Light Hydrocarbons in the Tropospheric Boundary Layer over Tropical Africa

Abstract

Fifty measurements of nonmethane hydrocarbons (NMHC) and several other trace gases were made over an equatorial rain forest in February 1988 as part of the DECAFE experiment. The measurements were made independently by two different laboratories. Each laboratory used its own sample containers, gas chromatographic measurement procedure, and calibration. Also, the altitudinal distribution of the samples differed. Apart from propene and i‐pentane for which a substantial difference in the absolute calibration existed between the two laboratories, the average results were very similar, and the vertical profiles were identical within the scatter of the data. For NMHC with longer atmospheric residence times (e.g., ethane and acetylene) the average results agreed within a few percent. In the boundary layer, only small gradients could be found. In all cases where a significant vertical gradient existed, there was an increase of the mixing ratios with increasing altitude. This can be explained by the different origin of the air masses at different altitudes. Above the boundary the trace gas mixing ratios decrease. The observed NMHC pattern can primarily be described as photochemically aged emissions from biomass burning. The observed depletion of the photochemically reactive NMHC also agrees with the occurrence of enhanced ozone levels in the boundary layer.