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Within the project EUropean Studies on Trace gases and Atmospheric CHemistry as acontribution to Large-scale Biosphere-atmosphere experiment in Amazonia(LBA-EUSTACH), we performed tower-based eddy covariance measurements ofO3 flux above an Amazonian primary rain forest at the end of the wetand dry season. Ozone deposition revealed distinct seasonal differences inthe magnitude and diel variation. In the wet season, the rain forest was aneffective O3 sink with a mean daytime (midday) maximum depositionvelocity of 2.3 cm s−1, and a corresponding O3 flux of−11 nmol m−2 s−1. At the end of the dry season, the ozonemixing ratio was about four times higher (up to maximum values of 80 ppb)than in the wet season, as a consequence of strong regional biomass burningactivity. However, the typical maximum daytime deposition flux was verysimilar to the wet season. This results from a strong limitation of daytimeO3 deposition due to reduced plant stomatal aperture as a response tolarge values of the specific humidity deficit. As a result, the averagemidday deposition velocity in the dry burning season was only 0.5 cm s−1. The large diel ozone variation caused large canopy storageeffects that masked the true diel variation of ozone deposition mechanismsin the measured eddy covariance flux, and for which corrections had to bemade. In general, stomatal aperture was sufficient to explain the largestpart of daytime ozone deposition. However, during nighttime, chemicalreaction with nitrogen monoxide (NO) was found to contribute substantiallyto the O3 sink in the rain forest canopy. Further contributions werefrom non-stomatal plant uptake and other processes that could not be clearlyidentified. Measurements, made simultaneously on a 22 years old cattle pasture enabledthe spatially and temporally direct comparison of O3 dry depositionvalues from this site with typical vegetation cover of deforested land insouthwest Amazonia to the results from the primary rain forest. The meanozone deposition to the pasture was found to be systematically lower thanthat to the forest by 30% in the wet and 18% in the dry season

Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia