Formation Features of Nitrous Acid in the Offshore Area of the East China Sea[END_REF] NOx and ozone (O3) concentrations were measured at 4 m a.g.l. by chemiluminescence and UV absorption analyzers, respectively, both of which have a detection limit of 0.5 ppb. The meteorological parameters such as temperature (T), relative humidity (RH), wind speed (WS), wind direction as well as CO2 concentrations at 0.46, 1.6, 3.4, 5.7, 8.5, 11.8, and 15.6 m a.g.l. were continuously monitored, while the solar radiation was measured at 15.6 m a.g.l. and measurement data were provided by the European Integrated Carbon Observation System (ICOS) station. All data are reported and discussed in Coordinated Universal Time (UTC) in this study, two hours later than local time.

Parametrization of OH concentration. The steady OH concentration ([OH]ss) could be estimated using Eq. 1, as in the study by Mermet et al.: [START_REF] Mermet | Atmospheric Reactivity of Biogenic Volatile Organic Compounds in a Maritime Pine Forest during the LANDEX Episode 1 Field Campaign[END_REF] [OH] ss = 1 × 10 5 +

Rad t ×K a,t 1×10 3 × [[OH] max -1 × 10 5 ] (Eq. 1)
Notably, 1 × 10 5 molecules cm -3 was retained as a typical value for OH concentration in forest environments at nighttime. [START_REF] Hens | Observation and Modelling of HOx Radicals in a Boreal Forest[END_REF][START_REF] Griffith | HO2 Radical Chemistry during PROPHET 2008 and CABINEX 2009 -Part 1: Measurements and Model Comparison[END_REF][START_REF] Petäjä | Sulfuric Acid and OH Concentrations in a Boreal Forest Site[END_REF][START_REF] Birmili | The Hohenpeissenberg Aerosol Formation Experiment (HAFEX): A Long-Term Study Including Size-Resolved Aerosol, H2SO4, OH, and Monoterpenes Measurements[END_REF][START_REF] Berresheim | Chemical Ionization Mass Spectrometer for Long-Term Measurements of Atmospheric OH and H2SO4[END_REF] The Radt (W m -2 ) was the solar irradiation above the forest canopy and Ka,t was the ratio between the solar irradiance values measured below and above the canopy as previously proposed by Mermet et al., [START_REF] Mermet | Atmospheric Reactivity of Biogenic Volatile Organic Compounds in a Maritime Pine Forest during the LANDEX Episode 1 Field Campaign[END_REF] whilst 1 × 10 3 W m -2 was the maximum of solar irradiation during the field campaign. Besides, 6 × 10 6 molecules cm -3 measured by Birmili et al. [START_REF] Birmili | The Hohenpeissenberg Aerosol Formation Experiment (HAFEX): A Long-Term Study Including Size-Resolved Aerosol, H2SO4, OH, and Monoterpenes Measurements[END_REF] and 3 × 10 6 molecules cm -3 reported by Hens et al. [START_REF] Hens | Observation and Modelling of HOx Radicals in a Boreal Forest[END_REF] were used as the high and lower limits of [OH]max, respectively. [START_REF] Mermet | Atmospheric Reactivity of Biogenic Volatile Organic Compounds in a Maritime Pine Forest during the LANDEX Episode 1 Field Campaign[END_REF] In addition, another empirical formula proposed by Ehhalt & Rohrer (2000) [START_REF] Ehhalt | Concentration and Sources of Atmospheric Nitrous Acid (HONO) at an Urban Site in Western China[END_REF] was widely used to estimate the [OH]ss concentration in the daytime: 15,44,52

[𝑂𝑂𝑂𝑂] 𝑠𝑠𝑠𝑠 = 𝑎𝑎(𝐽𝐽 𝑂𝑂1𝐷𝐷 ) 𝛼𝛼 (𝐽𝐽 𝑁𝑁𝑂𝑂2 ) 𝛽𝛽 𝑏𝑏𝑁𝑁𝑂𝑂 2 +1 𝑐𝑐𝑁𝑁𝑂𝑂 2 2 +𝑑𝑑𝑁𝑁𝑂𝑂 2 +1
(Eq. 2) If alkene ozonolysis dominates the production of OH radicals, notably during nighttime, [OH]ss concentration can be calculated using the following equation: [START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF]53 [𝑂𝑂𝑂𝑂] 𝑠𝑠𝑠𝑠 =

