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Size distribution of polycyclic aromatic hydrocarbon particulate emission factors from agricultural burning
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Atmospheric Environment 41 (2007) 2729–2739 www.elsevier.com/locate/atmosenv
Size distribution of polycyclic aromatic hydrocarbon particulate emission factors from agricultural burning Haleh Keshtkar, Lowell L. Ashbaugh Crocker Nuclear Laboratory, University of California, One Shields Avenue, Davis, CA 95616, USA Received 18 August 2006; received in revised form 22 November 2006; accepted 27 November 2006
Abstract Burning of agricultural waste residue is a common method of disposal when preparing land following crop harvest. This practice introduces volatile organic compounds, including polycyclic aromatic hydrocarbons (PAHs), into the atmosphere. This study examines the particle size distribution in the smoke emissions of two common agricultural waste residues (biofuels) in California, almond prunings and rice straw. The residues were burned in a combustion chamber designed specifically for this purpose, and the smoke emissions were collected on 10-stage MOUDI impactors for analysis of PAH and total particle mass. The results, in units of emission factors, show that combustion temperature is an important factor in determining the smoke particle PAH composition. Total PAH emissions from rice straw burns were 18.6 mg kg1 of fuel, while the emissions from almond prunings were lower at 8.03 mg kg1. The less volatile five- and six-ring PAH was predominately on smaller particles where it condensed in the early stages of combustion while the more volatile three- and four-ring PAH formed on larger particles as the smoke cooled. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polycyclic aromatic hydrocarbons; PAH; Particle size distribution; Agricultural smoke; Emission factors
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) originate from incomplete combustion of carbonaceous materials. Their sources include anthropogenic emissions such as motor vehicles, industrial processes, domestic heating, waste incineration and tobacco smoke and natural processes such as forest fires and volcanic eruptions. Volatile PAHs are found predominantly in the gas phase while PAHs with four or more rings are found mainly associated Corresponding author. Tel.: +1 530 752 2848;
fax: +1 535 752 4107. E-mail address: [email protected] (L.L. Ashbaugh). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.11.043
with fine particulate matter, typically 0.5 mm or smaller. Measurements of PAH in diesel and gasoline exhaust have found total emission factors in the range of 4–1000 mg kg1 of fuel, mainly associated with particles smaller than 0.2 mm. Jenkins et al. (1992) measured PAH emissions in agricultural smoke and found higher emission factors of 1–68 mg kg1 of fuel burned (less naphthalene). Burning of agricultural residues (biofuels) is a common technique for land preparation and disposal of crop and wood wastes. Concerns about these practices include acute health effects to people near the fires, global climate effects (Christopher et al., 1998) and regional visibility. The annual
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biomass burning in California amounts to 425,000 tons for orchards and 1,550,000 tons for crop residues. Rice and wheat straw, along with almond and walnut prunings, comprise 95% of the agricultural biomass openly burned in California (Jenkins et al., 1992). This practice introduces a considerable amount of pollutants into the atmosphere, major components being CO, particulate matter and hydrocarbons. Volatile organic compounds (VOC) and PAHs are emitted as well. Approximately, 4800 tons of particulate matter is emitted into the atmosphere annually from agricultural burning in California. Even though this figure represents only 1% of the total particulate matter emission in California, the temporal and spatial effect of this emission on air quality and public health is significant. Several studies have reported increased bronchial asthma in children living in the areas close to rice fields during the burn season (Jacobs et al., 1997; Katsumi et al., 2000). PAHs are highly lipid-soluble and are absorbed through the lung, skin or intestine. When PAHs enter the body, the enzyme system converts the nonpolar PAH into polar hydroxyl and epoxy derivatives (Hall, 1989). The enzyme systems that metabolize PAHs are widely distributed in the cells and tissues of humans and animals. The highest metabolizing capacity is in the liver, followed by the lung. PAHs exert their mutagenic and carcinogenic activity through biotransformation to chemically reactive intermediates which bind covalently to DNA and get permanently attached to DNA, alter DNA structure and lead to mutation (Graslund and Jernstrom, 1989). The primary objective of this work was to measure PAH emission factors as a function of particle size from burning almond prunings and rice straw. This paper is part of a larger study to examine the dependence of particle size on gas/ particle partitioning of PAH in smoke from agricultural waste biofuels.
2. Experimental methods 2.1. Combustor and sampling methods Gaseous and particulate matter measurements were carried out in a combustion chamber built at UC Davis for two agricultural fuels: almond prunings and rice straw residue collected from local farms in 1997–1998. Each fuel was tested in a single burn that was designed to mimic actual burn conditions as closely as possible. Table 1 shows the burn conditions for each of the two burns reported here. The combustion chamber incorporated a conveyor system for moving the fuel bed into the flame zone when operating the rice straw burn (a spreading fire). The conveyor system consists of two parts. The primary fuel conveyor, a flexible rubber belt, was used for transporting the fuel continuously to the secondary stainless steel belt on which it was burned. A single variable speed DC motor was used to operate both conveyor belts. The speed was adjusted to maintain the fire front under the hood at all times. The conveyor belt has the following dimensions: height 79 cm, width 46 cm, length of the primary fuel conveyor 3 m, and length of the secondary combustion conveyor 182 cm. The emission sampling section consists of a large (152 cm 152 cm) hood with adjustable height above the combustion section. The hood was connected to a 30.5 cm diameter stack of 305 cm length that houses a fan at the far end to create the air stream. The sampling ports were located about 30 cm upstream of the fan. Three sampling ports were used; one 10 cm port to house a reduced artifact dilution system (RADS) probe (Wall, 1996), one 6.4 cm port to house a photoelectric aerosol sensor (PAS) probe and one 2.5 cm port used to monitor temperature inside the stack. The air velocity inside the stack was set at approximately 11 m s1. The Reynolds number in the stack
Table 1 Sampling conditions for each agricultural fuel burn Fuel
Rice straw Almond pruning a
Burn date
1/12/2000 28/2/2001
Average particle concentration (mg m3) 28.2 16.4
Ambient air
Combustor stack
Dilution tunnela
Temp. (1C)
RH (%)
Temp. (1C)
Temp. (1C)
RH (%)
14 26
58 17
50 70
16 23
53 23
Average of the measurements over entire sampling period.
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was 4200,000, ensuring turbulent flow and adequate mixing of the smoke. The RADS diluted the combustion sample 35:1 before collection in a variety of samplers. This provided a sampling flow rate of 0.7 m3 min1 inside the dilution tunnel. Microprocessor controls of RADS maintained steady state conditions of stack sampling rate and dilution factor. Rice straw was burned in a spreading fire propagating against the main flow and almond pruning was burned in a pile fire to reflect the burn type in the field. Rice straw burning (spreading fire) was conducted by adjusting the conveyor belt speed to the fire spreading rate to maintain the fire stationary under the hood. The feed belt was marked in 0.45 m2 sections, each section 1 m long on the 0.45 m wide belt. Fuel for each section was weighed into a large tared container in an amount sufficient to provide the yield or loading rate (g m1) desired. The wet fuel loading rates were selected based on the reported yield of 3 ton acre1 (approximately 680 g m2) from field studies (Knutson and Miller, 1982). The fuel was loaded in a uniformly thick layer on the belt to a depth typical of that in the field, 75–100 mm but not exceeding 150 mm. The speed of the conveyor belt was adjusted to achieve a fire spreading rate of 1 m min1. Total weight of the moist fuel for the rice straw burn was 10.1 kg over a total sampling time of 50 min. For the almond pruning burn, the conveyor belt was removed and wood pieces of various sizes (4–20 cm diameter and 30–50 cm length) and thickness were arranged in a pile. Pieces of the wood were added to the pile during the sampling as needed. The fire was lit on the perimeters. A total of 11.5 kg of almond pruning was burned over a 65 min period. Four ports at the base of the RADS were connected to four separate sampling trains. Two ports were connected to 10 stage Micro Orifice Uniform Deposit Impactors (MOUDI) (Marple et al., 1991). Both MOUDIs were operated at 30 lpm and were matched for particle sizing characteristics. One MOUDI (F) was used for PAHs analysis and the other MOUDI (T) was used for organic carbon and elemental carbon analysis. Aluminum substrates of 47 mm diameter were used for sample collection in both MOUDI (F) and MOUDI (T) for both burn tests. A 37 mm Teflons filter was used as the after-filter for the rice straw burn and a 37 mm quartz filter was used as the after-filter for the almond prunings burn. A cyclone preceded MOUDI (F) to remove particles 42 mm.
