Investigating Smoke Emission Coefficients using MODIS Fire Radiative Energy and Smoke Aerosols

Smoke aerosols released from biomass burning have a great influence on air quality, weather, and climate. The total particular matter (TPM) in smoke aerosols has been demonstrated to be a linear function of fire radiative energy (FRE) that is the temporal integration of instantaneous fire radiative power (FRP) during a period of biomass burning. This function is called a top-down approach, in which a scalar constant, termed the smoke emission coefficient (Ce), is critical for estimating TPM from FRE directly. However, it is very challenging to quantify Ce appropriately from satellite observations. In this study, an innovative approach was put forward to explore Ce values by establishing relationships between FRE and TPM observed from MODIS in two typical regions as examples, i.e., the Contiguous United States (CONUS) in the temperate zone and Indonesia in the tropical zone. Specifically, we identified 700 isolated fire clusters and smoke plumes in Indonesia (from Jun 2012 to Jun 2018) and the CONUS (from Jun 2017 to Jun 2018) using the MOD/MYD09GA true-color composite images and then calculated FRE from MODIS active fire products (MOD/MYD14) and TPM from MODIS aerosol optical depth products (MOD/MYD04_L2) during the Terra ( 10:30 am) and Aqua ( 13:30pm) overpass times for each isolated fire cluster. The relationships between TPM and FRE were established to determine Ce values using an ordinary least square regression for various regions and fuel types, respectively. The results showed that FRE and TPM were strongly correlated (r² ≥ 0.66, p < 0.001) in all the cases while Ce varied in a wide range. The Ce was larger in peatland (31.376 g/MJ) than other fuel types (13.610 g/MJ) in Indonesia and it was larger in forests (10.699 g/MJ) than other vegetation types (4.019 g/MJ for savanna and 6.684 g/MJ for grassland) in the CONUS. Overall, this study improves our understanding of Ce variations and provides a simple way to estimate TPM directly from FRE.