Biomass burning emissions and vertical distribution of atmospheric methyl halides and other reduced carbon gases in the South Atlantic region

Abstract

The NASA TRACE A experiment (September – October 1992) investigated effects of dry season biomass burning emissions from both South America and southern Africa on the tropical South Atlantic troposphere. Whole air canister samples were collected aboard the NASA DC‐8 aircraft and analyzed for a wide range of nonmethane hydrocarbons (NMHCs) and halocarbons. Fast response in situ quantification of CH4, CO, and CO2 were also performed on the DC‐8. Sampling took place over Brazilian agricultural areas and southern African savanna where there was active biomass burning. The vertical distribution of the measured gases revealed that the concentrations of most hydrocarbons, methyl halides, CH4, CO, and CO2, were enhanced in the boundary layer of these regions principally as a result of biomass fires. Brazilian and African biomass burning emission ratios were calculated for CH3Br, CH3Cl, CH3I, and NMHCs relative to CO and CO2. Although both fire regions were dominated by efficient (flaming) combustion (CO/CO2 ratios <0.1), the Brazilian samples exhibited the lower (more flaming) CO/CO2 ratio of 0.037 compared with the African savanna value of 0.062. This difference in combustion efficiency was reflected in lower ratios versus CO2 for all correlated gases. However, the gases more closely associated with smoldering combustion (e.g., C3H8 and CH3Cl) exhibited significantly higher ratios relative to CO for the Brazilian fires, while the African samples exhibited higher values versus CO for compounds associated more closely with flaming combustion (e.g., C2H2). This variation in the trace gas ratios versus CO is most likely caused by different fuel characteristics. On the basis of the emission ratios obtained, the total biomass burning emission rates for savannas and worldwide were calculated for the hydrocarbons and methyl halides. From these it was estimated that roughly 25% and 20% of global CH3Cl and CH3Br emissions, respectively, derive from biomass burning but that the contribution of biomass burning to total CH3I emissions was not significant.

Origin of anthropogenic hydrocarbons and halocarbons measured in the summertime european outflow (on Crete in 2001)

“…We note that considerable variability in the emission ratios of CH 3 Br emitted by biomass burning processes has been reported in the literature, depending strongly on the bromine content of the fuel and on the proportion of flaming to smoldering combustion (Andreae and Merlet, 2001). However, even if we consider the upper limit of published CH 3 Br emission factors versus CO (Blake et al, 1996 and references therein), the 100 ppbv CO enhancement observed during this episode would lead to a CH 3 Br enhancement of about 1 pptv. This value, which represents the maximum expected impact, is only slightly higher than our analytical precision, which would explain why there is no significant enhancement of methyl bromide during the biomass burning episode.…”

Optimal estimation of the surface fluxes of methyl chloride using a 3-D global chemical transport model

“…Of our 20 independent observing sites, only 5 are tropical or sub-tropical (Barbados, Samoa, Hateruma, Mauna Loa, Mauna Kea), which implies the need for more tropical (preferably in situ) continuous observing sites in order to improve our understanding of the magnitudes and variabilities of the dominant tropical sources. Future inversions could also utilize short-term aircraft campaign data (e.g., Blake et al, 1996).…”

Crown fire emissions of CO2, CO, H2, CH4, and TNMHC from a dense Jack pine boreal forest fire

“…It is generally accepted that the higher-temperature flaming stage of combustion in vegetation fires is more efficient (producing higher proportions of carbon dioxide and water and correspondingly less partially oxidized products, e.g., carbon monoxide) than the lower-temperature smoldering phase of combustion (Crutzen and Andreae, 1990; Laursen et al, 1992; Blake et al, 1996). This can readily be explained by considering that the easily ignitable and well-aerated small size (high surface area to volume, thus, excellent heat transfer and combustion properties) fuels bum readily and flame significantly.…”