New emission factors for Australian vegetation fires measured using open-path Fourier transform infrared spectroscopy – Part 2: Australian tropical savanna fires

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 twenty-one 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 fourteen 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 subsetted 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 twenty-one 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 g kg−1 of carbon dioxide; 87 g kg−1 of carbon monoxide; 2.1 g kg−1 of methane; 0.11 g kg−1 of acetylene; 0.49 g kg−1 of ethylene; 0.08 g kg−1 of ethane; 1.57 g kg−1 of formaldehyde; 1.06 g kg−1 of methanol; 1.54 g kg−1 of acetic acid; 0.16 g kg−1 of formic acid; 0.53 g kg−1 of hydrogen cyanide; and 0.70 g kg−1 of ammonia.

Reduced biomass burning emissions reconcile conflicting estimates of the post-2006 atmospheric methane budget

“…It is also likely that this increased precipitation in these regions affects the fuel moisture content and in turn the combustion efficiency of the fires. However, while both CH 4 and CO emission factors, relative to burnt area or CO 2 , are expected to increase in response to a reduction in combustion efficiency 34 , 35 , there is currently no established relationship between combustion efficiency and the CH 4 /CO ratios. To the best of our knowledge, measurements tracking temporal changes in fire CH 4 /CO ratios indicate no coherent relationship between fire phase and CH 4 /CO variability on daily timescales 34 , 35 or any significant relationship between seasonal CH 4 /CO variability and combustion completeness 36 .…”

“…However, while both CH 4 and CO emission factors, relative to burnt area or CO 2 , are expected to increase in response to a reduction in combustion efficiency 34 , 35 , there is currently no established relationship between combustion efficiency and the CH 4 /CO ratios. To the best of our knowledge, measurements tracking temporal changes in fire CH 4 /CO ratios indicate no coherent relationship between fire phase and CH 4 /CO variability on daily timescales 34 , 35 or any significant relationship between seasonal CH 4 /CO variability and combustion completeness 36 . Ultimately, joint constraints on the temporal variability of CH 4 /CO, e.g., based on further in situ monitoring of fire CH 4 /CO or CH 4 and CO column measurements from upcoming TROPOMI satellite mission 37 , could be key to improving the accuracy of fire methane emission estimates derived from atmospheric CO constraints.…”

Fine Particle Emissions From Tropical Peat Fires Decrease Rapidly With Time Since Ignition

“…The experimental methodology involves coincident and collocated measurements of PM 2.5 and carbon monoxide (CO) in fresh smoke within a few meters of the burning peat in order to establish emission ratios (of PM 2.5 to CO). Emission factors of PM 2.5 can then be calculated by combining these emission ratios with emission factors of CO from the fires (see, e.g., Paton‐Walsh et al, ; Smith et al, , ; Stockwell et al, ). An aerosol monitor measured PM 2.5 concentrations (see section 2.1), while mole fractions of CO were measured with a Thermo Scientific Model 48i CO analyzer (see section 2.2).…”

“…The emission factor for CO (grams of CO emitted per kilogram of dry fuel burned) was calculated from in situ measurements of trace gas mole fractions using open‐path Fourier transform infrared spectroscopy. A full description of the use of this method for determining emission factors from biomass burning can be found in Smith et al (). Here we deployed a MIDAC M2000 series FTIR spectrometer to measure the spectra of an infrared lamp located 18–28 m from the spectrometer on two occasions (20 July 2016 and 27 July 2016) at Site 4.…”

Satellite observations of tropospheric ammonia and carbon monoxide: Global distributions, regional correlations and comparisons to model simulations

“…Calculating the observed NH 3 :CO correlations (emission ratios) for the pollutants and comparing them to the model simulations provides valuable insight on trace gas emission sources, and can be used to evaluate the performance of the chemistry transport model globally. Satellite observations of NH 3 enhancement ratio relative to CO in the fire regions can also be compared to recent enhancement ratios derived from in-situ measurements (Akagi et al, 2011), and ground-based remotely sensed values (PatonWalsh et al, 2014;Smith et al, 2014). The known NH 3 and CO ratio in regions of their common combustion sources is useful in determining NH 3 emissions from the better known CO emissions (Hegg et al, 1988).…”