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Wood smoke aerosols were tested for bounce in greased and ungreased impactor stages prior to the burn experiments. The tests indicated that coarse particles (size cut 41.8 mm) bounced (the ungreased stages in one MOUDI showed particle loss in comparison to greased stages in the other), but fine particles did not. Therefore, the first four stages ðd ae 41:8 mmÞ of the impactor were coated with silicon grease to avoid particle bounce during sampling. The third port was connected to an eight-stage Berner impactor (Berner et al., 1979) that was analyzed for PAH. The last port was connected to a cyclone (AIHL design) to remove particles larger than 2.5 mm and was followed by a union with four sampling ports. This manifold was used to collect samples on filters and denuders for additional PAH analysis. The data from these samplers and complete details of the combustion chamber design and operation are reported in Keshtkar (2003). 2.2. Sample preparation and analysis The MOUDI substrates and filters were weighed before and after sampling. MOUDI (T) substrates were shipped to Sunset Laboratory for OC/EC analysis. Sunset Laboratory uses the thermal optical transmission method for organic and elemental carbon analysis by thermal evolution/oxidation of CO2 followed by catalytic reduction to methane and detection by flame ionization. The split between ‘‘organic’’ and ‘‘elemental’’ carbon is determined by the transmission of laser light as the optical density of the filter increases (due to charring) and then returns to its starting point as carbonaceous material evolves and/or oxidizes (Birch and Cary, 1996). MOUDI (F) substrates were stored at 20 1C until analysis. Substrates were extracted, prepared, and analyzed as follows. In a 40 ml vial, 35 ml of 3:1 cyclohexane (CH):dichloromethane (DCM) mixture was added to each impactor substrate. The samples were then spiked with 1000 ng of a mixture of sixteen deuterium-labeled priority pollutant PAHs, 1000 ng of deuterium-labeled 1-nitropyrene and 3000 ng of deuterium-labeled tetracosane. The samples were then extracted with pulsed ultrasonication using a Ney proSONIK for 6 min (Wall, 1996). All of the extracts were concentrated to 500 ml using a metered stream of 99.9% pure nitrogen under a slight vacuum. A 200 ml aliquot of the 500 ml
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sample was then injected into an HP 1090 high performance liquid chromatograph for fractionation of each sample into non-polar, slightly polar and polar fractions for analysis of alkanes, PAHs and oxygenated PAHs, respectively. The separation was performed using a 4.6 mm diameter 250 mm length Whatman EQC 5 mm particle size, 60-A˚ pore size SI column at 35 1C temperature, pressure of 65 psi, 1.2 ml min1 flow rate and the UV detector wavelengths of 254, 340, 260, and 270 nm. The solvent gradient was programmed as 100% hexane for 5 min followed by a gradient of 5% min1 dichloromethane for 20 min and hold at 100% dichloromethane for 14 min. The alkane fraction was collected from 0 to 6 min, the PAH fraction was collected from 10 to 16 min, and the oxygenated PAH fraction was collected from 16 to 39 min after injection.
emission factors in g kg1 (for major carbon fractions) or mg kg1 (for PAH) of dry fuel. Emission factor ðEFÞ ¼
ðmspecies Þ ðV d Þ . ðV i Þ ðmf Þ
An emission factor was calculated for every PAH measured on each stage of the MOUDI impactors for all sampling runs. The resulting size distributions were then smoothed using the Twomey inversion algorithm (Twomey and Zalabsky, 1981; Winklmayr et al., 1990). The results are presented as size distributions of mass emission factors normalized to the total emission factors of the entire size range (dE/E d Log Dp). 3. Results and discussion 3.1. Particulate matter, organic and elemental carbon, and total PAH size distributions
2.3. Emission factors and size distributions The results are presented as emission factors derived from the mass measurements on the ten MOUDI stages and the after filter. The mass of each species measured on each stage (mspecies) was divided by the volume of air pulled through the impactor (Vi) during the time of burn (adjusted for 35:1 sample dilution in RADS) to obtain the average concentration inside the combustor duct. This result was multiplied by the volume of air that passed through the duct (Vd) during the burn to obtain the total mass per burn and was then divided by the mass of dry fuel burned (mf) to obtain
Table 2 shows the calculated emission factors for particulate matter, organic carbon and elemental carbon in each aerosol mode, where the nuclei mode is d ae o0:1 mm, the accumulation mode is 0:1 mmod ae o1:8 mm, and the coarse mode is d ae 41:8 mm. Total emission factors (summing all aerosol modes) of particulate matter (PM), organic carbon and elemental carbon were 7.22, 0.83 and 0.49 g kg1 for rice straw and 5.43, 2.37 and 0.17 g kg1 for almond pruning. Even though these results are for a single burn for each fuel, the total particulate matter results were in good agreement with those reported from open burning
Table 2 Particulate, organic and elemental carbon emission factors (g kg1) Nuclei mode d ae o0:1
Accumulation mode 0:1od ae o1:8
Coarse mode 1:8od ae
Almond pruning Particulate matter Organic carbon Elemental carbon Fraction of OC in PM (%) Fraction of EC in PM (%)
2.14 0.98 0.04 46.0 1.9
2.52 1.39 0.13 55.1 5.3
0.77 –a –a – –
Rice straw Particulate matter Organic carbon Elemental carbon Fraction of OC in PM (%) Fraction of EC in PM (%)
2.43 0.16 0.07 7 3
3.93 0.67 0.42 17 11
0.86 –b –b – –
a
Organic and elemental carbon were measured only for particles o1:8 mm aerodynamic size. Organic and elemental carbon were measured only for particles in the aerodynamic diameter size range ð0:0564d ae o1:8 mmÞ.
b
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of agricultural biomass of 2.0–8.6 g kg1 for rice straw and 4.8–6.2 g kg1 for almond prunings burns (Jenkins, 1996), and with other reports from various wood and biofuel burns from small stoves and fireplaces (Smith, 1987; Venkataraman and Rao, 2001; Rau, 1989; Hildemann et al., 1991). Also shown in Table 1 is the fraction of organic carbon and elemental carbon in particulate matter for each biofuel. Both fuels showed the largest fraction of particles in the accumulation mode. However, the emission factor for rice straw (3.93 g kg1) is higher than that for almond prunings (2.52 g kg1) in the accumulation mode. The emission factors in the nuclei mode are similar: 2.14 and 2.43 g kg1 for almond pruning and rice straw, respectively. In both biofuels about 88% of the particulate mass was smaller than 1 mm and can pass easily through the upper respiratory tract. This gives important information about health hazards of agricultural burning practices. The smoothed size distributions of particulate matter and total PAH compounds emission factors for rice straw and almond pruning burns are shown in Fig. 1. Each curve in Figs. 1–4 is normalized to the total emission factor for the species so that different species can be compared on the same scale. The size distribution of particulate matter mass from both biofuels is bimodal in the two burns we conducted, showing a small, broad peak in the 1.7–17 mm range and a much larger and narrower peak at 0.13–0.18 mm. Particle formation in the nucleation mode (0.01–0.1 mm) has been reported for aerosols from vehicle emission, coal combustion and wood burning (Hildemann et al., 1991). Particles in the larger coalescence mode (2–10 mm) have also been seen from coal combustion (Flagan and Freidlander, 1978; Neville et al., 1983). However, they could have been mechanically generated from dust, re-entrained soot or soil on fuel or pieces of biofuel itself. For the rice straw smoke stage 9 of the MOUDI impactor (0.1–0.18 mm) collected the largest mass of particles (34%), while for almond pruning the afterfilter ðd ae o0:056 mmÞ, with 30% of the total, had the largest mass. The size distribution of total PAH compounds is shifted towards smaller particles relative to the particulate matter results. This shift is more striking for almond pruning than for rice straw, where the PAH size distribution shows better agreement with the particulate matter size distribution. The peak at 0.02 mm in Fig. 1a for almond pruning represents the after-filter mass—it should
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Fig. 1. Size distribution of particulate matter and PAH compounds normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
not be inferred, though, that the actual size peak is at 0.02 mm. The lower limit used in the smoothing algorithm for the particle size range collected on the after-filter influences the shape of the smoothed distribution in the size range below 0.056 mm. The size distributions of organic and elemental carbon for rice straw and almond pruning burns are shown in Fig. 2. Emission factors of organic carbon in the accumulation and nuclei modes from almond prunings were 1.39 and 0.98 g kg1 while in rice straw they were 0.67 and 0.16 g kg1. Emission factors of elemental carbon in the nuclei mode were similar for both fuels while in the accumulation mode rice straw had higher emission factors. About 41% of the organic carbon and 23% of the
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Fig. 2. Size distribution of organic and elemental carbon normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
elemental carbon from the almond pruning burn was in particles smaller than 0.1 mm, while the values were 19% and 14%, respectively, for the rice straw burn. In the ultrafine and accumulation modes, organic carbon constituted 46% and 55%, respectively, of the mass of particles from the almond pruning burn, while for the rice straw burn only 7% and 17% of the mass of particles in the ultrafine and accumulation modes was organic carbon. These results are consistent with Rau (1989), who reported 50–60% organic carbon in fine particulate matter from air-starved (cool) wood burns in small stoves and 20–60% elemental carbon in fine particulate matter ðo2:5 mmÞ in open air (hot) burns.
