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Terpenoid emissions from heated needles of Pinus sylvestris and their potential influences on forest fires
Acta Ecologica Sinica 32 (2012) 33–37
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Terpenoid emissions from heated needles of Pinus sylvestris and their potential influences on forest fires Feng-Jun Zhao 1, Li-Fu Shu ⇑, Qiu-Hua Wang Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry; Forest Protection Laboratory of State Forestry Administration, Beijing, 100091, China
a r t i c l e
i n f o
Article history: Received 27 October 2010 Revised 14 April 2011 Accepted 10 June 2011
Keywords: Forest fire Fuel Terpenoids Volatile organic compound (VOC) Pinus sylvestris
a b s t r a c t It is assumed that terpenoids in biomass-derived fuels have important influences on forest fires due to their enormous flammability. The fires consuming terpenoid-rich fuels always burn violently and spread fast. But the mechanism how terpenoids influence occurrence and propagation of fires are little known. Some terpenoids are volatile organic compounds (VOC) as they are released from vegetation and litter in natural environment. Hence, they contribute to the characteristic composition of the ambient air. Many studies have reported terpenoid emissions in natural environment from different perspective. Nevertheless there are only a few studies concerning terpenoid emissions from heated fuels. The present study explored the differences in terpenoid emissions from needles of Pinus sylvestris var. mongolica under natural and heated conditions. Terpenoids were sampled on Tenax-TA and analyzed using Thermal Desorption–Gas Chromatography–Mass Spectrometry (TD–GC–MS). The results showed that the emission rate of terpenoid from P. sylvestris in natural environment was low (0.167 lg g1 h1 DW). However, terpenoid emissions dramatically increased at the temperature of 200 °C, with a major component, a-pinene. Within 15 min, the emission of terpenoids emitted by heated needles was up to 16.314 lg g1 DW for total and 10.321 lg g1 DW for a-pinene. These considerable emissions of terpenoids from heated needles will have great influences on occurrence and propagation of forest fires. Ó 2011 Ecological Society of China. Published by Elsevier B.V.
1. Introduction Forest fires are combustion of biomass-derived fuels, and usually result in significant property damage and casualties. Meanwhile, the forest fires also destroy the ecological environments. Despite differences in height, density and distribution, forest fuels are composed by cellulose (38–50%), hemicellulose (7–26%), lignin (23–34%), extractive (<15%) and mineral (<1%) [1]. Terpenoids and resin are main components of extractive [2]. Resin is non-volatile. Terpenoids are unsaturated hydrocarbons that belong to a family of natural products issued from plant secondary metabolism and made up of isoprene units (C5). Some of these compounds, such as the volatile monoterpenes (C10) and the semi-volatile sesquiterpenes (C15), are stored in specialized structures of plants. Monoterpenes and sesquiterpenes are volatile organic compounds (VOC) as they are released from vegetation and litter in natural environment [3]. Hence, they contribute to the characteristic composition of ambient air. Terpenoids in biomass-derived fuels have important influences on forest fires. Terpenoid concentration in leaf litter was positively ⇑ Corresponding author. 1
E-mail address: [email protected] (L.-F. Shu). Mobile: +13621114766.
correlated to flame height and negatively correlated to both flame residence time and ignition delay [4]. The fires consuming terpenoid-rich fuels always burn violently and spread fast. A good example is the fire occurred on February 7, 2009 in Australia [5]. It was reported that the fastest fires maintained forward rates of 10–12 km h1 for a number of hours, with peak fire-line intensities in the range of 100,000–150,000 kW m1. The main reason for so fast speed of fires is that in Australian forests more than 90% vegetation are eucalyptuses. It is well known that eucalyptuses are rich in terpenoids. In addition, eruptive fire phenomenon has been always observed in dense, terpenoid-rich species of vegetation [6]. One example of eruptive fire is the Palasca fire in Corsica (France). It was described that the fire originally appeared to be relatively minor, but turned into a sudden ignition of a large area of vegetation. In less than 1 min, the flames advanced extremely rapidly over an area of nearly 6 hectares, completely overwhelming some of the fire crew; two firefighters were killed and five others severely burned. After the sudden increase in speed, the propagation became normal and the fire intensity gradually diminished [7,8]. What is the main reason for the eruptive fire? Dold [9] considered that flammable gases accumulated in specific geographical zones (e.g. small valley, canyons) would burn rapidly and violently. And Peuch [10] thought that these flammable gases might be emitted
1872-2032/$ - see front matter Ó 2011 Ecological Society of China. Published by Elsevier B.V. doi:10.1016/j.chnaes.2011.06.002
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by terpenoid-rich fuels. Though researchers convince that the terpenoids have important influences on forest fires, up to now, little is known about the mechanism how terpenoids influence occurrence and propagation of fires. A particular feature of terpenoid-rich species is that they can release terpenoid-VOC [11]. The VOC emission are in close relation with temperature and radiation [12–14], and can be emitted at high levels under heated condition [15]. Convective and radiative effects from the fire front transited the heat fluxes from the combustion zone to the unburned fuels [16]. Then it is hypothesized that emissions from unburned terpenoid-rich fuels in fire environment will be high, and these abundant emissions will have great effects on occurrence and propagation of fires. Recently, a few studies have been operated to prove the hypothesis. Chetehouna [17] studied the terpenoid-VOC emission from heated vegetation at different temperature and concluded that the major peak of VOC emission from heated plant is about 175 °C. And the quantities of VOCs emitted by Rosmarinus officinalis shrubs under certain fire conditions are capable of creating an accelerating forest fire. Barboni [8] found that the maximum terpenoid-VOC emitted by P. laricio Poir., P. pinaster Ait. and C. monspeliensis L. were 147.9, 11.6 and 56.0 g m3 respectively. And the quantities of VOC emitted by P. laricio and C. monspeliensis were sufficiently high for eruptive fires to occur. P. sylvestris var. mongolica is an important timber species in Daxing’anling Mountains in northeast China. Its leaves and branches are rich in terpenoids [18]. The fires occurred in P. sylvestris forests always burn violently [19]. P. sylvestris has a wide distribution in the world, and it is called Scot pine in Europe. Many studies have reported terpenoid emissions from P. sylvestris needles in natural environment [20–25], and the major terpenoidVOC were monoterpenes, such as, a-pinene, 3-carene, b-pinene, with minor sesquiterpene, b-caryophyllene. However, there are few studies concerning the terpenoid emissions from heated needles of P. sylvestris. The aim of present study were: (i) to identify and quantify the terpenoids released from needles P. sylvestris under different temperature conditions; (ii) to compare the quantities of terpenoids from needles under different temperature conditions; (iii) to assess the potential influences of emission from heated needles on forest fires.