(α =
∑ 𝑘𝑘 (𝑂𝑂3+𝑉𝑉𝑂𝑂𝐶𝐶 𝑖𝑖 ) 𝛼𝛼 𝑖𝑖 [𝑂𝑂 3 ][𝑎𝑎𝑎𝑎𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎 𝑖𝑖 ] ∑ 𝑘𝑘 (𝑂𝑂𝑂𝑂+𝑉𝑉𝑂𝑂𝐶𝐶 𝑖𝑖 ) [𝑎𝑎𝑎𝑎𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎 𝑖𝑖 ] +𝑘𝑘 (𝑂𝑂𝑂𝑂+𝑂𝑂3) [𝑂𝑂 3 ] (Eq. 3)
where k(O3+VOC) is the rate constant for the reaction of O3 with alkenes with an OH yield of α; k(OH+VOC) and k(OH+O3) are the rate constants for OH reacting with alkenes and O3, respectively. The main alkenes are monoterpenes in the Landes forest region, but also consist of a small amount of isoprene and sesquiterpenes (Figures S1-S2). [START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF] In addition, a previous field observation in 2017 summer on the same site showed that the identified species including α-pinene, β-pinene, camphene, sabinene, myrcene, 2-carene, α-phellandrene, Δ 3 -carene, limonene, ocimene, α-terpinene, terpinolene account for ~80% of the total monoterpene concentrations, whilst β-caryophyllene represents ~40% of the total sesquiterpenes and ~90% of total sesquiterpene reactivity. [START_REF] Mermet | Atmospheric Reactivity of Biogenic Volatile Organic Compounds in a Maritime Pine Forest during the LANDEX Episode 1 Field Campaign[END_REF] Therefore, these identified species including their accounted percentages were selected for the calculations Eq.

3, whilst their reaction rate constants with OH radicals and O3 as well as their corresponding OH radical production yields were taken from previous studies. [54][55][56] Notably, OH production produced by the reactions of hydroperoxyl radical (HO2) with NO or O3 was neglected, since NO concentration was below the detection limit of NOx analyzer most of the time, and HO2 concentration was not measured in this study.

