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