DIURNAL CYCLES OF FORMIC AND ACETIC-ACIDS IN THE NORTHERN PART OF THE GUAYANA SHIELD, VENEZUELA

Formic and acetic acids were measured in a scrub-grass savanna and in a nearby semi-deciduous forest. Gaseous HCOOH and CH3COOH were collected using the mist-scrubber technique, and were determined using ion chromatography. A strong diurnal cycle was observed at both sites, with higher mixing ratios during daytime. Concentrations in the savanna were always higher than in the forest. Most of the time HCOOH/CH3COOH ratios greater than one were recorded at the savanna site, and ratios less than one at the forest site. Boundary-layer mixing ratios in the savanna region, derived from measurements during midday, are 1.3 +/- 0.4 ppbv and 0.7 +/- 0.3 ppbv for HCOOH and CH3COOH. Dry depositions velocities between 0.5 and 1 cm s-1 were estimated for the savanna region. Atmospheric residence times of < 3 days and > 5 days were estimated for the rainy and dry season, respectively.