High spatial and temporal resolution measurements of primary organics and their oxidation products over the tropical forests of Surinam

Tropical forests with emissions greater than 1015 g C of reactive hydrocarbons per year strongly affect atmospheric chemistry. Here we report aircraft-borne measurements of organics during March 1998 in Surinam, a largely unpolluted region which is optimally located to study chemical processes induced by tropical forest emissions. Isoprene and its degradation products methylvinyl ketone (MVK) and methacrolein (MACR) and possibly isoprene hydroperoxides (ISOHP), were measured in the nmol mol−1 volume mixing ratio (VMR) range, consistent with estimated emissions and model calculations. In addition, high VMRs of some non-isoprene-derived organics were measured, such as acetone (≈2–4 nmol mol1 up to 12 km altitude), an important source of HO and HO2 in the upper troposphere. Moreover, several masses were measured at significant mixing ratios which could not be identified by reference to previous field measurements or gas-phase isoprene chemistry. High VMRs, almost 0.4 nmol mol−1, were also recorded for a compound which is most likely dimethyl sulphide (DMS). If so, boundary layer loss of HO by reactions with hydrocarbons and their oxidation products strongly prolongs the lifetime of DMS, allowing its transport deep into the Amazon forest south of the intertropical convergence zone (ITCZ). We postulate greater sulphate production and deposition north than south of the (ITCZ) with possible consequences for cloud and ecosystem properties.