Emission Factors for Polycyclic Aromatic Hydrocarbons from Biomass

Environ. Sci. Technol. 1996, 30, 2462-2469

Emission Factors for Polycyclic Aromatic Hydrocarbons from Biomass Burning B R Y A N M . J E N K I N S , * ,† A. DANIEL JONES,‡ S C O T T Q . T U R N , †,§ A N D ROBERT B. WILLIAMS† Biological and Agricultural Engineering Department, University of California, Davis, California 95616, and Facility for Advanced Instrumentation, University of California, Davis, California 95616

Emission factors for 19 polycyclic aromatic hydrocarbons were measured during wind tunnel simulations of open burning for agricultural and forest biomass fuels including cereal grasses, agricultural tree prunings, and fir and pine wood (slash). Yields of total PAH varied from 5 to 683 mg kg-1 depending principally on burning conditions and to a lesser extent on fuel type. Barley straw and wheat straw loaded at 400-500 g m-2 emitted much higher levels of PAH, including benzo[a]pyrene, than other cereal and wood fuel types burning under more robust conditions. As anticipated, total PAH emission rates increased with increasing particulate matter emission rates and with declining combustion efficiency.

Introduction Open burning of biomass is a common technique for crop and forest residue disposal and land preparation and represents a considerable source of atmospheric pollutants. Principal pollutants emitted are CO, hydrocarbons, and particulate matter, with smaller amounts of NOx and SO2 (1). In addition are emissions of volatile organic compounds (VOC) and polycyclic aromatic hydrocarbons (PAH), many of which are toxic and some known or suspected carcinogens. In North America and Europe, deliberate agricultural and sylvicultural burning for the purposes of rapid residue and vegetation removal constitutes the largest share of emissions after wildfires. Land clearing operations constitute the major share in tropical regions. Recent reductions in atmospheric CO concentrations have been attributed in part to a slow down in the rate of tropical biomass burning (2). This source constitutes roughly 30% of known sources of atmospheric CO. The characteristics and mutagenic activities of biomass-derived PAH from a number * Author to whom correspondence should be addressed; telephone: 916-752-1422; fax: 916-752-2640; e-mail address: bmjenkins@ ucdavis.edu. † Biological and Agricultural Engineering Department. ‡ Facility for Advanced Instrumentation. § Present address: Hawaii Natural Energy Institute, University of Hawaii, Honolulu, HI 96822.

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of sources have been described, principally from stoves and furnaces burning wood and cereal straws, but including only limited work on open burning (3-9). Reported here are the results of several open burn simulations conducted in a combustion wind tunnel to measure emission factors for PAH emitted during burning of crop and forest residue biomass. These studies were intended to document mass emission rates of PAH species. No toxicity or mutagenicity studies were conducted. Fuels were selected on the basis of primary types open burned in California under prescribed conditions. Some of these types are commonly burned throughout the world. Eight fuels were evaluated: barley straw, corn stover, rice straw, wheat straw, almond and walnut tree prunings, and Douglas fir and Ponderosa pine slash. Rice and wheat straw, along with almond and walnut prunings comprise 95% of the agricultural biomass, exclusive of wildfires and prescribed forest fires, openly burned in the state of California (10).

Experimental Section Wind Tunnel. The wind tunnel used in the experiments was an open-circuit, forced-draft type 1.2 m wide. The design of the wind tunnel has been detailed by Jenkins et al. (11) and is not further described here. The fuels were burned in two types of fires: spreading fires and pile fires. All cereal straws and stovers were burned in spreading fires simulating open burns propagating against the wind. Forest materials were burned in pile fires and incorporated pieces ranging up to 150 mm in diameter. For spreading fires, the wind tunnel employed a system of conveyors for moving the fuel bed downstream at the same speed as the natural upwind spreading velocity so as to provide prolonged sampling intervals. Sampling was performed in the stack above the combustion test section approximately 10 m above the fire. An auxiliary floor made from sheet steel panels could be positioned immediately below the fuel conveyor extending from the entrance of the combustion test section past the fire. The auxiliary floor was used to test the effect of inhibiting ventilation from below, as in the case of a fire spreading through a fuel bed resting on the ground (11). An adjustable ceiling could be extended into the combustion test section from the upstream wall. The purpose of the ceiling was to sustain the well-developed inlet velocity profile up to the fire. The ceiling terminated just ahead of the fire so as to allow free buoyant plume expansion above and behind the fire. The ceiling and auxiliary floor were used only with spreading fires. Two configurations of the wind tunnel were evaluated to test the effect on the emission factors. The first employed both the floor and ceiling (designated CEWF for ceiling extended with floor); the second employed neither (designated CRNF for ceiling retracted, no floor). The wind speed for the CRNF configuration was set at about 2 m s-1 and for the CEWF configuration was set at 3-3.5 m s-1. Walnut prunings were burned in a pile situated underneath the stack. Almond, fir, and pine fuels were later burned on an open platform scale situated at the same position underneath the stack, which permitted continuous weight measurements. The blower was not operated during pile burns, instead air was admitted freely through the large side doors of the wind tunnel.