Fig. 3. Size distribution of three- and four-ring PAHs (molecular weight 178–228) normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
Elemental carbon is unimodal for almond prunings with a peak at 0.28 mm and organic carbon is bimodal with peaks at 0.2 and 0.02 mm. The bimodal distribution of organic carbon shows a good correlation with the bimodal particulate matter distribution. Organic and elemental carbon distributions for rice straw are both unimodal with peaks at 0.13 and 0.15 mm, respectively. However, no rice straw smoke results are available for OC/EC analysis in the particle size ranges 43:2 mm and o0:056 mm. Generally, the peaks in PM, OC and EC in smoke from almond prunings are at larger size ranges than those from rice straw. These data are consistent with other measurements of aerosol mass in emissions from combustion sources (Rau, 1989; Venkataraman et al., 1994).
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The mass median diameter of the particles was between 0.07 and 0.08 mm for organic carbon and elemental carbon and particulate matter from both biofuels. 3.2. Individual PAH size distributions
Fig. 4. Size distribution of five- and six-ring PAHs (molecular weight 252–278) normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
The samples were analyzed for 16 priority pollutant PAHs in this study. However, only PAHs with molecular weights 178 (phenanthrene) and higher were present in impactor samples for the two burns we conducted. More volatile compounds were either mainly in the gas phase or lost during sampling or storage. Small amounts of naphthalene were identified in some samples, but were not quantified. Anthracene was present in rice straw smoke samples but was not detected in almond pruning smoke samples. The size distributions of PAH emission factors from the two agricultural fuels are presented in Fig. 3 for three- and four-ring PAH and in Fig. 4 for five- and six-ring PAH. Tables 3 and 4 summarize the PAH emission factors for almond pruning smoke and rice straw smoke, respectively, and show the fractions in each particle size mode. Emission factors for the more volatile (three- and four-ring) PAH compounds were strikingly higher for rice straw than for almond pruning smoke. However, emission factors (in particulate matter phase) of less volatile PAH compounds (five- and six-rings) were comparable in both fuels. Thus, the ratio of the more volatile PAH compounds (three- and
Table 3 PAH emission factors from almond pruning smoke (mg kg1 of dry fuel) PAH
Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(k)fluoranthene Benzo(b)fluoranthene Benzo(a)pyrene Indeno[1,2,3-cd]pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Total PAH
Number of rings
3 3 4 4 4 4 5 5 5 6 5 6
Emission factor (mg kg1)
Fraction in particulate matter mode Nuclei
Accumulation
Coarse
2.05 – 1.29 1.44 0.37 0.46 0.49 0.34 0.43 0.44 0.18 0.55
0.25 – 0.19 0.18 0.55 0.52 0.55 0.52 0.58 0.33 0.54 0.44
0.47 – 0.46 0.44 0.45 0.48 0.45 0.48 0.42 0.67 0.46 0.56
0.28 – 0.35 0.38 0 0 0 0 0 0 0 0
8.03
0.33
0.48
0.2
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Table 4 PAH emission factors from rice straw smoke (mg kg1 of dry fuel) PAH
Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(k)fluoranthene Benzo(b)fluoranthene Benzo(a)pyrene Indeno[1,2,3-cd]pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Total PAH
Number of rings
3 3 4 4 4 4 5 5 5 6 5 6
Emission factor (mg kg1)
Fraction in particulate matter mode Nuclei
Accumulation
Coarse
6.95 1.75 2.47 2.43 0.68 0.67 1.31 0.51 0.67 0.44 0.20 0.52
0.33 0.85 0.47 0.50 0.23 0.40 0.44 0.48 0.38 0.38 0.55 0.37
0.55 0.06 0.33 0.34 0.54 0.47 0.48 0.37 0.62 0.62 0.47 0.63
0.12 0.09 0.20 0.16 0.22 0.13 0.07 0.15 0 0 0 0
18.62
0.44
0.44
0.12
four-rings) to less volatile PAH compounds (fiveand six-rings) for rice straw smoke is significantly higher than for almond prunings. Phenanthrene, anthracene, fluoranthene and pyrene constitute 73% of PAHs from rice straw smoke while phenanthrene, fluoranthene, pyrene and benzo[a]anthracene comprise 64% of all the PAHs from almond pruning smoke. The predominance of more volatile PAH compounds (three- and four-rings) in smoke from both biofuels agrees with the reported results for wood (Oanh et al., 1999) and biofuels (Rau, 1989; Venkataraman et al., 2002) combustion and is in contrast with the reported predominance of nonvolatile PAH compounds for coal fossil fuel combustion (Wornat and Sarofim, 1990). We measured total emission factors for PAH compounds of 8.03 mg kg1 for almond prunings and 18.62 mg kg1 for rice straw smoke. Jenkins (1996) reported total emission factors in the range of 5.04–49.29 mg kg1 for all PAH compounds for rice straw and 14.23 mg kg1 for almond pruning. He also reported an emission factor range for PAH (less naphthalene and 2-methylnaphthalene) of 1.41–9.70 mg kg1 for rice straw and 6.78 mg kg1 for almond pruning. Jenkins’ higher total PAH emission factor for rice straw (cereal crop) smoke is consistent with our higher particulate matter emission factor from rice straw compared to almond pruning (wood). The temperature inside the combustor duct at the sampling port was higher during the almond prunings burn (average of 70 1C while reaching 100 1C at times) than for rice straw (average of 50 1C
Table 5 Mass median diameter (mm) of particulate matter, organic and elemental carbon and PAH species from agricultural waste biofuel burning Species
Particulate matter Organic carbon Elemental carbon Phenanthrene Fluoranthene Pyrene Benzo[a]anthracene Chrysene Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Benzo(ghi)perylene Dibenzo[a,h]anthracene
Mass median diameter (mm) Rice straw
Almond pruning
0.08 0.07 0.08 0.13 0.11 0.10 0.15 0.12 0.11 0.11 0.12 0.12 0.12 0.09
0.07 0.07 0.06 0.40 0.55 0.70 0.09 0.09 0.09 0.09 0.09 0.13 0.12 0.09
with little variation throughout the sampling period). Also, the almond pruning burn had a more vigorous and robust flame. The particulate matter and PAH compounds emission factor trends are consistent with the previously reported results for the agricultural fuels (Jenkins, 1996). Higher combustion efficiency and lower emissions are expected from higher temperature combustion. Table 5 shows mass median diameters (MMAD) of particulate matter, organic and elemental carbon, and PAH compounds for our tests. The MMADs are in the range of 0.07–0.15 mm for all species in
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both fuels except for fluoranthene, phenanthrene and pyrene from almond prunings, which showed MMADs at 0.40, 0.55 and 0.70, respectively. Hildemann et al. (1991) reported a range of 0.07–0.15 mm for PAH MMADs for vehicular emissions. However, Venkataraman et al. (2002) reported MMADs ranging from 0.4 to 1.0 mm for all PAHs in a study on biofuel combustion. Figs. 3 and 4 show that all PAH compounds were present in the after-filter from almond pruning smoke, but only phenanthrene, anthracene, fluoranthene, pyrene and benzofluoranthenes were detected in the after-filter ðd ae o0:056Þ from rice straw smoke. The lower stages of the MOUDI sampler (below 1.8 mm) were below atmospheric pressure, so it’s possible that the more volatile PAH compounds are depleted in those stages due to evaporation. However, we found the more volatile PAH compounds in the smaller size ranges from both burns. The size distributions of phenanthrene, fluoranthene and pyrene exhibit a multimodal profile for both biofuels with three common peaks at 10, 1.9–2.2 and 0.07–0.11 mm and an additional peak at 0.3–0.5 mm for almond prunings. Anthracene was not detected in almond pruning smoke and in the rice straw smoke its size distribution was strikingly different from other semivolatile PAH compounds, with 77% of the mass in the after filter. For the rice straw burn benzo(a)anthracene, chrysene, benzo(k)fluoranthene and benzo(b)fluoranthene show bimodal distributions with a nucleation mode at 0.09–0.11 mm and a coalescence mode at 2.3 mm. However, these PAH compounds are unimodal in almond pruning with a broad peak at 0.13 mm. For both fuels, benzo(a)pyrene and 6ring PAH compounds all show unimodal distribution with broad peaks at 0.08–0.17 mm except for dibenzo(a,h)anthracene from almond pruning, which showed a bimodal profile with peaks at 0.06 and 0.28 mm as shown in Fig. 4. This bimodal profile is not an artifact of the smoothing, but results from very low concentrations of dibenzo(a,h)anthracene on the 0.1–0.18 mm MOUDI stage. Fig. 5 shows the size distributions of the concentrations of semivolatile three- and four-ring PAH compounds and non-volatile five- and six-ring PAH compounds per gram of particulate matter in almond pruning smoke and rice straw smoke. The relatively large peak at 1.3 mm pertaining to more volatile PAH compounds (molecular weight 178–228) from both biofuels is due to the very small concentration of particulate matter in this size
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Fig. 5. Concentration of three- and four-ring PAH species (lefthand side Y-axis) and five- and six-ring PAH species (right-hand side Y-axis) per gram of particulate matter with respect to particle size for smoke from: (a) almond prunings burn and (b) rice straw burn.
