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
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
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DOI：
10.5194/acp-11-11591-2011
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发表时间：
2011-11
影响因子：
6.3
通讯作者：
M. Wooster;P. Freeborn;Sally Archibald;Clive Oppenheimer;Gareth Roberts;Thomas E. L. Smith;N. Govender;Mike Burton;I. Palumbo
中科院分区：
文献类型：
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作者：
M. Wooster;P. Freeborn;Sally Archibald;Clive Oppenheimer;Gareth Roberts;Thomas E. L. Smith;N. Govender;Mike Burton;I. Palumbo
生物质燃烧排放系数对于量化植被火灾释放的微量气体至关重要。在这里，我们使用地面开放路径傅里叶变换红外(FTIR)光谱和相隔150-250米距离的红外源来评估南非克鲁格国家公园(KNP)的一系列大草原火灾的排放因子。利用与非线性最小二乘拟合法相结合的光谱正向模型，反演出了沿扩展开放路径的分子丰度。我们证明了水平路径横跨平流羽流宽度的示踪气柱数量的推导，并发现例如，在这里使用的相对较长的光路上，可以检测到0.01molmol1[10ppbv]的CO混合比的变化。虽然FTIR光谱可以检测到植物火灾烟雾中存在的数十种不同的化学物质，但我们的分析重点是在热解(CH2O)、燃烧(CO2)和阴燃(CO、CH4、NH3)过程中优先释放的五种关键燃烧产物。我们证明了这些稀树草原火的逆火、前火和剩余阴燃燃烧(RSC)阶段的这些气体对CO2和CO的严格约束排放比，然后可以计算出特定阶段的排放因子。前火和回火通常表现出相似的排放比和排放系数，但RSC阶段的排放比率和排放系数可能有很大不同。通过机载光学和热红外图像以及地面观察员报告确定了每个起火阶段的时间，机载红外图像还用于得出火灾辐射能(FRE)的估计值，从而能够计算每个阶段燃烧的燃料的相对数量，并确定“火灾平均”排放比率和排放系数。由于FRE的数据表明，绝大多数燃料是在这个阶段燃烧的，因此这些“火灾平均”指标主要是由头部燃烧贡献的。我们的火灾平均排放比率和二氧化碳和甲烷的系数与先前在同一地区进行的研究(例如使用空气中的烟羽样本)的结果非常一致。我们也同意过去的建议，即在广泛使用的数据库中，这种环境中的甲醛排放因子似乎被大大低估了，但没有证据支持Sinha等人的建议。(2003)在Andreae和Merlet(2001)和Akagi等人的作品中严重高估了氨的排放系数。(2011)。我们还测量到的CO和NH3排放比率和系数比通常报道的要高一些，这被解释为OP-FTIR地面技术对阴燃过程中的烟雾进行采样的比例高于空气采样等方法的一般情况。最后，我们的结果表明，燃烧动物(大象)粪便的贡献可能是某些KNP火灾排放特征的一个重要因素，遥感火灾温度提供有助于定制修正燃烧效率(MCE)和排放系数估计的信息的能力可能相当有限，至少在这种温度估计的普遍可用精度得到实质性改善之前是这样。OP-FTIR方法的一个局限性是它只能对近地面水平的烟雾进行采样，这可能会限制其在更强烈的火灾中的应用，因为大多数烟雾被释放到垂直上升的对流柱中。然而，即使在这种情况下，该方法也可能比目前通常进行的评估更好地评估RSC阶段的排放贡献。
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 groundbased open path Fourier transform infrared (FTIR) spectroscopy and an IR source separated by 150-250m distance. Molecular abundances along the extended open path are retrieved using a spectral forward model coupled to a nonlinear 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 stagespecific 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 Published by 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.