VOC emissions of smouldering combustion from ...

Atmospheric Environment 64 (2013) 339e348

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VOC emissions of smouldering combustion from Mediterranean wildfires in central Portugal Margarita Evtyugina*, Ana Isabel Calvo, Teresa Nunes, Célia Alves, Ana Patrícia Fernandes, Luís Tarelho, Ana Vicente, Casimiro Pio Centre for Environmental and Marine Studies (CESAM), Department of Environment, University of Aveiro, 3810-193 Aveiro, Portugal

h i g h l i g h t s < The CO, CO2, THC and VOC emissions from Mediterranean wildfires were studied. < The CO and CO2 emissions comprised 78 8% and 18 7% of the total carbon emitted. < Aromatic hydrocarbons were major components (w51%) of the identified VOC emissions. < Benzene and toluene were dominant aromatic compounds. < Isoprenoid emissions were dominated by isoprene and a-pinene.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 January 2012 Received in revised form 30 September 2012 Accepted 2 October 2012

Emissions of trace gases and C5eC10 volatile organic compounds (VOCs) from Mediterranean wildfires occurring in Portugal in summer 2010 were studied. Fire smoke was collected in Tedlar bags and analysed for CO, CO2, total hydrocarbons (THC) and VOCs. The CO, CO2 and THC emission factors (EFs) were 206 79, 1377 142 and 8.1 9 g kg1 biomass burned (dry basis), respectively. VOC emissions from Mediterranean wildfires were reported for the first time. Aromatic hydrocarbons were major components of the identified VOC emissions. Among them, benzene and toluene were dominant compounds with EFs averaging 0.747 0.303 and 0.567 0.422 g kg1 biomass burned (dry basis), respectively. Considerable amounts of oxygenated organic volatile compounds (OVOCs) and isoprenoids were detected. 2-Furaldehyde and hexanal were the most abundant measured OVOCs with EFs of 0.337 0.259 and 0.088 0.039 g kg1 biomass burned (dry basis), respectively. The isoprenoid emissions were dominated by isoprene (EF ¼ 0.207 0.195 g kg1 dry biomass burned) and a-pinene (EF ¼ 0.112 0.093 g kg1 dry biomass burned). Emission data obtained in this work are useful for validating and improving emission inventories, as well for carrying out modelling studies to assess the effects of vegetation fires on air pollution and tropospheric chemistry. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Wildfires Mediterranean forest Emission factors Trace gases VOCs

1. Introduction Wildfires represent an important source of gaseous and particulate compounds into the atmosphere with serious environmental impacts (Goldammer et al., 2009; Sutherland et al., 2005). Emissions from wildfires include greenhouse gases (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)), photochemically reactive compounds such as carbon monoxide (CO), volatile organic compounds (VOCs), nitrogen oxides (NOx), and fine and coarse particulate matter (PM2.5 and PM10) (Urbanski

* Corresponding author. E-mail address: [email protected] (M. Evtyugina). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.001

et al., 2009; and references therein). The greenhouse gases and PM directly influence climate (Menon et al., 2002; Ramanathan and Carmichael, 2008; Ramanathan et al., 2001; Randerson et al., 2006). Emissions of CO, CH4 and VOCs in combination with NOx lead to the photochemical production of secondary pollutants such as ozone (Bytnerowicz et al., 2010; Finlayson-Pitts and Pitts, 2000; Jaffe et al., 2008; Preisler et al., 2010; Trentmann et al., 2005) and secondary organic aerosol (SOA) (Kroll and Seinfeld, 2008; and references therein), which affect drastically the chemical composition of the atmosphere and provoke adverse effects on both human health and abiotic/biotic components of ecosystems (Brown and Smith, 2000; Goldammer et al., 2009). Detailed characterisation of the chemical composition and quantification of the amounts of trace gases and PM emitted from