Fig. 1. Sampling of VOC released from P. sylvestris in natural environment.
Fig. 2. Sampling of VOC emitted from heated needles of P. sylvestris (200 °C).
After sampling had been finished, Tenax-tubes with adsorbed volatiles were sealed and placed in an icebox until arrival at laboratory. 2.1.2. Sampling of VOC emitted from heated needles of P. sylvestris Needle samples were collected from P. sylvestris artificial forest in Daxing’anling Mountain too. The method for sampling of VOC emitted from heated needles was similar with that in natural environment (Fig. 2). The modification was that the Reynolds™ bag was changed into a vacuum oven (50 L) in order to heat needles at designed temperature. The inlet and outlet of the vacuum oven were connected with Teflon tubes. In first step, the vacuum oven was heated until the temperature was up to 200 °C. Then needles (about 15 g DW, dry weight) were put into the oven. Five minutes later, sampling process started. Sampling time was 10 min. The flow rate of pump was 0.1 L min1. After sampling process had been finished, Tenax-tubes with adsorbed volatiles were sealed and placed in an icebox. In total, nine samples were collected.
2. Materials and methods
2.2. TD–GC–MS
2.1. Sampling methods
VOC samples were analyzed with Thermal Desorption–Gas Chromatography–Mass Spectrometry (TD–GC–MS) (CDS 8000, US; Agilent 6890-5973 N GC-MSD, US). Firstly, volatiles in Tenaxtubes were released from the absorbent by heating the trap in a sample concentrator at 200 °C for 10 min, then carrier gas (Helium) flushed the volatiles toward a cold trap at 30 °C. In a second step, by ballistic heating of the cold trap to 250 °C, the volatiles were transferred into the analytical column (DB-624 capillary column, 30 m 0.25 mm 1.4 lm). The column was programmed to increase from 30 °C to 230 °C at 2 °C min1 with an isotherm at 230 °C for 10 min. The transfer line temperature was at 260 °C. The Helium pressure was 6 kPa. The MS was operated in the 70 eV EI ionization mode. Scanning was done from 29 to 500 amu.
2.1.1. Sampling of VOC released from P. sylvestris in natural environment VOC samples were collected in P. sylvestris artificial forest in Daxing’anling Mountain, planting spacing 2 m 2 m. The sampling was carried out on fine days, i.e., 6, 15, 24, 30 of July, 2009. It was always no wind or breeze on these days. Sampling time was at noon, due to highest air temperature of the day, 28–30 °C approximately. Every time, three samples were collected from three different trees. In total, 12 samples were collected. Closed sampling method was illustrated in following Fig. 1. The tree branches with 50–70 g (fresh weight) needles was carefully wrapped within a 18 L Reynolds™ oven bag, which releases and absorbs few volatiles. The branches were still attached to the tree when the VOC samples were collected. A Tenax-tube (15.0 0.3 cm; Chrompack, Middelburg, the Netherlands) containing Tenax-TA (60–80 mesh, Chrompack) was as volatile trap. A portable air sampler (QC-1; Beijing Municipal Institute of Labor Protection, China) served as a pump, with a flow rate 0.1 L min1. All parts were connected by Teflon tubes, which are not volatile. Sampling time was 30 min.
2.3. Identification and quantification The identification of individual components was based on: (i) comparison of their GC retention indices (RIcal) on non-polar column with those of literature (RIlit); (ii) computer matching on mass spectra of National Institute of Standards and Technology NIST WebBook (http://webbook.nist.gov/chemistry/).
