domain: sdu sde

text:
Dandan Li

Likun Xue
email: xuelikun@sdu.edu.cn

Wen Liang

Xinfeng Wang

Tianshu Chen

Abdelwahid Mellouki
email: mellouki@cnrs-orleans.fr

Jianmin Chen

Wenxing Wang

Characteristics and sources of nitrous acid in an urban atmosphere of northern China: Results from 1-yr continuous observations

Keywords: Nitrous acid, Seasonal variation, Heterogeneous conversion, Atmospheric oxidizing capacity, North China Plain

Nitrous acid (HONO) is a key reservoir of the hydroxyl radical (OH) and plays a central role in the atmospheric chemistry. To understand the sources and impact of HONO in the polluted atmosphere of northern China, continuous measurements of HONO and related parameters were conducted from September 2015 to August 2016 at an urban site in Ji'nan, the capital city of Shandong province. HONO showed well-defined seasonal and diurnal variation patterns with clear wintertime and nighttime concentration peaks. Elevated HONO concentrations (e.g., over 5 ppbv) were frequently observed with a maximum value of 8.36 ppbv. The HONO/NO X ratios of direct vehicle emissions varied in the range of 0.29%-0.87%, with a mean value of 0.53%. An average NO 2 -to-HONO nighttime conversion frequency (k het ) was derived to be 0.0068±0.0045 h -1 from 107 HONO formation cases. A detailed HONO budget analysis suggests an unexplained daytime missing source of 2.95 ppb h -1 in summer, which is about seven times larger than the homogeneous reaction of NO with OH. The effect of HONO on OH production was also quantified. HONO photolysis was the uppermost source of local OH radical throughout the daytime. This study provides the year-round

Introduction

Nitrous acid (HONO) is a key precursor of the hydroxyl radical (OH), one of the main tropospheric oxidants in the gas phase. Numerous field and modeling studies have shown that HONO photolysis contributes significantly to the OH sources not only in the early morning but also during the rest of the daytime (Acker et al., 2006b;[START_REF] Kleffmann | Daytime formation of nitrous acid: A major source of OH radicals in a forest[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]. This is mainly ascribed to the unexpectedly high concentrations of HONO during daytime which would have been kept at lower levels due to its rapid photolysis (R1).

Therefore, the knowledge of characteristics and sources of HONO is critical for a better understanding of the tropospheric oxidation chemistry processes. HONO + hυ → OH + NO (320 nm < λ < 400 nm) (R1)

So far, field observations of HONO have been carried out at remote, rural and urban areas. The reported ambient concentrations rang from several pptv up to 15 ppbv (e.g., [START_REF] Beine | Surprisingly small HONO emissions from snow surfaces at Browning Pass, Antarctica[END_REF][START_REF] Elshorbany | Oxidation capacity of the city air of Santiago, Chile[END_REF][START_REF] Zhou | Snowpack photochemical production of HONO: A major source of OH in the Arctic boundary layer in springtime[END_REF]). However, the potential sources that could explain the observed elevated daytime HONO are still under controversial discussion.

The well accepted HONO sources include direct emissions from vehicle exhaust [START_REF] Kurtenbach | Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel[END_REF] and homogeneous gas phase reaction of NO with OH (R2) [START_REF] Pagsberg | Kinetics of the gas phase reaction OH + NO(+M) →HONO(+M) and the determination of the UV absorption cross sections of HONO[END_REF].

NO + OH +M → HONO + M (R2)

Heterogeneous reactions of NO 2 occurring on wet surfaces (R3) have been also proposed as an important source of HONO according to both laboratory studies and field observations (e.g., [START_REF] Finlayson-Pitts | The heterogeneous hydrolysis of NO 2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism[END_REF]. Nonetheless, the source strength of reaction (R3) has not been accurately quantified and relies on the NO 2 concentrations, surface area density and water content [START_REF] Finlayson-Pitts | The heterogeneous hydrolysis of NO 2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism[END_REF]. These reactions could occur on various types of

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3 surfaces including ground, buildings, vegetation, and aerosol surfaces [START_REF] Liu | Evidence of aerosols as a media for rapid daytime HONO production over China[END_REF][START_REF] Vandenboer | Understanding the role of the ground surface in HONO vertical structure: High resolution vertical profiles during NACHTT-11[END_REF]. Up to now, the contribution of the ground surfaces to the overall production of HONO is still under discussion and subject to intensive research activity [START_REF] Wong | Vertical profiles of nitrous acid in the nocturnal urban atmosphere of Houston, TX[END_REF][START_REF] Zhang | Potential Sources of Nitrous Acid (HONO) and Their Impacts on Ozone: A WRF-Chem study in a Polluted Subtropical Region[END_REF].

2NO 2 + H 2 O → HONO + HNO 3(ads) (R3)

In addition, the heterogeneous reduction of NO 2 on soot particles, mineral dust, and surfaces containing organic substrates was also proposed as a source of HONO (R4) [START_REF] Ammann | Heterogeneous production of nitrous acid on soot in polluted air masses[END_REF]2005;[START_REF] Ma | SO 2 initiates the efficient conversion of NO 2 to HONO on MgO surface[END_REF], and these processes can be further photo-enhanced during the daytime [START_REF] George | Photoenhanced uptake of gaseous NO 2 on solid organic compounds: a photochemical source of HONO?[END_REF][START_REF] Monge | Light changes the atmospheric reactivity of soot[END_REF][START_REF] Ndour | Photoenhanced uptake of NO 2 on mineral dust: Laboratory experiments and model simulations[END_REF][START_REF] Stemmler | Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid[END_REF]. Although the heterogeneous NO 2 conversion on soot surfaces has high potential to produce HONO, it decreases rapidly with aging and is usually regarded to be less important for ambient HONO formation [START_REF] Han | Heterogeneous photochemical aging of soot by NO 2 under simulated sunlight[END_REF].

NO 2 + HC red → HONO +HC ox (R4)

Besides, some other HONO sources have also been proposed, including soil emissions [START_REF] Su | Soil nitrite as a source of atmospheric HONO and OH radicals[END_REF], photolysis of adsorbed nitric acid (HNO 3 ) and nitrate (NO 3 -) at UV wavelengths of 300 nm [START_REF] Zhou | Snowpack photochemical production of HONO: A major source of OH in the Arctic boundary layer in springtime[END_REF], and homogeneous reaction of NO 2 with HO 2 •H 2 O [START_REF] Li | Missing gas-phase source of HONO inferred from Zeppelin measurements in the troposphere[END_REF]. Despite the abovementioned significant progress, the 'missing' daytime source(s) of atmospheric HONO is still under exploration.

In comparison with sources, the sink pathways of HONO are relatively well established.

The chemical losses of HONO include the photolysis (R1) and reactions with OH radicals (R5). Moreover, HONO can be also removed through dry deposition on ground surfaces.

Budget analysis of HONO sources and sinks has been proved to be a robust method to examine the unknown sources and quantify their source strength [START_REF] Sörgel | Quantification of the unknown HONO daytime source and its relation to NO 2[END_REF]Su et al., 2008b).

HONO + OH → H 2 O + NO 2 (R5)
A number of field studies have been conducted to measure ambient HONO in the polluted urban and rural atmospheres of China during the last decade. High concentration

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4 levels and strong potential missing source(s) of HONO have been reported in some metropolises (e.g., Beijing, Shanghai and Guangzhou) and surrounding regions (e.g., [START_REF] Bernard | Measurements of nitrous acid (HONO) in urban area of Shanghai, China[END_REF][START_REF] Qin | An observational study of the HONO-NO 2 coupling at an urban site in Guangzhou City, South China[END_REF][START_REF] Tong | Exploring the nitrous acid (HONO) formation mechanism in winter Beijing: direct emissions and heterogeneous production in urban and suburban areas[END_REF]. However, most of these studies were mainly based on short-term intensive observations. While long-period measurements are necessary to support a holistic investigation of characteristics and sources of HONO, they are very scarce [START_REF] Hendrick | Four years of ground-based MAX-DOAS observations of HONO and NO 2 in the Beijing area[END_REF]. In the present study, we have carried out 1-yr continuous observations of HONO and related parameters at an urban site of Ji'nan city, which is located almost in the center of the North China Plain (NCP), the most polluted region of China with dense population and industries. A large amount of observational data and HONO formation cases provided an opportunity of a thorough examination of temporal variations, sources and impacts of HONO in this polluted urban atmosphere of northern China. In the following sections, we will first show the seasonal and diurnal variations of HONO and related species.