S0013-936X(95)00699-7 CCC: $12.00

 1996 American Chemical Society

TABLE 1

Average Fuel Compositions fuel type cereals barley straw

corn stover

C H N P K Ca Mg Cl S

44.85 5.85 0.77 0.12 2.50 0.26 0.11 0.20 0.13

44.78 5.88 0.58 0.08 1.76 0.17 0.38 0.36 0.06

ash volatiles fixed carbon

7.61 78.06 14.33 17.80

rice straw

woods Douglas fir

Ponderosa pine

walnut

Elemental Analysis (% dry weight) 38.06 44.28 49.16 5.28 5.77 6.24 0.72 0.62 0.49 0.07 0.05 0.03 1.71 1.89 0.29 0.27 0.11 0.28 0.20 0.11 0.12 0.48 0.29 0.03 0.09 0.18 0.02

51.64 6.24 0.27 0.02 0.17 0.35 0.03 0.01 0.02

52.41 6.20 0.30 0.02 0.16 0.35 0.06 0.00 0.02

48.23 6.00 0.60 0.04 0.30 1.33 0.28 0.13 0.04

6.12 77.33 16.55

Proximate Analysis (% dry weight) 18.59 9.38 1.33 68.77 76.24 82.28 12.64 14.38 16.39

0.55 82.95 16.50

1.22 81.51 17.27

3.67 82.96 13.37

17.97

Higher Heating Value (MJ/kg dry weight) 15.43 17.34 19.47

20.39

20.66

19.28

Sampling. Sampling was done from the top of the stack above the fire. Traverses on a 24-point grid across the stack were used to collect particulate and gas phase samples. PAH was collected and analyzed according to a modified California Air Resources Board (CARB) Method 429 (12) as detailed below. In addition to PAH, sampling was performed for total particulate matter, particle size distribution, primary gaseous pollutants, VOCs, and elemental characterization (source profiling) of particulate matter. The sampling method for total particulate matter and gas emissions was an in-stack method based on CARB Method 17 (12). Total particulate matter (PM) was sampled isokinetically from the stack flow through a non-size selective inlet onto 47 mm diameter Teflon-coated glass fiber filters (Pallflex type TX40HI20, Pallflex Products Corp., Putnam, CT). The PAH sample was pulled through the total PM inlet and filter, then through an ambient temperature water bath and back-up filter, to a foil-wrapped XAD-2 sorbent trap. The flow rate was kept isokinetic by matching inlet velocity to the stack velocity monitored with a hot film anemometer traversing with the probe inlets in the stack. Due to the large air dilution in the tunnel, stack temperatures at the point of sampling were in the range of only 10-50 K above ambient. Analysis. Filters and XAD-2 sorbents were Soxhlet extracted for 16 h using dichloromethane after the addition of perdeuterated PAH internal standards. Impingers were analyzed using liquid-liquid extraction (again using dichloromethane). Extracts were cleaned up using sequential column chromatographic steps with silica gel and alumina as column packing. PAH concentrations were determined using GC/MS and stable isotope dilution. Although CARB Method 429 specifically states that the method is not suitable for determining the partitioning of PAH among the different parts of the sampling train, individual components were analyzed separately to provide at least qualitative evaluations of the filter and sorbent fractions. Primary and backup filters were analyzed separately. Impinger analyses were also used to check for breakthrough, which was not observed to occur. In later samples, line rinses were combined with impinger extracts.

wheat straw

almond

Surrogate standards were added only to the XAD-2 sorbent modules. Selected monitoring of molecular ions for all target analytes and standards was performed instead of full scanning analysis. Two groups of ions were monitored, with the first group consisting of anthracene and all targets eluting earlier, and the second group consisting of all analytes eluting after anthracene. Low resolution MS analysis was performed on a VG Trio-2 quadrupole GC/MS system. Limits of detection were judged to range from 2-50 ng depending on sample. Target species included the 16 EPA priority PAH compounds plus 2-methylnaphthalene, benzo[e]pyrene, and perylene. Deuterated standards were not available for 2-methylnaphthalene at the time of method development; naphthalene-d8 was used as internal standard for this analyte. Fuels. These tests were carried out as a preliminary characterization of PAH emission rates from several different types of biomass and were not intended to develop detailed information pertaining to the formation mechanisms for PAH from biomass. A total of 30 tests were conducted on the eight fuels, from which trends relating to fuel and combustion conditions were observed. Properties of the fuels burned are listed in Table 1. Values shown are averages over all tests for each fuel type. For the wood fuels, the compositions are integrated by weight fraction over all size classes of individual fuel elements. No litter or duff was included in any of the slash burns. Although litter and duff can contribute in important ways to the emissions from prescribed burns and wildfires, these tests were intended to collect information on pure fuels only. The fuel loading rates (g m-2) for field crop residues were selected to be similar to published yields for each crop from field studies (13). Moisture contents and loading rates (spreading fires) are listed in Tables 2-4. Piles ranged in weight from 35 to 45 kg, with initial volumes around 4 m3. Spreading fires were replicated under each configuration of the wind tunnel. Rice straw, which constitutes approximately two-thirds of the crop residue burned in

VOL. 30, NO. 8, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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flaming and smoldering stages. The overall emission factors were later computed by mass averaging on the basis of the fuel weights involved in each fraction.