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Table 1 Terpenoid emissions from P. sylvestris needles under different conditions (BP: boiling point; FP: flash point; RIcal: calculated retention indices; RIlit: retention indices of literature; +, compounds identified). No. 1 2 3 4 5 6 7 8 9 10 A B C
Name
Formula
CAS
BP (°C)
FP (°C)
RIcal
RIlit
a-tricyclene a-pinene
D-limonene
C10H16 C10H16 C10H16 C10H16 C10H16 C10H16 C10H16
000508-32-7 007785-70-8 000079-92-5 000127-91-3 000123-35-3 013466-78-9 005989-27-5
150–152 155–156 159–160 165–167 167–168 168–169 175–176
26 32 36 31 37 46 48
923 935 952 980 990 1011 1030
924A 936B 950B 978B 982 B 1010B 1025B
Neo-allo-ocimene Longicyclene Longifolene
C10H16 C15H24 C15H24
007216-56-0 001137-12-8 000475-20-7
196–198 252–254 254–255
69 92 98
1130 1396 1439
1131C – 1402A
Camphene b-pinene b-myrcene 3-carene
Natural
Heated + + + +
+ + + +
+ +
+
+ + +
Literature [27]. Literature [17]. Literature [28].
The quantification of a-pinene was carried out by direct injection into sorbent tubes for analysis by TD–GC–MS. The calibration was done with masses ranging between 1.3 ng and 13 lg. Triplicate injections of standards were made of set of concentration for the curves of external calibration standard. The correlation coefficient of the linear regression of the external calibration curve was 0.96. The quantification of other terpenoids was a-pinene equivalent based on all ions from SM detector. According to Eq. (1) to calculate emission rates of different terpenoids released from needles in natural environments:
R¼
M1 V 60 R1 t M DW t
ð1Þ
where R the emission rate of different terpenoids from needles, lg g1 h1 DW, M1 the amount of different terpenoids in the Tenax-tube, lg, V the volume of Reynolds™ oven bag, 18 L, R1 the air flow rate of pump, 0.1 L min1, t the sampling time, 30 min, MDW the mass of needles, g. According to Eq. (2) to calculate the emission of different terpenoids from heated needles within 15 min:
E¼
M1 V R1 t M DW
ð2Þ
where E the emission of different terpenoids from heated needles within 15 min, lg g1 DW, M1 the amount of different terpenoids in Tenax-tube, lg, V the volume of vacuum oven, 50 L, R1 the air flow rate of pump, 0.1 L min1, t the sampling time, 10 min, MDW the mass of needles, g. 3. Results and discussion
released by P. sylvestris was 3-carene in southern Finland, but
a-pinene in northern Finland [12,22]. In general, the terpenoids identified in our study agree with the results in mentioned studies. However, there are a few differences. In our study, 3-carene did not be detected in the VOC released by P. sylvestris in natural environment. In addition, D-limonene was the sub-dominant monoterpene following a-pinene. And the sesquiterpenes (longicyclene and longifolene) detected in our experiment were different with literatures [12,22], in which b-caryophyllene was detected as sesquiterpene. The main reasons for these differences maybe lying in population origin, that is to say, P. sylvestris from different population origin release different terpenoids [25]. The terpenoids emitted by heated needles were all monoterpenes (Table 1). The similar results were reported in volatiles emitted from heated Rosmarinus officinalis plants [17], from sawdust of Norway spruce (Picea abies) when drying [15], and from eucalypt at different high temperature [13,14]. Terpenoids identified under natural and heated conditions are different. Firstly, sesquiterpenes were only detected under natural condition. As far as monoterepenes, b-myrcene was only detected under natural condition, and a-tricyclene, 3-carene and neo-allo-ocimene were only detected under heated condition. 3.2. Terpenoid emissions Terpenoid emission rates in natural environment were presented in Fig. 3. The total emission rate of terpenoids was 0.167 lg g1 h1 DW. The result is little lower than that of previous reported studies, i.e., 1.2 lg g1 h1 [3], 1.35 lg g1 h1 [22], 2.0 lg g1 h1 [20], 2.1 lg g1 h1 [21]. The emission rate of
3.1. Terpenoid identification 0.12
TD–GC–MS analysis of the VOC led to the identification of 10 compounds (Table 1): Eight monoterpenes (1–8 in Table 1), 2 sesquiterpenes (9–10 in Table 1). Analysis of the VOC released from needles of P. sylvestris. in natural environment enabled the identification of seven compounds: 5 monoterpenes (2–5, 7 in Table 1), 2 sesquiterpenes (9–10 in Table 1), with a-pinene as major component. The VOC emitted by heated needles (200 °C) were characterized by very high level of a-pinene, and 7 compounds identified were all monoterpenes (1–4, 6–8 in Table 1). The results of terpenoids released by P. sylvestris in natural environment are in accordance with the data in the literature concerning the VOC emissions of this species. Many studies reported that major compound of VOC released by P. sylvestris was a-pinene, followed by b-pinene, myrcene, 3-carene, and so on [20,23,26]. Whereas, some studies concluded that the major compound
0.1 0.08 0.06 0.04 0.02 0 1
2
3
4
5
6
7
Fig. 3. Emission rates of different terpenoids released by needles of P. sylvestires in natural environment (bars indicate the standard error) (n = 12) (1 a-pinene, 2 camphene,3 b-pinene, 4 b-myrcene, 5 D-limonene, 6 longicyclene, and 7 longifolene).