Then, several sources of HONO will be explored, including vehicle emission, nighttime heterogeneous formation and potential unknown daytime sources. We will finally evaluate the impacts of HONO photolysis on the primary OH sources and hence atmospheric oxidizing capacity.

Experimental

Site description

The measurements were conducted from September 1 st 2015 to August 31 st 2016 at an urban site of Ji'nan, the capital city of Shandong Province, with approximately 7 million A detailed description of the study site can be found elsewhere [START_REF] Wang | HONO and its potential source particulate nitrite at an urban site in North China during the cold season[END_REF].

Measurement techniques

HONO was measured by a commercial instrument of LOPAP (long path absorption photometer, QUMA GmbH, Germany). The LOPAP is a wet chemistry based real-time measurement device, with which HONO is sampled in an external sampling unit as a stable diazonium salt and is subsequently detected photo-metrically after conversion into an azodye in a long-path absorption tube of 2.4 m Teflon AF. The LOPAP is conceived as a 2-channel system to correct for the potential interferences. In channel 1, HONO as well as possible interfering gases are determined, while in channel 2 only the interfering gases are quantified.

The difference of both channels yields the HONO concentrations. A detailed description of the LOPAP instrument has been described in detail by [START_REF] Heland | A new instrument to measure gaseous nitrous acid (HONO) in the atmosphere[END_REF]. In the present study, the sampling gas flow and the peristaltic pump velocity were set to 1 L min -1 and 20 r min -1 during the whole measurement period. With these settings, the HONO collection efficiency was ensured above 99.99%. Zero air calibration by ultrapure nitrogen (purity of 99.999%) was performed for 30 min automatically at a time interval of 12 h 30 min. An experimental cycle of 8 days was calibrated twice manually by using a known concentration of nitrite (NO 2 -) standard solution. The detection limit of our measurements was 3 ppt at a time resolution of 30 s, with an accuracy of 10% and a precision of 1%. We note that although the LOPAP instrument may collect data in 30 s (or 1 min) intervals, the physical time resolution of the instrument is relatively longer, ca. 3 min.

The [START_REF] Xue | Source of surface ozone and reactive nitrogen speciation at Mount Waliguan in western China: new insights from the 2006 summer study[END_REF] .

Results and discussion

Data overview

Figure 2 shows an overview of the measured HONO, NO, NO 2 , PM 2.5 , PM 1.0 , J HONO , and meteorological parameters in the present study. During the 1-yr measurement period, the prevailing winds were from the east and southwest sectors, indicating the general influence of industrial emissions on the study site (see Fig. 1). The air temperature ranged from -15 ℃ to 39 ℃ with a mean value (±standard deviation) of 16±11 , ℃ and the relative humidity showed a clear seasonal variation pattern with higher levels in winter and summer. Markedly poor air quality was observed as expected. Throughout the 1-yr period, 137 haze episodes occurred with daily mass concentration of PM 2.5 exceeding the National Ambient Air Quality Standard (Class II: 75 µg m -3 ), including 9 severe polluted haze episodes with daily average PM 2.5 concentrations above 250 µg m -3 . In addition, elevated levels of NO X , i.e., up to 350 ppbv of NO and 108 ppbv of NO 2 , were also frequently recorded, possibly as a result of intensive vehicle emissions nearby the study site. Overall, these observations highlight the nature of our measurement station as a typical polluted urban environment in North China.

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Table 1 documents the measured levels of HONO, NOx, HONO/NOx ratios and the comparison with the results obtained previously elsewhere. The measured HONO mixing ratios in Ji'nan ranged from 17 pptv to 8.36 ppbv with a mean (±SD) value of 1.15±1.07 ppbv.

Elevated HONO concentrations were frequently observed during the measurement period, with the daily maximum values exceeding 2 ppbv and 4 ppbv on 156 and 50 days, respectively (see Fig. 2). The maximum hourly value of 7.39 ppbv was recorded in the early morning of 6 December 2015. Such high levels of ambient HONO indicate the intense sources of HONO and potentially strong atmospheric oxidizing capacity in urban Ji'nan. The nighttime average (18:00-06:00, LT) HONO concentration was 1.28±1.16 ppbv, compared to the daytime average value (6:00-18:00, LT) of 0.99±0.95 ppbv. In particular, the mean HONO mixing ratio around noontime (11:00-13:00, LT) was even as high as 0.76±0.61 ppbv, which is nearly the highest levels ever recorded in the urban atmospheres, and about 27% of the noontime HONO data were above 1.00 ppbv during the measurement period. This implies the existence of strong daytime sources of HONO in the atmosphere of Ji'nan, which will be further discussed in Section 3.4.

The seasonal variations of ambient HONO and related parameters are depicted in Fig. 3.

The highest concentrations of HONO occurred in winter (i.e., December-January), followed by spring (i.e., April-May), summer (especially August) and autumn, with seasonal mean (±SD) values of 1.71±1.62, 1.16±0.90, 1.12±0.93 and 0.78±0.60 ppbv, respectively. Overall, the seasonal variation of HONO coincided with that of NO 2 , an important precursor of HONO. Such measured seasonal pattern of HONO is different from those measured in Hong Kong [START_REF] Xu | Nitrous acid (HONO) in a polluted subtropical atmosphere: Seasonal variability, direct vehicle emissions and heterogeneous production at ground surface[END_REF] and Beijing [START_REF] Hendrick | Four years of ground-based MAX-DOAS observations of HONO and NO 2 in the Beijing area[END_REF], where the highest levels were found in the autumn season. The wintertime peak of ambient HONO in Ji'nan should be the result of the lower boundary layer height, weaker photolysis, and enhanced heterogeneous production of HONO given the more abundant NO 2 . The relatively higher springtime HONO mixing ratios might be related to some degree to the more intense heterogeneous reactions of NO 2 on the surface of mineral particles [START_REF] Nie | Asian dust storm observed at a rural mountain site in southern China: chemical evolution and heterogeneous photochemistry[END_REF], as indicated by the coincident higher concentrations of NO 2 and PM 2.5 (note that PM 10 was not measured in the present study). Indeed, the air quality of Ji'nan in the spring of 2016 was characterized by high levels The diurnal profiles of HONO and related supporting parameters are shown in Figure 4.

Overall, the diurnal variations of HONO in different seasons were similar, which dropped rapidly after sunrise and reached a minimum at around 15:00 LT, and then increased and peaked during the morning rush hours (an exception is the winter case that showed a concentration peak at midnight). The diurnal variation trend of HONO was similar to that of NO, owing to a variety of chemical and physical processes, and the similar nighttime profiles suggest that vehicle emissions may pose a significant effect on the measured HONO levels.

Such nighttime pattern was also found at Tung Chung, Hong Kong [START_REF] Xu | Nitrous acid (HONO) in a polluted subtropical atmosphere: Seasonal variability, direct vehicle emissions and heterogeneous production at ground surface[END_REF], a roadside site in Houston, U.S. [START_REF] Rappenglück | Radical precursors and related species from traffic as observed and modeled at an urban highway junction[END_REF] and in a tunnel in Wuppertal, Germany [START_REF] Kurtenbach | Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel[END_REF]. The average diurnal profiles of HONO/NO 2 ratio are also shown in Fig. 4f. The HONO/NO 2 ratio generally decreased after sunrise due to the increase of HONO photolysis, and then increased during the nighttime. An interesting finding was the second peak of HONO/NO 2 at around noontime in spring, summer and winter seasons. If the HONO sources during nighttime were the same as those at daytime, the minimum HONO/NO 2 ratios should be found at noon due to the strong photolysis of HONO. Thus, the higher ratios at noontime indicated the existence of additional daytime sources of HONO.

