Abstract
Abstract. The 2015–2016 strong El Niño event has had a dramatic impact on the amount of Indonesian biomass burning, with the El Niño-driven drought further desiccating the already-drier-than-normal landscapes that are the result of decades of peatland draining, widespread deforestation, anthropogenically driven forest degradation and previous large fire events. It is expected that the 2015–2016 Indonesian fires will have emitted globally significant quantities of greenhouse gases (GHGs) to the atmosphere, as did previous El Niño-driven fires in the region. The form which the carbon released from the combustion of the vegetation and peat soils takes has a strong bearing on its atmospheric chemistry and climatological impacts. Typically, burning in tropical forests and especially in peatlands is expected to involve a much higher proportion of smouldering combustion than the more flaming-characterised fires that occur in fine-fuel-dominated environments such as grasslands, consequently producing significantly more CH4 (and CO) per unit of fuel burned. However, currently there have been no aircraft campaigns sampling Indonesian fire plumes, and very few ground-based field campaigns (none during El Niño), so our understanding of the large-scale chemical composition of these extremely significant fire plumes is surprisingly poor compared to, for example, those of southern Africa or the Amazon.Here, for the first time, we use satellite observations of CH4 and CO2 from the Greenhouse gases Observing SATellite (GOSAT) made in large-scale plumes from the 2015 El Niño-driven Indonesian fires to probe aspects of their chemical composition. We demonstrate significant modifications in the concentration of these species in the regional atmosphere around Indonesia, due to the fire emissions.Using CO and fire radiative power (FRP) data from the Copernicus Atmosphere Service, we identify fire-affected GOSAT soundings and show that peaks in fire activity are followed by subsequent large increases in regional greenhouse gas concentrations. CH4 is particularly enhanced, due to the dominance of smouldering combustion in peatland fires, with CH4 total column values typically exceeding 35 ppb above those of background “clean air” soundings. By examining the CH4 and CO2 excess concentrations in the fire-affected GOSAT observations, we determine the CH4 to CO2 (CH4 ∕ CO2) fire emission ratio for the entire 2-month period of the most extreme burning (September–October 2015), and also for individual shorter periods where the fire activity temporarily peaks. We demonstrate that the overall CH4 to CO2 emission ratio (ER) for fires occurring in Indonesia over this time is 6.2 ppb ppm−1. This is higher than that found over both the Amazon (5.1 ppb ppm−1) and southern Africa (4.4 ppb ppm−1), consistent with the Indonesian fires being characterised by an increased amount of smouldering combustion due to the large amount of organic soil (peat) burning involved. We find the range of our satellite-derived Indonesian ERs (6.18–13.6 ppb ppm−1) to be relatively closely matched to that of a series of close-to-source, ground-based sampling measurements made on Kalimantan at the height of the fire event (7.53–19.67 ppb ppm−1), although typically the satellite-derived quantities are slightly lower on average. This seems likely because our field sampling mostly intersected smaller-scale peat-burning plumes, whereas the large-scale plumes intersected by the GOSAT Thermal And Near infrared Sensor for carbon Observation – Fourier Transform Spectrometer (TANSO-FTS) footprints would very likely come from burning that was occurring in a mixture of fuels that included peat, tropical forest and already-cleared areas of forest characterised by more fire-prone vegetation types than the natural rainforest biome (e.g. post-fire areas of ferns and scrubland, along with agricultural vegetation).The ability to determine large-scale ERs from satellite data allows the combustion behaviour of very large regions of burning to be characterised and understood in a way not possible with ground-based studies, and which can be logistically difficult and very costly to consider using aircraft observations. We therefore believe the method demonstrated here provides a further important tool for characterising biomass burning emissions, and that the GHG ERs derived for the first time for these large-scale Indonesian fire plumes during an El Niño event point to more routinely assessing spatiotemporal variations in biomass burning ERs using future satellite missions. These will have more complete spatial sampling than GOSAT and will enable the contributions of these fires to the regional atmospheric chemistry and climate to be better understood.
Highlights
The 2015–2016 strong El Niño event, which is ongoing in the tropical Pacific at the time of writing, has had a dramatic impact on the amount of landscape burning occurring across large parts of Indonesia
In combination with fire radiative power (FRP) and atmospheric carbon monoxide data taken from the new Copernicus Atmosphere Service Global Fire Assimilation System (CAMSGFAS, 2016), we demonstrate the Indonesian fires are occurring in peatland-dominated landscapes that explain certain characteristics of the noted emission ratio (ER), which are themselves important in determining the so-called emission factors representative of the combustion processes occurring in these very large-scale landscape fires
This would enable the characterisation of certain aspects of the chemical makeup of these large-scale El Niño-driven fires for the first time, which, in 1997–1998, were responsible for the largest release of fire-emitted greenhouse gases (GHGs) seen worldwide, and which are believed to be of a magnitude not seen since that period anywhere on Earth
Summary
The 2015–2016 strong El Niño event, which is ongoing in the tropical Pacific at the time of writing, has had a dramatic impact on the amount of landscape burning occurring across large parts of Indonesia. At present there are no known field-based studies of emission makeup, certainly none conducted during El Niño years where the dry conditions may promote different combustion behaviour than occurs under more normal meteorological and fuel moisture conditions, and none where the constituents of the large-scale plumes that most likely contain the bulk of the emitted gases (and aerosols) are assessed The latter point is important because whilst groundbased sampling can measure emission makeup close to the source, including in the field under real landscape combustion conditions, such an approach is, by necessity, limited to capturing smoke from individual fire locations, usually from smaller fires. Knowledge of the relative amounts of these two phases of combustion are known to exert strong controls on the relative emissions of many other compounds (e.g. Yokelson et al, 1996; Cofer et al, 1998; Lee et al, 2010), and if we can better understand the relative CO2 and CH4 emission makeup of the large-scale plumes emanating from these fires, it may provide useful information to better estimate the type of combustion occurring and potentially the overall emission characteristics beyond the two species observed
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