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Abstract. Landscape fires are a significant contributor to atmospheric burdens of greenhouse gases and aerosols. Although many studies have looked at biomass burning products and their fate in the atmosphere, estimating and tracing atmospheric pollution from landscape fires based on atmospheric measurements is challenging due to the large variability in fuel composition and burning conditions. Stable carbon isotopes in biomass burning (BB) emissions can be used to trace the contribution of C3 plants (e.g., trees or shrubs) and C4 plants (e.g. savanna grasses) to various combustion products. However, there are still many uncertainties regarding changes in isotopic composition (also known as fractionation) of the emitted carbon compared to the burnt fuel during the pyrolysis and combustion processes. To study BB isotope fractionation, we performed a series of laboratory fire experiments in which we burned pure C3 and C4 plants as well as mixtures of the two. Using isotope ratio mass spectrometry (IRMS), we measured stable carbon isotope signatures in the pre-fire fuels and post-fire residual char, as well as in the CO2, CO, CH4, organic carbon (OC), and elemental carbon (EC) emissions, which together constitute over 98 % of the post-fire carbon. Our laboratory tests indicated substantial isotopic fractionation in combustion products compared to the fuel, which varied between the measured fire products. CO2, EC and residual char were the most reliable tracers of the fuel 13C signature. CO in particular showed a distinct dependence on burning conditions; flaming emissions were enriched in 13C compared to smouldering combustion emissions. For CH4 and OC, the fractionation was opposite for C3 emissions (13C-enriched) and C4 emissions (13C-depleted). This indicates that while it is possible to distinguish between fires that were dominated by either C3 or C4 fuels using these tracers, it is more complicated to quantify their relative contribution to a mixed-fuel-fire based on the δ13C signature of emissions. Besides laboratory experiments, we sampled gases and carbonaceous aerosols from prescribed fires in the Niassa special Reserve (NSR) in Mozambique, using an unmanned aerial system (UAS)-mounted sampling set-up. We also provide a range of C3 : C4 contributions to the fuel and measured the fuel isotopic signatures. While both OC and EC were useful tracers of the C3 to C4 fuel ratio in mixed fires in the lab, we found particularly OC to be depleted compared to the calculated fuel signal in the field experiments. This suggests that either our fuel measurements were incomprehensive and underestimated the C3 : C4 ratio in the field, or that other processes caused this depletion. Although additional field measurements are needed, our results indicate that C3 vs C4 source ratio estimation is possible with most BB products, albeit with varying uncertainty ranges.

Abstract. Landscape fires, predominantly found in the frequently burning global savannas, are a substantial source of greenhouse gases and aerosols. The impact of these fires on atmospheric composition is partially determined by the chemical breakup of the constituents of the fuel into individual emitted chemical species, which is described by emission factors (EFs). These EFs are known to be dependent on, amongst other things, the type of fuel consumed, the moisture content of the fuel, and the meteorological conditions during the fire, indicating that savanna EFs are temporally and spatially dynamic. Global emission inventories, however, rely on static biome-averaged EFs, which makes them ill-suited for the estimation of regional biomass burning (BB) emissions and for capturing the effects of shifts in fire regimes. In this study we explore the main drivers of EF variability within the savanna biome and assess which geospatial proxies can be used to estimate dynamic EFs for global emission inventories. We made over 4500 bag measurements of CO2, CO, CH4, and N2O EFs using a UAS and also measured fuel parameters and fire-severity proxies during 129 individual fires. The measurements cover a variety of savanna ecosystems under different seasonal conditions sampled over the course of six fire seasons between 2017 and 2022. We complemented our own data with EFs from 85 fires with locations and dates provided in the literature. Based on the locations, dates, and times of the fires we retrieved a variety of fuel, weather, and fire-severity proxies (i.e. possible predictors) using globally available satellite and reanalysis data. We then trained random forest (RF) regressors to estimate EFs for CO2, CO, CH4, and N2O at a spatial resolution of 0.25∘ and a monthly time step. Using these modelled EFs, we calculated their spatiotemporal impact on BB emission estimates over the 2002–2016 period using the Global Fire Emissions Database version 4 with small fires (GFED4s). We found that the most important field indicators for the EFs of CO2, CO, and CH4 were tree cover density, fuel moisture content, and the grass-to-litter ratio. The grass-to-litter ratio and the nitrogen-to-carbon ratio were important indicators for N2O EFs. RF models using satellite observations performed well for the prediction of EF variability in the measured fires with out-of-sample correlation coefficients between 0.80 and 0.99, reducing the error between measured and modelled EFs by 60 %–85 % compared to using the static biome average. Using dynamic EFs, total global savanna emission estimates for 2002–2016 were 1.8 % higher for CO, while CO2, CH4, and N2O emissions were, respectively, 0.2 %, 5 %, and 18 % lower compared to GFED4s. On a regional scale we found a spatial redistribution compared to GFED4s with higher CO, CH4, and N2O EFs in mesic regions and lower ones in xeric regions. Over the course of the fire season, drying resulted in gradually lower EFs of these species. Relatively speaking, the trend was stronger in open savannas than in woodlands, where towards the end of the fire season they increased again. Contrary to the minor impact on annual average savanna fire emissions, the model predicts localized deviations from static averages of the EFs of CO, CH4, and N2O exceeding 60 % under seasonal conditions.

