NO x emissions from African savanna fires

Authors

  • J. P. Lacaux,

  • R. Delmas,

  • C. Jambert,

  • T. A. J. Kuhlbusch


Abstract

Fire of Savanna/Dynamique et Chimie Atmosphèrique en Forêt Equatoriale (FOS/DECAFE-91) and Southern African Fire-Atmosphere Research Initiative (SAFARI-92) were two multidisciplinary experiments organized in Africa to determine the gas and aerosol emissions due to prescribed savanna fires. These two experiments took place in two types of savannas which have very different ecological properties: Lamto, Ivory Coast (FOS/DECAFE-91), in the Guinean area, is wet (80% moisture content) and dense (9 to 10 t ha−1) and has little litter (2 to 5% of total biomass); Kruger Park, in South Africa (SAFARI-92), on the contrary, is dry (10 to 20% moisture) and has a low biomass (3 to 6 t ha−1) and 40% of its total biomass in the form of litter. The experimental strategy used to determine the emission ratios ΔCO/ΔCO2 and ΔNOx/ΔCO2 was to measure at ground level, in the same volume of the plume above the fire, the CO, CO2 and NOx concentrations. The mean carbon content of the two savannas was similar (about 43%), and the ΔCO/ΔCO2 ratio, the indicator of the burning process, was comparable in back and head fires with mean ratios between 4.5 and 6.1%. These ΔCO/ΔCO2 ratios were characteristic of intense flaming combustion with the formation of mainly fully oxidized compounds for both savannas. The ΔCO/ΔCO2 ratio for the head fires can be divided into two distinct phases: a flaming period and a smoke period which immediately follows. Nitrogen concentration of the two savannas varied from as low as 0.3% in Lamto to 0.80% of dry mass in the Kruger Park in the Faai plot. In this study we could clearly identify a linear dependency between the nitrogen concentration of the dry and wet savannas of Africa and the NOx/CO2 emission ratio, ΔNOx/ΔCO2 = 0.66 N% − 0.01, with a correlation coefficient of 0.93, statistically significant at a confidence level better than 99%. This result enables the quantification of the emissions of NOx from African savanna burning by measuring the nitrogen concentration of the vegetation and monitoring the type of burn.

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