Impact of tropical biomass burning emissions on the diurnal cycle of upper tropospheric CO2 retrieved from NOAA 10 satellite observations

Four years (July 1987 to June 1991) of monthly mean upper tropospheric CO₂ mixing ratios over the tropics are retrieved from the observations, at 0730 LT (day) and 1930 LT (night) local time, of the meteorological satellite NOAA 10. Analysis of night minus day differences (N-DD) shows large diurnal variations of CO₂, of the order of 2-3 ppm, during months and over regions affected by biomass burning. The patterns of these diurnal variations are in very good agreement with the diurnal and seasonal variations of biomass burning activity. We interpret them as the signal of CO₂ plumes being rapidly uplifted by fire-induced convection into the upper troposphere during the daytime peak of fire activity and then being rapidly dispersed at night by large-scale atmospheric transport. The upper air CO₂ diurnal cycle closely follows the seasonal distribution of burned areas from the European Space Agency's monthly Global Burnt Scar satellite product, which is recognized as yielding reasonable estimates of burned areas for large and presumably intense fires. The largest N-DD values are found in 1990 over southern Africa in agreement with the reported interannual variability of fire activity. However, the magnitude of these extreme signatures, on the order of 5 ppm locally, is larger than what can reasonably be expected from either in situ observations or from simulations, suggesting some contamination of the N-DD retrieval by fire emission products other than CO₂. It is concluded from a detailed sensitivity analysis that the presence of high-altitude and large optical depth aerosols, or of elevated tropospheric ozone concentrations, as are often encountered in fire plumes, may significantly contaminate the retrieved CO₂ signal (by up to 2-3 ppm for extreme events). The possible contaminating effects of undetected fire-induced thin cirrus (optical depths less than 0.05 at 14 μm) are also quantified.