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

Abstract

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 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.

Methane gas emissions from savanna fires: what analysis of local burning regimes in a working West African landscape tell us

“…It is not surprising that our values had a lower MCE and higher CH 4 EF than the biome means because we based our values on emissions from "wooded mesic sa-vannas" as opposed to the "grassland" values used in most savanna biome estimates. Wooded savannas contain small trees, shrubs and leaf litter, which tend to reduce MCE and increase methane EF values (e.g., Vernooij et al, 2021) due to the fraction of ligneous fuel in the mix.…”

“…In fact, there are not a lot of data in the literature on fine fuel mixtures (the primary fuel for savanna fires) used to estimate EFs in Africa, although the amount of woody vegetation clearly affects emissions (Korontzi, 2005;van Leeuwen and van der Werf, 2011). In the Brazilian Cerrado, for example, Vernooij et al (2021) found that the seasonal effect on methane EF was stronger in more woody savanna vegetation, with late dry season (LDS) fires having 20 % lower EF than early dry season (EDS) ones in shrub-dominated areas.…”

Stable carbon isotopic composition of biomass burning emissions – implications for estimating the contribution of C3 and C4 plants

“…During the field experiments we filled single-polypropylene fitted Tedlar bags (SKC, type 232-01) with fresh smoke using a UAS-based (DJI, Matrice 200) sampling system. The UAS sampling methodology was described in detail in Vernooij et al (2021). In short, we filled 1L Tedlar bags with fresh smoke at altitudes of roughly 15 meters over the fire.…”

An Unmanned Aerial System (UAS) based methodology for measuring biomass burning emission factors

“…Aircraft measurements maybe biased towards flaming emissions, since they sample lofted emissions that typically result from higher intensity combustion, (airplane), where as ground or tower based measuremetns maybe biased towards residual smouldering combustion (RSC) sampling since the smoke from higher intensity burns is lofted out of reach. Unmanned Aerial Systems (UAS) have the potential to offer a low-cost and versatile solution for sampling different locations within a freshly emitted, dense smoke plume (Aurell et al, 2021;Vernooij et al, 2021). UAS-compatible (i.e.…”