Diurnal cycles and seasonal variation of isoprene and its oxidation products in the tropical savanna atmosphere

Using the proton transfer reaction mass spectrometry (PTR-MS) technique, isoprene and its oxidation products were measured in a productive woodland savanna (Calabozo site, during the wet and dry seasons) and a less productive grassland savanna (La Gran Sabana, Parupa site). The measured protonated masses in the PTR-MS, postulated compounds, and daytime average volume mixing ratios at the Calabozo site during the wet season are: 69 isoprene (1.62 nmol/mol), 71 methyl vinyl ketone + methacrolein (0.98 nmol/mol), 83 3-methyl furan + unsaturated C-5-hydroxycarbonyls (0.12 nmol/mol), and 101 isoprene hydroperoxides (0.16 nmol/mol). Significant diurnal cycles of the hydrocarbon concentrations were observed, with distinct characteristics between sites and seasons. Two times lower levels of isoprene were observed during the dry season. At the Parupa site measured concentrations of all masses were about three times lower than at the Calabozo site during the wet season, and significant transport of isoprene from upwind forests was observed. Comparison with a photochemical box model revealed that surface deposition is likely a significant sink for isoprene and its oxidation products. An isoprene source of 2.1-3.2 x 10(6) molec/cm(3)/s and an HO concentration of 4.1- 6.0 x 10(5) molec/cm(3) averaged over 24 hours were needed to match the observed mixing ratios. Assuming a mixed boundary layer of 1500 m and an isoprene source half the strength during the 5 months dry season, a global emission of isoprene to the atmosphere from tropical savannas between 53 and 79 Tg C/yr can be calculated from our results if the Calabozo site is representative