Moreover, the HONO/NO 2 ratios increased with solar radiation (e.g., J HONO ), implying that the additional sources may be related to the solar radiation intensity. We will further discuss the potential daytime sources of HONO in Section 3.4.

Contribution of vehicle emissions

As our study site is close to several major roads of large traffic fleet, it is necessary to evaluate the contribution of vehicle emissions to the measured HONO concentrations. The HONO/NO X ratio was usually chosen to derive the emission factor of HONO in the freshly emitted plumes [START_REF] Kurtenbach | Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel[END_REF]. In order to ensure the fresh air masses, the Table 2 summarizes the estimated emission factors of HONO/NOx for the 12 vehicular emission plumes. The average ∆NO/∆NO X ratio of the selected plumes was 94%, indicating that the air masses were indeed freshly emitted. The correlation coefficients (r 2 ) of HONO with NOx varied case by case and were in the range of 0.58-0.96, which may be due to the inevitable mixing of vehicle plumes with other air masses and/or heterogeneous conversion of NO 2 on soot particles and ground surface. The derived ∆HONO/∆NO X ratios varied in the range of 0.19%-0.87%, with an average value (±SD) of 0.53%±0.20%. This is comparable to the emission factors obtained in Santiago, Chile (0.8%; [START_REF] Elshorbany | Oxidation capacity of the city air of Santiago, Chile[END_REF] and Wuppertal, Germany (0.3-0.8%; [START_REF] Kurtenbach | Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel[END_REF], but is substantially lower than those derived in Guangzhou (with a minimum value of 1.4%; [START_REF] Qin | An observational study of the HONO-NO 2 coupling at an urban site in Guangzhou City, South China[END_REF] and Houston, U.S. (1.7%; Rappengluck et al., 2013). The emission factors should be dependent on the types of vehicle engines, fuels and catalytic converters [START_REF] Kurtenbach | Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel[END_REF]. The variance in the HONO/NOx ratios derived from different metropolitan areas highlights the necessity of examining the vehicular emission factors of HONO in the target city in the future studies. In the present study, the average HONO/NOx value of 0.53% was adopted as the emission factor in urban Ji'nan, and was used to estimate the contributions of traffic 1) with a rough assumption that the observed NOx at our site was mainly emitted from vehicles.

HONO = NO × 0.0053 (E1)
Where, HONO emis is the HONO concentration arising from the direct vehicle emissions. The calculated HONO emis levels contributed on average 12%, 15%, 18% and 21% of the whole measured nighttime HONO concentrations in urban Ji'nan in spring, summer, autumn and winter, respectively. (3) the meteorological conditions, especially surface winds, should be stable. Figure 5 presents an example of the heterogeneous HONO formation case occurring on 6-7 September, 2015. In this case, the HONO mixing ratios increased rapidly after sunset from 0.08 ppbv to 0.78 ppbv. Since the HONO concentrations and HONO/NO 2 almost increased linearly throughout the night, the slope fitted by the least linear regression for HONO/NO 2 ratios against time can be taken as the conversion frequency of NO 2 -to-HONO (k het ; also referred to as C HONO in other studies). During the 1-yr period, a total of 107 cases were finally selected. Such a large set of cases facilitated a more robust statistical analysis of the heterogeneous formation of HONO.

Heterogeneous conversion of NO

As our study site is close to the traffic roads, it is necessary to subtract the contribution from direct vehicle emissions. The emission ratio of HONO/NO X derived in Section 3.2 was

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then used to adjust the HONO concentrations by Eq. ( 2). The NO 2 -to-HONO conversion frequency can be computed by Eq. ( 3), by assuming that the increase of HONO/NO 2 ratio was caused by the heterogeneous conversion (Su et al., 2008a;[START_REF] Xu | Nitrous acid (HONO) in a polluted subtropical atmosphere: Seasonal variability, direct vehicle emissions and heterogeneous production at ground surface[END_REF].

HONO = HONO -NO × 0.0053 (E2) k = [ ! "#$$ ](& ' ) [! ' ](& ' ) ( [ ! "#$$ ](& ' ) [! ' ](& ) ) ( ' ( ) ) (E3)
The k het values derived from the 107 cases showed a large variability, from 0.0013 h -1 to 0.0194 h -1 , with a mean value of 0.0068±0.0045 h -1 . These results are well within the range of k het obtained previously from other urban areas. For example, the k het in Ji'nan is comparable to that derived at an urban site of Shanghai (0.007 h -1 ; [START_REF] Wang | Long-term observation of atmospheric nitrous acid (HONO) and its implication to local NO 2 levels in Shanghai, China[END_REF], and less than those in Guangzhou (0.016 h -1 ; [START_REF] Qin | An observational study of the HONO-NO 2 coupling at an urban site in Guangzhou City, South China[END_REF], Milan (0.012 h -1 ; [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] and Kathmandu (0.014 h -1 ; [START_REF] Yu | Observations of high rates of NO 2 -HONO conversion in the nocturnal atmospheric boundary layer in Kathmandu, Nepal[END_REF]. Figure 6 provides the seasonal variation of the NO 2 -to-HONO conversion rate in Ji'nan. Clearly, the largest average k het was found in winter with a value of 0.0073±0.0044 h -1 . This should be ascribed to the higher S/V surface density within the shallower boundary layer in the wintertime. Moreover, weak correlation (R = 0.07) between k het and aerosol surface density was also found, which suggests that the efficient heterogeneous formation of HONO may be independent on the aerosol surface.

The uptake coefficient of NO 2 on various surfaces to yield HONO (* +, ' →.,+, ) is a key parameter with large uncertainty in the air quality models to simulate HONO and OH radicals [START_REF] Zhang | Potential Sources of Nitrous Acid (HONO) and Their Impacts on Ozone: A WRF-Chem study in a Polluted Subtropical Region[END_REF]. The overall * +, ' →.,+, on the bulk surface of ground and particles can be estimated from Eq. ( 4). Where, > +, ' is the mean molecular velocity of NO 2 (370 m s -1 ); S/? @ and S/? A are the surface area to volume ratio (m -1 ) for both aerosol and ground, respectively. Considering the land use of the study site, the ground was treated as an uneven surface, and a factor of 2.2 per unit ground surface measured by [START_REF] Voogt | Complete urban surface temperatures[END_REF] was adopted to calculate the total

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12 active surface. Hence, S/? A can be calculated by Eq. ( 5), where H is the mixing layer height and was obtained from the European Centre for Medium-Range Weather Forecasts (ECMRWF, ERA-Interim; http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/).

The calculated uptake coefficients for the 107 cases varied in a wide range from 6.1×10 -8 to 1.7×10 -5 , whilst the majority (5%-95% percentiles) fell in a narrower range of 1.1×10 -7 to 4.5×10 -6 . The mean γ ΝΟ2 value was 1.4±2.4×10 -6 . Current laboratory studies have reported the range of γ 01 ' →2101 from 10 -6 to 10 -5 on the ground surface [START_REF] Kurtenbach | Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel[END_REF][START_REF] Vandenboer | Understanding the role of the ground surface in HONO vertical structure: High resolution vertical profiles during NACHTT-11[END_REF] and from 10 -7 to 10 -5 on the aerosol surface [START_REF] Ndour | Photoenhanced uptake of NO 2 on mineral dust: Laboratory experiments and model simulations[END_REF][START_REF] Wong | Vertical profiles of nitrous acid in the nocturnal urban atmosphere of Houston, TX[END_REF]. Obviously, the uptake coefficient in different orders of magnitude would definitely lead to different assessment of the importance of heterogeneous HONO sources [START_REF] Li | Impacts of HONO sources on the photochemistry in Mexico City during the MCMA-2006/MILAGO Campaign[END_REF]. The average uptake coefficient obtained from such a large set of samples could serve as a reference for modeling studies to simulate ambient HONO and atmospheric oxidation processes in the urban atmospheres of North China. Furthermore, the total area of ground surface is much larger than the reactive surface provided by aerosols, suggesting that the heterogeneous reactions of NO 2 on ground surface may play a dominant role. It should be noted that the exact uptake coefficients of NO 2 on ground and aerosol surfaces are variable and should be different, and the present analysis simplified this process by treating the ground and aerosol surfaces the same. Our derived uptake coefficients can be regarded as an equivalent γ NO2 on the bulk surface of ground and particles.