RESULTS AND DISCUSSION

Measurements of meteorological conditions and trace gases. ) and the measured HONO, NOx, and O3 concentrations, for which diel variation pattern was consistent during the campaign. [START_REF] Kammer | Observation of Nighttime New Particle Formation over the French Landes Forest[END_REF] Except for some rainy and cloudy days (i.e., 6 th , 21 st , and 22 nd July), the weather remained sunny with strong solar irradiation, indicating favorable conditions for intense photochemical activities. The air masses within the forest canopy were relatively stable, with wind speed < 1 m s -1 , suggesting a major influence of local emissions. The ambient temperature ranged from 11 ℃ to 35 ℃ with a mean value (± standard deviation) of 22 ± 6 ℃ and an average RH of 74 ± 21 %. Figure 1C shows that the average concentration of NO2 was 1.6 ± 0.9 ppb, while the concentration of NO was below the detection limit (i.e., < 0.5 ppb) most of the time, but showing a peak in the morning, with a maximum value of 5.6 ppb at 5:00 on 10 th July. O3 concentration appeared with a mean value of 28 ± 19 ppb, with an average maximum of 49 ppb at 12:00 (Figure 2). The HONO concentration ranged from 4 ppt to 1.1 ppb, with a mean value of 96 ± 107 ppt, which is slightly higher than typical values of 30 to 90 ppt previously observed in forest environments. [START_REF] Acker | Atmospheric Variation of Nitrous Acid at Different Sites in Europe. Environ. Canopy Surface as a Source for Tropospheric Nitrous Acid[END_REF]17,57,58 The diel profiles of meteorological conditions and trace gases are depicted in Figure 2, all of which showing clear diurnal variations. The CO2 concentrations measured at different heights from 0.46 m a.g.l. to 15.6 m a.g.l. was used as a proxy to estimate the decoupled layer height (DLH). [START_REF] Zha | Vertical Characterization of Highly Oxygenated Molecules (HOMs) below and above a Boreal Forest Canopy[END_REF] The temperature, O3 concentration and photolysis frequencies (i.e., JO3, JNO2, and JHONO) followed the pattern of the solar irradiation, while the RH and NO2 concentrations exhibited opposite diel profiles. The mean NO2 concentration started to increase from 0.9 ± 0.14 ppb at 14:00 to 2.3 ± 1.3 ppb at 21:00, and stayed constant during the night. Consistent with the increase of NO2 photolysis rate (Figure 2F) and the expansion of boundary layer height in the early morning, the concentration of NO2 quickly declined. The average HONO concentrations in the daytime (UTC time, 7:00-19:00) and nighttime (UTC time, 20:00-next day 6:00) were 60 ± 32 ppt and 139 ± 72 ppt, respectively. HONO measurements in the Finnish boreal forest also showed higher HONO concentrations at night, 57 while several other forest measurements exhibited the opposed trend due to unknown daytime HONO sources. [START_REF] Acker | Atmospheric Variation of Nitrous Acid at Different Sites in Europe. Environ. Canopy Surface as a Source for Tropospheric Nitrous Acid[END_REF]17,59 Interestingly, the mean HONO concentration quickly rose in the evening from 53 ± 29 ppt at 18:00 to 300 ± 209 ppt at 20:00 (Figure 2D), and the fastest increasing case was on 14 th July, where the HONO concentration increased from 94 ppt to 1.1 ppb within 2 hours (Figure 1D). It is worth pointing out that such a fast rise has not yet been reported from previous HONO measurements. [START_REF] Acker | Atmospheric Variation of Nitrous Acid at Different Sites in Europe. Environ. Canopy Surface as a Source for Tropospheric Nitrous Acid[END_REF]17,57,59 Impact of HONO and O3 photolysis and alkene ozonolysis on the primary OH production.

Firstly, as shown in Figure 3A, the low and high [OH]ss concentrations calculated by Eq.1 are in the ranges of 1-10 × 10 5 molecules cm -3 and 1-20 × 10 5 molecules cm -3 , with the average values of 2.9 × 10 5 molecules cm -3 and 4.8 × 10 5 molecules cm -3 , respectively. The maximum concentration of [OH]ss is shown at 11:00 owing to the strongest solar irradiation within the forest canopy, consistent with the maximum JNO2, JHONO and JO3 (Figures 2E and2F). Notably, all photolysis frequencies below the forest canopy were calculated by the measured ones multiplying Ka,t (i.e., the ratio between the solar irradiance values measured below and above the canopy). Similarly, the

[OH]ss expressed by the NO2 concentrations and photolysis frequencies of O3 and NO2 (i.e., calculated by Eq. 2) also showed the maxima at 11:00, with a concentration of 2.4 × 10 6 molecules cm -3 . It is worth noting that solar irradiation under the forest canopy was weak (Figure 2F) in the time period of 17:00 to the next day 9:00, both of which suggesting that alkene ozonolysis might have been the main source of OH radicals. Therefore, [OH]ss is calculated by means of Eq. 3, considering the competition between OH production and removal processes leading to the [OH]ss formation. Different to the result calculated by Eq. 1, a greater [OH]ss concentration in the range of 4-8 × 10 5 molecules cm -3 was observed during the time period from 17:00 to 24:00, that results from the subsequent monoterpene ozonolysis. As a result, the average [OH]ss concentration (7.4 × 10 5 molecule cm -3 ) calculated by Eqs. 2-3 is higher than those (low average value: 2.9 × 10 5 molecule cm -3 , high average value: 4.8 × 10 5 molecule cm -3 ) by Eq. 1.

It has been widely accepted that HONO photolysis is an important source of OH radicals in the atmosphere, especially in urban and suburban regions. [START_REF] Elshorbany | Oxidation Capacity of the City Air of Santiago, Chile[END_REF]52,60 In addition, O3 photolysis in the presence of water vapor is the most important source of OH radicals globally (reactions 1-3) during daytime. 61

O3 + hv → O2 + O( 1 D) (hv < 320 nm) (1) O( 1 D) + H2O → 2 OH kO(1D)+H2O = 2.2 × 10 -10 cm -3 molecule -1 s -1 at T = 298K (2)
O( 1 D) + M → O( 3 P) + M kO(1D)+M = 2.9 × 10 -11 × [M] s -1 at T = 298K ( 3
)
where M is N2 or O2.