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M. Evtyugina et al. / Atmospheric Environment 64 (2013) 339e348

wildfires are critical to the establishment of sustainable environmental strategies. Characterisation of primary VOC emissions is needed to accurately initialise photochemical models for wildfire impact assessment from local to global scales. The knowledge about fire smoke composition is predominately focused on the emissions of CO, CO2, trace gases and PM and available for the African savannah fires (Korontzi et al., 2003), Mexico forest, savannah and agricultural residues (Yokelson et al., 2011). In the last decade the understanding of the emission processes and the databases covering the emission of VOCs have been improved considerably. Andreae and Merlet (2001) summarised emission factors (EFs) of more than 100 trace species for tropical, extratropical, savannah and grassland fires. VOC emissions have been also characterised in field smoke plumes from temperate forests, sage scrub and savannah over North America and southern Africa (Friedli et al., 2001; Sinha et al., 2003, 2004). Gouw et al. (2006) have studied the VOC composition of merged and aged forest fire plumes from western Canada. Shirai et al. (2003) have estimated the emission of selected VOCs from tropical savannah burning in northern Australia. Yokelson et al. (2007a, 2008) presented EFs of trace gases and 48 VOCs for tropical forest fires. Urbanski et al. (2009) reviewed emission factor data from wildfires of a large number of species for boreal/temperate/tropical geographic areas, as well for forest and savannah/rangeland vegetation groups. In the last decade improvements in instrumentation and sampling methods (e.g. airborne FTIR, airborne aerosol mass spectrometers) and a focus on biomass burning in different regions (e.g. Mexico e in MILAGRO (Yokelson et al., 2011), boreal regions in ARCTAS-B (Simpson et al., 2011), prescribed burning in the U.S. (Burling et al., 2011)) have greatly expanded knowledge of VOC emissions from biomass burning in different parts of the world. Akagi et al. (2011) provide the most recent and comprehensive review of biomass burning emission studies. Overall, EFs, especially from field studies are available for Brazil, Mexico and Canada (Ferek et al., 1998; Simpson et al., 2011; Yokelson et al., 2007a,b, 2011). Nevertheless, emission information for wildfires in Europe and Central Asia is extremely scarce. There are only a few reports on trace gases and PM emissions for Mediterranean wildfires (Alves et al., 2010a, 2011a,b; Vicente et al., 2011)

and measurements of VOC emission factors for Southern Europe are not available. The Mediterranean regions are characterised by hot and dry summers, high diversity of plant species and unusual geographical/topographical variability related to the presence of a jagged coastline and many mountain ranges, often rather steep. These features give rise to a situation of high fire risk. In the Mediterranean regions, the duration, severity and coincidence of heat waves and wildfires have significantly increased in the last decade (Hoinka et al., 2009 and references therein; Piñol et al., 1998). Dangerous fire weather conditions in 2003 and 2005 have contributed significantly to an increased number of fires and burned areas in Southern Europe. In 2010 the total burned area in Europe was about 245 000 ha estimated (until 3rd September 2010), and nearly 60% of this area located in Portugal (Schmuck et al., 2011). In this study the VOC emissions from wildfires occurring in central Portugal during summer 2010 have been assessed. Emission factors for about 30 VOCs from Mediterranean wildfires were determined for the first time. 2. Experimental 2.1. Description of fire locations Currently, 38% of the Portuguese territory (about 3.4 Mha), is covered by forest and woodlands (U.N. FAO, 2010). Maritime pine (Pinus pinaster), cork oak (Quercus suber) and eucalyptus (Eucalyptus globulus) are the three most abundant species occupying about 75% of the total forest area. According to the Portuguese Forest Services (DGRF), in 2010, the burnt forest area was more than 130 000 ha. During the fire season, in 2010, smoke samples from wildfire episodes were collected in central Portugal (Fig. 1, Table 1). This sparsely populated region is characterised by roughness relief. Most of the vegetation cover is evergreen, drought resistant, and pyrophitic, being composed predominantly of maritime pine and eucalyptus. Agricultural areas of central Portugal consist of diverse crops which include vineyards and olive groves. The most abundant species burned during each wildfire event are listed in Supplementary Table A.1.

Fig. 1. Sampling locations and MODIS active fire detection from 25 July to 13 August of 2010 obtained through the platform Global Fire Information Management System (GFIMS) (http://geonetwork4.fao.org/firemap/).


341

Table 1 Characterisation of wildfire episodes over Portugal in summer 2010. Plume

Date

Sampling location

Latitude

Longitude

Altitude (m.a.s.l.)