TABLE 2

Average Experimental Conditions for Cereals barley straw

corn stover

rice straw

wheat straw

fuel moisture (% w.b.) 7 9 9 7 470 890 605 370 loading rate (g m-2) ambient air temp (°C) 19 23 29 24 relative humidity (%) 67 20 41 68 3.85 8.52 10.77 2.38 burning rate (g s-1, d.b.) overall air-fuel ratio 993 596 431 1909 118 123 30 heat release rate (kW m-1) 47 stack gas temp (°C) 33 41 54 34 -1 0.41 0.48 0.89 0.32 fire spread rate (m min )

TABLE 3

Experimental Conditions for Agricultural Woods almond prunings

fuel moisture (% w.b.) ambient air temp (°C) relative humidity (%) burning rate (g s-1, d.b.) Overall air-fuel ratio heat release rate (kW) stack gas temp (°C)

walnut prunings

flaming

flaming stoked

flaming

flaming stoked

18 12 69 9.22 607 171 37

18 17 60 13.21 361 242 58

33 11 58 7.51 679 137 32

33 18 29 12.66 340 238 50

TABLE 4

Experimental Conditions for Forest Woods Douglas fir slash Ponderosa pine slash flaming flaming late flame low rate high rate flaming and smolder fuel moisture (% w.b.) 30 30 24 ambient air temp (°C) 24 29 26 relative humidity (%) 25 14 37 4.44 14.08 21.56 burning rate (g s-1, d.b.) overall air-fuel ratio 898 230 234 heat release rate (kW) 86 278 434 stack gas temp (°C) 38 73 87

30 23 3.24 1359 74 46

California, was subjected to a somewhat more detailed analysis, with four replicates under the CEWF configuration and six under the CRNF configuration. Almond and walnut prunings were tested first by allowing the fire to burn to completion. A second test was then conducted in which the fire was stoked with fresh fuel during sampling. Stoking is often done during field burning of these fuels. Douglas fir was tested in two different pile arrangements: the first a random piling of fuel elements, the second an ordered crib arrangement. The second pile arrangement ignited and burned substantially faster than the first. Although combustion efficiencies for the two types of pile configurations were similar (see later), differences were noted in the total PAH production, possibly due to differences in flame structure or the mass of fuel involved in smoldering. The two fire types are later referred to as low flaming and high flaming, respectively. The pine fires, like the almond and walnut fires, were conducted using random piling only, and an attempt was made to capture both the flaming and smoldering stages. Two burns were required to accomplish this, as filter loading was rapid and filter and sorbent assemblies could not be changed fast enough during a single test. The first test was carried out during the ignition and flaming stages of the burn, the second during the late

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Results and Discussion Burning Conditions. Average burning conditions for the fuels are listed in Tables 2-4. Fuel moistures for the cereals were 7-9% wet basis at air-dry equilibrium moisture. Loading rates for wheat and barley straw were set at 400500 g m-2 to match field rates. Corn stover and rice straw have higher yields and were loaded at 900 and 600 g m-2, respectively. Average burning rates were higher for corn stover and rice straw largely as a result of the higher loading rates and greater heat release rates. Overall air-fuel ratios for the cereals ranged from 400 for the more robust fires to about 2000 for the weaker fires under lighter fuel loads. Fuel moistures for the wood fuels were higher than for the cereals, ranging between 18 and 33% wet basis, which is typical for these fuels when burned in the field. Overall air-fuel ratios ranged from 200 to 1300 depending on stage and intensity of burn. PAH Emissions. Results of all tests are listed in Tables 5 (cereals) and 6 (woods). Listed are emission factors (µg kg-1) for the 19 PAH species analyzed, expressed on the basis of the mass of PAH emitted per unit dry mass of fuel burned. For the cereals, the results are segregated by wind tunnel configuration (CEWF and CRNF). Average values of emission factors are shown for each configuration. Arithmetic averages are shown for the two different burning conditions in the almond, walnut, and fir fires. The results for the pine fires have been mass averaged as mentioned above. Also shown in the tables is the total PAH emission factor for the sum of the 19 species, along with a second sum excluding naphthalene and 2-methylnaphthalene for reasons noted below. A large number of species other than the 19 targeted were likely present, but these were not quantitatively analyzed. Ramdahl and Becher (14) identified 30 PAH derivatives, not examined here, from wood burning in a small air-tight wood stove, which amounted to a total emission of 28 mg kg-1. For barley straw burned in a stoker-fed furnace, 25 PAH derivatives were identified yielding a total of 11 mg kg-1. The results for naphthalene and 2-methylnaphthalene are somewhat uncertain. Despite extensive washing by Soxhlet extraction with methanol, dichloromethane, and hexane, several batches of XAD-2 yielded substantial amounts of naphthalene and 2-methylnaphthalene in field blank analyses. In separate studies, we have observed that acidic vapors (perhaps including NO2) can cause depolymerization of the XAD-2 polymer, yielding an assortment of aromatic byproducts that interfere with analysis of naphthalene and alkylnaphthalenes. No definite trend exists in comparisons of NOx emission levels with the two naphthalenes. Nor is there a clear correlation between emissions of 2-methylnaphthalene and naphthalene, although with one exception there is a general trend of mutually increasing concentrations. Naphthalene was also measured by a separate technique (collection on dual sorbent bed followed by thermal desorption onto GC) as part of the VOC analysis. A few tests show large discrepancies between the VOC and PAH analyses. Below 200 mg kg-1, however, the results are quite comparable. These results indicate that high naphthalene concentrations cannot in general be classified as artifactual. Naphthalene has been observed to constitute a major share of total PAH