Daytime HONO budget analysis

In this section, we examine the potential unknown source(s) of daytime HONO by a detailed budget analysis. Equation ( 6) summarizes the main factors affecting the ambient concentrations of HONO. Even though taking a mean noontime [HONO] level of 1 ppbv, a value of 6×10 -5 ppb s -1 was derived [START_REF] Dillon | Chemical evolution of the Sacramento urban plume: Transport and oxidation[END_REF][START_REF] Sörgel | Quantification of the unknown HONO daytime source and its relation to NO 2[END_REF], which is much smaller compared to L phot (1×10 -3 ppb s -1 ).

The noontime data (11:00-14:00 LT) with the strongest solar radiation were chosen to calculate the unknown HONO source strength based on Eq. ( 7). Here the d[HONO]/dt was approximated by ∆HONO/∆t, which is the difference of the measured HONO concentrations every 10 minutes [START_REF] Sörgel | Quantification of the unknown HONO daytime source and its relation to NO 2[END_REF]. Where, l .,+, and l +, ' are the photolysis frequencies of HONO and NO 2 (s -1 ), respectively.

P EFGF HF = L J + L 12D2101 + L B J + ∆[2101] ∆ -P 12D01 -P (E7) = [HONO] \J 2101 + K 12D2101 [OH] + ^ ! ;$#_`a 2 b + ∆[2101] ∆ -K 12D01 [OH][NO] - ∆[01 c ]×d.e5% ∆ [OH] = a(J 1 ) h ) i (J 01 ' ) j k01 ' D4 01 ' ' DB01 ' D4 (
Direct measurements of l .,+, and l +, ' were made in this study except for the period from December 2015 to May 2016. For the reaction of HONO with OH, a rate constant m ,.D.,+, of 6.0×10 -12 cm 3 molecules s -1 was taken from [START_REF] Atkinson | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I -gas phase reactions of Ox, HOx, NOx and SOx species[END_REF]. The OH mixing ratios were expressed by the NO 2 concentrations and photolysis frequencies of O 3 and NO 2 , as shown in Eq. ( 8) [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]. In the present study, the calculated daily peak OH concentrations were in the range of 0.3-2×10 7 molecules cm -3 , which are comparable to those

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14 measured in the polluted atmospheres of northern China [START_REF] Lu | Missing OH source in a suburban environment near Beijing: observed and modelled OH and HO 2 concentrations in summer 2006[END_REF]. Nonetheless, it should be noted that the OH calculation from such empirical equation may be subject to some uncertainty. V ZMW was calculated by assuming a HONO daytime dry deposition velocity of 2 cm s -1 and an effective mixing height of 200 m. Due to the rapid photolysis of HONO at daytime, most of HONO cannot reach the height above 200 m [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]. m ,.D+, is the rate constant for the reaction of OH with NO, using a value of 9.8×10 -12 cm 3 molecules -1 s -1 from [START_REF] Atkinson | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I -gas phase reactions of Ox, HOx, NOx and SOx species[END_REF]. The emission source strength was estimated from the HONO/NO X emission ratio of 0.53% as determined in Sec. 3.2.

Figure 7 shows the average contributions of all the source and sink terms to the HONO budget in August 2016, when accurate J values observations were available and elevated daytime HONO were observed. An unknown source L QRSRTUR was clearly the dominant part, accounting for over 80% of the HONO production. An average P unknown value of 2.95 ppb h -1 was derived, which is more than 7 times greater than that of the homogeneous formation rate (L ,.D+, , 0.40 ppb h -1 ). The major loss pathway of HONO was the photolysis with a mean V WXTY value of 2.80 ppb h -1 , followed by dry deposition (V ZMW , 0.49 ppb h -1 ), and V ,.D.,+, was very small and almost less than 3% of V WXTY . The unknown source strength of daytime HONO in Ji'nan is higher than those derived in Santiago, Chile (1.69 ppb h -1 ; [START_REF] Elshorbany | Oxidation capacity of the city air of Santiago, Chile[END_REF], Beijing (1.83 ppb h -1 ; [START_REF] Hou | Comparison of atmospheric nitrous acid during severe haze and clean periods in Beijing, China[END_REF], and Houston, US (0.61 ppb h -1 ; Wong et al., 2012). Some studies have reported much lower L QRSRTUR obtained from a rural site in Guangzhou, China (0.76 ppb h -1 ; [START_REF] Li | Exploring the atmospheric chemistry of nitrous acid (HONO) at a rural site in Southern China[END_REF], a mountain site in Hohenpeissenberg, Germany (0.40 ppb h -1 ; Acker et al., 2006), and a forest site in Julich, Germany (0.50 ppb h -1 ; [START_REF] Kleffmann | Daytime formation of nitrous acid: A major source of OH radicals in a forest[END_REF].

We further explored the potential unknown daytime source(s) of HONO based on our measurement data. According to the laboratory studies, heterogeneous reactions of NO 2 on wet surfaces should be an important contributor to the ambient HONO concentrations, and the reaction rate is first order in NO 2 . It has been also proposed that these heterogeneous reactions can be photo-enhanced [START_REF] Stemmler | Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid[END_REF]. Thus, the strength of the unknown HONO source (P unknown ) can be expressed by equation (E9), if the heterogeneous reactions were the major HONO sources.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT L QRSRTUR ∝ l +, ' * [pq = ] * r s t u (E9)
Correlation analysis between P unknown and related parameters has been widely adopted to diagnose the potential HONO sources (e.g., Su et al., 2008b). The NO 2 concentration ([NO 2 ]) is usually used as an indicator of the heterogeneous reactions on the ground surface since the ground surface to volume ratio (S/V g ) can be assumed to be constant for the well-mixed boundary layer at noontime, whilst [NO 2 ]*[S/V a ] can be taken as a proxy for the HONO formation on the aerosol surface. JNO 2 *[NO 2 ] and JNO 2 *[NO 2 ]*[S/V a ] can be used to infer the photo-enhanced heterogeneous reactions on ground and aerosol surfaces. Figure 8 shows the scatter plots of the calculated P unknown versus the abovementioned four indicators for the summer case (i.e., August 2016) when accurate J value measurements were available. P unknown showed a moderate correlation with [NO 2 ] with a correlation coefficient (R) of 0.55, and it was significantly improved after J NO2 *[NO 2 ] was considered (R=0.76). When aerosol surface density was taken into account, however, the correlation became even weaker with R values of 0.40 and 0.43. This suggests that the photo-enhanced heterogeneous reactions of NO 2 on the ground surface played a major role in the daytime HONO formation in Ji'nan in summer.

Impact on the primary OH sources

Photolysis of HONO presents an important primary source of OH in the atmosphere. The from O 3 photolysis can be calculated by Equation ( 12) (Su et al., 2008b).

P 12 (O w ) = 2J 1 ) h × [O w ] / (1 + k w [M]/k = [H = O]) (E12)
Figure 9 shows the daytime profiles of OH production rates from photolysis of HONO and O 3 in the summer (i.e., August 2016) period when accurate J value measurements were available. Clearly, photolysis of HONO dominated the daytime OH production in urban Ji'nan. The mean L ,. (vqpq) was 1.88 ppb h -1 , almost 3 times higher than L ,. (q w ).

Furthermore, in contrast to most of the earlier studies which suggested that the contribution of HONO photolysis was mainly concentrated in the early morning and neglected at noon, photolysis of HONO presents the dominant OH contributor throughout the daytime at our study site. Even though at noontime in summer, high contributions of HONO photolysis were still found. These results demonstrate the significant role of HONO in the atmospheric oxidizing capacity in the urban atmosphere of Ji'nan.