General vegetation type

Number of samples

MCEa

1 2 3 4 5 6 7

25/07/2010 26/07/2010 26/07/2010 27/07/2010 03/08/2010 06/08/2010 06/08/2010

40 350 4200 N 40 430 4500 N 40 460 3100 N 40 590 4700 N 40 430 01.1400 N 40 420 18.7600 N 40 450 48.9800 N

7 410 1700 W 8 210 4500 W 8 230 4900 W 8 250 1400 W 8 290 06.4800 W 8 300 04.3400 W 8 110 19.0000 W

384 231 227 244 173 162 275

Bush Mixed forest Eucalyptus forest Hardwood forest Mixed forest Mixed forest Eucalyptus forest

1 2 1 4 1 1 1

0.65 0.78e0.79 0.87 0.83e0.94 0.85 0.89 0.79

8 9 10

07/08/2010 11/08/2010 13/08/2010

Mangualde Sever do Vouga Dornelas Rebordelo Albergaria-a-Velha Alvergaria-a-Velha Chão do Coto (Oliveira de Frades) Sobrosa (São Pedro do Sul) Junqueira (Vale de Câmbra) Vila Nova de Tazem

40 460 48.1800 N 40 480 37.9000 N 40 300 56.4200 N

8 090 40.3400 W 8 200 37.7000 W 7 410 48.2600 W

282 246 374

Pinewood forest Mixed forest Mixed forest dominated by pine

6 2 6

0.76e0.84 0.87e0.91 0.64e0.84

a

MCE e Modified combustion efficiency.

2.2. Sample collection Twenty five gas samples from 10 wildfires were collected (Table 1). Additionally, supplemental air sampling was carried out in four representative locations not affected by fire plumes to obtain background levels. Sampling was carried out at 1.5 m above ground near the fire under smoky conditions, at a distance of 10e200 m downwind of the flame-front. The importance of plume collection from real fires must be highlighted since laboratory experiments and even prescribed burnings do not represent neither the severity of wildfire events, nor their emissions. A prescribed burning can be defined as a controlled application of open-fire to wildland fuels under specified environmental conditions which allows the fire to be confined to a predetermined area and to produce the required intensity and spread rate. Prescribed fires are only allowed under specific conditions, depending upon available resources, time of year, weather and desired results. However, for some compounds, EFs from prescribed burning can be reasonably extrapolated to wildfire conditions if the approximate MCE of the wildfire is known (Akagi et al., 2011). Unlike other sources, wildfire emissions are difficult to measure due to the unpredictability in space and time, dangerousness and, many times, inaccessibility of these events. Prompt availability and fast response are required to collect smoke samples from unexpected fire events, which frequently are located in areas of difficult access. Truly representative sampling is extremely hard to accomplish. The experimental approach most suitable for sampling emissions from wildfire smoke plumes, which during strong flaming combustions and especially under calm wind conditions are injected at high altitudes, is the use of very expensive aircraft means. In Portugal, where aerial means are used for fire fighting, other concurrent aeronautical activities are prohibited. Consequently, the sample collection is only possible at ground level. These samplings are carried out predominantly during the smouldering phase though it is often possible to assess the flaming phase benefiting from steep terrain declivities in mountain catchments under conditions of slope winds. Sampling equipments were configured to collect simultaneously gas and particle phase emissions and assembled in a portable box that could be moved easily as needed. Gas-phase compounds from the smoke plumes were collected, at a flow rate of 1 L min1, into 25-L Tedlar bags previously fluxed with pure N2, using a Teflonlined diaphragm pump connected to a needle valve and a calibrated rotametre. Quartz filters (4.7 cm diameter) and a glass sphere filled U-tube, immersed in an ice bath were used to remove, respectively, particles and water vapour before gas sampling. In highly polluted air the surface of quartz filters is quickly saturated by organics during sampling (Kirchstetter et al., 2001). Since essentially water-soluble organics, polar compounds and semi-