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temperature and no turbulence conditions. It is inferred that the conjugation of accumulation and flammability may be capable of causing a gas phase flame propagation, which together with dry vegetation and an adjacent specific terrain may result in unusual fast fire propagation. 4. Conclusion The present study focused on the terpenoid emissions from P. sylvestris needles under different temperature conditions. Several conclusions can be drawn as follows: Fig. 4. Emissions of the terpenoids emitted by heated needles of P. sylvestires (200 °C) (bars indicate the standard error) (n = 9) (1 a-tricyclene, 2 a-pinene, 3 camphene, 4 b-pinene, 5 3-carene, 6 D-limonene, and 7 neo-allo-ocimene).
a-pinene was highest among all terpenoids, 0.077 lg g1 h1, followed by D-limonene, 0.043 lg g1 h1. Terpenoid emissions are in close relation with temperature [12]. Along with the temperature increases, the terpenoid emissions increase. The highest emission will appear at a specific temperature, as far as Rosmarinus officinalis the specific temperature is approximately 175 °C [17]. When the temperature is over the specific temperature, the emission will decrease due to the thermal degradation and/or polymerisation of the terpenic compounds. The boiling points of terpenoids, particularly monoterpenes, are below 200 °C (see Table 1). So in our experiment, 200 °C was designed as the heated temperature, which is in coincidence with that in many previous studies [13–15]. Terpenoid emissions from heated needles within 15 min were present in Fig. 4. The total emission is 16.314 lg g1 DW. The similar result was found in terpenoid emissions from Norway spruce (Picea abies) sawdust during drying, 13–250 lg g1 [15] depending on the temperature (140 °C, 170 °C or 200 °C) of the drying medium and the final moisture content of the sawdust. The emission rate of a-pinene was biggest, 10.321 lg g1, followed by b-pinene, 2.189 lg g1, camphene, 1.699 lg g1. 3.3. Potential influences on forest fires The results indicated that the terpenoids emitted by heated needles are markedly higher than that by needles in natural environment. The emission rate of terpenoids released by P. sylvestris in natural environment was only 0.167 lg g1h1. However, 16.314 lg of total monoterpenes and 10.321 lg of a-pinene would be emitted into air within 15 min by 1 g heated needles. In our experiment, the moisture content of fresh needles was 53.57%. The mass of total fresh needles of one tree of P. sylvestris was 11.2146 kg approximately [29]. Hence, the terpenoid emissions per tree within 15 min under heated condition (200 °C) can be calculated as follows:
16:314 lg g1 11:2146 kg 53:57% ¼ 98:00 mg Terpenoids are extremely flammable, with high calorific value, relatively low Flash Point (FP) and Low Flammability Limit (LFL), for instance, a-pinene shows a calorific value of 45 MJ kg1 [30], a FP of 32 °C, and a LFL of 0.7% (v/v) (see Table 1). LFL values of a-pinene can also be expressed in grams per cubic metre of air at 298 K and 100 kPa (1 bar), 38.4 g m3 [8]. Depending on the results, the LFL of terpenoids emitted by heated needles of P. sylvestris will be obtained easily in fire environment. The ignition and burning of these terpenoids will contribute to the acceleration of an ongoing spreading fire. In addition, being heavier than air, these emitted terpenoids will be retained in the accumulated gas in certain geographical zones (e.g., small valleys, canyons) under high
(i) TD–GC–MS analysis of the VOC led to the identification of 10 terpenoid compounds: 8 monoterpenes and 2 sesquiterpenes. five monoterpenes and 2 sesquiterpenes were detected in the VOC released from P. sylvestris. in natural environment. However, the 7 terpenoid compounds of VOC emitted by heated needles (200 °C) were all monoterpenes. The major compounds of VOC present were a-pinene both under natural and heated conditions. (ii) The terpenoid emission rate of terpenoids released by P. sylvestris in natural environment was low, 0.167 lg g1h1 DW. However, the terpenoid emission emitted by heated needles was considerable, 16.314 lg g1 DW for total and 10.321 lg g1 DW for a-pinene, within 15 min, at temperature of 200 °C. (iii) It is concluded that the terpenoids emitted by heated needles are markedly higher than that by needles in natural environment. Because of high calorific value, relatively low FP and LFL, the ignition and burning of emissions emitted by unburned terpenoid-rich fuels in fire environment will contribute to the acceleration of an ongoing spreading fire. In addition, terpenoid emissions can be retained in the accumulated gases in topographically uneven areas (e.g. canyons or valleys) under high temperature and no turbulence conditions. The combination of concentrations above the LFL and uneven geographic areas can lead to situations with a high ignition potential, and thus increase the risk of eruptive fire.
