The Role of Bushfires in the Formation and Fate of Dioxins

Robert Black (2011). The Role of Bushfires in the Formation and Fate of Dioxins PhD Thesis, School of Medicine, The University of Queensland.

       
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Author Robert Black
Thesis Title The Role of Bushfires in the Formation and Fate of Dioxins
School, Centre or Institute School of Medicine
Institution The University of Queensland
Publication date 2011-11
Thesis type PhD Thesis
Supervisor Prof. Jochen Mueller
Mick Meyer
Lukas Van Zwieten
Total pages 138
Total colour pages 33
Total black and white pages 105
Language eng
Subjects 050206 Environmental Monitoring
039901 Environmental Chemistry (incl. Atmospheric Chemistry)
Abstract/Summary The burning of biomass during bushfires and crop fires is thought to contribute more than 80 % of the Australian National dioxin emissions budget, making it the most important source of PCDD/PCDF emissions by far. Two methodologies have been used to measure emission factors (EFs); laboratory based simulations and field studies. Reported emission factors vary by two orders of magnitude. Measuring PCDD/PCDF emissions from open burning of biomass has many potential sources of inconsistency. Questions that need to be asked include: are the methods used producing accurate results; are artefacts formed during sampling; is the fuel reproducible, and how much heating of soil contributes to emissions? This study examines these questions through a series of experiments. To examine the effects of methodology (and the need to transport fuel to laboratory burn facilities), a series of tests using the same fuels were conducted using a portable field sampler and a laboratory burn facility to allow comparisons of PCDD/PCDF emissions under controlled conditions. Field sampling and laboratory simulations were in good agreement, emission values for forest fuels were, 0.52 (range: 0.40-0.79), 0.59 (range: 0.18-1.2) and 0.75 (range: 0.27-1.2) µg TEQ (t fuel consumed)-1 for the in-field, over a brick hearth, and burn facility experiments respectively, and were not significantly different (P>005). Similarly, experiments with sugarcane showed very good agreement with EFAir of 1.1 (range: 0.40-2.2), 1.5 (range: 0.84-2.2) and 1.7 (range: 0.34-4.4) µg TEQ (t fuel consumed)-1 for in-field, over a brick hearth, open field and burn facility respectively, and were not significantly different (P>005). Therefore, no significant changes in emissions of PCDD/PCDF could be attributed to fuel storage or transport or measurement method. A comparison of these results with published emission factors (EFs) assessed four types of biomass and 111 EFs. Statistical analysis showed no systematic differences in EFs are apparent between biomass types or fire classes. New EFs were proposed and application of these EFs to national emissions of PCDD/PCDF for global estimates from open burning will lower previous estimates of PCDD/PCDF releases to air and to land by 85% and 90%, respectively. For some countries, the ranking of their major sources will be changed and open burning of biomass will become less significant than previously thought. Formation of artefacts during sampling of emissions of PCDD/PCDF from open combustion was examined. Experiments where sampler hood and inlet temperatures were varied by adjusting hood position over the combustion zone. EFs ranged from 0.04 µg TEQ (t fuel)-1 at the lowest maximum hood temperature (185 ºC) to 15 µg TEQ (t fuel)-1 at the highest maximum hood temperature (598 ºC). The increase in hood temperature also resulted in a clear change in the PCDD/PCDF congener and homologue profile of the emissions. At the lowest temperature the PCDD/PCDF ratio was 50, whereas at the highest temperature this ratio has fallen to 0.53. When the sampler hood and inlet temperatures were kept in the normal operating range of <200 ºC, emission factors were comparable to those observed in many previous studies in Australia with emissions where PCDD was predominant. A series of laboratory experiments examined the potential for soil to contribute to emissions during bush fires. Mass labelled PCDD/PCDF was added to soil known to contain native PCDD/PCDF. Soil was heated between 150ºC and 400oC. Both native and mass labelled PCDD/PCDF was released from the soil when heated beyond 200 ºC. Release of the mass labelled compounds was linearly related to temperature with up to 9 % of the labelled PCDD/PCDF being released into the air stream at 400 ºC. The release of the native PCDD/PCDF was much greater. Above 300 ºC, emission of 1, 2, 3, 7, 8-Cl5DD were much higher than the mass labelled Cl5DF, Cl6DF and Cl6DDcongeners. Field experiments used the same spiked soil during open burning of biomass. After each fire the concentration of all mass labelled PCDD/PCDF congeners in the surface soil decreased. However, the concentration of some native 2378 chlorinated congeners increased in soils after fire, indicating formation. Mass labelled PCDD/PCDF congeners were detected in all air and ash samples demonstrating release from the soil, As the origin of these mass labelled congeners was restricted to the soil, this gives clear evidence that soil contributes to release of PCDD/PCDF to air and ash, and that emissions are not simply a reflection of biomass residues. Overall, the results from this research demonstrate that the two measurement methodologies used give results that are reproducible and not significantly different. Soil, when heated by fire events has the potential to contribute to the release of toxicologically significant PCDD/PCDF during bushfires, and that bushfires are a significant source of PCDD/PCDF release to the environment. However, the new EFs proposed for the UNEP Toolkit sub-category 6a will lower previous estimates of global PCDD/PCDF releases to air and land by 85% and 90% respectively. Application of this data to Australian emissions means that contribution of open burning will be reduced from >80% to 42 % of total PCDD/PCDF emissions.
Keyword Bushfire
forest fire
open burning
emission factor
biomass combustion
persistent organic pollutants
PCDD/PCDF
polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans
Additional Notes colour: 1,18, 20, 24, 25, 26, 33, 34, 46, 47, 48, 49, 50, 51, 52, 57, 58, 61, 65, 66, 67, 71, 72, 77, 78, 79, 94, 96, 97, 100, 101, 104, 113 landscape: 37, 38, 74, 75, 76

 
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