The absolute total diel OH reactivity, however, did not change significantly. Interestingly, the diel maximum of OH reactivity during the El Niño event occurred at sunset instead of, under normal conditions, early afternoon. Here we present a comparison of the OH reactivity diel cycle from November 2015, i.e., extreme drought and elevated temperatures associated with strong El Niño conditions, with November 2012, a “normal” El Niño Southern Oscillation (ENSO)-neutral period. Total OH reactivity is a directly measureable quantity that gives the reaction frequency of OH radicals with all reactive species in the atmosphere in s−1. The diverse VOCs emitted can be significant for plants’ carbon budgets, influence ozone and particle production, and through their reactivity impact OH concentrations. How tropical forests react to such extreme events in terms of volatile organic compound (VOC) emissions is of interest as the frequency of these events is predicted to increase through climate change. The 2015/16 El Niño event caused unprecedented drought and warming in the Amazon basin. We present a temperature-dependent parameterization of OH reactivity that could be applied in future models of the OH sink to further reduce our knowledge gaps in tropical forest OH chemistry. Biomass burning increased total OH reactivity by 2.7 s-1 to 9.5 s-1. Precipitation caused short-term spikes in total OH reactivity, 30 which were followed by below-normal OH reactivity for several hours. The effects of different environmental parameters on the OH sink were investigated, and quantified, where possible. Seasonal differences in total OH reactivity were observed, with the lowest daytime average and standard deviation of 19.9 ± 6.2 s-1 during a wet-dry transition season with frequent precipitation, 23.7 ± 6.5 s-1 during the wet season, and the highest average OH reactivities during two dry season observation periods with 28.1 ± 7.9 s-1 and 29.1 ± 10.8 s-1, respectively. By day, total OH reactivity decreased towards higher altitudes with strongest vertical gradients observed around noon during 25 the dry season (-0.026 s-1 m-1), while the gradient was inverted at night. These findings show that OVOCs were until now an underestimated contributor to the OH sink above the Amazon forest. In terms of seasonal average OH reactivity, isoprene accounted for 23-43 % the total, oxygenated VOCs (OVOCs) for 22-40 %, while monoterpenes, sesquiterpenes, and green leaf volatiles combined were responsible for 9-14 %. By considering a wide range of previously unaccounted for VOCs, which we identified by PTR-ToF-MS, the unattributed fraction was with an overall average of 19 % 20 within the measurement uncertainty of ~35 %. We present the first total OH reactivity and 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 and one transition season in 2018-2019. However, the OH sink above tropical forests is poorly understood, as past studies revealed large unattributed fractions of total OH reactivity. The tropical forests are Earth's largest source of biogenic volatile organic compounds (BVOCs) and thus also the 15 largest atmospheric sink region for the hydroxyl radical (OH). Até agora não foram feitas medidas de perfilamento até estas alturas na floresta Amazônica. Isso possibilitará encontrar novos resultados relacionados a processos de transporte dentro e acima do dossel. Nessa pesquisa estão sendo monitoradas as concentrações de 5 gases traços (H 2 O, CO 2, O 3, NO, NO 2), em 8 níveis diferentes, entre 0,05 e 79,3 metros. RESUMO Em 2011, a torre mais alta de toda Amazônia foi erguida no sitio ATTO (Amazon Tall Tower Observatory) (02☀8'38,8''S, 58★9'59,5''W), com a finalidade de se fazer medidas atmosféricas. Never before there have been made profile measurements up to that height in the Amazonian rainforest. In 2011 the currently highest atmospheric research tower of Amazonia was erected at the ATTO site (02☀8'38,8''S, 58★9'59,5''W) (whereas ATTO stands for Amazon Tall Tower Observatory), which is monitoring concentration gradients regarding 5 trace gases (H 2 O, CO 2, O 3, NO, NO 2) from 8 different heights between 0,05 m and 79,3 m, which enables the possibility to get new results regarding transport processes in and above the canopy.
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