Acknowledgments The present study was financially supported by National Natural Science Foundation of China (30872037 and 31070587) and Open Project Program of State Key Laboratory of Fire Science, University of Science and Technology of China (HZ2008-KF08). References [1] T. Barboni, G. Pellizzaro, B. Arca, et al., Analysis and origins of smoke from ligno-cellulosic fuels, Journal of Analytical and Applied Pyrolysis 89 (1) (2010) 60–65. [2] M. Cannac, T. Barboni, L. Ferrat, et al., Oleoresin flow and chemical composition of corsican pine (Pinus nigra subsp. laricio) in response to prescribed burnings, Forest Ecology and Management 257 (4) (2009) 1247–1254. [3] J. Rinne, H. Hakola, T. Laurila, et al., Canopy scale monoterpene emissions of Pinus sylvestris dominated forests, Atmospheric Environment 34 (7) (2000) 1099–1107. [4] E. Ormeño, B. Céspedes, I.A. Sánchez, et al., The relationship between terpenes and flammability of leaf litter, Forest Ecology and Management 257 (2) (2009) 471–482. [5] L. McCaw, T. Bannister, A. Sullivan, et al., Weather and fire behaviour during the Victorian bushfires of 7 February, 2009, Environment International 35 (2) (2009) 342–352. [6] N. Raffalli, C. Picard, F. Giroud, Safety and awareness of people involved in forest fires suppression, in: Fire Research and Wildland Fire Safety: Proceedings of IV International Conference on Forest Fire Research 2002 Wildland Fire Safety Summit, Coimbra, Portugal, 2002. [7] J.W. Dold, A. Simeoni, A. Zinoviev, et al., The Palasca fire, September 2000: Eruption or Flashover? Recent Forest Fire Related Accidents in Europe, Viegas, 2009, pp. 54–64.
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Terpenoid emissions from heated needles of Pinus sylvestris and their potential influences on forest fires Feng-Jun Zhao 1, Li-Fu Shu ⇑, Qiu-Hua Wang Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry; Forest Protection Laboratory of State Forestry Administration, Beijing, 100091, China
a r t i c l e
i n f o
Article history: Received 27 October 2010 Revised 14 April 2011 Accepted 10 June 2011
Keywords: Forest fire Fuel Terpenoids Volatile organic compound (VOC) Pinus sylvestris
a b s t r a c t It is assumed that terpenoids in biomass-derived fuels have important influences on forest fires due to their enormous flammability. The fires consuming terpenoid-rich fuels always burn violently and spread fast. But the mechanism how terpenoids influence occurrence and propagation of fires are little known. Some terpenoids are volatile organic compounds (VOC) as they are released from vegetation and litter in natural environment. Hence, they contribute to the characteristic composition of the ambient air. Many studies have reported terpenoid emissions in natural environment from different perspective. Nevertheless there are only a few studies concerning terpenoid emissions from heated fuels. The present study explored the differences in terpenoid emissions from needles of Pinus sylvestris var. mongolica under natural and heated conditions. Terpenoids were sampled on Tenax-TA and analyzed using Thermal Desorption–Gas Chromatography–Mass Spectrometry (TD–GC–MS). The results showed that the emission rate of terpenoid from P. sylvestris in natural environment was low (0.167 lg g1 h1 DW). However, terpenoid emissions dramatically increased at the temperature of 200 °C, with a major component, a-pinene. Within 15 min, the emission of terpenoids emitted by heated needles was up to 16.314 lg g1 DW for total and 10.321 lg g1 DW for a-pinene. These considerable emissions of terpenoids from heated needles will have great influences on occurrence and propagation of forest fires. Ó 2011 Ecological Society of China. Published by Elsevier B.V.
1. Introduction Forest fires are combustion of biomass-derived fuels, and usually result in significant property damage and casualties. Meanwhile, the forest fires also destroy the ecological environments. Despite differences in height, density and distribution, forest fuels are composed by cellulose (38–50%), hemicellulose (7–26%), lignin (23–34%), extractive (<15%) and mineral (<1%) [1]. Terpenoids and resin are main components of extractive [2]. Resin is non-volatile. Terpenoids are unsaturated hydrocarbons that belong to a family of natural products issued from plant secondary metabolism and made up of isoprene units (C5). Some of these compounds, such as the volatile monoterpenes (C10) and the semi-volatile sesquiterpenes (C15), are stored in specialized structures of plants. Monoterpenes and sesquiterpenes are volatile organic compounds (VOC) as they are released from vegetation and litter in natural environment [3]. Hence, they contribute to the characteristic composition of ambient air. Terpenoids in biomass-derived fuels have important influences on forest fires. Terpenoid concentration in leaf litter was positively ⇑ Corresponding author. 1
E-mail address: [email protected] (L.-F. Shu). Mobile: +13621114766.
