PTR-TOF-MS eddy covariance measurements of isoprene and monoterpene fluxes from an Eastern Amazonian rainforest

Abstract. Biogenic volatile organic compounds (BVOCs) are important components of the atmosphere due to their contribution to atmospheric chemistry and biogeochemical cycles. Tropical forests are the largest source of the dominant BVOC emissions (e.g. isoprene and monoterpenes). In this study, we report isoprene and total monoterpene flux measurements with a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) using the eddy covariance (EC) method at the Tapajos National Forest (−2.857°S, −54.959°W), a primary rainforest in eastern Amazonia. Measurements were carried out from 1–16 June 2014, during the wet to dry transition season. During the measurement period, the measured daytime (06:00–18:00 LT) average isoprene mixing ratios and fluxes were 1.15 ± 0.60 ppb and 0.55 ± 0.71 mg C m−2 h−1, respectively, whereas the measured daytime average total monoterpenes mixing ratios and fluxes were 0.14 ± 0.10 ppb and 0.20 ± 0.25 mg C m−2 h−1, respectively. Midday (10:00–14:00 LT) average isoprene and total monoterpenes mixing ratios were 1.70 ± 0.49 ppb and 0.24 ± 0.05 ppb, respectively whereas midday average isoprene and monoterpene fluxes were 1.24 ± 0.68 mg C m−2 h−1 and 0.46 ± 0.22 mg C m−2 h−1, respectively. Isoprene and total monoterpene emissions in Tapajos were correlated with ambient temperature and solar radiation. Significant correlation with sensible heat flux, SHF (r2 = 0.77), was also observed. Measured isoprene and monoterpene fluxes were strongly correlated with each other (r2 = 0.93). The MEGAN2.1 model could simulate most of the observed diurnal variations (r2 = 0.7 to 0.8) but declined a little later in the evening for both isoprene and total monoterpene fluxes. The results also demonstrate the importance of site-specific vegetation emission factors (EFs) for accurately simulating BVOC fluxes in regional and global BVOC emission models.