Halocarbons over the rainforest

Airborne measurements of the halogenated trace gases methyl chloride, methyl bromide and chloroform were conducted over the Atlantic Ocean and 1000 km of pristine tropical rainforest in Suriname and French Guyana (3–6 • N, 51–59 • W) in October 2005. In the boundary layer (0–1.4 km), maritime air masses initially low in forest hy-5 drocarbons, advected over the forest by southeasterly trade winds, were measured at various distances from the coast. Since the organohalogens presented here have relatively long atmospheric lifetimes (0.4–1.0 years) in comparison to the transport times (1–2 days), emissions will accumulate in air traversing the rainforest. The distributions of methyl chloride, methyl bromide and chloroform were analyzed as a function 10 of forest contact time and the respective relationship used to determine fluxes from the rainforest during the long dry season. Emission fluxes have been calculated for methyl chloride and chloroform as 9.4 (±4.0 2σ) and 0.34 (±0.14 2σ) µg m −2 h −1 , respectively. No significant flux from the rainforest was observed for methyl bromide within the limits of these measure-15 ments. The flux of methyl chloride was in general agreement with the flux measured over the same region in March 1998 during the LBA Claire project using a different analytical system. This confirms that the rainforest is a strong source for methyl chloride and suggests that this emission is relatively uniform throughout the year. In contrast the chloroform flux derived here is a factor of three less than previous measurements 20 made in March 1998 suggesting a pronounced ecosystem variation. The differences in chloroform fluxes could not be attributed to either temperature or rainfall changes. The global extrapolation of the derived fluxes led to 1.5 (±0.6 2σ) Tg yr −1 for methyl chloride , which is in the range of the missing source postulated by previous model studies and 55 (±22 2σ) Gg yr −1 for chloroform.