By Yuqi Hu, Nieves Fernandez-Anez, and Guillermo Rein
Haze and Peatlands
Haze is defined as the large-scale accumulation of smoke at low altitudes in the atmosphere from long-term smouldering peat fires1. These fires release a large fraction of atmospheric pollutant emissions worldwide. Haze events that periodically occurred in Southeast Asia, especially in 1997, 2006, 2009, 2013, 2015 and 2016, are an example of how acute and recurrent these episodes are2. However, Southeast Asia haze is only an example of the magnitude of the problem. The USA and Russia are among other areas suffering from haze1.
The haze events evolved from smouldering peat fires have been identified as a worldwide problem that must be addressed.
Peatlands, resulting from the accumulation of partially decayed vegetation, are important terrestrial ecosystems that cover 4 million km2 of the Earth’s surface (3% of the land surface worldwide) and are the most widespread of all wetland types. Peat accumulation only takes place in water-saturated environments, ensuring the high moisture content of this bulk material and protecting it from burning. However, natural droughts or human activities lower the water table in peatlands, which reduces the moisture content and renders them vulnerable to smouldering fires3.
Effects of Haze Events
Haze migrates over long distances via wind, forming transboundary environmental pollution (regional air quality deterioration) and causing international disputes among neighbouring countries. The effects of haze are diverse, ranging from inducing surges of respiratory emergencies in the population to disrupting shipping and aviation routes for long periods — weeks, and even months1,4. Haze severely affects local transportation, construction, tourism and agriculture-based industries, leading to vast economic losses.
Aerosol imaging by NASA satellite in October 1997 showing vast smoke haze released by smouldering peat fires in Indonesia.
Haze exposes people to high concentrations of various pollutants. Little is known about the health implications associated with haze, but it is characterised by higher concentrations of particulate matter (PM) than flaming fires. Exposure to PM constitutes a substantial human health risk, causing or aggravating cardiovascular cases or aggravating cardiovascular arrhythmias, affecting the central nervous and reproductive systems, and causing cancer5. PM10 (particles with an aerodynamic diameter ≤ 10 µm) can be inhaled and deposited in the lungs, inducing a systemic inflammatory response1, 5. PM2.5 can exacerbate respiratory diseases such as chronic bronchitis, emphysema and asthma and increase the rate of respiratory and heart problems, as well as lung cancer cases6. Epidemiologic studies also have shown a positive correlation relevance between PM2.5 uptake and increased morbidity and mortality5, 7.
Epidemiological studies conducted in Indonesia, Malaysia, Singapore and the USA reveal the health issues that arise from haze: 527 haze-related deaths, 298,125 cases of asthma, 58,095 cases of bronchitis and more than 1.446 million cases of acute respiratory infection were reported in Indonesia during the 1997 Southeast Asia haze7. In Malaysia, this haze increased total all-cause mortality by 22%, and non-traumatic mortality among the population aged 65–74 by 72%8. In Singapore, the 1997 haze event led to an increase of 5,588 cases of asthma, bronchitis, emphysema and pneumonia9.
In the USA, asthma-related visits accounted for 44% of all respiratory events (2,081 cases) considered during the 2008 North Carolina severe peat fire event, and heart failure for 33% of all cardiac events (1,817 cases)10.
What is Smouldering?
Haze is the result of smouldering peat fires. Smouldering megafires are the largest and longest-burning fires on Earth2. They destroy essential peatland ecosystems, and are responsible for 15% of annual global greenhouse gas emissions11. This is the same amount attributed to the worldwide fleet of road vehicles, and yet it is not accounted for in global carbon budgets.
Smouldering is the slow, low-temperature, flame-less form of combustion12. The process of smouldering ignition requires the supply of heat, and is governed by heat transfer and kinetics, with the oxygen supply rate playing a secondary role. Above a critical threshold of heat supply, the increase of temperature initiates endothermic pyrolysis, which is followed by the onset of oxidation of char. When the heat released by oxidation is high enough to balance the heat required for the peat endothermic processes (heat loss, pyrolysis, drying and preheating of fuel), propagation occurs and the fire might become self-sustaining12.
If ignition occurs on the top surface of the peat bed, a self-sustained smouldering peat front will spread both laterally and in depth2, 13. Each location of a burning peat layer sees the successive arrival of four distinct thermal and chemical waves that form the structure of a smouldering front: preheating, drying, pyrolysis and oxidation sub-fronts12.
Schematic diagram of the lateral and in-depth spreads of a smouldering peat13 (left); smouldering combustion of coal briquettes (right). Photo: JB Nielsen, 2006.