VOCs contribute to positive artifact related to the use of quartz filters (Chow et al., 2008; Salma et al., 2007), the contribution of studied non-polar/poorly polar VOCs (C5eC11) may be considered as negligible. It was also verified the minimal loss of VOCs by water condensation in the U-tube, because most of studied compounds are insoluble or poorly soluble in water (Cerqueira et al., 2003). A more detailed description of this experimental setup was reported previously by Gonçalves et al. (2011). The gas samples were stored in opaque bags to minimise the impact of U.V. radiation during transportation to laboratory, where collected smoke was “resampled” into stainless steel adsorbent tubes aiming to concentrate VOCs (>C5). The gaseous content in the bags was passed through steel tubes filled with solid adsorbents (200 mg of 60e80 mesh Carbopack B and 200 mg of 60e80 mesh Carbopack C from Supelco). Two adsorbent tubes were placed in series to assure complete retention of hydrocarbons on the adsorbents. It was found that 1e32% of all identified VOCs were trapped in the second tube. Before re-sampling, all tubes were cleaned by passage of a pure helium stream at a flow rate of 30e40 mL min1 for 60 min at 300 C. Re-sampling was performed at a flow rate of 200 mL min1 for 10 min, corresponding to sample volumes of 2 L. Smoke particles were collected on quartz fibre filters using a tripod high-volume sampler/impactor (Tisch Environmental Inc.). The PM sampling system was described earlier by Gonçalves et al. (2011). 2.3. Analytical methodologies The >C5 VOCs were desorbed and analysed by a thermal desorption method (TD) on a Trace Ultra (Thermo scientific) gas chromatograph (GC) equipped with a thermal desorption injector ATD-50 (Perkin Elmer) and a flame ionisation detector (FID). The TD-GC-FID conditions, detection limits and precisions determined for different VOCs are listed in appendix (Table A.2). CO, CO2 and THC (as methane equivalent) were analysed by online connection of the Tedlar sample bags to a non-dispersive infrared analyser (Environnment, MIR 9000) and an automatic FID analyser (Dyna-FID, model SE-310), respectively. Each gas analyser was calibrated with appropriate gas on zero and span points. A thermal-optical transmission technique (Alves et al., 2010b) was used to analyse the total particulate carbon (TC) collected on the quartz fibre filters. It should be noted that the carbonaceous content is solely used in the calculation of VOC EFs and will not be discussed further in this paper. 2.4. Calculations The modified combustion efficiency (MCE) defined as the ratio between the carbon emitted from the fire as CO2 and the carbon


3. Results and discussion 3.1. General features During a wildland fire event, flaming and smouldering combustions occur simultaneously and often in close proximity. Thus, the smoke samples represent a mixture of emissions from both combustion phases (Andreae and Merlet, 2001). The MCE values observed in this study ranged from 0.64 to 0.94 (average 0.82 0.08) suggesting the dominance of samples from smouldering combustion (Table 1). CO2 and CO are the most abundant gases released by biomass burning, accounting for more than 95% of the total carbon emitted (Radojevic, 2003). In this study, CO2 and CO emissions represented 78.3 8.1% and 18.4 7.1% of the total carbon emitted, respectively. Fig. 2 compares CO, CO2 and THC emission factors obtained in this work with results from a wildfire in a mixed-evergreen forest and 2009 summer wildfires in Portugal (Alves et al., 2011a,b) and with data reported for boreal, tropical, extratropical and savannah fires (Andreae and Merlet, 2001; Simpson et al., 2011). The CO emission factors (dry basis) measured in this study, including the flaming samples, ranged from 67 to 383 g kg1 (average 206 79 g kg1) which are similar to results from a mixed-evergreen forest fires (170 83 g kg1) and other wildfires (231 77 g kg1) in Portugal in 2009 (Alves et al., 2011a,b). However, these values were approximately two-three times higher than CO emissions from boreal (113 72 g kg1),

-1

EF, g kg dry biomass burned

100 0 2000

CO2

1500

1000

500

0 200

THC

100 20

10

Tropical forestd

Savannah and grasslandd

Extratropical forestd

Boreal forestc

Portugal 2010 (this study)

Wirldfires in

0 Wirlfires in

where [x] is the concentration of species x in the smoke, and [CO2], [CO], [THC], and [TC] are the mass concentrations of CO2, CO, THC (as carbon equivalent) and TC in the smoke, respectively. Biomass burning emissions can also be expressed as emission ratios (ERs). ERs are calculated as the ratio between the excess mixing ratio of species x and the excess mixing ratio of a reference species (Andreae and Merlet, 2001), (Table A.3). Taking into account that the majority of smoke samples were collected during smouldering combustion (Table 1), CO has been selected as the reference species. ERs and EFs are inter-related; the ratio (ERx * MWx)/EFx, where MWx is the molecular weight of x, expressed in kg of carbon burned per mole CO generated, is a constant for fires with similar vegetation and fire behaviour (Friedli et al., 2001).

