Field determination of biomass burning emission ratios and factors via open-path FTIR spectroscopy and fire radiative power assessment: headfire, backfire and residual smouldering combustion in African savannahs

Abstract

Abstract. Biomass burning emissions factors are vital to quantifying trace gases releases from vegetation fires. Here we evaluate emissions factors for a series of savannah fires in Kruger National Park (KNP), South Africa using ground-based open path Fourier transform infrared (FTIR) spectroscopy and an infrared lamp separated by 150–250 m distance. Molecular abundances along the extended open path are retrieved using a spectral forward model coupled to a non-linear least squares fitting approach. We demonstrate derivation of trace gas column amounts for horizontal paths transecting the width of the advected plume, and find, for example, that CO mixing ratio changes of ~0.001 μmol mol−1 (~10 ppbv) can be detected across the relatively long optical paths used here. We focus analysis on five key compounds whose production is preferential during the pyrolysis (CH2O), flaming (CO2) and smoldering (CO, CH4, NH3) fire phases. We demonstrate that well constrained emissions ratios for these gases to both CO2 and CO can be derived for the backfire, headfire and residual smouldering combustion stages of these savannah fires, from which stage-specific emission factors can then be calculated. Headfires and backfires in general show similar emission ratios and emission factors, but those of the residual smouldering combustion stage can differ substantially (e.g., ERCH4/CO2 up to ~7 times higher than for the flaming stages). The timing of each fire stage was identified via airborne optical and thermal IR imagery and ground-observer reports, with the airborne IR imagery also used to derive estimates of fire radiative energy, thus allowing the relative amount of fuel burned in each stage to be calculated and the "fire averaged" emission ratios and emission factors to be determined. The derived "fire averaged" emission ratios are dominated by the headfire contribution, since the vast majority of the fuel is burned in this stage. Our fire averaged emission ratios and factors for CO2 and CH4 agree with those from published studies conducted in the same area using airborne plume sampling, and we concur with past suggestions that emission factors for formaldehyde in this environment appear substantially underestimated in widely used databases. We also find the emission ratios and factors for CO and NH3 to be somewhat higher than most other estimates, however, we see no evidence to support suggestions of a major overestimation in the emission factor of ammonia. Our data also suggest that the contribution of burning animal (elephant) dung can be a significant factor in the emissions characteristics of certain KNP fires, and indicate some similarities between the time series of fire brightness temperature and modified combustion efficiency (MCE) that supports suggestions that EO-derived fire temperature estimates maybe useful when attempting to remotely classify fire activity into its different phases. We conclude that ground-based, extended open path FTIR spectroscopy is a practical and very effective means for determining emission ratios, emission factors and modified combustion efficiencies at open vegetation fire plumes, allowing these to be probed at temporal and spatial scales difficult to explore using other ground-based approaches. Though we limited our study to five key emissions products, open path FTIR spectroscopy can detect dozens of other species, as has been demonstrated during previous closed-path FTIR airborne deployments in the same study area.