correlated to flame height and negatively correlated to both flame residence time and ignition delay [4]. The fires consuming terpenoid-rich fuels always burn violently and spread fast. A good example is the fire occurred on February 7, 2009 in Australia [5]. It was reported that the fastest fires maintained forward rates of 10–12 km h1 for a number of hours, with peak fire-line intensities in the range of 100,000–150,000 kW m1. The main reason for so fast speed of fires is that in Australian forests more than 90% vegetation are eucalyptuses. It is well known that eucalyptuses are rich in terpenoids. In addition, eruptive fire phenomenon has been always observed in dense, terpenoid-rich species of vegetation [6]. One example of eruptive fire is the Palasca fire in Corsica (France). It was described that the fire originally appeared to be relatively minor, but turned into a sudden ignition of a large area of vegetation. In less than 1 min, the flames advanced extremely rapidly over an area of nearly 6 hectares, completely overwhelming some of the fire crew; two firefighters were killed and five others severely burned. After the sudden increase in speed, the propagation became normal and the fire intensity gradually diminished [7,8]. What is the main reason for the eruptive fire? Dold [9] considered that flammable gases accumulated in specific geographical zones (e.g. small valley, canyons) would burn rapidly and violently. And Peuch [10] thought that these flammable gases might be emitted
1872-2032/$ - see front matter Ó 2011 Ecological Society of China. Published by Elsevier B.V. doi:10.1016/j.chnaes.2011.06.002
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by terpenoid-rich fuels. Though researchers convince that the terpenoids have important influences on forest fires, up to now, little is known about the mechanism how terpenoids influence occurrence and propagation of fires. A particular feature of terpenoid-rich species is that they can release terpenoid-VOC [11]. The VOC emission are in close relation with temperature and radiation [12–14], and can be emitted at high levels under heated condition [15]. Convective and radiative effects from the fire front transited the heat fluxes from the combustion zone to the unburned fuels [16]. Then it is hypothesized that emissions from unburned terpenoid-rich fuels in fire environment will be high, and these abundant emissions will have great effects on occurrence and propagation of fires. Recently, a few studies have been operated to prove the hypothesis. Chetehouna [17] studied the terpenoid-VOC emission from heated vegetation at different temperature and concluded that the major peak of VOC emission from heated plant is about 175 °C. And the quantities of VOCs emitted by Rosmarinus officinalis shrubs under certain fire conditions are capable of creating an accelerating forest fire. Barboni [8] found that the maximum terpenoid-VOC emitted by P. laricio Poir., P. pinaster Ait. and C. monspeliensis L. were 147.9, 11.6 and 56.0 g m3 respectively. And the quantities of VOC emitted by P. laricio and C. monspeliensis were sufficiently high for eruptive fires to occur. P. sylvestris var. mongolica is an important timber species in Daxing’anling Mountains in northeast China. Its leaves and branches are rich in terpenoids [18]. The fires occurred in P. sylvestris forests always burn violently [19]. P. sylvestris has a wide distribution in the world, and it is called Scot pine in Europe. Many studies have reported terpenoid emissions from P. sylvestris needles in natural environment [20–25], and the major terpenoidVOC were monoterpenes, such as, a-pinene, 3-carene, b-pinene, with minor sesquiterpene, b-caryophyllene. However, there are few studies concerning the terpenoid emissions from heated needles of P. sylvestris. The aim of present study were: (i) to identify and quantify the terpenoids released from needles P. sylvestris under different temperature conditions; (ii) to compare the quantities of terpenoids from needles under different temperature conditions; (iii) to assess the potential influences of emission from heated needles on forest fires.
Fig. 1. Sampling of VOC released from P. sylvestris in natural environment.
Fig. 2. Sampling of VOC emitted from heated needles of P. sylvestris (200 °C).
After sampling had been finished, Tenax-tubes with adsorbed volatiles were sealed and placed in an icebox until arrival at laboratory. 2.1.2. Sampling of VOC emitted from heated needles of P. sylvestris Needle samples were collected from P. sylvestris artificial forest in Daxing’anling Mountain too. The method for sampling of VOC emitted from heated needles was similar with that in natural environment (Fig. 2). The modification was that the Reynolds™ bag was changed into a vacuum oven (50 L) in order to heat needles at designed temperature. The inlet and outlet of the vacuum oven were connected with Teflon tubes. In first step, the vacuum oven was heated until the temperature was up to 200 °C. Then needles (about 15 g DW, dry weight) were put into the oven. Five minutes later, sampling process started. Sampling time was 10 min. The flow rate of pump was 0.1 L min1. After sampling process had been finished, Tenax-tubes with adsorbed volatiles were sealed and placed in an icebox. In total, nine samples were collected.
2. Materials and methods
2.2. TD–GC–MS
2.1. Sampling methods
VOC samples were analyzed with Thermal Desorption–Gas Chromatography–Mass Spectrometry (TD–GC–MS) (CDS 8000, US; Agilent 6890-5973 N GC-MSD, US). Firstly, volatiles in Tenaxtubes were released from the absorbent by heating the trap in a sample concentrator at 200 °C for 10 min, then carrier gas (Helium) flushed the volatiles toward a cold trap at 30 °C. In a second step, by ballistic heating of the cold trap to 250 °C, the volatiles were transferred into the analytical column (DB-624 capillary column, 30 m 0.25 mm 1.4 lm). The column was programmed to increase from 30 °C to 230 °C at 2 °C min1 with an isotherm at 230 °C for 10 min. The transfer line temperature was at 260 °C. The Helium pressure was 6 kPa. The MS was operated in the 70 eV EI ionization mode. Scanning was done from 29 to 500 amu.
2.1.1. Sampling of VOC released from P. sylvestris in natural environment VOC samples were collected in P. sylvestris artificial forest in Daxing’anling Mountain, planting spacing 2 m 2 m. The sampling was carried out on fine days, i.e., 6, 15, 24, 30 of July, 2009. It was always no wind or breeze on these days. Sampling time was at noon, due to highest air temperature of the day, 28–30 °C approximately. Every time, three samples were collected from three different trees. In total, 12 samples were collected. Closed sampling method was illustrated in following Fig. 1. The tree branches with 50–70 g (fresh weight) needles was carefully wrapped within a 18 L Reynolds™ oven bag, which releases and absorbs few volatiles. The branches were still attached to the tree when the VOC samples were collected. A Tenax-tube (15.0 0.3 cm; Chrompack, Middelburg, the Netherlands) containing Tenax-TA (60–80 mesh, Chrompack) was as volatile trap. A portable air sampler (QC-1; Beijing Municipal Institute of Labor Protection, China) served as a pump, with a flow rate 0.1 L min1. All parts were connected by Teflon tubes, which are not volatile. Sampling time was 30 min.