range. The less volatile PAH compounds in almond pruning smoke are exclusively in the ultrafine size range with a peak at 0.075 mm. This peak in the almond pruning smoke is about 50% larger than that of the rice straw smoke. The peak at about 2.5 mm in rice straw smoke, which is completely absent in almond pruning smoke, is due to the presence of Benzo(k)fluoranthene and Benzo(b)fluoranthene in the 1.8–3.2 mm size range and is enhanced by the very small concentration of particulate matter in that range (Fig. 1). The condensation process depends on the vapor pressure of the condensing species, its concentration in the gas phase, and the number concentration and size of the particles (i.e. the available surface area).
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Condensation will occur earlier for the less volatile and more plentiful species. At any stage of combustion, condensation preferentially occurs on ultrafine particles as they are in greater numbers and offer a higher surface to mass ratio. However, condensation of more volatile PAH compounds is inhibited on smaller particles due to the Kelvin effect. This effect is less important for PAH compounds with low volatility. Therefore, compounds with low volatility preferentially condense on smaller particles in the early stages of combustion while more volatile compounds mainly condense on larger particles at later stages as the smoke cools. Our almond pruning burn was relatively hotter (see Table 1) and more efficient than the rice straw burn, producing lower emissions of particulate matter and PAH compounds. However, almond pruning smoke contained more ultrafine particles ðo0:1 mmÞ, providing a larger surface to mass ratio for less volatile PAH compounds to condense on in the early stages of condensation. The higher volatility PAH compounds partitioned into the particle phase relatively less or remained in the gas phase. Our rice straw burn was cooler and less efficient than the almond pruning burn, with a steady, more consistent smoke throughout the burn. It resulted in higher particulate PAH emission factors. Both rice straw and almond pruning showed predominance of semivolatile PAH compounds in particles larger than 0.3 mm relative to non-volatile PAH compounds. However, in almond pruning, where there were lower emission factors of PAH compounds, all of the PAH compounds in particles larger than 0.3 mm were the more volatile PAH (only phenanthrene, fluoranthene and pyrene). 4. Conclusions For the two burns we conducted, one each for rice straw and almond prunings, combustion temperature was the most important factor in determining the smoke particle PAH composition. Burn temperature depends primarily on the combustion air supply, but is also influenced by the amount of fuel burning, fuel moisture content and type of burn (in this study pile fire versus spreading fire). Hot flames generally produce a lower amount of particulate matter as well as lower organic pollutants such as polycyclic aromatic hydrocarbons. However, flames with higher temperatures tend to produce relatively more particles in the ultrafine range. In our study
rice straw, with a lower burn temperature, was more polluting than almond prunings in terms of particulate matter with most of the difference in concentration coming from particles in the accumulation mode (0.1–1.8 mm). The rice straw burn also produced more PAHs than the almond pruning burn, with most of the difference coming from three- and four-ring compounds. However, the almond pruning burn generated more particles in the ultrafine range with the after-filter having the largest concentration of particulate matter of any single stage, providing a larger surface area for the condensation of PAH compounds. This resulted in a relatively larger fraction of individual PAH compounds (four-rings and higher) in the ultrafine range. The ratio of more volatile PAHs (three- and four-rings) to less volatile (five- and six-rings) is 1.8 times higher in rice straw. The less volatile five- and six-ring PAH compounds were predominately on smaller particles where they condensed in the early stages of combustion, while the more volatile threeand four-ring PAH compounds formed on larger particles as the smoke cooled. Although this study is based on a single trial for each burn, the results are consistent with other research studies and show that open burning of agricultural residues is a significant source of particulate matter and PAHs in the respirable particle size range below 1 mm. As a result this practice is important in terms of acute health effects for people near the fires. The smoke particles also adversely affect regional visibility as this size range efficiently scatters and absorbs visible light. Acknowledgments The University of California, Davis Crocker Nuclear Laboratory provided funding to construct the combustion chamber, and the Department of Agricultural and Biological engineering designed and built it. All the equipment and media preparation prior to sampling and the sample analytical work was carried out under the guidance of Dr. Stephen Wall and Diamon Pon at the Environmental Health Laboratory Branch of the California Department of Health Services. References Berner, A., Lurzer, C., Pohl, F., Preining, O., Wagner, P., 1979. The size distribution of the urban aerosol in Vienna. The Science of the Total Environment 13, 245–261.
ARTICLE IN PRESS H. Keshtkar, L.L. Ashbaugh / Atmospheric Environment 41 (2007) 2729–2739 Birch, M.E., Cary, R.A., 1996. Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust. Aerosol Science and Technology 25, 221–241. Christopher, S.A., Wang, M., Berendes, T.A., Welch, R.M., Yang, S.K., 1998. 1985 biomass burning season in South America: satellite remote sensing of fires, smoke and regional radiative energy budget. Journal of Applied Meteorology 37 (7), 661–678. Flagan, R.C., Friedlander, S.K., 1978. Particle formation in pulverized coal combustion: a review. In: Shaw, D.T. (Ed.), Recent Developments in Aerosol Science. Wiley, New York. Graslund, A., Jernstrom, B., 1989. DNA-carcinogen interaction: covalent DNA adducts of Benzo[a]pyrene, 7,8-dihydroxodiol 9,10-epoxides studied by biochemical and biophysical techniques. Quarterly Review of Biophysics 22, 1–37. Hall, M., 1989. Relative contribution of various forms of Cytochrome P450 to the metabolism of Benzo[a]pyrene by human liver microsomes. Carcinogenesis 10, 1815–1821. Hildemann, L.M., Markowski, G.R., Jones, M.C., Cass, G.R., 1991. Submicrometer aerosol mass distribution of emissions from boilers, fireplaces, automobiles, diesel trucks and meat cooking operations. Aerosol Science and Technology 14, 138–152. Jacobs, J., Kreutzer, R., Smith, D., 1997. Rice burning and asthma hospitalization, Butte County, California, 1983–1992. Environmental Health Perspectives 105, 980–985. Jenkins, B.M., 1996. Atmospheric pollutant emission factors from open burning of agricultural and forest biomass by wind tunnel simulations. Final Report on California Air Resources Board Contract No. A932-126, Vols. 1–3. CARB, Sacramento, CA. Jenkins, B.M., Turn, S.Q., Williams, R.B., 1992. Atmospheric emissions from agricultural burning in California: determination of burn fractions, distribution factors, and crop-specific contributions. Agriculture, Ecosystems and Environment 38, 313–330. Katsumi, T., Hasegawa, S., Numata, O., Yazaki, S., Matsunaga, M., Boku, N., Hiura, M., Ino, H., 2000. Influence of emission from rice straw burning on bronchial asthma in children. Pediatrics international 42, 143–150. Keshtkar, H., 2003. Characterization of polycyclic aromatic hydrocarbons (PAH) in particulate and gaseous emissions from agricultural burning. Proquest Dissertations and Theses 2003, Section 0029, Part 0749, 140pp. (Ph.D. dissertation). Publication Number: AAT 3097441, United States, CA, University of California, Davis, 2003.