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configuration

rep

1.06 137 066 0.69 162 018 149 542 0.71 12 665 0.64 9 441 11 053 1.19 11 665 0.38 3 601 7 633 0.50 1 210 0.46 1 539 1 330 0.20 9 857 0.06 7 081 0.86 14 453 0.57 7 185 0.42 9 644 0.36 3 454 0.85 0.36 0.24 3 368 0.21 3 252 0.21 7 867 0.25 14 030 0.62 7 989 0.55 7 462 0.35 7 328 0.19 3 949 0.54 0.54 0.84 44 736 0.59 44 056 44 396 0.49 669 311 0.41 26 666 347 988

PMb Nap (128)

Acy (152)

Ace (154)

Fle (166)

4 226 19 562 345 4 632 2 696 24 598 1 035 1 922 3 461 22 080 690 3 277 2 138 2 446 19 665 2 304 1 726 384 16 208 1 950 1 932 1 415 17 936 2 127 7 858 555 1 355 265 863 61 27 49 4 360 308 691 157 899 527 857 106 883 467 400 64 891 497 629 85 840 1 061 68 446 633 884 56 406 27 001 425 1 220 1 219 15 406 274 921 742 10 970 661 566 703 12 730 372 595 375 1.16 0.56 1.05 0.53 259 409 21 131 258 433 22 166 507 789 75 228 437 857 48 289 1 941 2 568 180 34 2 195 2 703 197 9 933 1 293 91 143 888 1 056 79 109 0.95 0.82 0.87 0.76 657 702 260 360 2 148 1 124 393 246 1 402 913 326 303 508 3 572 0 551 981 618 27 115 745 2 095 13 333

2-mNap (142) 22 403 19 442 20 923 14 589 12 949 13 769 2 152 626 1 389 1 871 1 775 1 823 1 442 1 383 2 517 1 989 1 833 531 0.29 438 524 849 992 2 940 2 178 1 320 1 010 0.77 5 653 4 764 5 208 3 372 2 582 2 977

Ph (178) 5 362 2 593 3 977 2 107 1 939 2 023 33 89 61 271 362 316 247 232 368 416 316 91 0.29 54 72 137 173 571 401 235 206 0.88 1 322 1 230 1 276 1 090 650 870

An (178) 3 318 2 382 2 850 1 144 2 362 1 753 1 050 221 635 1 173 758 966 450 434 614 449 487 85 0.17 114 185 285 426 829 711 425 289 0.68 7 316 5 992 6 654 1 314 1 095 1 205

Fla (202) 2 433 1 812 2 122 7 622 2 440 5 031 1 040 210 625 1 028 787 908 341 311 363 332 337 22 0.06 88 139 248 374 650 584 347 232 0.67 4 474 3 267 3 870 1 121 1 022 1 071

Py (202) 1 380 850 1 115 1 075 1 213 1 144 345 77 211 63 285 174 94 36 216 130 119 76 0.63 27 35 70 62 347 327 145 150 1.04 2 364 2 188 2 276 297 358 327

B[a]A (228)

B[b]F (252)

B[k]F (252)

B[a]P (252)

B[e]P (252) 809 375 592

24 11 20 17 0.86

24 12 20 16 0.80

23 32 39 52 44 46 107 53 206 31 70 10 76 0.31 1.08 1 070 1 160 1 021 186 1 046 673

46

43

283 4 836

567 9 672

682 361 522

total 10 209 561 10 225 420 10 217 490 71 724 57 318 64 521 565 119 484 6 558 283 63 021 8 092 7 717 7 904 15 408 11 507 49 287 28 309 26 128 17 027 0.65 5 044 5 279 11 452 18 136 19 636 18 426 12 995 6 711 0.52 79 180 72 004 75 592 682 746 35 266 359 006

Per B[ghi]P Ind D[ah]A (252) (276) (276) (278)

1 702 2 653 684 852 1 210 229 1 086 1 989 681 632 706 232 1 394 2 321 683 742 958 231 1 359 2 257 539 739 1 076 1 553 2 718 491 903 1 040 1 456 2 488 515 821 1 058 482 18 459 8 195 28 634 22 433 4 158 146 162 303 37 83 4 314 9 311 4 249 14 336 11 258 2 081 76 29 61 11 390 6 233 18 30 6 116 165 30 64 81 18 40 11 252 200 189 63 128 13 138 92 87 42 77 5 136 117 76 42 72 9 88 84 81 12 28 7 0.64 0.72 1.06 0.29 0.39 0.74 34 37 21 18 22 2 37 69 13 17 29 5 75 126 30 33 53 10 79 129 30 41 56 17 383 310 360 175 171 34 443 192 103 244 352 67 175 144 93 88 114 22 186 97 135 97 128 25 1.06 0.68 1.45 1.10 1.13 1.10 2 542 2 908 790 1 043 1 521 302 2 109 989 533 276 484 996 2 326 1 949 662 659 1 003 649 447 506 274 237 144 377 136 326 78 221 16 412 321 300 158 182 8

