A quantitative link between CO2 emissions from tropical vegetation fires and the daily tropospheric excess (DTE) of CO2 seen by NOAA-10 (1987-1991)

Monthly mean mid-tropospheric CO2 columns over the tropics are retrieved from evening and morning observations of NOAA-10 (1987-1991). We find that the difference between these two columns ("Daily Tropospheric Excess'', DTE) increases up to 3 ppm over regions affected by fires. At regional scale over Africa, America, and Australia, the variations of the DTE are very similar to those of independently derived biomass burning CO2 emissions. A strong correlation (R-2 similar to 0.8) is found between regional mean DTE and fire CO2 emissions values from the Global Fire Emissions Database (GFEDv2) even though the two products span over periods ten years apart from each other. The DTE distribution over Africa indicates that the southern hemisphere experiences 20% more fire activity during El Nino conditions than during La Nina conditions and the reverse for the northern hemisphere. Such an African dipole of ENSO-related fire variability is comparable to changes analyzed from GFEDv2 CO2 emission maps. However, the estimated one sigma uncertainty on the DTE remains close to this DTE ENSO signal. The physical mechanism linking DTE with emissions is not fully elucidated. Hot convective fire plumes injecting CO2 into the troposphere during the afternoon peak of fire activity, seen by the satellite at 1930 LT, and then being diluted by large scale atmospheric transport, before the next satellite pass at 0730 LT, could explain the tight observed relationship between DTE and CO2 emissions. We conclude that DTE data can be very useful to quantitatively reconstruct fire emission patterns before the ATSR and MODIS era when better quality fire count and burned area data became available.