Les incendies de savane qui se produisent en Afrique subsaharienne représentent plus de 60 % de l'étendue mondiale des incendies, dont plus de la moitié dans l'hémisphère sud, contribuant à environ29 % des émissions mondiales dues aux incendies. En nous appuyant sur l'expérience acquise dans la réduction des émissions dues aux incendies de savane dans les savanes du nord de l'Australie sujettes aux incendies grâce à la mise en œuvre d'une méthodologie de réduction des émissions de « brûlage de savane » accréditée au niveau international, nous explorons les opportunités et les défis associés à l'application d'une approche similaire pour encourager la réduction des émissions dans les savanes d'Afrique australe sujettes aux incendies. Nous montrons d'abord que pour une région focale couvrant sept pays contigus, au moins 80% de l'étendue annuelle des grands incendies de savane (>250 ha) et des émissions se produisent dans des conditions météorologiques relativement sévères de fin de saison sèche (LDS), principalement dans des zones peu habitées. Nous évaluons ensuite la faisabilité d'adapter la méthodologie australienne de réduction des émissions par le biais d'études exploratoires sur le terrain sur le site du patrimoine mondial de Tsodilo Hills, dans le nord-ouest du Botswana, et dans la réserve spéciale de Niassa, dans le nord du Mozambique. Notre évaluation démontre que l'application d'une méthode de réduction des émissions de combustion dans la savane axée sur la mise en œuvre d'un brûlage de début de saison sèche (EDS) stratégiquement situé pour réduire l'étendue des feux de forêt LDS et les émissions résultantes répond à des critères techniques clés, notamment : les combustibles fins LDS ont tendance à être nettement supérieurs aux combustibles EDS compte tenu des apports saisonniers de litière de feuilles ; les incendies LDS ont tendance à être beaucoup plus graves et à brûler plus de combustibles ; les facteurs d'émission de méthane et d'oxyde nitreux sont essentiellement équivalents dans les périodes EDS et LDS dans des conditions de combustible durci. Au cours de la discussion, nous examinons les principaux défis et mises en garde associés à la mise en œuvre qui doivent être abordés pour faire progresser le développement de méthodes de brûlage de savane qui encouragent la gestion durable des incendies, réduisent les émissions et soutiennent les moyens de subsistance des communautés dans les savanes d'Afrique australe dominées par les feux de forêt. Los incendios de sabana que ocurren en el África subsahariana representan más del 60% de la extensión mundial de incendios, de los cuales más de la mitad ocurre en el hemisferio sur, contribuyendo con ~29% de las emisiones mundiales de incendios. Aprovechando la experiencia en la reducción de las emisiones de incendios de sabana en las sabanas del norte de Australia propensas a incendios mediante la implementación de una metodología de reducción de emisiones de "quema de sabana" acreditada internacionalmente, exploramos oportunidades y desafíos asociados con la aplicación de un enfoque similar para incentivar la reducción de emisiones en las sabanas del sur de África propensas a incendios. En primer lugar, mostramos que para una región focal que abarca siete países contiguos, al menos el 80% de la extensión y las emisiones anuales de grandes incendios de sabana (>250 ha) se producen en condiciones climáticas de incendios de estación seca tardía (LDS) relativamente severas, predominantemente en áreas escasamente habitadas. Luego evaluamos la viabilidad de adaptar la metodología australiana de reducción de emisiones a través de estudios de campo exploratorios en el sitio del Patrimonio Mundial de Tsodilo Hills en el noroeste de Botswana y la Reserva Especial de Niassa en el norte de Mozambique. Nuestra evaluación demuestra que la aplicación de un método de reducción de emisiones de quema de sabana centrado en la realización de la quema de la estación seca temprana (EDS) estratégicamente ubicada para reducir la extensión de los incendios forestales Sud y las emisiones resultantes cumple con los criterios técnicos clave, que incluyen: los combustibles finos Sud tienden a ser notablemente mayores que los combustibles EDS dadas las