Landscape fires, often referred to as biomass burning (BB), emit substantial amounts of (greenhouse) gases and aerosols into the atmosphere each year. Frequently burning savannas, mostly in Africa, Australia, and South America are responsible for over 60 % of total BB carbon emissions. Compared to many other sources of emissions, fires have a strong seasonality. Previous research has identified the mitigation potential of prescribed fires in savanna ecosystems; by burning cured fuels early in the dry season when landscape conditions still provide moist buffers against fire spread, fires are in general smaller, patchier, and less intense. While it is widely accepted that burned area (BA) and the total carbon consumed are lower when fires are ignited early in the dry season, little is known about the intraseasonal variability of emission factors (EFs). This is important because potentially, higher EFs in the early dry season (EDS) could offset some of the carbon benefits of EDS burning. Also, a better understanding of EF intraseasonal variability may improve large-scale BB assessments, which to date rely on temporally static EFs. We used a sampling system mounted on an unmanned aerial vehicle (UAV) to sample BB smoke in the Estação Ecológica Serra Geral do Tocantins in the Brazilian states of Tocantins and Bahia. The protected area contains all major Cerrado vegetation types found in Brazil, and EDS burning has been implemented since 2014. Over 800 smoke samples were collected and analysed during the EDS of 2018 and late dry season (LDS) of 2017 and 2018. The samples were analysed using cavity ring-down spectroscopy, and the carbon balance method was used to estimate CO₂, CO, CH₄, and N₂O EFs. Observed EF averages and standard deviations were 1651 (±50) g kg⁻¹ for CO₂, 57.9 (±28.2) g kg⁻¹ for CO, 0.97 (±0.82) g kg⁻¹ for CH₄, and 0.096 (±0.174) g kg⁻¹ for N₂O. Averaged over all measured fire prone Cerrado types, the modified combustion efficiency (MCE) was slightly higher in the LDS (0.961 versus 0.956), and the CO and CH₄ were 10 % and 2.3 % lower in the LDS compared to the EDS. However, these differences were not statistically significant using a two-tailed t test with unequal variance at a 90 % significance level. The seasonal effect was larger in more wood-dominated vegetation types. N₂O EFs showed a more complex seasonal dependency, with opposite intraseasonal trends for savannas that were dominated by grasses versus those with abundant shrubs. We found that the N₂O EF for the open Cerrado was less than half the EF suggested by literature compilations for savannas. This may indicate a substantial overestimation of the contribution of fires in the N₂O budget. Overall, our data imply that in this region, seasonal variability in greenhouse gas emission factors may offset only a small fraction of the carbon mitigation gains in fire abatement programmes.

中文翻译：

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巴西塞拉多地区生物质燃烧产生的温室气体排放因子的季节内变化

每年都会被称为生物质燃烧（BB）的景观火向大气排放大量（温室）气体和气溶胶。频繁燃烧的稀树草原，主要在非洲，澳大利亚和南美，占BB碳排放总量的60％以上。与许多其他排放源相比，火灾具有强烈的季节性。先前的研究已经确定了稀树草原生态系统中明火的缓解潜力；通过在干旱季节早期燃烧已固化的燃料，此时景观条件仍然可以防止火势蔓延，因此火势一般较小，不规则且强度较小。尽管人们普遍认为，在旱季初期点火时，燃烧面积（BA）和总碳消耗量较低，关于排放因子（EFs）的季节内变异性知之甚少。这很重要，因为在干旱早期（EDS）较高的EF可能会抵消EDS燃烧产生的某些碳效益。此外，对EF季节内变异的更好理解可能会改善大规模BB评估，迄今为止，BB评估依赖于时间静态EF。我们使用了安装在无人机（UAV）上的采样系统，对巴西Tocantins和巴伊亚州的EstaçãoEcológicaSerra Geral do Tocantins中的BB烟进行采样。该保护区包含巴西发现的所有主要塞拉多植被类型，并且自2014年以来实施了EDS燃烧。在2018年的EDS以及2017年和2018年的旱季（LDS）期间，收集并分析了800多个烟雾样品。使用腔衰荡光谱分析₂，CO，CH ₄和N ₂ O EF。观察到的EF平均值和标准偏差分别为1651（ ±50）克千克^-1为CO ₂，57.9（ ±28.2）克千克^-1为CO，0.97（ ±0.82）克千克^-1为CH ₄，和0.096（ ±0.174）对于N ₂ O为g kg ^-1。在所有测得的容易发火的Cerrado类型的平均值上，LDS的修正燃烧效率（MCE）略高（0.961对0.956），CO和CH ₄与EDS相比，LDS分别降低了10％和2.3％。然而，使用两尾t 检验在90％显着性水平上具有不等方差的这些差异在统计学上并不显着。在以木材为主的植被类型中，季节效应更大。N ₂ O EFs具有更复杂的季节依赖性，以草为主的热带稀树草原与灌木丛丰富的热带稀树草原的季节内趋势相反。我们发现，开放式塞拉多的N ₂ O EF小于稀树草原文献汇编所建议的EF的一半。这可能表明对N ₂中火的贡献进行了高估。O预算。总体而言，我们的数据表明，在该地区，温室气体排放因子的季节性变化可能仅抵消了消火计划中碳减排收益的一小部分。