Seasonal Patterns of Biogenic Volatile Organic Compounds above a Central Amazonian Rainforest During the Wet to Dry Season Transition

The Amazon rainforest influences precipitation processes by emitting biogenic volatile organic compounds (BVOCs) that contribute to the formation secondary organic aerosols and cloud condensation nuclei. Differences in the timing and amount of precipitation between the wet and dry season in the Amazon can be pronounced, and the role of BVOCs in seasonal precipitation dynamics is uncertain. To improve our understanding of surface conditions that influence precipitation events, we quantified BVOC concentrations using proton transfer reaction-mass spectrometry (PTR-MS) during the wet-to-dry season transition in a central Amazonian rainforest as part of the GoAmazon initiative. Of the compounds identified by PTR-MS, methanol made up over half of the average atmospheric composition above the forest canopy, followed by isoprene, acetone, and acetaldehyde (11-13%) and monoterpenes, methyl vinyl ketone, and methyl ethyl ketone (3-5%). Ambient levels of BVOCs exhibited characteristic diurnal patterns that were more pronounced during the wet season, when maximum mid-day isoprene and monoterpene mixing ratios averaged 6 and 1 ppbv (parts per billion volume), respectively. Precipitation events strongly influenced emissions of BVOCs and contributed to ozone enhancements of up to 25 ppbv from downdrafts that preceded mesoscale convective storms. Results demonstrate the potential for stronger positive surface-atmosphere feedbacks during the wet season that may play an important role in the propagation of precipitation events across the Amazon Basin.