Measurements of reactive chlorocarbons over the Surinam tropical rain forest: indications for strong biogenic emissions

Abstract. Contrary to the understanding of the emissions and chemical behavior of halocarbons from anthropogenic sources (e.g. CFCs and HCFCs), the biogeochemistry of naturally emitted halocarbons is still poorly understood. We present measurements of chloromethane (methyl chloride, CH₃Cl), trichloromethane (chloroform, CHCl₃), dichloromethane (CH₂Cl₂), and tetrachloroethylene (C₂Cl₄) from air samples taken over the Surinam rainforest during the 1998 LBA/CLAIRE campaign. The samples were collected in stainless steel canisters on-board a Cessna Citation jet aircraft and analyzed in the laboratory using a gas chromatograph equipped with FID and ECD. The chlorocarbons we studied have atmospheric lifetimes of ~1 year or less, and appear to have significant emissions from natural sources including oceans, soils and vegetations, as well as biomass burning. These sources are primarily concentrated in the tropics (30º N-30º S). We detected an increase as a function of latitude of methyl chloride, chloroform, and tetrachloroethylene mixing ratios, in pristine air masses advected from the Atlantic Ocean toward the central Amazon. In the absence of significant biomass burning sources, we attribute this increase to biogenic emissions from the Surinam rainforest. From our measurements, we deduce fluxes from the Surinam rainforest of 7.6±1.8 μg CH₃Cl m⁻² h⁻¹, 1.11±0.08g CHCl₃ μm⁻² h⁻¹, and 0.36±0.07 μg C₂Cl₄ m⁻² h⁻¹. Extrapolated to a global scale, our emission estimates suggest a large potential source of 2 Tg CH₃Cl yr⁻¹ from tropical forests, which could account for the net budget discrepancy (underestimation of sources), as indicated previously. In addition, our estimates suggest a potential emission of 57±17 Gg C₂C₄ yr⁻¹ from tropical forest soils, equal to half of the currently missing C₂Cl₄ sources. We hypothesize that the extensive deforestation over the last two decades relates to the observed global downward trend of atmospheric methyl chloride.