Chr (228)

68 269 60 706 64 487 56 921 46 151 51 536 99 961 2 095 51 028 6 073 5 295 5 684 4 711 3 793 7 833 5 717 5 513 1 735 0.31 1 417 1 768 3 078 3 668 9 706 8 768 4 734 3 596 0.76 33 787 25 799 29 793 12 926 7 620 10 273

total less Nap, 2-mNap

a Blank indicates not detected, treated as zero for averaging. Mol wt given in parentheses. Abbreviations: Nap (naphthalene), 2-mNap (2-methylnaphthalene), Acy (acenaphthylene), Ace(acenaphthene), Fle (fluorene), Ph (phenanthrene), An (anthracene), Fla (fluoranthene), Py (pyrene), B[a]A (benz[a]anthracene), Chr (chyrsene), B[b]F (benzo[b]fluoranthene), B[k]F (benzo[k]fluoranthene), B[a]P (benzo[a]pyrene), Per (perylene), B[ghi]P (benzo[ghi]perylene), Ind (indeno[1,2,3-cd]pyrene), D[ah]A (dibenz[ah]anthracene). b Total particulate matter emission factor (% dry fuel). c Coefficient of variation.

barley CEWF 1 2 av CRNF 1 2 av corn CEWF 1 2 av CRNF 1 2 av rice CEWF 1 2 3 4 av SD COVc CRNF 1 2 3 4 5 6 av SD COV wheat CEWF 1 2 av CRNF 1 2 av

fuel

PAH Emission Factors (µg/kg) for Cerealsa

TABLE 5

5 2

1

19 39 37

6 61 8 35 24 248 25 136 50 103 9 56 48 0.36 0.50

0.75 0.43

0.51 0.38

Blank indicates not detected, treated as zero for averaging. Mol wt given in parentheses. Abbreviations: Nap (naphthalene), 2-mNap (2-methylnaphthalene), Acy (acenaphthylene), Ace (acenaphthene), Fle (fluorene), Ph (phenanthrene), An (anthracene), Fla (fluoranthene), Py (pyrene), B[a]A [benz[a]anthracene), Chr (chrysene), B[b]f (benzo[b]fluoranthene) B[k]F (benzo[k]fluoranthene), B[a]P (benzo[a]pyrene), B[e]P (benzo[e]pyrene), Per (perylene), B[ghi]P (benzo[ghi]perylene), Ind (indeno[1,2,3-cd]pyrene), D[ah]A (dibenz[ah]anthracene). b Total particulate matter emission factor (% dry fuel).

a

7 975 5 588 6 781 9 881 7 134 8 508 23 434 5 443 14 439 6 464 20 589 9 731 16 161 12 306 14 233 27 931 22 170 25 050 48 755 12 406 30 581 16 711 69 663 28 956 3

24 10 17 30 7 18 99 9 54 27 13 24 38 18 28 12 56 45 50 59 27 43

165 248 206 80 77 78 335 99 217 80 168 100 243 185 214 54 66 60 396 104 250 93 184 114 197 74 2 451 447 634 518 159 18 1 627 192 415 376 178 46 2 039 319 524 447 1 772 1 228 2 778 494 1 329 925 1 670 630 1 207 253 1 261 1 021 1 721 929 1 993 374 1 295 973 3 650 1 427 6 967 1 271 2 859 2 413 1 387 288 908 175 673 525 2 518 857 3 938 723 1 766 1 469 1 224 428 1 934 325 843 613 4 007 1 521 4 789 772 3 039 2 574 1 868 680 2 594 429 1 351 1 066 3 066 2 268 2 667 1 180 942 1 061 3 607 1 229 2 418 775 3 521 1 410 161 128 145 2 203 1 757 1 980 4 370 779 2 575 1 316 5 418 2 265 8 025 6 590 7 307 15 846 13 279 14 563 20 951 6 184 13 567 8 931 43 656 16 960 0.48 0.62

almond flaming stoked/flaming av walnut flaming stoked flaming av fir low flame high flame av pine flaming late flame/smolder mass av

total less Nap, 2-mNap total B[a]A Chr B[b]F B[k]F B[a]P B[e]P Per B[ghi]P Ind D[ah]A (228) (228) (252) (252) (252) (252) (252) (276) (276) (278) Py (202) Fla (202) An (178) Ph (178) Fle (166) Ace (154) fuel

configuration

PMb

Nap (128)

2-mNap (142)

