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 10(15) g C of reactive hydroca rbons per year strongly affect atmospheric chemistry. Here we report aircra ft-borne measurements of organics during March 1998 in Surinam, a largely u npolluted region which is optimally located to study chemical processes ind uced by tropical forest emissions. Isoprene and its degradation products me thylvinyl ketone (MVK) and methacrolein (MACR) and possibly isoprene hydrop eroxides (ISOHP), were measured in the nmol mol(-1) volume mixing ratio (VM R) range, consistent with estimated emissions and model calculations. In ad dition, high VMRs of some non-isoprene-derived organics were measured, such as acetone (approximate to 2-4 nmol mol(1) up to 12 km altitude), an impor tant source of HO and HO2 in the upper troposphere. Moreover, several masse s 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 whi ch is most likely dimethyl sulphide (DMS). If so, boundary layer loss of HO by reactions with hydrocarbons and their oxidation products strongly prolo ngs the lifetime of DMS, allowing its transport deep into the Amazon forest south of the intertropical convergence zone (ITCZ). We postulate greater s ulphate production and deposition north than south of the (ITCZ) with possi ble consequences for cloud and ecosystem properties. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.