2.3. Identification and quantification The identification of individual components was based on: (i) comparison of their GC retention indices (RIcal) on non-polar column with those of literature (RIlit); (ii) computer matching on mass spectra of National Institute of Standards and Technology NIST WebBook (http://webbook.nist.gov/chemistry/).
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Table 1 Terpenoid emissions from P. sylvestris needles under different conditions (BP: boiling point; FP: flash point; RIcal: calculated retention indices; RIlit: retention indices of literature; +, compounds identified). No. 1 2 3 4 5 6 7 8 9 10 A B C
Name
Formula
CAS
BP (°C)
FP (°C)
RIcal
RIlit
a-tricyclene a-pinene
D-limonene
C10H16 C10H16 C10H16 C10H16 C10H16 C10H16 C10H16
000508-32-7 007785-70-8 000079-92-5 000127-91-3 000123-35-3 013466-78-9 005989-27-5
150–152 155–156 159–160 165–167 167–168 168–169 175–176
26 32 36 31 37 46 48
923 935 952 980 990 1011 1030
924A 936B 950B 978B 982 B 1010B 1025B
Neo-allo-ocimene Longicyclene Longifolene
C10H16 C15H24 C15H24
007216-56-0 001137-12-8 000475-20-7
196–198 252–254 254–255
69 92 98
1130 1396 1439
1131C – 1402A
Camphene b-pinene b-myrcene 3-carene
Natural
Heated + + + +
+ + + +
+ +
+
+ + +
Literature [27]. Literature [17]. Literature [28].
The quantification of a-pinene was carried out by direct injection into sorbent tubes for analysis by TD–GC–MS. The calibration was done with masses ranging between 1.3 ng and 13 lg. Triplicate injections of standards were made of set of concentration for the curves of external calibration standard. The correlation coefficient of the linear regression of the external calibration curve was 0.96. The quantification of other terpenoids was a-pinene equivalent based on all ions from SM detector. According to Eq. (1) to calculate emission rates of different terpenoids released from needles in natural environments:
R¼
M1 V 60 R1 t M DW t
ð1Þ
where R the emission rate of different terpenoids from needles, lg g1 h1 DW, M1 the amount of different terpenoids in the Tenax-tube, lg, V the volume of Reynolds™ oven bag, 18 L, R1 the air flow rate of pump, 0.1 L min1, t the sampling time, 30 min, MDW the mass of needles, g. According to Eq. (2) to calculate the emission of different terpenoids from heated needles within 15 min:
E¼
M1 V R1 t M DW
ð2Þ
where E the emission of different terpenoids from heated needles within 15 min, lg g1 DW, M1 the amount of different terpenoids in Tenax-tube, lg, V the volume of vacuum oven, 50 L, R1 the air flow rate of pump, 0.1 L min1, t the sampling time, 10 min, MDW the mass of needles, g. 3. Results and discussion
released by P. sylvestris was 3-carene in southern Finland, but
a-pinene in northern Finland [12,22]. In general, the terpenoids identified in our study agree with the results in mentioned studies. However, there are a few differences. In our study, 3-carene did not be detected in the VOC released by P. sylvestris in natural environment. In addition, D-limonene was the sub-dominant monoterpene following a-pinene. And the sesquiterpenes (longicyclene and longifolene) detected in our experiment were different with literatures [12,22], in which b-caryophyllene was detected as sesquiterpene. The main reasons for these differences maybe lying in population origin, that is to say, P. sylvestris from different population origin release different terpenoids [25]. The terpenoids emitted by heated needles were all monoterpenes (Table 1). The similar results were reported in volatiles emitted from heated Rosmarinus officinalis plants [17], from sawdust of Norway spruce (Picea abies) when drying [15], and from eucalypt at different high temperature [13,14]. Terpenoids identified under natural and heated conditions are different. Firstly, sesquiterpenes were only detected under natural condition. As far as monoterepenes, b-myrcene was only detected under natural condition, and a-tricyclene, 3-carene and neo-allo-ocimene were only detected under heated condition. 3.2. Terpenoid emissions Terpenoid emission rates in natural environment were presented in Fig. 3. The total emission rate of terpenoids was 0.167 lg g1 h1 DW. The result is little lower than that of previous reported studies, i.e., 1.2 lg g1 h1 [3], 1.35 lg g1 h1 [22], 2.0 lg g1 h1 [20], 2.1 lg g1 h1 [21]. The emission rate of
3.1. Terpenoid identification 0.12
TD–GC–MS analysis of the VOC led to the identification of 10 compounds (Table 1): Eight monoterpenes (1–8 in Table 1), 2 sesquiterpenes (9–10 in Table 1). Analysis of the VOC released from needles of P. sylvestris. in natural environment enabled the identification of seven compounds: 5 monoterpenes (2–5, 7 in Table 1), 2 sesquiterpenes (9–10 in Table 1), with a-pinene as major component. The VOC emitted by heated needles (200 °C) were characterized by very high level of a-pinene, and 7 compounds identified were all monoterpenes (1–4, 6–8 in Table 1). The results of terpenoids released by P. sylvestris in natural environment are in accordance with the data in the literature concerning the VOC emissions of this species. Many studies reported that major compound of VOC released by P. sylvestris was a-pinene, followed by b-pinene, myrcene, 3-carene, and so on [20,23,26]. Whereas, some studies concluded that the major compound
0.1 0.08 0.06 0.04 0.02 0 1
2
3
4
5
6
7
Fig. 3. Emission rates of different terpenoids released by needles of P. sylvestires in natural environment (bars indicate the standard error) (n = 12) (1 a-pinene, 2 camphene,3 b-pinene, 4 b-myrcene, 5 D-limonene, 6 longicyclene, and 7 longifolene).