Acy (152) 9

PAH Emission Factors (µg/kg) for Woodsa

TABLE 6 2466


in other studies with biomass (15). A very high naphthalene emission for one test (wheat straw CRNF-1) is suspicious however and is likely an artifact of the sampling. The problem of naphthalene contamination of XAD-2 resin has recently been acknowledged by CARB in revised protocols for method 429 (16). Total PAH emissions shown in Tables 5 and 6 for all fuel types and fire configurations range from 5 to 683 mg kg-1 dry fuel or 2-100 mg kg-1 exclusive of naphthalene and 2-methylnaphthalene. For the wheat straw test giving the highest total emission rate (CRNF-1), naphthalene comprises nearly all (98%) of the PAH determined. When naphthalene is excluded, the emission factor is 12 mg kg-1 for this test. Variability in both the total and species-specific emission rates is apparent. Corn stover (with the exception of CEWF1) and rice straw yield relatively low emission rates for PAH compared to the other cereal residues. This is believed to be due to the higher loading rates and denser fuel bed structures leading to improved ignition and flame structure compared with barley and wheat straw. One test in corn giving high PAH emission rate (CEWF-1) had fuel left on the wind tunnel conveyor overnight. All other tests loaded fuel directly from bales onto the conveyor at the time of burning. Moisture samples were collected from the fuel on the conveyor prior to burning, but did not yield substantially different moisture contents as compared to other tests with the same fuel. However, surface moisture readily evaporates under handling, and surface moisture may have been responsible for the change in burning and emission rates observed. The increase in total PAH for this test indicates sensitivity to the burning conditions. The two wind tunnel configurations used with spreading fires in the cereal fuels also suggest influences of flame structure and ventilation on the total PAH emission rate. For wheat and barley straw, with poorer ignition due to the lower loading and heat release rates, the CEWF configuration yields a higher PAH emission rate than the lower velocity CRNF configuration. At higher wind speeds with the CEWF configuration, the flames appeared shorter and blown over, leading to earlier flamelet extinction and greater emission of unburned pyrolysis products. The opposite trend was observed in the case of corn (if CEWF-1 is excluded for reasons mentioned above) and rice. The higher oxygen availability in the flame regions (11) for the CEWF configuration appears to have resulted in more complete combustion under these more vigorous burning conditions with higher loading rates. The results for rice straw in Table 5 also suggest some differences due to the fuel source. Rice straw alone reveals this because fuel from two different seasons was used. The first two tests in the CEWF configuration and the first four tests in the CRNF configuration used straw from the same rice variety as the last two tests in each configuration but harvested 1 year earlier (all materials were stored dry under cover to protect them from decomposition). The straw in all 10 tests was burned at the same moisture content and loading rate. In general, the later batch of straw yields higher amounts of PAH compared to the earlier batch. The difference is not due to an increase in any single species, but instead a trend toward increasing emission rate is observed across species, especially for the CRNF configuration. Differences in the fire spreading rate were also observed between the two sources. These are thought to be related to differences in the chlorine concentrations of

the fuels, possibly resulting from differential weathering and leaching of chlorine and alkali metals under natural precipitation/condensation. The halogens are known fire retardants due to their involvement in termination of chain branching reactions. Although for most fuels there is an insufficient number of replicates to quantify the variability other than reporting the results for individual experiments, in the case of rice straw the standard deviation and coefficient of variation (COV) have been computed for each configuration. The data for the two fuel batches have been pooled, although as noted above there appear to be differences in the emission rates due to fuel source. The results are listed in Table 5. Individual COV ranges from 0.06 to 1.16 (standard deviation greater than mean) for the CEWF configuration and from 0.31 to 1.45 for the CRNF configuration. The variation in general is large, even though fuel moisture, loading rate, fuel bed structure, and wind speed were rather closely controlled. Differences in species-specific and total emission rates between the CEWF and CRNF configurations were also tested by pooling results from all cereal fuels for each configuration. Ordinarily, an F-test on the pooled results could be used to test for significant differences. However, homogeneity of variance cannot be demonstrated among fuel types. A non-parametric Kruskal-Wallis test was used instead, resulting in no significant differences (R ) 0.05) found between configurations with the exception of 2-methylnaphthalene, which fails for R ) 0.025. Differences in PAH emission rates appear to be related to changes in configuration as noted above but cannot be confirmed with the limited number of experiments conducted. A more exhaustive set of experiments (11) did reveal significant differences in total particulate matter emission rate between configurations, but this only qualitatively supports the trends observed here for PAH. The more vigorous burning conditions also appear to have led to lower PAH emission rates for the wood fuels. Emission levels are reduced in both almond and walnut tests where stoking was conducted. The almond fuel yields somewhat lower emissions overall, possibly as a result of reduced fuel moisture. The high flaming stage of the Douglas fir test also yielded lower emission rates for PAH than the low flaming stage. Similarly, the flaming stage of the Ponderosa pine tests yielded lower emission levels than the late flame and smolder stages, based on the mass proportion of fuel involved in each stage. Species-Specific Emission Factors. The less robust fires in general released greater amounts of all species of PAH compared with the more vigorous fires. This is especially noticeable for perylene, benzo[ghi]perylene, indeno[1,2,3cd]pyrene, and dibenz[ah]anthracene, which were largely undetected in samples obtained from the wood fires. Correlations among species within three classes of compounds were observed (17), distinguished largely by molecular weight and number of rings in the molecule, except for benzo[ghi]perylene: (I) acenaphthylene, fluorene, phenanthrene, anthracene (II) fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[ghi]perylene (III) benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[e]pyrene, indeno[1,2,3-cd]pyrene There is little correlation among species between groups. A principal components analysis (PCA) performed on the data confirms the CEWF-1 test in corn to be substantially

