EXCHANGE OF SHORT‐CHAIN ALDEHYDES BETWEEN AMAZONIAN VEGETATION AND THE ATMOSPHERE: Fid-Bau Portal

EXCHANGE OF SHORT‐CHAIN ALDEHYDES BETWEEN AMAZONIAN VEGETATION AND THE ATMOSPHERE

Rottenberger, S. / Kuhn, U. / Wolf, A. / Schebeske, G. / Oliva, S. T. / Tavares, T. M. / Kesselmeier, J.

As a part of the LBA‐EUSTACH (EUropean Studies on Trace Gases and Atmospheric CHemistry as a contribution of the Large‐scale Biosphere–Atmosphere experiment in Amazonia) project, the exchanges of formaldehyde (HCHO) and acetaldehyde (CH₃CHO) between Amazonian vegetation and the atmosphere were investigated by branch enclosures and compared with gradient measurements during the wet‐to‐dry transition and dry‐to‐wet‐transition periods at a remote forest site in Brazil, 1999. Branch enclosure measurements of several tree species showed emission as well as deposition of short‐chain aldehydes, but fluxes were clearly dominated by deposition during both seasons. This bidirectional exchange was found to depend mainly on the actual ambient concentrations of these compounds and to exhibit a compensation point below 0.6 ppb of the compound in air with deposition velocities between 0.16 and 0.21 cm/s during the wet‐to‐dry season. During the dry‐to‐wet season, the deposition velocities and the compensation point increased. Under the clean air conditions of the wet‐to‐dry season, the major pathway for the aldehyde uptake was via leaf stomata. For HCHO, a mesophyll resistance of the same order of magnitude as the stomatal resistance contributed to the total leaf surface resistance, whereas the mesophyll resistance for CH₃CHO was small, allowing a rapid uptake. This finding indicates a major contribution of metabolic consumption processes in addition to physical and chemical processes to the overall resistance. During the dry‐to‐wet period, when ambient air concentrations substantially increased, we found indications for an additional deposition to the leaf cuticle. Vertical gradient measurements of ambient air concentrations in and above the canopy closely agreed with the branch enclosure studies and confirmed that the forest acts rather as a sink than as a direct source for HCHO and CH₃CHO. Diel courses of ambient air concentrations and ratios of HCHO and CH₃CHO above the canopy suggest photochemical oxidation of biogenically or pyrogenically emitted precursor compounds as the major sources for short‐chain aldehydes in the tropical atmosphere.