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Knutson, J., Miller, G.E., 1982. Agricultural residues (biomass) in Californiayfactors affecting utilization. Leaflet No. 21303, Cooperative Extension, University of California, Berkeley. Marple, V.A., Rubow, K.L., Behm, S.M., 1991. A Microorifice uniform deposit impactor (MOUDI): description, calibration and use. Aerosol Science and Technology 14, 434–446. Neville, M., McCarthy, J.F., Sarofim, A.F., 1983. Size fractionation of submicrometer coal combustion aerosol for chemical analysis. Atmospheric Environment 17, 2599–2604. Oanh, N.T.K., Reutergardh, L., Dung, N.T., 1999. Emission of polycyclic aromatic hydrocarbons and particulate matter from domestic combustion of selected fuels. Environmental Science and Technology 33, 2703–2709. Rau, J.A., 1989. Composition and size distribution of residential wood smoke particles. Aerosol Science and Technology 10, 181–192. Smith, K.R., 1987. Biofuels, Air Pollution and Health—A Global Review. Plenum Press, New York and London. Twomey, S.A., Zalabsky, R.A., 1981. Multi-filter technique for examination of the size distribution of the natural aerosol in the submicrometer range. Environmental Science and Technology 15, 177–184. Venkataraman, C., Rao, M.U.G., 2001. Emission factors of carbon monoxide and size-resolved aerosols from biofuel combustion. Environmental Science and Technology 35, 2100–2107. Venkataraman, C., Lyons, J.M., Friedlander, S.K., 1994. Size distribution of polycyclic aromatic hydrocarbons and elemental carbon 1. Sampling, measurement methods and source characterization. Environmental Science and Technology 28, 555–562. Venkataraman, C., Negi, G., Sardar, S.B., Rastogi, R., 2002. Size distribution of polycyclic aromatic hydrocarbons in aerosol emissions from biofuel combustion. Aerosol Science 33, 503–518. Wall, S., 1996. Improved methods for PAH combustion source sampling. Final Report on California Air Resources Board Contract No. A932-098. Winklmayr, W., Wang, H.C., John, W., 1990. Adaptation of the Twomey algorithm to the inversion of cascade impactor data. Journal of Aerosol Science and Technology 13, 322–331. Wornat, M.J., Sarofim, A.F., 1990. Char- and aerosol-associated polycyclic aromatic compounds from coal pyrolysis. Aerosol Science and Technology 12, 832–841.
Atmospheric Environment 41 (2007) 2729–2739 www.elsevier.com/locate/atmosenv
Size distribution of polycyclic aromatic hydrocarbon particulate emission factors from agricultural burning Haleh Keshtkar, Lowell L. Ashbaugh Crocker Nuclear Laboratory, University of California, One Shields Avenue, Davis, CA 95616, USA Received 18 August 2006; received in revised form 22 November 2006; accepted 27 November 2006
Abstract Burning of agricultural waste residue is a common method of disposal when preparing land following crop harvest. This practice introduces volatile organic compounds, including polycyclic aromatic hydrocarbons (PAHs), into the atmosphere. This study examines the particle size distribution in the smoke emissions of two common agricultural waste residues (biofuels) in California, almond prunings and rice straw. The residues were burned in a combustion chamber designed specifically for this purpose, and the smoke emissions were collected on 10-stage MOUDI impactors for analysis of PAH and total particle mass. The results, in units of emission factors, show that combustion temperature is an important factor in determining the smoke particle PAH composition. Total PAH emissions from rice straw burns were 18.6 mg kg1 of fuel, while the emissions from almond prunings were lower at 8.03 mg kg1. The less volatile five- and six-ring PAH was predominately on smaller particles where it condensed in the early stages of combustion while the more volatile three- and four-ring PAH formed on larger particles as the smoke cooled. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polycyclic aromatic hydrocarbons; PAH; Particle size distribution; Agricultural smoke; Emission factors
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) originate from incomplete combustion of carbonaceous materials. Their sources include anthropogenic emissions such as motor vehicles, industrial processes, domestic heating, waste incineration and tobacco smoke and natural processes such as forest fires and volcanic eruptions. Volatile PAHs are found predominantly in the gas phase while PAHs with four or more rings are found mainly associated Corresponding author. Tel.: +1 530 752 2848;
fax: +1 535 752 4107. E-mail address: [email protected] (L.L. Ashbaugh). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.11.043
with fine particulate matter, typically 0.5 mm or smaller. Measurements of PAH in diesel and gasoline exhaust have found total emission factors in the range of 4–1000 mg kg1 of fuel, mainly associated with particles smaller than 0.2 mm. Jenkins et al. (1992) measured PAH emissions in agricultural smoke and found higher emission factors of 1–68 mg kg1 of fuel burned (less naphthalene). Burning of agricultural residues (biofuels) is a common technique for land preparation and disposal of crop and wood wastes. Concerns about these practices include acute health effects to people near the fires, global climate effects (Christopher et al., 1998) and regional visibility. The annual
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biomass burning in California amounts to 425,000 tons for orchards and 1,550,000 tons for crop residues. Rice and wheat straw, along with almond and walnut prunings, comprise 95% of the agricultural biomass openly burned in California (Jenkins et al., 1992). This practice introduces a considerable amount of pollutants into the atmosphere, major components being CO, particulate matter and hydrocarbons. Volatile organic compounds (VOC) and PAHs are emitted as well. Approximately, 4800 tons of particulate matter is emitted into the atmosphere annually from agricultural burning in California. Even though this figure represents only 1% of the total particulate matter emission in California, the temporal and spatial effect of this emission on air quality and public health is significant. Several studies have reported increased bronchial asthma in children living in the areas close to rice fields during the burn season (Jacobs et al., 1997; Katsumi et al., 2000). PAHs are highly lipid-soluble and are absorbed through the lung, skin or intestine. When PAHs enter the body, the enzyme system converts the nonpolar PAH into polar hydroxyl and epoxy derivatives (Hall, 1989). The enzyme systems that metabolize PAHs are widely distributed in the cells and tissues of humans and animals. The highest metabolizing capacity is in the liver, followed by the lung. PAHs exert their mutagenic and carcinogenic activity through biotransformation to chemically reactive intermediates which bind covalently to DNA and get permanently attached to DNA, alter DNA structure and lead to mutation (Graslund and Jernstrom, 1989). The primary objective of this work was to measure PAH emission factors as a function of particle size from burning almond prunings and rice straw. This paper is part of a larger study to examine the dependence of particle size on gas/ particle partitioning of PAH in smoke from agricultural waste biofuels.
2. Experimental methods 2.1. Combustor and sampling methods Gaseous and particulate matter measurements were carried out in a combustion chamber built at UC Davis for two agricultural fuels: almond prunings and rice straw residue collected from local farms in 1997–1998. Each fuel was tested in a single burn that was designed to mimic actual burn conditions as closely as possible. Table 1 shows the burn conditions for each of the two burns reported here. The combustion chamber incorporated a conveyor system for moving the fuel bed into the flame zone when operating the rice straw burn (a spreading fire). The conveyor system consists of two parts. The primary fuel conveyor, a flexible rubber belt, was used for transporting the fuel continuously to the secondary stainless steel belt on which it was burned. A single variable speed DC motor was used to operate both conveyor belts. The speed was adjusted to maintain the fire front under the hood at all times. The conveyor belt has the following dimensions: height 79 cm, width 46 cm, length of the primary fuel conveyor 3 m, and length of the secondary combustion conveyor 182 cm. The emission sampling section consists of a large (152 cm 152 cm) hood with adjustable height above the combustion section. The hood was connected to a 30.5 cm diameter stack of 305 cm length that houses a fan at the far end to create the air stream. The sampling ports were located about 30 cm upstream of the fan. Three sampling ports were used; one 10 cm port to house a reduced artifact dilution system (RADS) probe (Wall, 1996), one 6.4 cm port to house a photoelectric aerosol sensor (PAS) probe and one 2.5 cm port used to monitor temperature inside the stack. The air velocity inside the stack was set at approximately 11 m s1. The Reynolds number in the stack
Table 1 Sampling conditions for each agricultural fuel burn Fuel
Rice straw Almond pruning a
Burn date
1/12/2000 28/2/2001
Average particle concentration (mg m3) 28.2 16.4
Ambient air
Combustor stack
Dilution tunnela
Temp. (1C)
RH (%)
Temp. (1C)
Temp. (1C)
RH (%)
14 26
58 17
50 70
16 23
53 23
Average of the measurements over entire sampling period.
