Arctic fires and smouldering combustion: influence of soil and air temperature on fire spread

Abstract

Peat fires smoulder for long periods (weeks to months) and releasing large amount of ancient carbon that have been stored for millennia in the organic soils. Recent wildfires in Arctic regions have burned unprecedented swaths of land, demonstrating a detrimental change in the arctic fire regime and highlighting the vulnerability of these biomes to climate change. This work aims to experimentally study Arctic peat fires in the lab scale by using an experimental rig with adjustable air temperature and bottom boundary of the peat fire which imitate the condition of permafrost in the Arctic. The initial temperature of the peat sample varied from -13 to 18°C, and the moisture content (MC) was varied from 50 to 120% in dry-mass basis. The experimental results show that smouldering can be sustained with soil temperatures below the freezing point of water. The range of condition temperature in this study was found to insignificantly affect spread rate but have profound effect on the depth of burn, increasing by up to 66% as bottom boundary decreased from 21 to -7°C. We found that the critical moisture content of ignition under cold condition in this work is between 110 and 120% (dry-mass basis), and is lower than the literature in room temperature (160%). At high moisture content (≥100% MC), smouldering was weakly spreading under air temperature of ~12°C, initial peat temperature of -11°C, and bottom boundary of -7°C. However, spread rate significantly increased as the air and bottom boundary temperatures were increased to 22°C, demonstrating overwintering fires which often found in the Arctic when peat fires were considered to be extinguished only to resurface when warmer season arrives. This study is the first experimental work on smouldering Arctic wildfires with findings that can improve our understanding on the effect of cold temperatures on the smouldering dynamics of peat fires, and presents a novel methodology to investigate Arctic fires at laboratory scale.