Emissions from Smouldering Peat Fires
The combustion reaction in smouldering is characteristically incomplete, producing a smoke composed of a rich and complex mixture of gases and aerosols1. Compared with the emissions derived from flaming wildland fires, smouldering combustion typically yields a higher conversion of the fuel to toxic compounds, although it occurs more slowly12.
The most-abundant carbon-containing species from peat fires is CO2, followed by CO and CH4. The major trace gases include ethane (C2H6), ethene (C2H4), acetylene (C2H2), propene (C3H6), formaldehyde (HCHO), methanol (CH3OH), acetic acid (CH3COOH), formic acid (HCOOH), glycolaldehyde (C2H4O2), phenol (C6H5OH), furan (C4H4O) and ammonia (NH3)1.
Most of the peat fire aerosols are in the PM2.5 size range, and these particles are primarily composed of organic carbon and trace amount of black carbon, which is characteristic for haze1. A range of volatile organic compounds (VOCs), including BTEX (benzene, toluene, ethylbenzene and xylene) and polycyclic aromatic hydrocarbons (PAHs), can also be enriched in the particles, making the PM a complex mixture of organic and inorganic substances1.
A typical haze particle and its main components.
How Can Peat Fires Be Mitigated?
The ideal of research on haze is to advance the science and create the technology that will reduce the burden of smouldering fires. Despite their importance, how smouldering fires ignite, spread or extinguish is not yet understood, which impedes the development of any successful mitigation strategy. Megafires are routinely fought across the globe with techniques that were developed for flaming fires, and are ineffective for smouldering.
Moreover, the burning of deep peat affects older soil carbon that has not been part of the active carbon cycle for centuries to millennia, and thus creates a positive feedback to the climate system. Further work may turn the challenges faced by research on smouldering into opportunities and create pathways for novel mitigation technologies in accurate prevention, quick detection systems and simulation-driven suppression strategies.
The topic of peat fires is inherently interdisciplinary, since geoscientists study the ecosystem and the soil chemistry of peatlands, while combustion experts like fire protection professionals study smouldering phenomena. This has been a challenge to previous work and led to a fragmented science topic, but is also an opportunity worth embracing. A research approach that relates to thermofluids and mechanical engineering but studies a geoscience topic would be ideal.
Yuqi Hu, Nieves Fernandez-Anez, and Guillermo Rein are with Department of Mechanical Engineering, Imperial College London, UK
References
1Y. Hu, N. Fernandez-Anez, T.E.L. Smith, and G. Rein. Review of emissions from smouldering peat fires and their contribution to regional haze episodes, International Journal of Wildland Fire, 2018 (in press).
2B. Poulter, N.L. Christensen, and P.N. Halpin, Carbon emissions from a temperate peat fire and its relevance to interannual variability of trace atmospheric greenhouse gases, Journal of Geophysical Research: Atmospheres, vol. 111, no. D6, 2006.
3Y. Hu, N. Fernandez-Anez, T.E.L. Smith, and G. Rein. Review of emissions from smouldering peat fires and their contribution to regional haze episodes, International Journal of Wildland Fire, 2018 (in press).
4A. Hinwood and C. Rodriguez, Potential health impacts associated with peat smoke: a review, Journal of the Royal Society of Western Australia, vol. 88, pp. 133-138, 2005.
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6O. Kunii, et al., The 1997 haze disaster in Indonesia: its air quality and health effects, Archives of Environmental Health: An International Journal, vol. 57, no. 1, pp. 16-22, 2002.
8A. G. Rappold, et al., Peat bog wildfire smoke exposure in rural North Carolina is associated with cardiopulmonary emergency department visits assessed through syndromic surveillance, Environmental Health Perspectives, vol. 119, no. 10, p. 1415, 2011.
10D. Glover and T Jessup, Indonesia's fires and haze: the cost of catastrophe. IDRC, 2006.
11N. Sastry, Forest fires, air pollution, and mortality in Southeast Asia, Demography, vol. 39, no. 1, pp. 1–23, 2002.
12G. Rein, Smoldering Combustion, in SFPE Handbook of FIre Protection Engineering, M.J. Hurley, D.T. Gottuk, J.R. Hall Jr., K. Harada, E.D. Kuligowski, M. Puchovsky, C.J. Wieczorek, eds., New York: Springer, 2015, pp. 581–603.
13X. Huang and G. Rein, Smouldering combustion of peat in wildfires: Inverse modelling of the drying and the thermal and oxidative decomposition kinetics, Combustion and Flame, vol. 161, no. 6, pp. 1,633–1,644, 2014.