200

Portugal 2009b

½x y Cfuel ; Mbiomass ½CO2 þ ½CO þ ½THC þ ½TC Mx

300

Mixed-evergreen forest (Portugal)a

EFx ¼

CO

400

-1

where DCO2 ¼ [CO2]plume [CO2]background and DCO ¼ [CO]plume [CO]background. According to Akagi et al. (2011) MCE for smouldering combustion varies over the large range of 0.65e0.85. Pure flaming combustion has an MCE near 0.99 (Chen et al., 2007; Yokelson et al., 1996). MCE below 0.9 indicates predominance of smouldering combustion. EFs were calculated using the carbon balance mass methodology (Andreae and Merlet, 2001), i.e. were obtained from the ratio between the mass concentrations of a species (x) and the carbon emitted in the plume. To express EF to units of grams of species x emitted per kg of dry biomass burned, this ratio was multiplied by the mass fraction of carbon in the fuel (Cfuel), which typically varies from 45 to 55% (Susott et al., 1996) and was taken as 48% according to Silva et al. (2008):


MCE ¼ DCO2 =ðDCO2 þ DCOÞ;

500

-1

emitted from the fire as CO2 and CO as a total in relation to background was calculated to evaluate the combustion phase by the following equation:


342

Fig. 2. Comparison between emission factors of CO, CO2 and THC for fires in Portugal and values published in the literature (a e Alves et al., 2011a; b e Alves et al., 2011b; c e Simpson et al., 2011; d e Andreae and Merlet, 2001). Note that extratropical forests include both boreal and temperate ecosystems.

savannah (65 20 g kg1), tropical (104 8 g kg1) and extratropical (107 37 g kg1) forest fires reported by Andreae and Merlet (2001) and Simpson et al. (2011). EFCO2 registered for wildfires in Portugal in 2010 ranged between 1029 and 1655 g kg1 (averaging 1377 142 g kg1, dry basis) that were slightly lower than 1616 180, 1613 95, 1580 90 and 1569 131 g kg1 reported by Simpson et al. (2011) and Andreae and Merlet (2001) for fires in boreal, savannah, tropical and extratropical forests, respectively. EFTHC in this study accounted for 1.3 9.0% of the total carbon emitted. Highly variable EFs, averaging 8.1 9.0 g kg1, were observed for THC. These values are of the same order as those referred by Andreae and Merlet (2001) for tropical and extratropical forests. The EFHTC measured in this study was substantially different from those reported for Portuguese wildfires in 2009 (Alves et al., 2011b). Good correlations between the EF of carbon oxides and MCE were obtained in the present study: ¼ 1919 MCE þ 172 (r2 ¼ 0.953), EFCO2 EFCO ¼ 1056 MCE þ 1060 (r2 ¼ 0.995). In contrast, THC showed more variable EFs without any clear dependence on the combustion phase (Fig. 3). High and variable EFs for CO and THC were found by Chen et al. (2007) during the smouldering phase from laboratory burning of wildland fuels with substantial moisture content. A low correlation between MCE and EFs of THC from laboratory measurements of pure smouldering combustion of duff, organic soils, and large diameter woody fuels was observed (Bertschi et al., 2003; Yokelson et al., 1997). The notable variability of THC

M. Evtyugina et al. / Atmospheric Environment 64 (2013) 339e348 400

Table 2 Plume average concentration compared to background average concentration of VOCs.