Summary

Highly time-resolved continuous field observations of HONO, related air pollutants and meteorological parameters were performed at an urban site of Ji'nan in North China, for a year from September 2015 to August 2016. The measured mean concentration of HONO was 1.15 ppbv with a maximum level of 8.36 ppbv. The ambient HONO concentrations presented a seasonal variation with the highest level in winter as well as elevated concentrations in spring (April-May) and summer (August). Well-defined diurnal cycles of HONO with concentration peaks in the early morning and valleys in the afternoon were found for all the four seasons. Direct emissions from vehicle exhaust posed a large contribution to the ambient HONO, with an average emission ratio ∆HONO/∆NO X of 0.53%. During the nighttime, the heterogeneous conversion of NO 2 on the ground surface is an important source of HONO.

The average conversion frequency of NO 2 to HONO was derived as 0.0068 h -1 from over hundred cases. At daytime, a missing HONO source with an average strength of 2.95 ppb h -1 was derived in summer, which was about seven times larger than the gas phase reactions. Our One-year continuous measurements of HONO were made at a typical urban site in northern China.

Seasonal and diurnal variations, vehicle emission factors, heterogeneous formation, and daytime sources were examined based on a large observational data set.

A strong missing daytime source was needed to explain the measured HONO concentrations.

HONO photolysis is the dominant OH source throughout the daytime.

  population and 1.6 million automobiles. The site is located in the central campus of Shandong University (36º40'N, 117º03'E), a typical urban area surrounded by massive buildings and condensed population and close to several main traffic roads (Fig.1). Large-scale industries, including steel plants, thermal power plants, cement plants, oil refineries and chemical plants in suburban areas are the major industrial emission sources of local air pollution in Ji'nan, and are mainly distributed in the northeast and southwest of the site. By observing the spikes of sulfur dioxide (SO 2 ) concentrations under northeasterly and/or southwesterly winds, we know that the site was affected by the local industrial emission sources. All the measurements were carried out on the rooftop of a six-floor teaching building, around 22 m above the

  influenced by several dust storms and urban dust (http://www.sdein.gov.cn /dtxx/hbyw/201605/t20160512_294758.html). Besides, the elevated HONO levels in August under the condition of intense solar radiation suggests the presence of strong HONO sources as well as the important contributions of HONO as a potential source of OH radicals to the atmospheric oxidation chemistry.

  were adopted to select the cases: (a) only data in the morning rush hours (6:00-8:30 LT) in winter (e.g., November-February) were used; (b) NO/NO X>0.7; (c) good correlation between HONO and NO X ; (d) short duration of the plumes (<2 hours). Rush hours are the prominent period with strong traffic emission and thus greatest contribution of vehicle exhaust to HONO concentrations. Furthermore, during the winter early morning rush hours when the solar radiation is weak and boundary layer height is relatively stable, the derived HONO/NOx is less interfered by atmospheric photochemical reactions and mixing with the air masses aloft. Criteria (b) was used as an indicator for identifying the freshly emitted plumes. Criteria (c) and (d) further confirmed that the increase of HONO was mainly attributed to direct emissions instead of heterogeneous reactions of NO 2 . The slopes of the scatter plot of HONO versus NO X can be considered as the emission ratios. With such strict selection criteria, a total of twelve cases were screened out to estimate the vehicle emission factors of HONO in urban Ji'nan.

  ambient nocturnal HONO levels (Equation

  2 to HONO during nighttimeIt has been widely accepted that the heterogeneous reactions of NO 2 on wet surfaces present an important formation pathway of ambient HONO, and generally make a dominant contribution at nighttime (e.g.,[START_REF] Finlayson-Pitts | The heterogeneous hydrolysis of NO 2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism[END_REF]. To investigate the heterogeneous production of HONO in Ji'nan, a number of nighttime HONO formation cases were identified to estimate the NO 2 -to-HONO conversion frequency. The selected cases should meet the following criteria: (1) only the nighttime data in the absence of sunlight (i.e., 20:00-05:59 LT in autumn and winter and 20:00-04:59 LT in spring and summer) were used considering the fast HONO loss via photolysis and potential existence of unknown HONO sources at daytime; (2) both HONO concentrations and HONO/NO 2 ratios increased steadily during the target case;

  photolysis and gas phase reactions at noontime. The average daytime wind speed in Ji'nan in the present study was 1.7 m s -1 , so that at a HONO lifetime of about 15 min at noontime, the horizontal transport had to occur within 1.6 km (in which no large-scale pollution sources) to reach the site.[START_REF] Dillon | Chemical evolution of the Sacramento urban plume: Transport and oxidation[END_REF] proposed a parameterization for the dilution by background air to estimate the magnitude of vertical transport (T V = k (dilution) ([HONO]-[HONO] background ).

  E8) (α = 0.83, β = 0.19, a = 4.1×10 9 , b = 140, c = 0.41, and d = 1.7)

  photolysis (O 1 D+H 2 O), another important OH source, based on the concurrent observations of HONO, O 3 , J HONO and J O1D in summer. The other primary OH sources, such as photolysis of peroxides and ozonolysis reactions of alkenes, are generally not very important in urban areas, especially at daytime, and were not considered in the present study. We also don't consider the primary sources of HO 2 and RO 2 radicals (such as photolysis of OVOCs) due to the lack of measurement data for these radical precursors. The net OH production rate from HONO photolysis (L ,. (vqpq) RMY ) was calculated by the source strength subtracting the sink terms due to reactions (R2) and (R5)(Equations 10 and 11). The OH production rate

  HONO) F = P 12 (HONO) -k 01D12 [NO][OH] -k 2101D12 [HONO][OH] (E11)

  the photo-enhanced heterogeneous reaction of NO 2 on the ground surface may be a major source of daytime HONO in summer. Photolysis of HONO presents the predominant OH contributor not only in the early morning but also throughout the daytime in urban Ji'nan, and hence plays a vital role in the atmospheric oxidation and ozone formation in the polluted urban atmosphere of northern China.

  (18:00-06:00, LT); D: daytime (06:00-18:00, LT) 1: Elshorbany et al. (2009); 2: Acker et al. (2006a); 3: Yu et al. (2009); 4: Bernard et al. (2016); 5: Qin et al. (2009); 6: Tong et al. (2015); 7: Su et al. (2008a); 8: Alicke et al. (2002); 9: Li et al. (2012); 10: this study.

Figure 1 .

 1 Figure 1. Locations of Ji'nan and the sampling site. The left map is color-coded by the anthropogenic NOx emissions (Zhang et al., 2009), while the right is color-coded by the geographical height. The large industrial sources are labeled with different colors, including steel plants (light blue), thermal power plants (pink), cement plants (red), oil refineries (grey) and chemical plants (green).

Figure 2 .Figure 3 .Figure 4 .Figure 5 .

 2345 Figure 2. Time series of HONO, NO, NO 2 , PM 2.5 , PM 1.0 , J HONO , temperature (T), relative humidity (RH) and surface wind in Ji'nan from September 2015 to August 2016. The data gap is mainly due to the maintenance of the instruments.

Figure 6 .Figure 7 .

 67 Figure 6. Seasonal variation of the NO 2 -to-HONO conversion frequency (k het ) in Ji'nan.

Figure 8 .Figure 9 .

 89 Figure 8. Scatter plots of the unknown daytime HONO source strength (P unknown ) with (a) NO 2 , (b) NO 2 *(S/V) a , (c) NO 2 *J NO2 , and (d) NO 2 *J NO2 *(S/V) a during August 2016.

Table 1 .

