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

摘要

Biomass burning emissions factors are vital to quantifying trace gas releasefrom vegetation fires. Here we evaluate emissions factors for a series ofsavannah fires in Kruger National Park (KNP), South Africa using ground-basedopen path Fourier transform infrared (FTIR) spectroscopy and an IR sourceseparated by 150–250 m distance. Molecular abundances along the extendedopen path are retrieved using a spectral forward model coupled to anon-linear least squares fitting approach. We demonstrate derivation of tracegas column amounts for horizontal paths transecting the width of the advectedplume, and find for example that CO mixing ratio changes of~0.01 μmol mol?1 [10 ppbv] can be detected across therelatively long optical paths used here. Though FTIR spectroscopy can detectdozens of different chemical species present in vegetation fire smoke, wefocus our analysis on five key combustion products released preferentiallyduring the pyrolysis (CH₂O), flaming (CO₂) and smoldering (CO,CH₄, NH₃) processes. We demonstrate that well constrained emissionsratios for these gases to both CO₂ and CO can be derived for thebackfire, headfire and residual smouldering combustion (RSC) stages of thesesavannah fires, from which stage-specific emission factors can then becalculated. Headfires and backfires often show similar emission ratios andemission factors, but those of the RSC stage can differ substantially. Thetiming of each fire stage was identified via airborne optical and thermal IRimagery and ground-observer reports, with the airborne IR imagery also usedto derive estimates of fire radiative energy (FRE), allowing the relativeamount of fuel burned in each stage to be calculated and "fire averaged"emission ratios and emission factors to be determined. These "fireaveraged" metrics are dominated by the headfire contribution, since the FREdata indicate that the vast majority of the fuel is burned in this stage. Ourfire averaged emission ratios and factors for CO₂ and CH₄ agreewell with those from prior studies conducted in the same area using e.g.airborne plume sampling. We also concur with past suggestions that emissionfactors for formaldehyde in this environment appear substantiallyunderestimated in widely used databases, but see no evidence to supportsuggestions by Sinha et al. (2003) of a major overestimation in theemission factor of ammonia in works such as Andreae and Merlet (2001) andAkagi et al. (2011). We also measure somewhat higher CO and NH₃emission ratios and factors than are usually reported for this environment,which is interpreted to result from the OP-FTIR ground-based techniquesampling a greater proportion of smoke from smouldering processes than isgenerally the case with methods such as airborne sampling. Finally, ourresults suggest that the contribution of burning animal (elephant) dung canbe a significant factor in the emissions characteristics of certain KNPfires, and that the ability of remotely sensed fire temperatures to provideinformation useful in tailoring modified combustion efficiency (MCE)and emissions factor estimates maybe rather limited, at least until thegenerally available precision of such temperature estimates can besubstantially improved. One limitation of the OP-FTIR method is its abilityto sample only near-ground level smoke, which may limit application at moreintense fires where the majority of smoke is released into a verticallyrising convection column. Nevertheless, even in such cases the methodpotentially enables a much better assessment of the emissions contribution ofthe RSC stage than is typically conducted currently.