Seasonality of greenhouse gas emission factors from biomass burning in the Brazilian Cerrado

Abstract. 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 early in the dry season when the vegetation has not fully cured, fires are in general patchier and burn less intense. While it is widely accepted that burned area and the total carbon consumed is lower when fires are ignited early in the dry season, little is known about the seasonality of emission factors (EF) of greenhouse gases. 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 seasonality 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) and cavity ring-down spectroscopy to estimate CO2, CO, CH4, and N2O EFs 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 was implemented on a large scale since 2014. We collected and analyzed over 800 smoke samples during the EDS and late dry season (LDS). Averaged over all measurements, the modified combustion efficiency (MCE) was slightly higher in the LDS (0.976 vs. 0.972) and the CH4 and CO EFs were 13 % and 15 % lower in the LDS compared to the EDS. This seasonal effect was larger in more wood-dominated vegetation types. N2O EFs showed a more complex seasonal dependency, with opposite seasonal 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 of those reported so far in the BB literature for savannas. This may indicate a substantial overestimation of the contribution of fires in the N2O budget. Overall, our data implies 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 programs.