Abstract
Abstract. Savanna fires contribute approximately 40–50% of total global annual biomass burning carbon emissions. Recent comparisons of emission factors from different savanna regions have highlighted the need for a regional approach to emission factor development, and better assessment of the drivers of the temporal and spatial variation in emission factors. This paper describes the results of open-path Fourier transform infrared (OP-FTIR) spectroscopic field measurements at 21 fires occurring in the tropical savannas of the Northern~Territory, Australia, within different vegetation assemblages and at different stages of the dry season. Spectra of infrared light passing through a long (22–70 m) open-path through ground-level smoke released from these fires were collected using an infrared lamp and a field-portable FTIR system. The IR spectra were used to retrieve the mole fractions of 14 different gases present within the smoke, and these measurements used to calculate the emission ratios and emission factors of the various gases emitted by the burning. Only a handful of previous emission factor measures are available specifically for the tropical savannas of Australia and here we present the first reported emission factors for methanol, acetic acid, and formic acid for this biome. Given the relatively large sample size, it was possible to study the potential causes of the within-biome variation of the derived emission factors. We find that the emission factors vary substantially between different savanna vegetation assemblages; with a majority of this variation being mirrored by variations in the modified combustion efficiency (MCE) of different vegetation classes. We conclude that a significant majority of the variation in the emission factor for trace gases can be explained by MCE, irrespective of vegetation class, as illustrated by variations in the calculated methane emission factor for different vegetation classes using data sub-set by different combustion efficiencies. Therefore, the selection of emission factors for emissions modelling purposes need not necessarily require detailed fuel type information, if data on MCE (e.g. from future spaceborne total column measurements) or a correlated variable were available. From measurements at 21 fires, we recommend the following emission factors for Australian tropical savanna fires (in grams of gas emitted per kilogram of dry fuel burned), which are our mean measured values: 1674 ± 56 g kg−1 of carbon dioxide; 87 ± 33 g kg−1 of carbon monoxide; 2.1 ± 1.2 g kg−1 of methane; 0.11 ± 0.04 g kg−1 of acetylene; 0.49 ± 0.22 g kg−1 of ethylene; 0.08 ± 0.05 g kg−1 of ethane; 1.57 ± 0.44 g kg−1 of formaldehyde; 1.06 ± 0.87 g kg−1 of methanol; 1.54 ± 0.64 g kg−1 of acetic acid; 0.16 ± 0.07 g kg−1 of formic acid; 0.53 ± 0.31 g kg−1 of hydrogen cyanide; and 0.70 ± 0.36 g kg−1 of ammonia. In a companion paper, similar techniques are used to characterise the emissions from Australian temperate forest fires.
Highlights
Estimates of mean annual carbon emissions from worldwide savanna and grassland fires amount to ∼ 1 Pg C per year (2000–2009), or approximately a mean of 40–50 % of total global annual biomass burning carbon emissions
As the back-burn fire line burns away from the open-path Fourier transform infrared (OP-FTIR) path, most of the smoke crossing the infrared beam will have been emitted from the steadily increasing area of smouldering-phase combustion, with contributions from both the flaming combustion of the head fire and the smouldering area behind the head fire
This equates to the area of “flaming combustion” and the associated “smoking zone” of smouldering combustion located close behind the fire front, as detailed in the OP-FTIR African savannah measurements of Wooster et al (2011)
Summary
Estimates of mean annual carbon emissions from worldwide savanna and grassland fires amount to ∼ 1 Pg C per year (2000–2009), or approximately a mean of 40–50 % of total global annual biomass burning carbon emissions (van der Werf et al, 2010). In tropical Australia, net carbon emissions from fires are considered negligible as the carbon lost to burning is approximately balanced by the carbon sequestered during regrowth (Haverd et al, 2013). Emissions of non-CO2 gases from savanna fires, such as nitrous oxide and methane, whose emissions are not sequestered by regrowth, comprise 2–4 % of total Australian national greenhouse gas emissions (DCCEE, 2010), and are included in the accountable greenhouse gas inventory. At state or territory scales, pyrogenic emissions of CO2, CH4, and N2O can be far more significant, with emissions from fires comprising nearly 50 % of the Northern Territory’s accountable carbon emissions (DCCEE, 2010)
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