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. The tropical forests are Earth's largest source ofbiogenic volatile organic compounds (BVOCs) and thus also the largestatmospheric sink region for the hydroxyl radical (OH). However, the OH sinkabove tropical forests is poorly understood, as past studies have revealed largeunattributed fractions of total OH reactivity. We present the first total OHreactivity and volatile organic compound (VOC) measurements made at the Amazon Tall Tower Observatory(ATTO) at 80, 150, and 320 m above ground level, covering two dry seasons,one wet season, and one transition season in 2018–2019. By considering a wide rangeof previously unaccounted for VOCs, which we identified by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS), theunattributed fraction was with an overall average of 19 % within themeasurement uncertainty of ∼ 35 %. In terms of seasonalaverage OH reactivity, isoprene accounted for 23 %–43 % of the total and oxygenated VOCs (OVOCs) for 22 %–40 %, while monoterpenes, sesquiterpenes,and green leaf volatiles combined were responsible for 9 %–14 %. Thesefindings show that OVOCs were until now an underestimated contributor to theOH sink above the Amazon forest. By day, total OH reactivity decreased towards higher altitudes withstrongest vertical gradients observed around noon during the dry season(−0.026 s−1 m−1), while the gradient was inverted at night.Seasonal differences in total OH reactivity were observed, with the lowestdaytime average and standard deviation of 19.9 ± 6.2 s−1 duringa wet–dry transition season with frequent precipitation; 23.7 ± 6.5 s−1 during the wet season; and the highest average OH reactivitiesduring two dry-season observation periods with 28.1 ± 7.9 s−1and 29.1 ± 10.8 s−1, respectively. The effects of differentenvironmental parameters on the OH sink were investigated, and quantified,where possible. Precipitation caused short-term spikes in total OHreactivity, which were followed by below-normal OH reactivity for severalhours. Biomass burning increased total OH reactivity by 2.7 to9.5 s−1. We present a temperature-dependent parameterization of OHreactivity that could be applied in future models of the OH sink to furtherreduce our knowledge gaps in tropical-forest OH chemistry.

Total OH reactivity over the Amazon rainforest: variability with temperature, wind, rain, altitude, time of day, season, and an overall budget closure