 1 Overview of the measured HONO and NOx levels in urban Ji'nan and comparison with other studies.

	Location	Time	HONO(ppb) N D	NO 2 (ppb) N D	NO X (ppb) N D	HONO/NO 2 N D	HONO/NO X N D	Ref.
	Santiago, Chile (urban)	Mar-Jun 2005	3.00	1.50	30.0	20.0	200.0	40.0	0.100	0.075	0.015	0.038	1
	Rome, Italy (urban)	May-Jun 2001	1.00	0.15	27.2	4.0	51.2	4.2	0.037	0.038	0.020	0.024	2
	Kathmandu, Nepal (urban)	Jan-Feb 2003	1.74	0.35	17.9	8.6	20.1	13.0	0.097	0.041	0.087	0.027	3
	Shanghai, China (urban)	Oct 2009	1.50	1.00	41.9	30.0	/	/	0.038	0.032	/	/	4
	Guangzhou, China (urban)	Jun 2006	3.5	2.00	20.0	30.0	/	/	0.175	0.067	/	/	5
	Beijing, China (urban)	Oct-Nov 2014	1.75	0.93	37.6 35.3.0	94.5	53.4	0.047	0.026	0.019	0.017	6
	Xinken, China (suburban)	Oct-Nov 2004	1.30	0.80	34.8	30.0	37.8	40.0	0.037	0.027	0.034	0.020	7
	Milan, Italy (suburban)	May-Jun 1998	0.92	0.14	33.2	18.3	117.5	23.4	0.028	0.008	0.008	0.006	8
	Backgarden, China (rural)	Jul 2006	0.95	0.24	16.5	4.5	20.9	5.5	0.057	0.053	0.045	0.043	9
		Sep 2015-Aug 2016	1.28	0.99	31.0	25.8	46.4	40.6	0.079	0.056	0.040	0.035	10
		Sep-Nov 2015 (autumn)	0.87	0.66	25.4	23.2	38.	37.5	0.049	0.034	0.034	0.022	10
	Ji'nan, China (urban)	Dec 2015-Feb 2016 (winter)	2.15	1.35	41.1	34.6	78.5	64.8	0.056	0.047	0.034	0.031	10
		Mar-May 2016 (spring)	1.24	1.04	35.8	25.8	47.3	36.0	0.046	0.052	0.035	0.041	10
		Jun-Aug 2016 (summer)	1.20	1.01	22.5	19.0	29.1	25.8	0.106	0.079	0.060	0.049	10
	N: nighttime

Table 2 .

 2 The emission ratios ∆HONO/∆NO X of fresh vehicle plumes.

	Date	Local Time	∆NO/∆NO X	R 2	∆HONO/∆NO X (%)
	11/03/2015	06:08-07:45	0.89	0.91	0.29
	11/05/2015	06:00-07:30	0.92	0.84	0.63
	11/17/2015	06:00-07:30	1	0.95	0.75
	11/20/2015	06:00-07:15	0.92	0.72	0.59
	12/06/2015	06:00-07:48	0.83	0.75	0.87
	12/08/2015	06:02-07:30	0.95	0.61	0.58
	12/25/2015	06:00-07:30	0.94	0.72	0.30
	12/31/2015	06:00-07:30	0.96	0.94	0.47
	01/02/2016	06:00-07:30	1	0.61	0.46
	01/20/2016	06:44-07:48	0.92	0.96	0.71
	01/21/2016	06:26-07:56	0.94	0.58	0.54
	01/27/2016	07:00-08:12	0.89	0.77	0.19

Acknowledgments

We thank Chuan Yu, Ruihan Zong, Dr. Zheng Xu and Dr. Long Jia for their contributions to the filed study. We are grateful to the European Centre for Medium-Range Weather Forecasts for sharing the boundary layer height data and to the National Center Atmospheric Research for providing the TUV model. This work was funded by the National Natural Science Foundation of China (No. 41505111 and 91544213), the National Key Research and Development Programme of the Ministry of Science and Technology of China (No. 2016YFC0200500), the Natural Science Foundation of Shandong Province (ZR2014BQ031), the Qilu Youth Talent Program of Shandong University, and the Jiangsu Collaborative Innovation Center for Climate Change.


Fabrice Pierron

may come

The iosipescu in-plane shear test : modelization and experimental procedure for composites


Keywords: thermoporosimetry, confinement, crystallisation, nanoporous materials

The thermal behaviour of carbon tetrachloride confined in silica gels of different porosity was studied by differential Scanning Calorimetry. Both the melting and the phase transition at low temperature were measured and found to be inextricably dependant upon the degree of confinement. The amount of solvent was varied through two sets of experiments, sequential addition and original progressive evaporation allowing the measurement of the DSC signals for the various transitions as a function of the amount of CCl 4 . These experiments allowed the determination of transition enthalpies in the confined state which in turn allowed the determination of the exact quantities of solvent undergoing the transitions. A clear correlation was found between the amounts of solvent undergoing the two transitions (both free and confined) demonstrating that the formation of the adsorbed layer t does not interfere with the second transition. The thickness of this layer and the porous volumes of the two silica samples were measured and found to be in very close agreement with the values determined by gas sorption.

Crystallization of carbon tetrachloride in confined geometries

Adil Meziane 1 , Jean-Pierre E. Grolier 2 , Mohamed Baba 2 and Jean-Marie Nedelec

Introduction

The peculiar behaviour of liquids in confined geometry has attracted a lot of interest in particular during the past ten years. A comprehensive review has been published in 2001 [1].

The case of water [2,3] is particularly relevant because of the numerous works dealing with water and also because of obvious practical applications. The revival in the interest in transitions in confined geometry undoubtedly comes from the considerable progress in the preparation of nanoporous materials with controlled pore size and with spatially controlled pore distribution and connectivity. In this context discovery of MCM type materials [4] has played an important role. The use of organized molecular systems (surfactants) to limit spatially the condensation of alkoxide precursors is now common and has been extended to various systems and various pore organizations. The availability of such porous materials with controlled porosity, and to some extent with tuneable porosity, has lead to an increased interest for the study of crystallisation in confined geometry. Practical interest of liquids in porous materials is also very widespread and the case of oil recovery is a major example. The chemistry of water in clouds is also greatly affected by confinement effects.

More importantly, the research devoted to the preparation of nanocrystals with a good control of both crystal size and size distribution has been incredibly expanding in the last twenty years [5]. In particular semi-conducting nanocrystals or quantum dots have been the subject of many research papers [6,7] due to the possible observation in these materials of a direct quantum effect correlated to the size of the crystals.

Porous materials appeared to be ideal candidates for the preparation of such nanocrystals, utilizing the pores as nanoreactors where the crystallization of the desired material could be confined. In particular abundant examples concerning MCM-41 and SBA-15 mesoporous silicas templates can be found in the literature, see [8,9] for instance. Another very interesting example of crystallization in confined geometries is Biomineralization [10,11].

Biomineralization is a complex process in which the solution conditions, organic template, and crystal confinement coordinate to yield nanostructured composite materials with controlled morphology and mechanical and structural properties. Over the past few decades, research has examined various aspects of this mineralization process both by characterizing those found in nature and by creating synthetic composites.

Another field in which crystallization in confined geometries play a major role is polymer science. Numerous examples demonstrate how the confinement can modify the kinetics of crystallization of polymers and also the morphology of the crystals [12].

All these selected examples demonstrate how crucial it is to get information concerning crystallization in confined geometries. In particular the energetic of crystallization in confining media is not well documented.

The well known modification of the freezing point temperature of liquids in confined geometry has led to the development of characterization techniques for the measurement of porosity in solids. Such techniques are based upon the Gibbs-Thomson equation [START_REF] Gibbs | Collected works[END_REF][START_REF] Thomson | [END_REF] which relates the shift ∆T of the crystallization temperature to the pore size of the confining material according to [15]:

p m p s m SL p R H k R H T Cos T T T ∆ ≈ ∆ = - = ∆ ρ θ σ 0 0 . 2 (Equation 1)
where T p is the melting temperature of a liquid confined in a pore of radius R p , T 0 is the normal melting temperature of the liquid, σ SL is the surface energy of the solid/liquid interface, θ the contact angle, ∆H m is the melting enthalphy, ρ S the density of the solid and k a constant.

The measurement of ∆T by calorimetry or NMR technique leads to thermoporosimetry [16] and NMR cryoporometry [17] respectively. The advantages of both techniques have been discussed extensively [18].

