J. Geophys. Res.

We measured atmospheric gas-phase volatile organic compounds (VOCs) at the end of the wet and end of the dry season at a tropical rainforest site in Rondonia, Brazil, using various sampling techniques such as trapping on different adsorbents or cryogenic trapping combined with appropriate analysis techniques. The measuring sites were located inside the forest of a biological reserve near Ji-Parana. Sampling was performed from 3 May 1999 to 17 May 1999 and from 24 September 1999 to 2 November 1999 during the "wet-to-dry season transition'' and "dry-to-wet season transition'' periods in Rondonia, respectively. Samples were obtained at the canopy top close to the potential sources/sinks for these compounds as well as above the forest. We report the measured concentrations of a large number of different VOCs and their oxidation products, such as isoprenoids, organic acids, carbonyls, aromatics, and alcohols. The most prominent VOCs present in air over the last part of the wet season were isoprene, formaldehyde, and formic acid, with mixing ratios of each ranging up to several parts per billion (ppb). Methyl vinyl ketone as well as methacrolein, both oxidation products of isoprene, ranged around 1 ppb. The sum of the measured monoterpene concentrations was below 1 ppb. At the end of the dry season, the amount of C-1-C-2 organic acids and C-1-C-2 aldehydes increased significantly up to 17 and 25 ppb, respectively, which is thought to result significantly from vegetation fire emissions. High methanol concentrations also support this scenario. At the same time, however, atmospheric mixing ratios of biogenic compounds such as isoprene increased up to 30 ppb near the crown region and well above 10 ppb at 10-20 m over the forest, whereas monoterpene species seem to decrease. We discuss seasonal development of the vegetation and climatological factors to be responsible for such concentration pattern. The results give an impression about the variability and concentration of VOCs during the different seasons