entradas estacionales de hojarasca; los incendios Sud tienden a ser significativamente más severos y queman más combustibles; los factores de emisión de metano y óxido nitroso son esencialmente equivalentes en los períodos EDS y LDS en condiciones de combustible curado. En el debate, consideramos los desafíos clave de implementación asociados y las advertencias que deben abordarse para avanzar en el desarrollo de métodos de quema de sabanas que incentiven la gestión sostenible de incendios, reduzcan las emisiones y apoyen los medios de vida comunitarios en las sabanas del sur de África dominadas por los incendios forestales. Savanna fires occurring in sub-Saharan Africa account for over 60% of global fire extent, of which more than half occurs in the Southern Hemisphere contributing ~29% of global fire emissions. Building on experience in reducing savanna fire emissions in fire-prone north Australian savannas through implementation of an internationally accredited 'savanna burning' emissions abatement methodology, we explore opportunities and challenges associated with the application of a similar approach to incentivise emissions reduction in fire-prone southern African savannas. We first show that for a focal region covering seven contiguous countries, at least 80% of annual savanna large fire (>250 ha) extent and emissions occur under relatively severe late dry season (LDS) fire-weather conditions, predominantly in sparsely inhabited areas. We then assess the feasibility of adapting the Australian emissions abatement methodology through exploratory field studies at the Tsodilo Hills World Heritage site in north-west Botswana, and the Niassa Special Reserve in northern Mozambique. Our assessment demonstrates that application of a savanna burning emissions abatement method focused on the undertaking of strategically located early dry season (EDS) burning to reduce LDS wildfire extent and resultant emissions meets key technical criteria, including: LDS fine fuels tend to be markedly greater than EDS fuels given seasonal leaf litter inputs; LDS fires tend to be significantly more severe and combust more fuels; methane and nitrous oxide emission factors are essentially equivalent in EDS and LDS periods under cured fuel conditions. In discussion we consider associated key implementation challenges and caveats that need to be addressed for progressing development of savanna burning methods that incentivise sustainable fire management, reduce emissions, and support community livelihoods in wildfire-dominated southern African savannas. تمثل حرائق السافانا التي تحدث في أفريقيا جنوب الصحراء الكبرى أكثر من 60 ٪ من نطاق الحرائق العالمية، والتي يحدث أكثر من نصفها في نصف الكرة الجنوبي مما يساهم بنحو 29 ٪ من انبعاثات الحرائق العالمية. بناءً على الخبرة في الحد من انبعاثات حرائق السافانا في السافانا المعرضة للحرائق في شمال أستراليا من خلال تنفيذ منهجية معتمدة دوليًا للحد من انبعاثات "حرق السافانا"، نستكشف الفرص والتحديات المرتبطة بتطبيق نهج مماثل لتحفيز الحد من الانبعاثات في السافانا الجنوبية المعرضة للحرائق. نظهر أولاً أنه بالنسبة لمنطقة محورية تغطي سبعة بلدان متجاورة، يحدث ما لا يقل عن 80 ٪ من حرائق السافانا السنوية الكبيرة (>250 هكتارًا) والانبعاثات في ظل ظروف طقس حريق شديدة نسبيًا في أواخر موسم الجفاف (LDS)، في الغالب في المناطق ذات الكثافة السكانية المنخفضة. ثم نقوم بتقييم جدوى تكييف المنهجية الأسترالية لخفض الانبعاثات من خلال الدراسات الميدانية الاستكشافية في موقع التراث العالمي تسوديلو هيلز في شمال غرب بوتسوانا، ومحمية نياسا الخاصة في شمال موزمبيق. يوضح تقييمنا أن تطبيق طريقة خفض انبعاثات حرق السافانا التي تركز على القيام بحرق في وقت مبكر من موسم الجفاف (EDS) في موقع استراتيجي لتقليل مدى حرائق الغابات LDS والانبعاثات الناتجة عنها تفي بالمعايير الفنية الرئيسية، بما في ذلك: يميل الوقود الناعم LDS إلى أن يكون أكبر بشكل ملحوظ من وقود EDS بالنظر إلى مدخلات فضلات الأوراق الموسمية ؛ تميل حرائق LDS إلى أن تكون أكثر حدة بشكل ملحوظ وتحترق المزيد من الوقود ؛ عوامل انبعاث الميثان وأكسيد النيتروز مكافئة بشكل أساسي في فترات EDS و LDS في ظل ظروف الوقود المعالجة. في المناقشة، ننظر في تحديات التنفيذ الرئيسية المرتبطة بها والمحاذير التي يجب معالجتها للتقدم في تطوير طرق حرق السافانا التي تحفز الإدارة المستدامة للحرائق، وتقلل الانبعاثات، وتدعم سبل عيش المجتمع في السافانا الجنوبية التي تهيمن عليها حرائق الغابات.