different from the other tests in its higher emission rates for heavier compounds, and relatively lower emission rates for intermediate species. Exclusion of this test from the pooled data results in a less distinct grouping of species among the categories above. PCA conducted in this case shows barley and wheat to group differently compared to the other fuels. The differences in all cases are thought to be related more to the burning conditions as influenced by loading rate or, in the case of the CEWF-1 experiment for corn, to moisture content. Partitioning of the species between particle and gas phases was observed but not always in equilibrium amounts (17). For molecular weight 200 and above, most PAH was found in the filter fraction. Equilibrium partitioning as determined from the vapor pressures of the individual species was observed more often with the cereal fuels than with the woods. The wood fires in particular produced higher levels of gas-phase PAH (as indicated by concentrations in the sorbent relative to the filter), due probably to the higher temperatures at the sampling point and shorter post-flame residence times prior to sampling under the more intense flaming conditions. Influence of Burning Conditions. The emission of PAH is due to incomplete combustion of the fuel. As seen above, the less vigorous burning conditions yielded higher emission levels of PAH, which can for these experiments be related in part to lower combustion efficiencies. The combustion efficiency is defined as

fCO2 fCO2,complete

)1-

1 CfC

(

∑

i)CO,THC

fi

WC Wi

+ fPMCpC + y′aCaC

)

(1)

The fi are the dimensionless emission factors of the species i, Wi are the molecular weights, CpC is the carbon mass fraction of the particulate matter, y′a is the mass fraction of ash remaining, CaC is the carbon mass fraction of the ash, and CfC is the carbon mass fraction for the fuel. The subscripts refer to carbon monoxide (CO), total hydrocarbons (THC), particulate matter (PM), carbon dioxide (CO2), and ash (a). The emission factor identified as fCO2,complete is the theoretical CO2 emission factor if all fuel carbon were converted to CO2:

fCO2,complete )

WCO2 C WC fC

(2)

Fuel carbon is also partitioned to CO, hydrocarbons, particulate matter, and ash. An increase in PAH would be expected to accompany a decline in combustion efficiency, although competing alternate emission products (e.g., CO, other hydrocarbons) makes the dependence less than straightforward. Figure 1a illustrates the relationship between combustion efficiency and PAH emission factor (in this case, the total less naphthalene and 2-methylnaphthalene) for the cereal fuels. A similar plot is shown in Figure 2a for the wood fuels. There exists for the cereals a general trend toward increased PAH emission with declining combustion efficiency, although there is apparently some influence due to fuel type. There is only a small increase for rice straw in comparison to wheat and barley straw. The high PAH emission (100 mg kg-1) for the CEWF-1 corn stover test at high combustion efficiency (93%) is somewhat an anomaly, in that although the fire produced


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FIGURE 1. PAH emission factor (excluding naphthalene and 2-methylnaphthalene) in comparison to (a) combustion efficiency and (b) total particulate matter emission factor for cereals under different wind tunnel configurations. Open symbols: CEWF configuration; filled symbols: CNRF configuration. Fuel type: barley (triangles), corn (circles), rice (squares), wheat (diamonds).

trend is difficult to observe. Most tests, as is typical of flaming combustion conditions, yielded relatively high combustion efficiencies (93-97%). The low efficiency for the unstoked fire with walnut wood is uncertain and may be partially the result of the method used for this fuel to monitor CO2 emission rate. Both the low flame experiment with fir and the late flame and smolder stage of the pine experiments show substantially elevated emission factors for PAH, even with a high indicated combustion efficiency for the fir test. There is also some correspondence between total PAH emission and total particulate matter emission, as shown in Figure 1b for the cereals and in Figure 2b for the woods. Such correspondence is to be expected on the basis of the reduced combustion efficiency under high PM emission conditions and on the basis of the role of PAH in soot formation (18). No direct attempt was made to determine the mechanisms influencing the correspondence between PAH and particle emission. There is also some possibility that the iron in the steel walls of the wind tunnel enhanced the soot and PAH formation or reduced the rate of destruction (19). However, the role of wall iron in the reactions of combustion products in the wind tunnel is thought to be rather limited. The role of metal salts originating from the fuel has not been determined, however, and may be important (20). Scanning electron micrographs of filter samples from cereal crop residues frequently reveal a background of KCl particles. Potassium concentrations routinely exceed 1% dry matter in cereals (range in cereals tested is 1.7-2.5%), and chlorine concentrations are typically 0.2-0.5% dry matter, occasionally higher.

