Sources and sinks of formic, acetic, and pyruvic acids over central Amazonia. 2. Wet season

Abstract

At 150-m altitude in the boundary layer the daytime average concentrations were 430 ± 225, 340 ± 155, and 25 ± 15 ppt for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. These values were fivefold lower than those observed in the 1985 dry season. Concentrations measured near canopy top were not significantly different from boundary layer values (P = 0.10), while concentrations in the lower canopy were significantly less. Concentrations in the free troposphere (5 km) were lower than in the boundary layer and averaged 170 ± 40, 210 ± 40, and 15 ± 15 ppt for HCOOH, CH3COOH and CH3C(O)COOH, respectively. Fivefold enhancements of CH3C(O)COOH concentrations were observed in convective outflows at 5- to 6-km altitudes. Model results indicate that direct emissions from vegetation can account for most of the concentrations observed in the canopy. These emissions from vegetation can account for most of the concentrations observed in the canopy. These emissions peak during the daytime hours, and 24-hour average upward fluxes at canopy top are 4.4 × 109, 3.7 × 109, and 2.8 × 108 molecules cm-2 s-1 for HCOOH, CH3COOH, and CH3C(O)COOH, respectively. However, direct emissions from vegetation can account for only a small fraction of the observed carboxylic acid concentrations in the boundary layer, suggesting a large contribution from atmospheric sources. -from Authors