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was 4200,000, ensuring turbulent flow and adequate mixing of the smoke. The RADS diluted the combustion sample 35:1 before collection in a variety of samplers. This provided a sampling flow rate of 0.7 m3 min1 inside the dilution tunnel. Microprocessor controls of RADS maintained steady state conditions of stack sampling rate and dilution factor. Rice straw was burned in a spreading fire propagating against the main flow and almond pruning was burned in a pile fire to reflect the burn type in the field. Rice straw burning (spreading fire) was conducted by adjusting the conveyor belt speed to the fire spreading rate to maintain the fire stationary under the hood. The feed belt was marked in 0.45 m2 sections, each section 1 m long on the 0.45 m wide belt. Fuel for each section was weighed into a large tared container in an amount sufficient to provide the yield or loading rate (g m1) desired. The wet fuel loading rates were selected based on the reported yield of 3 ton acre1 (approximately 680 g m2) from field studies (Knutson and Miller, 1982). The fuel was loaded in a uniformly thick layer on the belt to a depth typical of that in the field, 75–100 mm but not exceeding 150 mm. The speed of the conveyor belt was adjusted to achieve a fire spreading rate of 1 m min1. Total weight of the moist fuel for the rice straw burn was 10.1 kg over a total sampling time of 50 min. For the almond pruning burn, the conveyor belt was removed and wood pieces of various sizes (4–20 cm diameter and 30–50 cm length) and thickness were arranged in a pile. Pieces of the wood were added to the pile during the sampling as needed. The fire was lit on the perimeters. A total of 11.5 kg of almond pruning was burned over a 65 min period. Four ports at the base of the RADS were connected to four separate sampling trains. Two ports were connected to 10 stage Micro Orifice Uniform Deposit Impactors (MOUDI) (Marple et al., 1991). Both MOUDIs were operated at 30 lpm and were matched for particle sizing characteristics. One MOUDI (F) was used for PAHs analysis and the other MOUDI (T) was used for organic carbon and elemental carbon analysis. Aluminum substrates of 47 mm diameter were used for sample collection in both MOUDI (F) and MOUDI (T) for both burn tests. A 37 mm Teflons filter was used as the after-filter for the rice straw burn and a 37 mm quartz filter was used as the after-filter for the almond prunings burn. A cyclone preceded MOUDI (F) to remove particles 42 mm.
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Wood smoke aerosols were tested for bounce in greased and ungreased impactor stages prior to the burn experiments. The tests indicated that coarse particles (size cut 41.8 mm) bounced (the ungreased stages in one MOUDI showed particle loss in comparison to greased stages in the other), but fine particles did not. Therefore, the first four stages ðd ae 41:8 mmÞ of the impactor were coated with silicon grease to avoid particle bounce during sampling. The third port was connected to an eight-stage Berner impactor (Berner et al., 1979) that was analyzed for PAH. The last port was connected to a cyclone (AIHL design) to remove particles larger than 2.5 mm and was followed by a union with four sampling ports. This manifold was used to collect samples on filters and denuders for additional PAH analysis. The data from these samplers and complete details of the combustion chamber design and operation are reported in Keshtkar (2003). 2.2. Sample preparation and analysis The MOUDI substrates and filters were weighed before and after sampling. MOUDI (T) substrates were shipped to Sunset Laboratory for OC/EC analysis. Sunset Laboratory uses the thermal optical transmission method for organic and elemental carbon analysis by thermal evolution/oxidation of CO2 followed by catalytic reduction to methane and detection by flame ionization. The split between ‘‘organic’’ and ‘‘elemental’’ carbon is determined by the transmission of laser light as the optical density of the filter increases (due to charring) and then returns to its starting point as carbonaceous material evolves and/or oxidizes (Birch and Cary, 1996). MOUDI (F) substrates were stored at 20 1C until analysis. Substrates were extracted, prepared, and analyzed as follows. In a 40 ml vial, 35 ml of 3:1 cyclohexane (CH):dichloromethane (DCM) mixture was added to each impactor substrate. The samples were then spiked with 1000 ng of a mixture of sixteen deuterium-labeled priority pollutant PAHs, 1000 ng of deuterium-labeled 1-nitropyrene and 3000 ng of deuterium-labeled tetracosane. The samples were then extracted with pulsed ultrasonication using a Ney proSONIK for 6 min (Wall, 1996). All of the extracts were concentrated to 500 ml using a metered stream of 99.9% pure nitrogen under a slight vacuum. A 200 ml aliquot of the 500 ml
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sample was then injected into an HP 1090 high performance liquid chromatograph for fractionation of each sample into non-polar, slightly polar and polar fractions for analysis of alkanes, PAHs and oxygenated PAHs, respectively. The separation was performed using a 4.6 mm diameter 250 mm length Whatman EQC 5 mm particle size, 60-A˚ pore size SI column at 35 1C temperature, pressure of 65 psi, 1.2 ml min1 flow rate and the UV detector wavelengths of 254, 340, 260, and 270 nm. The solvent gradient was programmed as 100% hexane for 5 min followed by a gradient of 5% min1 dichloromethane for 20 min and hold at 100% dichloromethane for 14 min. The alkane fraction was collected from 0 to 6 min, the PAH fraction was collected from 10 to 16 min, and the oxygenated PAH fraction was collected from 16 to 39 min after injection.
emission factors in g kg1 (for major carbon fractions) or mg kg1 (for PAH) of dry fuel. Emission factor ðEFÞ ¼
ðmspecies Þ ðV d Þ . ðV i Þ ðmf Þ
An emission factor was calculated for every PAH measured on each stage of the MOUDI impactors for all sampling runs. The resulting size distributions were then smoothed using the Twomey inversion algorithm (Twomey and Zalabsky, 1981; Winklmayr et al., 1990). The results are presented as size distributions of mass emission factors normalized to the total emission factors of the entire size range (dE/E d Log Dp). 3. Results and discussion 3.1. Particulate matter, organic and elemental carbon, and total PAH size distributions
2.3. Emission factors and size distributions The results are presented as emission factors derived from the mass measurements on the ten MOUDI stages and the after filter. The mass of each species measured on each stage (mspecies) was divided by the volume of air pulled through the impactor (Vi) during the time of burn (adjusted for 35:1 sample dilution in RADS) to obtain the average concentration inside the combustor duct. This result was multiplied by the volume of air that passed through the duct (Vd) during the burn to obtain the total mass per burn and was then divided by the mass of dry fuel burned (mf) to obtain
Table 2 shows the calculated emission factors for particulate matter, organic carbon and elemental carbon in each aerosol mode, where the nuclei mode is d ae o0:1 mm, the accumulation mode is 0:1 mmod ae o1:8 mm, and the coarse mode is d ae 41:8 mm. Total emission factors (summing all aerosol modes) of particulate matter (PM), organic carbon and elemental carbon were 7.22, 0.83 and 0.49 g kg1 for rice straw and 5.43, 2.37 and 0.17 g kg1 for almond pruning. Even though these results are for a single burn for each fuel, the total particulate matter results were in good agreement with those reported from open burning
Table 2 Particulate, organic and elemental carbon emission factors (g kg1) Nuclei mode d ae o0:1
Accumulation mode 0:1od ae o1:8
Coarse mode 1:8od ae
Almond pruning Particulate matter Organic carbon Elemental carbon Fraction of OC in PM (%) Fraction of EC in PM (%)
2.14 0.98 0.04 46.0 1.9
2.52 1.39 0.13 55.1 5.3
0.77 –a –a – –
Rice straw Particulate matter Organic carbon Elemental carbon Fraction of OC in PM (%) Fraction of EC in PM (%)
2.43 0.16 0.07 7 3
3.93 0.67 0.42 17 11
0.86 –b –b – –
a
Organic and elemental carbon were measured only for particles o1:8 mm aerodynamic size. Organic and elemental carbon were measured only for particles in the aerodynamic diameter size range ð0:0564d ae o1:8 mmÞ.
