In this study we report aircraft-borne measurements of organic species made during March 1998 in Surinam, an unpolluted region on the northeast coast of South America Measurements included the following: CO by tunable diode laser; a wide variety of organics including acetone, acetonitrile, and isoprene by proton transfer mass spectrometry (PTR-MS); and nonmethane hydrocarbon measurements by gas chromatography-flame ionization detection. Here we compare the standard deviation of the natural logarithm of the mixing ratio (Sigma_lnX) to the estimated lifetime of these species. This relationship has been used to support identification of masses measured by the PTR-MS; ascertain the consistency and quality of hydrocarbon measurement data; and to provide information concerning sinks of important trace species. A selection of the data is used to indirectly determine an average HO concentration of 2.0¿105 molecules cm-3 along the back trajectory for air encountered during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Cooperative LBA Airborne Regional Experiment (LBA-CLAIRE) measurement campaign between 0--1 km over the tropical rain forest. The lower than expected HO concentration derived could have been caused by significant atmospheric or oceanic photochemical production of acetone and MEK along the back trajectory. ¿ 2000 American Geophysical Union