Then we compared the OH production rates from the photolysis of HONO and O3, and alkene ozonolysis. While considered here as the main OH production pathways yet some others such as aldehyde photolysis or RC(O)O2 with HO2 reaction may also contribute. 62 The OH production rates from HONO photolysis (POH (HONO)), O3 photolysis in the presence of water vapor (POH (O 1 D)), and alkene ozonolysis (POH (alkene ozonolysis)) were calculated using equations 4-6, respectively: 52,54,61

P OH (HONO) = J HONO [HONO]
(Eq. 4)

P OH (O 1 D) = 2J�O 1 D�[O 3 ]k O1D+H2O [H 2 O] k O1D+H2O [H 2 O] + k O1D+M [M]
(Eq. 5)

P OH (alkene ozonolysis) = ∑ k O3+alkene i α i [alkene i ][O 3 ]
(Eq. 6)

As depicted in Figure 3B, POH (O 1 D) showed a maximum value of 1.6 × 10 6 molecules cm - 3 s -1 at 11:00 and a daily average value of 3.1 × 10 5 molecules cm -3 s -1 . However, POH (HONO) was much less, showing a maximum value of 3.7 × 10 5 molecules cm -3 s -1 at 11:00 and a mean production rate of 7.6 × 10 4 molecules cm -3 s -1 , respectively. On the contrary, as shown in Figure 3B, alkene ozonolysis contributes significantly to the OH radical formation from 18:00 to 24:00 with a mean production rate of 9.1 × 10 6 molecules cm -3 s -1 , therefore, showing a daily average production rate of 4.4 × 10 6 molecules cm -3 s -1 , higher than POH (O 1 D) and POH (HONO). However, it should be noted that even if alkene ozonolysis contributed significantly to OH formation, [OH]ss still stayed at a relatively low level as a result of an active OH chemistry during this period, confirmed by the formation of alkene-derived oxygenated VOCs. [START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF] All in all, this result shows POH (HONO) was insignificant in this area, only accounting for 1.6% of total daily OH production rate from these different formation pathways, probably due to the low concentration of HONO in the daytime and relatively weak solar irradiation below the forest canopy compared to that above the forest canopy.

HONO budget analysis from 18:00 to 24:00. As discussed above, surprisingly, a daily high but unexplained HONO peak was observed from 18:00 to 24:00. Therefore, to identify the potential sources, the HONO budget was investigated during this period. Because the photolysis frequencies below the forest canopy were negligible (Figures 2E and2F), the photochemical sources (i.e., NO3¯/HNO3 photolysis, photosensitized oxidation of NO2, etc.) and photolysis of HONO were not considered. As a result, the main factors affecting the ambient HONO concentration from 18:00 to 24:00 were summarized by Eq. 7:

𝑑𝑑[𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂] 𝑑𝑑𝑑𝑑 = 𝑃𝑃 ℎ𝑎𝑎𝑑𝑑 + 𝑃𝑃 𝑂𝑂𝐻𝐻+𝑁𝑁𝑂𝑂 + 𝑃𝑃 𝑎𝑎𝑒𝑒𝑒𝑒𝑠𝑠 + 𝑃𝑃 𝑢𝑢𝑎𝑎𝑘𝑘𝑎𝑎𝑢𝑢𝑢𝑢𝑎𝑎 -𝐿𝐿 𝑑𝑑𝑎𝑎𝑑𝑑 -𝐿𝐿 𝑂𝑂𝐻𝐻+𝐻𝐻𝑂𝑂𝑁𝑁𝑂𝑂 (Eq. 7)
where, d[HONO]/dt is the observed change rate of HONO concentration; HONO sources include heterogeneous conversion of NO2 to HONO (Phet), gas-phase reaction of OH and NO (POH+NO), direct emission (Pemis) and unknown sources including some processes influencing HONO concentration (Punknown), while the loss pathways include dry deposition of HONO (Ldep) and reaction between OH and HONO (LOH+HONO).