As first proposed by Kuhn [19] in the 1950's, thermoporosimetry can also be of great value for soft networks characterization like polymeric gels [20]. In this case, the confinement is created by the meshes defining the 3-dimensional polymer network. The study of polymer architecture modification by thermoporosimetry requires knowledge of the behaviour of liquids able to swell these organic materials. We recently developed reference porous materials for calibration of thermoporosimetry with various solvents [21,22]. In our systematic work, we observed that some solvents presenting a low temperature phase transition in the solid state offered even more interest [23]. Indeed, this transition is also affected by the confinement and is an interesting alternative to the use of liquid to solid transition since it is usually much more energetic. From a practical point of view the use of these transitions does not change the procedure requiring the calibration of the technique with samples of known porosity. But from a fundamental point of view, this observation raises some questions about the underlying thermodynamics. The objective of this paper is to discuss the transitions of carbon tetrachloride in confined geometry because CCl 4 is an effective solvent for polymer swelling and also presents this solid state phase transition as observed before. [24,25].

Theoretical considerations

According to Equation (1), the shift of the transition temperature of a confined liquid ∆T is inversely proportional to the radius of the pore in which it is confined. In fact it is well known that not all the solvent takes part in the transition and that a significant part of it remains adsorbed on the surface of the pore. The state of this adsorbed layer has been discussed extensively in the case of water. Consequently, the radius measured by application of the Gibbs-Thomson equation should be written R=R p -t where t is the thickness of the adsorbed layer leading to a reformulation [7] of Equation 1as 2)

t T H k R m p + ∆ ∆ = (Equation
The value of t can be determined by the calibration procedure using materials of various pore sizes and this is the traditionally adopted procedure. The problem in doing so is that the underlying hypothesis is that the thickness of the adsorbed layer t does not vary with pore size. For small pores, the error on t can lead to large errors on the measurement of R p .

We proposed an alternative method to measure t by adding sequentially various amounts of liquid in the porous material [26]. As stated before this layer t represents the part of the solvent which does not crystallize. For solvents like CCl 4 which exhibit a further transition at low temperature the behaviour of this adsorbed layer is an open question. Does this solvent participate in the second transition? Is a new adsorbed layer created on the top of the first one? To get further insight into these questions we studied the behaviour of CCl 4 in mesoporous silica gels in this paper as described in the following section.

Experimental section

Mesoporous silica gels

Mesoporous monolithic silica gels (2.5 mm × 5.6 mm diameter cylinders) were prepared by the acid catalysed hydrolysis and condensation of a silicon alkoxide, following procedures reviewed elsewhere [START_REF] Hench | Sol-gel silica : processing, properties and technology transfer[END_REF]. Careful control of the aging time performed at 900°C allowed the production of samples with controlled textural properties. In this study two samples (A and B)

with different textural properties (Specific Surface Area (SSA), total pore volume (V p ) and pore size distribution (PSD)) were used. The textural characteristics of the samples were determined by N 2 sorption.

Gas sorption measurements

Textural data of the silica gels were determined on a Quantachrome Autosorb 1 apparatus.

The instrument permits a volumetric determination of the isotherms by a discontinuous static method at 77.4 K. The adsorptive gas was nitrogen with a purity of 99.999%. The cross sectional area of the adsorbate was taken to be 0.162 nm 2 for SSA calculations purposes. Prior to N 2 sorption, all samples were degassed at 100°C for 12 h under reduced pressure. The masses of the degassed samples were used in order to estimate the SSA. The BET [START_REF] Brunauer | [END_REF] SSA was determined by taking at least 4 points in the 0.05<P/P 0 <0.3 relative pressure range. The pore volume was obtained from the amount of nitrogen adsorbed on the samples up to a partial pressure taken in the range 0.994<P/P 0 <0.999. Pore size distributions were calculated from the desorption isotherm by the BJH method [29]. The mean pore radius R av was calculated according to

BET p av S V R 2 =
(Equation 3) corresponding to a cylindrical shape for the pores which is also the underlying hypothesis in equation (1).

Textural data for the two samples are displayed in Table 1. In this table, the modal pore diameter R p is also shown. This value fairly matches the R av derived from S BET measurement with cylindrical shape assumption thus confirming the validity of the hypothesis on the pore shape.

Sample SSA (m 2 /g)

Vp (cm 3 /g) R av (nm) R p (nm) A 183 1,

DSC measurements

A Mettler-Toledo DSC821 instrument calibrated (both for temperature and enthalpy) with metallic standards (In, Pb, Zn) and with n-heptane was used to record the thermal curves. It was equipped with an intracooler set allowing a scanning range of temperature between -70 and 600 °C. About 10 or 20 mg of the studied material was introduced into an aluminium DSC pan to undergo an appropriate temperature program. To allow the system to be in an equilibrium state, a slow freezing rate is required [30]. A rate of -0.7 °C/min was chosen.

Other slower cooling rates were tested which did not show any significant discrepancy. CCl 4 (Aldrich) of HPLC quality was used without any supplementary purification.

Results and discussion

Thermal behavior of free CCl 4

Bulk CCl 4 was studied before and its thermal phase transitions were well characterized [24,25]. It exhibits a complex thermal transitions system as shown in Figure 1.

-

(R) (M) (Liquid) (Liquid) (Liquid) (M) (M) (R) (R) (FCC) (FCC) Heat Flow (mW.g -1 )
T (°C) As it is cooled down, liquid CCl 4 crystallizes into Face-Centered-Cubic phase (FCC) which follows a phase transition upon further cooling to a Rhombohedral one (R) which, in turn, transforms to Monoclinic crystalline structure (M) around -48°C. Heating the (M) phase leads to (R) in a reversible way but upon heating (R) melts directly without transforming into the (FCC) phase. Observing the transition heat values (Figure 1), it can be pointed out that the R to-liquid transition releases an enthalpy (13.6 J/g) equivalent to the total heat liberated by the liquid-to-FCC (9.6 J/g) together with the FCC-to-R (3.8 J/g). Takei et al. [24] showed that both solid-to-solid and liquid-to-solid transitions of CCl 4 were strongly dependent on the average pore size of the material in which the liquid is confined. In particular, they demonstrated that the FCC-to-R transition is no longer observed when the pore radius is smaller than 16.5 nm, which is the case for our silica samples (see Table 1).

Because of the complex behaviour of CCl 4 upon cooling, we chose to use the heating of the solvent to limit the study to the M-to-R and R-to liquid transitions.

The two transitions were studied for CCl 4 confined in the two porous samples A and B.

Thermal behaviour of CCl 4 confined in sample A

The objective is to get quantitative information on the solvent undergoing both transitions (both confined and free solvent). In order to do so, we performed sequential addition of precise quantities of CCl 4 in the sample as described in [26]. Briefly, a known mass of silica gel (about 20 mg) is set in the DSC pan which is sealed. A small hole is drilled in the cover allowing further injection of known masses of carbon tetrachloride. This procedure allows a precise control of the added mass of solvent. After each thermal cycle, a new injection is performed.

For the first time to our knowledge, we also performed some experiments in the reverse way, by progressively evaporating the solvent starting from a large excess. This was performed by inert gas flushing in the DSC pan at 25 °C. The subsequent evaporation of the solvent is controlled by the flushing time. Obviously in this case we do not know the remaining mass of CCl 4 , but we can calculate it from the measured enthalpies.

The thermograms recorded for various quantities of CCl 4 added to sample A are shown in Figure 2. In this figure, 4 peaks can be observed which are labelled from 1 to 4 starting from low temperature to room temperature. The assignment of all peaks is presented in Table 2. In Figure 2, it can be seen that for small quantities of added CCl 4 , no transition is observed. This first step corresponds to the creation of the adsorbed layer t onto the surface of the porous silica gel. For higher amounts of CCl 4 , peak 3 appears at a temperature shifted with respect to the normal melting temperature of solid CCl 4 . At about the same time, peak 1 also appears corresponding to the M-to-R transition for the confined solvent. The intensities of these two peaks increase upon further addition of solvent until they remain constant coinciding with the appearance of peaks 2 and 4 corresponding to excess free solvent. A plot of the heats corresponding to peaks 3 and 4 (H3 and H4) as a function of the mass of CCl 4 added (m CCl4 ) is presented in Figure 4. The different steps are clearly observable. The point where H3 is different from zero corresponds to the end of the creation of the adsorbed layer allowing the determination of the quantity of solvent (m t ) participating in the formation of this layer. At a given point, H3 remains constant and this point corresponds to the total filling of the pores (H3 Max ) thus allowing the determination of the porous volume of the sample (see section 3.4). To measure the amounts of CCl 4 involved in each transition precisely, we need to know the transition enthalpy at the given temperature. These values are known for free solvent transiting at regular temperatures but not for confined solvent which undergoes transitions at lower temperature. From Figure 4, we can measure H3 Max at the point where all pores are filled in the constant part of the curve. In this case the enthalpy corresponds to a mass of solvent equal to the total mass added (m vp ) minus the mass required for the creation of the adsorbed layer (m t ) namely m=m vp -m t . We can then deduce the enthalpy of melting per gram for the confined solvent ∆H3 = 13.67 J.g -1 .