b
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of agricultural biomass of 2.0–8.6 g kg1 for rice straw and 4.8–6.2 g kg1 for almond prunings burns (Jenkins, 1996), and with other reports from various wood and biofuel burns from small stoves and fireplaces (Smith, 1987; Venkataraman and Rao, 2001; Rau, 1989; Hildemann et al., 1991). Also shown in Table 1 is the fraction of organic carbon and elemental carbon in particulate matter for each biofuel. Both fuels showed the largest fraction of particles in the accumulation mode. However, the emission factor for rice straw (3.93 g kg1) is higher than that for almond prunings (2.52 g kg1) in the accumulation mode. The emission factors in the nuclei mode are similar: 2.14 and 2.43 g kg1 for almond pruning and rice straw, respectively. In both biofuels about 88% of the particulate mass was smaller than 1 mm and can pass easily through the upper respiratory tract. This gives important information about health hazards of agricultural burning practices. The smoothed size distributions of particulate matter and total PAH compounds emission factors for rice straw and almond pruning burns are shown in Fig. 1. Each curve in Figs. 1–4 is normalized to the total emission factor for the species so that different species can be compared on the same scale. The size distribution of particulate matter mass from both biofuels is bimodal in the two burns we conducted, showing a small, broad peak in the 1.7–17 mm range and a much larger and narrower peak at 0.13–0.18 mm. Particle formation in the nucleation mode (0.01–0.1 mm) has been reported for aerosols from vehicle emission, coal combustion and wood burning (Hildemann et al., 1991). Particles in the larger coalescence mode (2–10 mm) have also been seen from coal combustion (Flagan and Freidlander, 1978; Neville et al., 1983). However, they could have been mechanically generated from dust, re-entrained soot or soil on fuel or pieces of biofuel itself. For the rice straw smoke stage 9 of the MOUDI impactor (0.1–0.18 mm) collected the largest mass of particles (34%), while for almond pruning the afterfilter ðd ae o0:056 mmÞ, with 30% of the total, had the largest mass. The size distribution of total PAH compounds is shifted towards smaller particles relative to the particulate matter results. This shift is more striking for almond pruning than for rice straw, where the PAH size distribution shows better agreement with the particulate matter size distribution. The peak at 0.02 mm in Fig. 1a for almond pruning represents the after-filter mass—it should
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Fig. 1. Size distribution of particulate matter and PAH compounds normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
not be inferred, though, that the actual size peak is at 0.02 mm. The lower limit used in the smoothing algorithm for the particle size range collected on the after-filter influences the shape of the smoothed distribution in the size range below 0.056 mm. The size distributions of organic and elemental carbon for rice straw and almond pruning burns are shown in Fig. 2. Emission factors of organic carbon in the accumulation and nuclei modes from almond prunings were 1.39 and 0.98 g kg1 while in rice straw they were 0.67 and 0.16 g kg1. Emission factors of elemental carbon in the nuclei mode were similar for both fuels while in the accumulation mode rice straw had higher emission factors. About 41% of the organic carbon and 23% of the
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Fig. 2. Size distribution of organic and elemental carbon normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
elemental carbon from the almond pruning burn was in particles smaller than 0.1 mm, while the values were 19% and 14%, respectively, for the rice straw burn. In the ultrafine and accumulation modes, organic carbon constituted 46% and 55%, respectively, of the mass of particles from the almond pruning burn, while for the rice straw burn only 7% and 17% of the mass of particles in the ultrafine and accumulation modes was organic carbon. These results are consistent with Rau (1989), who reported 50–60% organic carbon in fine particulate matter from air-starved (cool) wood burns in small stoves and 20–60% elemental carbon in fine particulate matter ðo2:5 mmÞ in open air (hot) burns.
Fig. 3. Size distribution of three- and four-ring PAHs (molecular weight 178–228) normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
Elemental carbon is unimodal for almond prunings with a peak at 0.28 mm and organic carbon is bimodal with peaks at 0.2 and 0.02 mm. The bimodal distribution of organic carbon shows a good correlation with the bimodal particulate matter distribution. Organic and elemental carbon distributions for rice straw are both unimodal with peaks at 0.13 and 0.15 mm, respectively. However, no rice straw smoke results are available for OC/EC analysis in the particle size ranges 43:2 mm and o0:056 mm. Generally, the peaks in PM, OC and EC in smoke from almond prunings are at larger size ranges than those from rice straw. These data are consistent with other measurements of aerosol mass in emissions from combustion sources (Rau, 1989; Venkataraman et al., 1994).
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The mass median diameter of the particles was between 0.07 and 0.08 mm for organic carbon and elemental carbon and particulate matter from both biofuels. 3.2. Individual PAH size distributions
Fig. 4. Size distribution of five- and six-ring PAHs (molecular weight 252–278) normalized emission factors from: (a) almond prunings burn and (b) rice straw burn.
The samples were analyzed for 16 priority pollutant PAHs in this study. However, only PAHs with molecular weights 178 (phenanthrene) and higher were present in impactor samples for the two burns we conducted. More volatile compounds were either mainly in the gas phase or lost during sampling or storage. Small amounts of naphthalene were identified in some samples, but were not quantified. Anthracene was present in rice straw smoke samples but was not detected in almond pruning smoke samples. The size distributions of PAH emission factors from the two agricultural fuels are presented in Fig. 3 for three- and four-ring PAH and in Fig. 4 for five- and six-ring PAH. Tables 3 and 4 summarize the PAH emission factors for almond pruning smoke and rice straw smoke, respectively, and show the fractions in each particle size mode. Emission factors for the more volatile (three- and four-ring) PAH compounds were strikingly higher for rice straw than for almond pruning smoke. However, emission factors (in particulate matter phase) of less volatile PAH compounds (five- and six-rings) were comparable in both fuels. Thus, the ratio of the more volatile PAH compounds (three- and
Table 3 PAH emission factors from almond pruning smoke (mg kg1 of dry fuel) PAH
Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(k)fluoranthene Benzo(b)fluoranthene Benzo(a)pyrene Indeno[1,2,3-cd]pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Total PAH
Number of rings
3 3 4 4 4 4 5 5 5 6 5 6
Emission factor (mg kg1)
Fraction in particulate matter mode Nuclei
Accumulation
Coarse
2.05 – 1.29 1.44 0.37 0.46 0.49 0.34 0.43 0.44 0.18 0.55
0.25 – 0.19 0.18 0.55 0.52 0.55 0.52 0.58 0.33 0.54 0.44
0.47 – 0.46 0.44 0.45 0.48 0.45 0.48 0.42 0.67 0.46 0.56
0.28 – 0.35 0.38 0 0 0 0 0 0 0 0
8.03
0.33
0.48
0.2
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Table 4 PAH emission factors from rice straw smoke (mg kg1 of dry fuel) PAH
Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(k)fluoranthene Benzo(b)fluoranthene Benzo(a)pyrene Indeno[1,2,3-cd]pyrene Dibenzo(ah)anthracene Benzo(ghi)perylene Total PAH
Number of rings
3 3 4 4 4 4 5 5 5 6 5 6
Emission factor (mg kg1)
Fraction in particulate matter mode Nuclei
Accumulation
Coarse
6.95 1.75 2.47 2.43 0.68 0.67 1.31 0.51 0.67 0.44 0.20 0.52
0.33 0.85 0.47 0.50 0.23 0.40 0.44 0.48 0.38 0.38 0.55 0.37
0.55 0.06 0.33 0.34 0.54 0.47 0.48 0.37 0.62 0.62 0.47 0.63
0.12 0.09 0.20 0.16 0.22 0.13 0.07 0.15 0 0 0 0
18.62
0.44
0.44
0.12
four-rings) to less volatile PAH compounds (fiveand six-rings) for rice straw smoke is significantly higher than for almond prunings. Phenanthrene, anthracene, fluoranthene and pyrene constitute 73% of PAHs from rice straw smoke while phenanthrene, fluoranthene, pyrene and benzo[a]anthracene comprise 64% of all the PAHs from almond pruning smoke. The predominance of more volatile PAH compounds (three- and four-rings) in smoke from both biofuels agrees with the reported results for wood (Oanh et al., 1999) and biofuels (Rau, 1989; Venkataraman et al., 2002) combustion and is in contrast with the reported predominance of nonvolatile PAH compounds for coal fossil fuel combustion (Wornat and Sarofim, 1990). We measured total emission factors for PAH compounds of 8.03 mg kg1 for almond prunings and 18.62 mg kg1 for rice straw smoke. Jenkins (1996) reported total emission factors in the range of 5.04–49.29 mg kg1 for all PAH compounds for rice straw and 14.23 mg kg1 for almond pruning. He also reported an emission factor range for PAH (less naphthalene and 2-methylnaphthalene) of 1.41–9.70 mg kg1 for rice straw and 6.78 mg kg1 for almond pruning. Jenkins’ higher total PAH emission factor for rice straw (cereal crop) smoke is consistent with our higher particulate matter emission factor from rice straw compared to almond pruning (wood). The temperature inside the combustor duct at the sampling port was higher during the almond prunings burn (average of 70 1C while reaching 100 1C at times) than for rice straw (average of 50 1C