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temperature and no turbulence conditions. It is inferred that the conjugation of accumulation and flammability may be capable of causing a gas phase flame propagation, which together with dry vegetation and an adjacent specific terrain may result in unusual fast fire propagation. 4. Conclusion The present study focused on the terpenoid emissions from P. sylvestris needles under different temperature conditions. Several conclusions can be drawn as follows: Fig. 4. Emissions of the terpenoids emitted by heated needles of P. sylvestires (200 °C) (bars indicate the standard error) (n = 9) (1 a-tricyclene, 2 a-pinene, 3 camphene, 4 b-pinene, 5 3-carene, 6 D-limonene, and 7 neo-allo-ocimene).
a-pinene was highest among all terpenoids, 0.077 lg g1 h1, followed by D-limonene, 0.043 lg g1 h1. Terpenoid emissions are in close relation with temperature [12]. Along with the temperature increases, the terpenoid emissions increase. The highest emission will appear at a specific temperature, as far as Rosmarinus officinalis the specific temperature is approximately 175 °C [17]. When the temperature is over the specific temperature, the emission will decrease due to the thermal degradation and/or polymerisation of the terpenic compounds. The boiling points of terpenoids, particularly monoterpenes, are below 200 °C (see Table 1). So in our experiment, 200 °C was designed as the heated temperature, which is in coincidence with that in many previous studies [13–15]. Terpenoid emissions from heated needles within 15 min were present in Fig. 4. The total emission is 16.314 lg g1 DW. The similar result was found in terpenoid emissions from Norway spruce (Picea abies) sawdust during drying, 13–250 lg g1 [15] depending on the temperature (140 °C, 170 °C or 200 °C) of the drying medium and the final moisture content of the sawdust. The emission rate of a-pinene was biggest, 10.321 lg g1, followed by b-pinene, 2.189 lg g1, camphene, 1.699 lg g1. 3.3. Potential influences on forest fires The results indicated that the terpenoids emitted by heated needles are markedly higher than that by needles in natural environment. The emission rate of terpenoids released by P. sylvestris in natural environment was only 0.167 lg g1h1. However, 16.314 lg of total monoterpenes and 10.321 lg of a-pinene would be emitted into air within 15 min by 1 g heated needles. In our experiment, the moisture content of fresh needles was 53.57%. The mass of total fresh needles of one tree of P. sylvestris was 11.2146 kg approximately [29]. Hence, the terpenoid emissions per tree within 15 min under heated condition (200 °C) can be calculated as follows:
16:314 lg g1 11:2146 kg 53:57% ¼ 98:00 mg Terpenoids are extremely flammable, with high calorific value, relatively low Flash Point (FP) and Low Flammability Limit (LFL), for instance, a-pinene shows a calorific value of 45 MJ kg1 [30], a FP of 32 °C, and a LFL of 0.7% (v/v) (see Table 1). LFL values of a-pinene can also be expressed in grams per cubic metre of air at 298 K and 100 kPa (1 bar), 38.4 g m3 [8]. Depending on the results, the LFL of terpenoids emitted by heated needles of P. sylvestris will be obtained easily in fire environment. The ignition and burning of these terpenoids will contribute to the acceleration of an ongoing spreading fire. In addition, being heavier than air, these emitted terpenoids will be retained in the accumulated gas in certain geographical zones (e.g., small valleys, canyons) under high
(i) TD–GC–MS analysis of the VOC led to the identification of 10 terpenoid compounds: 8 monoterpenes and 2 sesquiterpenes. five monoterpenes and 2 sesquiterpenes were detected in the VOC released from P. sylvestris. in natural environment. However, the 7 terpenoid compounds of VOC emitted by heated needles (200 °C) were all monoterpenes. The major compounds of VOC present were a-pinene both under natural and heated conditions. (ii) The terpenoid emission rate of terpenoids released by P. sylvestris in natural environment was low, 0.167 lg g1h1 DW. However, the terpenoid emission emitted by heated needles was considerable, 16.314 lg g1 DW for total and 10.321 lg g1 DW for a-pinene, within 15 min, at temperature of 200 °C. (iii) It is concluded that the terpenoids emitted by heated needles are markedly higher than that by needles in natural environment. Because of high calorific value, relatively low FP and LFL, the ignition and burning of emissions emitted by unburned terpenoid-rich fuels in fire environment will contribute to the acceleration of an ongoing spreading fire. In addition, terpenoid emissions can be retained in the accumulated gases in topographically uneven areas (e.g. canyons or valleys) under high temperature and no turbulence conditions. The combination of concentrations above the LFL and uneven geographic areas can lead to situations with a high ignition potential, and thus increase the risk of eruptive fire.
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