30

this work (left axis) Alves et al., 2011 (right axis) Sinha et al., 2003 (left axis)

200

10

-1 EFTHC, g kg

-1 EFTHC, g kg

300

20

100

0

0 0.5

0.6

0.7

0.8

0.9

343

1.0

MCE Fig. 3. Emission factors of THC as a function of MCE for the Mediterranean wildfires (this study, Alves et al., 2011b) and for savannah fires (Sinha et al., 2003).

emissions obtained in the present study for wildfires of 2010, and the significant difference in comparison with values obtained in the previous wildfire season in Portugal mainly arises from dissimilar proportions of flaming and smouldering samples. The most important factors that influence the flaming/smouldering ratio, and therefore EFs, include weather conditions (wind velocity, ambient temperature, relative humidity), biomass properties (structure, moisture content, size) and topography (slope and profile). In turn, the biomass type can affect the speciation profile of emissions. Hence, flaming combustion dominates in savannah, grass-shrub fires and tropical fires occurring during dry season, while smouldering combustion is more important for boreal forests (van Leeuwen and van der Werf, 2011 and references therein). The importance of smouldering in boreal forest fires is related to the burning of duff and organic soils which tend to be consumed by smouldering combustion (Bertschi et al., 2003; Yokelson et al., 1997). The average concentrations of identified VOCs in the smoke samples were compared to background air (Table 2). Taking into account that smoke samples were collected near the flame front, under conditions of rapid mixing and fast gradient changes in space and time, a wide variation of concentrations for almost all VOCs took place. The detected concentrations of VOCs in the smoke were significantly higher than measured background levels. Therefore, all compounds listed in Table 2 may be considered as emitted from wildfires. Toluene, 2-furaldehyde, isoprene, n-hexane, a-pinene and m,p-xylenes were the most abundant among the identified compounds. The highest background mixing ratios were registered for isoprene, toluene and m,p-xylenes. It should be noted that the analytical system did not allow good separation of benzene and cyclohexane. Their content was conventionally expressed as a sum but quantified as benzene. Benzene is the dominant aromatic VOC in fire plumes representing an environmental concern whether due to its known carcinogenic effect on human health (U.S. EPA, 1995) or as a precursor of PAH (Warnatz et al., 1999). For these reasons, it was decided to present semiquantitative EFs of benzene, which correspond to upper limits of the true emissions (Table 3). It is known, that large amount of oxygenated organic volatile compounds (OVOCs) is emitted by wildfires. In previously published studies the emission of methanol, formaldehyde, acetic acid, formic acid, glycolaldehyde, phenol and furan were reported as dominant OVOCs (Akagi et al., 2011, and references therein). However, these compounds were not assessed in this study owing the unavailability of analytical equipment for their measurement. The positive correlation of the excess mixing ratios of CO versus total identified and the most abundant individual VOCs confirmed

Compound

Plume averagea (ppbv)

CO2c COc THCc Isoprene 2,2- Dimethylbutane Methyl acetate 2-Methylpentane n-Hexane Benzene 2,2,4-Trimethylpentane n-Heptane Propyl acetate Toluene Hexanal 2-Furaldehyde Chlorobenzene Ethylbenzene m,p-Xylene Styrene o-Xylene Nonane Isopropylbenzene a-Pinene Camphene Benzaldehyde 1,3,5-Trimethylbenzene b-Pinene n-Decane a-Phellandrene 3-Carene Limonene Eucalyptol n-Butylbenzene g-Terpinene 3-Methyl benzaldehyde Naphthalene

685 61 7.9 27 2.3 4.6 7.4 17 90 1.7 8.0 1.7 52 4.6 37 0.40 5.0 12 6.5 6.4 2.0 0.58 13 3.8 3.6 1.4 1.5 1.2 1.3 0.071 6.7 4.9 0.85 2.4 0.053 0.89

a b c

220 59 7.6 27 1.7 5.8 7.3 8 82 2.7 7.4 1.3 48 8.0 44 0.78 5.0 10 10.6 5.6 2.2 0.67 18 4.3 4.2 1.2 1.8 1.5 2.6 0.012 9.3 8.6 0.72 2.8 0.52 1.03

Plume max. (ppbv)

Background averageb (ppbv)

1136 195 33 136 7 18 32 31 280 9 25 5 182 24 151 3 18 38 35 21 7 3 76 13 14 4 7 5 10 0.082 40 31 3 9 1 4

406 0.02 n/a 4.01 0.31 0.015 0.91 0.44 0.78 0.81 0.41 0.26 3.95 0.076