Conclusions

FIGURE 2. PAH emission factor (excluding naphthalene and 2-methylnaphthalene) in comparison to (a) combustion efficiency and (b) total particulate matter emission factor for woods under different flaming conditions. Open symbols: flaming (unstoked) or low flame and smoldering; filled symbols: high rate flaming or stoked (refer to Table 6). Fuel types: almond (triangles), walnut (circles), fir (squares), pine (diamonds).

higher total hydrocarbon and particulate matter emissions than the other corn stover fires, it produced lower amounts of CO and overall generated a higher combustion efficiency. The data for the wood fuels are more limited, and a general

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Wind tunnel simulations of open burning of eight biomass types show a wide range in PAH emission factors. Total PAH emissions were found to vary between 5 and 683 mg kg-1 dry fuel for the 16 EPA priority compounds plus 2-methylnaphthalene, benzo[e]pyrene, and perylene. The highest value of 683 mg kg-1 was composed primarily (98%) of naphthalene, which may have been an artifact of the sampling and analysis procedure, possibly due to breakdown of the XAD-2 sorbent. Excluding naphthalene and 2-methylnaphthalene, the total PAH emission factor varied from 1.4 to 100 mg kg-1 for all fuel types. The PAH emission rates are dependent on fuel and burning conditions. Barley and wheat straw loaded at 400 g m-2 yielded higher emission factors for PAH than corn stover, rice straw, or any of the wood fuels. The high emission rate for the former two fuels is likely due to the poor ignition and fire spread under lighter loading. These fires also showed the effect of wind speed on weaker flame structures, giving higher PAH emissions at higher wind speed, the opposite result of the fires in corn and rice at roughly the same moisture content. Lower flaming rates and smoldering for the wood fuels were also found to yield proportionately higher emission rates of PAH than during more vigorous flaming conditions. Overall, weak flaming and smoldering contributed an additional 70% to the total PAH load from Ponderosa pine, even though only 30% of the total fuel mass was involved in this later stage of the fire. Rice straw from two separate sources but of the same variety and burned under the same conditions yielded on average different PAH emission factors for each source. The PAH emission factors for all fuels increase with decreasing combustion efficiency, although the variation

is rather wide. These preliminary evaluations suggest some interesting influences due to fuel composition as well as burning conditions that remain for future investigation.

Acknowledgments This work was supported under a grant from the California Air Resources Board (CARB). The support of CARB is gratefully acknowledged.

Literature Cited (1) Levine, J. S., Ed. Global biomass burning; MIT Press: Cambridge, MA, 1991. (2) Khalil, M. A. K. EOS 1995, 76 (36), 353-354. (3) Bjorseth, A.; Ramdahl, T. Handbook of polycyclic aromatic hydrocarbons; Marcel Dekker: New York, 1985. (4) Ramdahl, T.; Alfheim, I.; Rustad, S.; Olsen, T. Chemosphere 1982, 11 (6), 601-611. (5) Ramdahl, T.; Moller, M. Chemosphere 1983, 12 (1), 23-34. (6) Nielsen, P. A.; Grove, A.; Olsen, H. Chemosphere 1992, 24 (9), 1317-1330. (7) Tan, Y. L., Quanci, J. F. Atmos. Environ. 1992, 26A (6), 11771181. (8) Elomaa, M.; Saharinen, E. J. Appl. Polym. Sci. 1991, 42(10), 28192824. (9) Mast, T. J.; Hsieh, D. P. H.; Seiber, J. N. Environ. Sci. Technol. 1984, 18 (5), 338-348. (10) Jenkins, B. M.; Turn, S. Q.; Williams, R. B. Agric. Ecosyst. Environ. 1992, 38, 313-330.

(11) Jenkins, B. M.; Kennedy, I. M.; Turn, S. Q.; Williams, R. B.; Hall, S. G.; Teague, S.; Chang, D. P. Y.; Raabe, O. G. Environ. Sci. Technol. 1993, 27 (9), 1763-1775. (12) California Air Resources Board. Stationary source test methods. CARB: Sacramento, CA, 1987. (13) Knutson, J.; Miller, G. E. Agricultural residues (biomass) in California ... factors affecting utilization; Leaflet No. 21303; Cooperative Extension, University of California: Berkeley, 1982. (14) Ramdahl, T.; Becher, G. Anal. Chim. Acta 1982, 144, 83-91. (15) Nielsen, P. A.; Jensen, L. Chemosphere 1991, 23 (6), 723-735. (16) California Air Resources Board. Stationary source test methods, workshop package; CARB: Sacramento, CA, 1995. (17) Jenkins, B. M.; Jones, A. D.; Turn, S. Q.; Williams, R. B. Atmos. Environ. (in press). (18) Bartok, W.; Sarofim, A. F. Fossil fuel combustion; John Wiley and Sons: New York, 1991. (19) Feitelberg, A. S.; Longwell, J. P.; Sarofim, A. F. Comb. Flame 1993, 92, 241-253. (20) You, J. H.; Chiang, P. C.; Chang, K. T.; Chang, S. C. J. Hazard. Mater. 1994, 36, 1-17.

Received for review September 22, 1995. Revised manuscript received March 13, 1996. Accepted May 1, 1996.X ES950699M X

Abstract published in Advance ACS Abstracts, July 1, 1996.


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Emission Factors for Polycyclic Aromatic Hydrocarbons from Biomass

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