Figure 4 shows the average rates of POH+NO, Phet, LOH+HONO, Ldep, and the sum of Pemis and Punknown during the period of 18:00-24:00, and the details on their calculations were described in the Supplemental Materials Text S1. First, the LOH+HONO and POH+NO rates were negligible compared to other formation and loss pathways. Both Phet and Ldep rates were influenced by the DLH, which was around 6 m from 18:00 to 24:00, based on the separation of vertical CO2 concentrations (Figure 2A) and temperature inversion (Figure 5). Notably, DLH includes stable decoupled layer height (SDL) and convective decoupled layer height (CDL) in the present study.

Although the adopted value of DLH can impact the rates of Phet and Ldep, their ratio (i.e., Phet/Ldep) remained constant. Pemis was not available in this study, but the sum of Pemis and Punknown were calculated based on the Eq. 7. As shown in Figure 4, the known HONO sources, i.e., gas phase reaction of NO with OH radicals and heterogeneous conversion of NO2 on surfaces, cannot explain the fast increase of HONO (0.7-1.0 × 10 6 molecules cm -3 s -1 ) during the nightfall (Figure 2D).

Interestingly, there is a temperature inversion layer formed at the ground level (~ 6 m) at ~ 17:00 (Figure 5), caused likely by radiation cooling. 63,64 This temperature inversion can yield a stable decoupled layer (SDL) close to the ground and retain the emissions from the ground and the vegetation within this thin layer, [START_REF] Zha | Vertical Characterization of Highly Oxygenated Molecules (HOMs) below and above a Boreal Forest Canopy[END_REF]65 concomitantly to the rapid increases of HONO (Figure 2D), NO2 (Figure 2C) and monoterpene concentrations (Figure S2). However, the NO concentration stayed low from 18:00 to 24:00 due to the rapid consumption by peroxy radicals formed from the terpene ozonolysis and O3, producing organic nitrates (see Figure S3) [START_REF] Li | Terpenes and Their Oxidation Products in the French Landes Forest: Insights from Vocus PTR-TOF Measurements[END_REF] and NO2, 61 respectively.

However, HONO concentration did not increase significantly at 17:00, due to its remaining strong photolysis (Figure 2E). Oswald et al. 22 showed that HONO emissions from soils were greater when soil temperature increases (20-30 ℃), but less as soil water content increases (> 10% of water holding capacity). In this study, the gaseous HONO level showed strong positive and negative correlations with the ambient temperature (Figure S4A) and RH (Figure S5A), respectively. This is suggesting that soil emission can be potentially an important HONO source within the Landes forest. For example, the highest HONO peak detected during the field campaign might be related to the relatively higher temperature and lower RH in the evening on July 14 th (Figure 1). The exchange of NOx and HONO between the various surfaces present (such as vegetation cover) and the atmosphere may be bi-directional, which cannot be distinguished in this study.

Afterwards, a weak convective decoupled layer (CDL) was created at the ground level after 20:00 and remained stable at 22:00 until 6:00 of the next day (Figures 5D and5E). Notably, the correlation between HONO and temperature became weaker (Figure S4B) but HONO concentration still showed a strong negative correlation with ambient RH (Figure S5B), suggesting that increasing RH likely enhanced the deposition loss of gaseous HONO in addition to its negative influence on HONO emission from soil. Therefore, both the created weak CDL at the ground level and the increasing RH (Figure 1B) probably led to the fast depletion of gaseous HONO from 20:00 to 24:00 (Figure 2D). However, the NO2 concentration stayed constant and NO concentration showed an increasing trend during the night, which were probably due to their deposition rate (i.e., NO2, 3-8 × 10 -3 m s -1 ; NO, 1-2 × 10 -3 m s -1 ) 66 lower than HONO (2 × 10 -2 m s -1 ), and less active O3 chemistry. Interestingly, many isoprene-, monoterpene-, and sesquiterpene-derived oxidation products also show a similar high peak during this period (Figure S6 and the study of Li et al.