For the M-to-R transition, the situation is different. Because of the overlapping of peaks 1 and 2 we can only use the sum H1+H2. If we plot the evolution of H3 and H4 as a function of (H1+H2) we obtained the curves presented in Figure 5. It is worth noting that the points corresponding to desorption experiments complete nicely the points corresponding to sequential addition of solvent (empty and full symbols respectively).

The point where H4 differs from zero corresponds to the H1 Max value corresponding to the totality of solvent undergoing the transition (in this case H2=0). The enthalpy of transition ∆H1 can then be deduced ∆H1= 27.22 J.g -1 . Knowing ∆H1 and ∆H3, we can now calculate the masses of solvent which undergo the various transitions for all points. Figure 6 presents the correlation between these masses M3 and M1 (the indexes correspond to the different peaks). Progressive filling (•) and desorption (○) experiments. The line y=x is also plotted.

A clear correlation is observed between the confined solvent which undergoes the R-tliquid and the M-to-R transitions. This correlation is observed both for addition and evaporation experiments. This clearly confirms that all solvent undergoing the first transition also undergoes the second one.

This observation is further confirmed by the plot of Figure 7 showing the correlation between M2 and M4, the masses of free solvent which undergo the transitions 2 and 4. M2 is determined through the following equation:

( )

2 1 2 1 2 H H H H M Max ∆ - + = Equation (4)
where H1 Max is the enthalpy required for the M-to-R transition of the liquid totally filling the pores (see Figure 5) and ∆H2=46.6 J.g -1 the specific enthalpy for the M-to-R transition of free CCl 4 . Once again a clear correlation is observed between the two quantities confirming that all the solvent which has crystallised outside the pores undergoes the second transition at a regular temperature (no confinement). These conclusions also demonstrate that the layer t remains adsorbed and does not participate in the low temperature transition.

Thermal behaviour of CCl 4 confined in sample B

The same experiments and calculations were applied to sample B which presents smaller pores i.e. higher confinement.

The thermograms recorded for sample B filled with CCl 4 upon progressive evaporation are displayed in Figure 8. Because of the higher degree of confinement, the two peaks 1 and 2 are well resolved and can be discriminated.

-60 -50 -40 -30 -20

Heat Flow (a.u.)

T (°C) Following the same procedure, we can plot the evolution of H3 and H4 as a function of m CCl4 as performed in Figure 9. The plot of H1 and H2 as a function of m CCl4 (not shown here) can also be performed.

From these curves, ∆H1 and ∆H3 for sample B can be derived (∆H1=22.19 J.g -1 and ∆H3=10.13 J.g -1 ). Together with the known values of ∆H2 (46.6 J.g -1 ) and ∆H4 (25.07 J.g -1 ) they allow the calculation of the quantities of solvent which undergo the different transitions for the various experiments.

Calculation of porous volumes and thicknesses of adsorbed layers

Considering the curves of Figure 4 and 9, we can measure the mass of solvent corresponding to total filling of the pore m vp . The porous volume of the gel can then be calculated according to: ρ being the density of CCl 4 . We took the value at -20°C ( ρ=10.85 kmol.m -3 ) [31].

From the same figures, we can also measure the mass of the adsorbed layer m t , the thickness of this layer can be calculated according to: SSA being the specific surface area of the silica sample given in Table 1.

The results are summarized in Table 3.

Sample

V p (cm 3 .g -1 ) V N2 (cm The calculated value of V p are in very good agreement with the value measured by nitrogen sorption, the error is less than 2%. The values of t determined for samples A and B are also in good agreement with average value given in [25] after calibration procedure with samples of various pore size.

All calculations were performed with a constant value of ρ CCl4 measured at -20°C.

Obviously no information can be found in the literature for densities of carbon tetrachloride at lower temperatures since it is usually solid at these temperatures. Nevertheless using the value at -20°C, the error must be small. Furthermore, with the validity of such an approach demonstrated, we can now consider the exact porous volume to calculate the exact density of the confined solvent at various low temperatures.

Conclusions

The thermal behavior of carbon tetrachloride confined in two mesoporous silica gels of different porosity was studied. The two transitions (solid to liquid and Monoclinic to Rhombohedral) were measured and are affected by the confinement. The enthalpies of these two transitions were determined for the first time at the temperatures corresponding to confined solvent. Using these enthalpies, a clear correlation has been shown between the solvent undergoing the first and the second transitions. Consequently, the adsorbed layer which is created during the intrusion of CCl 4 inside the porosity of the silica gels is kept constant and does not participate in the two transitions. The thickness of this layer was measured for both samples and is found to be slightly dependant on the pore radius. Finally the porous volumes of the silica gels have been measured and the values agree very closely with those derived from nitrogen sorption isotherm. It has been demonstrated that using porous samples of known porosity (measured by mercury intrusion porosimetry or gas sorption analysis) could allow the measurement of thermodynamical data of confined liquids (Enthalpy of transition, density,….).

Figure 1 :

 1 Figure 1: DSC thermogram of pure CCl 4 showing the different transitions.

2 :

 2 Labelling of the different peaks observed in the DSC curves.

Figure 3

 3 Figure 3 presents the DSC curves recorded upon desorbing the CCl 4 by gas flushing. As can

Figure 2 :

 2 Figure 2: Thermograms recorded for various amount of CCl 4 added to sample A.

Figure 3 :

 3 Figure 3: Thermograms recorded for various flushing times for sample A filled with CCl 4 .

Figure 4 :

 4 Figure 4: Evolution of H3 (circles) and H4 (squares) as a function of the mass of CCl 4

Figure 5 :

 5 Figure 5: Evolution of H3 (circles) and H4 (squares) for progressive filling (full symbols) and

Figure 6 :

 6 Figure 6: Evolution of the mass of confined solvent undergoing transition M-to-R (M1) as a

Figure 7 :

 7 Figure 7: Correlation between the masses of free solvent undergoing the R-to-liquid (M2)

Figure 8 :

 8 Figure 8: Thermograms recorded for various flushing times for sample B filled with CCl 4 .

Figure 9 :

 9 Figure 9: Evolution of H3 (circles) and H4 (squares) as a function of the mass of CCl 4

Figure 10 :Scheme 1 :

 101 Figure 10: Correlation between M3 and M1 (circles) and M2 and M4 (squares) for

3 *

 3 1 Laboratoire de Photochimie Moléculaire et Macromoléculaire, UMR CNRS 6005

2 Laboratoire de Thermodynamique des Solutions et des Polymères, UMR CNRS 6003 3 Laboratoire des Matériaux Inorganiques, UMR CNRS 6002 TransChiMiC Ecole Nationale Supérieure de Chimie de Clermont Ferrand & Université Blaise Pascal, 24 Avenue des landais 63177 Aubière Cedex, FRANCE.

Table 1 :

 1 Porous characteristics of the silica gels samples.

			327	14,5	14,25
	B	166	0,991	11,9	8,7

Acknowledgements

Financial support from the French ANR under project Nanothermomécanique (ACI Nanosciences N°108) is gratefully acknowledged. The authors would like to thank A. Gordon and Pr S. Turrell for careful reading of the paper.

Contact author for correspondence and return of proofs:

e-mail : j-marie.nedelec@univ-bpclermont.fr