Table 5 Mass median diameter (mm) of particulate matter, organic and elemental carbon and PAH species from agricultural waste biofuel burning Species
Particulate matter Organic carbon Elemental carbon Phenanthrene Fluoranthene Pyrene Benzo[a]anthracene Chrysene Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene Indeno[1,2,3-cd]pyrene Benzo(ghi)perylene Dibenzo[a,h]anthracene
Mass median diameter (mm) Rice straw
Almond pruning
0.08 0.07 0.08 0.13 0.11 0.10 0.15 0.12 0.11 0.11 0.12 0.12 0.12 0.09
0.07 0.07 0.06 0.40 0.55 0.70 0.09 0.09 0.09 0.09 0.09 0.13 0.12 0.09
with little variation throughout the sampling period). Also, the almond pruning burn had a more vigorous and robust flame. The particulate matter and PAH compounds emission factor trends are consistent with the previously reported results for the agricultural fuels (Jenkins, 1996). Higher combustion efficiency and lower emissions are expected from higher temperature combustion. Table 5 shows mass median diameters (MMAD) of particulate matter, organic and elemental carbon, and PAH compounds for our tests. The MMADs are in the range of 0.07–0.15 mm for all species in
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both fuels except for fluoranthene, phenanthrene and pyrene from almond prunings, which showed MMADs at 0.40, 0.55 and 0.70, respectively. Hildemann et al. (1991) reported a range of 0.07–0.15 mm for PAH MMADs for vehicular emissions. However, Venkataraman et al. (2002) reported MMADs ranging from 0.4 to 1.0 mm for all PAHs in a study on biofuel combustion. Figs. 3 and 4 show that all PAH compounds were present in the after-filter from almond pruning smoke, but only phenanthrene, anthracene, fluoranthene, pyrene and benzofluoranthenes were detected in the after-filter ðd ae o0:056Þ from rice straw smoke. The lower stages of the MOUDI sampler (below 1.8 mm) were below atmospheric pressure, so it’s possible that the more volatile PAH compounds are depleted in those stages due to evaporation. However, we found the more volatile PAH compounds in the smaller size ranges from both burns. The size distributions of phenanthrene, fluoranthene and pyrene exhibit a multimodal profile for both biofuels with three common peaks at 10, 1.9–2.2 and 0.07–0.11 mm and an additional peak at 0.3–0.5 mm for almond prunings. Anthracene was not detected in almond pruning smoke and in the rice straw smoke its size distribution was strikingly different from other semivolatile PAH compounds, with 77% of the mass in the after filter. For the rice straw burn benzo(a)anthracene, chrysene, benzo(k)fluoranthene and benzo(b)fluoranthene show bimodal distributions with a nucleation mode at 0.09–0.11 mm and a coalescence mode at 2.3 mm. However, these PAH compounds are unimodal in almond pruning with a broad peak at 0.13 mm. For both fuels, benzo(a)pyrene and 6ring PAH compounds all show unimodal distribution with broad peaks at 0.08–0.17 mm except for dibenzo(a,h)anthracene from almond pruning, which showed a bimodal profile with peaks at 0.06 and 0.28 mm as shown in Fig. 4. This bimodal profile is not an artifact of the smoothing, but results from very low concentrations of dibenzo(a,h)anthracene on the 0.1–0.18 mm MOUDI stage. Fig. 5 shows the size distributions of the concentrations of semivolatile three- and four-ring PAH compounds and non-volatile five- and six-ring PAH compounds per gram of particulate matter in almond pruning smoke and rice straw smoke. The relatively large peak at 1.3 mm pertaining to more volatile PAH compounds (molecular weight 178–228) from both biofuels is due to the very small concentration of particulate matter in this size
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Fig. 5. Concentration of three- and four-ring PAH species (lefthand side Y-axis) and five- and six-ring PAH species (right-hand side Y-axis) per gram of particulate matter with respect to particle size for smoke from: (a) almond prunings burn and (b) rice straw burn.
range. The less volatile PAH compounds in almond pruning smoke are exclusively in the ultrafine size range with a peak at 0.075 mm. This peak in the almond pruning smoke is about 50% larger than that of the rice straw smoke. The peak at about 2.5 mm in rice straw smoke, which is completely absent in almond pruning smoke, is due to the presence of Benzo(k)fluoranthene and Benzo(b)fluoranthene in the 1.8–3.2 mm size range and is enhanced by the very small concentration of particulate matter in that range (Fig. 1). The condensation process depends on the vapor pressure of the condensing species, its concentration in the gas phase, and the number concentration and size of the particles (i.e. the available surface area).
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Condensation will occur earlier for the less volatile and more plentiful species. At any stage of combustion, condensation preferentially occurs on ultrafine particles as they are in greater numbers and offer a higher surface to mass ratio. However, condensation of more volatile PAH compounds is inhibited on smaller particles due to the Kelvin effect. This effect is less important for PAH compounds with low volatility. Therefore, compounds with low volatility preferentially condense on smaller particles in the early stages of combustion while more volatile compounds mainly condense on larger particles at later stages as the smoke cools. Our almond pruning burn was relatively hotter (see Table 1) and more efficient than the rice straw burn, producing lower emissions of particulate matter and PAH compounds. However, almond pruning smoke contained more ultrafine particles ðo0:1 mmÞ, providing a larger surface to mass ratio for less volatile PAH compounds to condense on in the early stages of condensation. The higher volatility PAH compounds partitioned into the particle phase relatively less or remained in the gas phase. Our rice straw burn was cooler and less efficient than the almond pruning burn, with a steady, more consistent smoke throughout the burn. It resulted in higher particulate PAH emission factors. Both rice straw and almond pruning showed predominance of semivolatile PAH compounds in particles larger than 0.3 mm relative to non-volatile PAH compounds. However, in almond pruning, where there were lower emission factors of PAH compounds, all of the PAH compounds in particles larger than 0.3 mm were the more volatile PAH (only phenanthrene, fluoranthene and pyrene). 4. Conclusions For the two burns we conducted, one each for rice straw and almond prunings, combustion temperature was the most important factor in determining the smoke particle PAH composition. Burn temperature depends primarily on the combustion air supply, but is also influenced by the amount of fuel burning, fuel moisture content and type of burn (in this study pile fire versus spreading fire). Hot flames generally produce a lower amount of particulate matter as well as lower organic pollutants such as polycyclic aromatic hydrocarbons. However, flames with higher temperatures tend to produce relatively more particles in the ultrafine range. In our study
rice straw, with a lower burn temperature, was more polluting than almond prunings in terms of particulate matter with most of the difference in concentration coming from particles in the accumulation mode (0.1–1.8 mm). The rice straw burn also produced more PAHs than the almond pruning burn, with most of the difference coming from three- and four-ring compounds. However, the almond pruning burn generated more particles in the ultrafine range with the after-filter having the largest concentration of particulate matter of any single stage, providing a larger surface area for the condensation of PAH compounds. This resulted in a relatively larger fraction of individual PAH compounds (four-rings and higher) in the ultrafine range. The ratio of more volatile PAHs (three- and four-rings) to less volatile (five- and six-rings) is 1.8 times higher in rice straw. The less volatile five- and six-ring PAH compounds were predominately on smaller particles where they condensed in the early stages of combustion, while the more volatile threeand four-ring PAH compounds formed on larger particles as the smoke cooled. Although this study is based on a single trial for each burn, the results are consistent with other research studies and show that open burning of agricultural residues is a significant source of particulate matter and PAHs in the respirable particle size range below 1 mm. As a result this practice is important in terms of acute health effects for people near the fires. The smoke particles also adversely affect regional visibility as this size range efficiently scatters and absorbs visible light. Acknowledgments The University of California, Davis Crocker Nuclear Laboratory provided funding to construct the combustion chamber, and the Department of Agricultural and Biological engineering designed and built it. All the equipment and media preparation prior to sampling and the sample analytical work was carried out under the guidance of Dr. Stephen Wall and Diamon Pon at the Environmental Health Laboratory Branch of the California Department of Health Services. References Berner, A., Lurzer, C., Pohl, F., Preining, O., Wagner, P., 1979. The size distribution of the urban aerosol in Vienna. The Science of the Total Environment 13, 245–261.
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