Flux estimates of isoprene, methanol and acetone from airborne PTR-MS measurements over the tropical rainforest during the GABRIEL 2005 campaign

Abstract

Abstract. Tropical forests are a strong source of biogenic volatile organic compounds (BVOCs) to the atmosphere and such emissions can impact the atmospheric oxidation capacity. Here we present airborne and ground-based BVOC measurements performed during the long dry season in October 2005 during the GABRIEL (Guyanas Atmosphere-Biosphere exchange and Radicals Intensive Experiment with the Learjet) project, which covered a large area of the northern Amazonian rainforest (6–3° N, 50–59° W). The vertical (35 m to 10 km) and diurnal (09:00–16:00) profiles of selected BVOCs like isoprene, its oxidation products methacrolein and methyl vinyl ketone, methanol and acetone, measured by PTRMS (Proton Transfer Reaction Mass Spectrometry), have been used to empirically estimate their emission fluxes from the forest canopy on a regional scale. The mixed layer isoprene emission flux, inferred from the airborne measurements above 300 m, is 4.1 mg isoprene m−2 h−1 whereas the surface flux is 7.3 mg isoprene m−2 h−1 after compensating for chemistry. This surface flux is in general agreement with previous tropical forest studies. Mixed layer fluxes of 0.8 mg methanol m−2 h−1 and 0.35 mg acetone m−2 h−1 were found. The BVOC measurements were compared with fluxes and mixing ratios simulated with a single-column model (SCM). The isoprene flux inferred from the measurements is substantially smaller than that simulated with an SCM implementation of MEGAN (Model of the Exchange of Gases between the Atmosphere and Nature) though consistent with global emission estimates. The exchanges of methanol and acetone can be reasonably well described using a compensation point approach.