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Abstract:
A method to constrain carbon dioxide (CO2) emissions from open biomass burning
by using satellite observations of co-emitted species and a chemistry-transport model
(CTM) is proposed and applied to the case of wildfires in Siberia. CO2 emissions
5 are assessed by means of an emission model assuming a direct relationship between
the biomass burning rate (BBR) and the Fire Radiative Power (FRP) derived
from the MODIS measurements. The key features of the method are (1) estimating
the FRP-to-BBR conversion factors () for different vegetative land cover types by assimilating
the satellite observations of co-emitted species into the CTM, (2) optimal
10 combination of the estimates of derived independently from satellite observations of
different species (CO and aerosol in this study), and (3) estimation of the diurnal cycle
of the fire emissions directly from the FRP measurements. Values of for forest and
grassland fires in Siberia and their uncertainties are estimated by using the IASI carbon
monoxide (CO) retrievals and the MODIS aerosol optical depth (AOD) measurements
15 combined with outputs from the CHIMERE mesoscale chemistry transport model. The
constrained CO emissions are validated through comparison of the respective simulations
with the independent data of ground based CO measurements at the ZOTTO
site. Using our optimal regional-scale estimates of the conversion factors (which are
found to be in agreement with the earlier published estimates obtained from local mea20
surements of experimental fires), the total CO2 emissions from wildfires in Siberia in
2012 are estimated to be in the range from 262 to 477 TgC, with the optimal (maximum
likelihood) value of 354 TgC. Sensitivity test cases featuring different assumptions regarding
the injection height and diurnal variations of emissions indicate that the derived
estimates of the total CO2 emissions in Siberia are robust with respect to the modelling
25 options (the different estimates vary within less than 10% of their magnitude). The
obtained CO2 emission estimates for several years are compared with the independent
estimates provided by the GFED3.1 and GFASv1.0 global emission inventories. It
is found that our “top-down” estimates for the total annual biomass burning CO2 emis-sions in the period from 2007 to 2011 in Siberia are by factors of 2.3 and 1.7 larger than
the respective bottom-up estimates; these discrepancies cannot be fully explained by
uncertainties in our estimates. There are also considerable differences in the spatial
distribution of the different emission estimates; some of those differences have a sys-5 tematic character and require further analysis.
