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| Titel |
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 |
| VerfasserIn |
M. J. Wooster, P. H. Freeborn, S. Archibald, C. Oppenheimer  , G. J. Roberts, T. E. L. Smith, N. Govender, M. Burton, I. Palumbo |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 22 ; Nr. 11, no. 22 (2011-11-22), S.11591-11615 |
| Datensatznummer |
250010208
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| Publikation (Nr.) |
 copernicus.org/acp-11-11591-2011.pdf |
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| Zusammenfassung |
| Biomass burning emissions factors are vital to quantifying trace gas release
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 IR source
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
~0.01 μmol mol−1 [10 ppbv] can be detected across the
relatively long optical paths used here. Though FTIR spectroscopy can detect
dozens of different chemical species present in vegetation fire smoke, we
focus our analysis on five key combustion products released preferentially
during the pyrolysis (CH2O), flaming (CO2) and smoldering (CO,
CH4, NH3) processes. 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 (RSC) stages of these
savannah fires, from which stage-specific emission factors can then be
calculated. Headfires and backfires often show similar emission ratios and
emission factors, but those of the RSC stage can differ substantially. 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 (FRE), allowing the relative
amount of fuel burned in each stage to be calculated and "fire averaged"
emission ratios and emission factors to be determined. These "fire
averaged" metrics are dominated by the headfire contribution, since the FRE
data indicate that the vast majority of the fuel is burned in this stage. Our
fire averaged emission ratios and factors for CO2 and CH4 agree
well with those from prior studies conducted in the same area using e.g.
airborne plume sampling. We also concur with past suggestions that emission
factors for formaldehyde in this environment appear substantially
underestimated in widely used databases, but see no evidence to support
suggestions by Sinha et al. (2003) of a major overestimation in the
emission factor of ammonia in works such as Andreae and Merlet (2001) and
Akagi et al. (2011). We also measure somewhat higher CO and NH3
emission ratios and factors than are usually reported for this environment,
which is interpreted to result from the OP-FTIR ground-based technique
sampling a greater proportion of smoke from smouldering processes than is
generally the case with methods such as airborne sampling. Finally, our
results suggest that the contribution of burning animal (elephant) dung can
be a significant factor in the emissions characteristics of certain KNP
fires, and that the ability of remotely sensed fire temperatures to provide
information useful in tailoring modified combustion efficiency (MCE)
and emissions factor estimates maybe rather limited, at least until the
generally available precision of such temperature estimates can be
substantially improved. One limitation of the OP-FTIR method is its ability
to sample only near-ground level smoke, which may limit application at more
intense fires where the majority of smoke is released into a vertically
rising convection column. Nevertheless, even in such cases the method
potentially enables a much better assessment of the emissions contribution of
the RSC stage than is typically conducted currently. |
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