(2020)). This was caused by high concentrations of terpenes and O3 within the SDL at the ground level during the period from 17:00 to 20:00. After 20:00, the created CDL started to enhance the dry deposition of these oxidation products, whilst O3 concentration decreases fast, both of which caused the fast loss of these oxidation products. During the period from 00:00 to 7:00, the gaseous HONO concentration stayed constant, indicating the sinks and sources of HONO reached an equilibrium.

Notably, the slope fitted by the linear regression for HONO/NO2 ratios against time was generally used as the conversion frequency of NO2-to-HONO (Phet) during the nighttime. [START_REF] Acker | Atmospheric Variation of Nitrous Acid at Different Sites in Europe. Environ. Canopy Surface as a Source for Tropospheric Nitrous Acid[END_REF]15,16,[START_REF] Cui | Formation Features of Nitrous Acid in the Offshore Area of the East China Sea[END_REF]52,60,67,[START_REF] Kleffmann | Measured and Simulated Vertical Profiles of Nitrous Acid -Part I: Field Measurements[END_REF] However, sometimes the creation of a SDL at the ground level can efficiently promote the concentrations of primary pollutants including HONO after sunset, which will overestimate the heterogeneous conversion rate of NO2-to-HONO. Therefore, chemical formation pathways, direct emissions and meteorological processes should all be taken into consideration to better understand the variations of atmospheric species during field observations.

CONCLUSION

In the atmosphere, HONO is present at trace levels, typically up to a few ppb, but plays an important role in the tropospheric chemistry due to its photolysis, producing one of the major oxidants (i.e., OH radicals). [START_REF] Alicke | Impact of Nitrous Acid Photolysis on the Total Hydroxyl Radical Budget during the Limitation of Oxidant Production/Pianura Padana Produzione Di Ozono Study in Milan[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] Harris | Observations of Nitrous Acid in the Los Angeles Atmosphere and Implications for Predictions of Ozone-Precursor Relationships[END_REF][START_REF] Xue | Oxidative Capacity and Radical Chemistry in the Polluted Atmosphere of Hong Kong and Pearl River Delta Region: Analysis of a Severe Photochemical Smog Episode[END_REF] In the present study, HONO as well as meteorological conditions and other measured trace gases showed regular variations during the campaign except for rainy days. The daily mean HONO concentration was 96 ± 107 ppt and exhibited a large increase from 19:00 to 22:00.

Terpene ozonolysis contributed significantly to OH radical formation with an average value of 4.4 × 10 6 molecules cm -3 s -1 , higher than O3 photolysis in the presence of water vapor and HONO photolysis, due to the active alkene ozonolysis chemistry after sunset. However, the daily OH formation rate from HONO photolysis was only around 7.6 × 10 4 molecules cm -3 s -1 as a result of low HONO concentrations and relative weak solar irradiation below the forest canopy.

Interestingly, HONO peaked daily from 18:00-24:00, up to 1 ppb, which was not yet reported for forested environments and could not be explained by the usually reported chemical formation pathways. The creation of an SDL at the ground level retaining HONO produced from the soil within the thick layer leads to a fast HONO increase. Afterwards, both the created CDL and increasing RH enhanced HONO deposition resulting in its fast loss. Overall, the local meteorological conditions are highlighted in this work to be critical parameters to be taken into account as they significantly influence air pollutants at the ground level.

  0.83, β = 0.19, a = 4.1 × 10 9 , b = 140, c = 0.41, and d = 1.7)

  Figure 1 displays an overview of meteorological conditions (measured at 3.4 m a.g.l. except for solar radiation measured at 15.6 m a.g.l.

FrenchFiguresFigure 1 .

 1 Figures

Figure 2 .

 2 Figure 2. Average diel variations of (A-B) meteorological parameters and CO2 concentrations at

Figure 3 .

 3 Figure 3. (A) The average steady-state OH concentration and (B) production rates from photolysis

Figure 4 .

 4 Figure 4. HONO formation and loss rates from different pathways during the measurement period from 5 th to 25 th July, 2018.

Figure 5 .

 5 Figure 5. (A) Diurnal variations of vertical temperature and (B-E) the vertical average temperature

Acknowledgments and Data

The authors would like to thank the PRIMEQUAL programme for financial support (ADEME, convention #1662C0024). This study has also been carried out with financial support from the

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