Abstract
The hydroxyl radical (OH) removes most atmospheric pollutants from air. The loss frequency of OH radicals due to the combined effect of all gas-phase OH reactive species is a measureable quantity termed total OH reactivity. Here we present total OH reactivity observations in pristine Amazon rainforest air, as a function of season, time-of-day and height (0–80 m). Total OH reactivity is low during wet (10 s−1) and high during dry season (62 s−1). Comparison to individually measured trace gases reveals strong variation in unaccounted for OH reactivity, from 5 to 15% missing in wet-season afternoons to mostly unknown (average 79%) during dry season. During dry-season afternoons isoprene, considered the dominant reagent with OH in rainforests, only accounts for ∼20% of the total OH reactivity. Vertical profiles of OH reactivity are shaped by biogenic emissions, photochemistry and turbulent mixing. The rainforest floor was identified as a significant but poorly characterized source of OH reactivity.
Highlights
The hydroxyl radical (OH) removes most atmospheric pollutants from air
Measurements were conducted in Central Amazonia within the framework of the Brazilian-German ATTO (Amazonian Tall Tower Observatory) project[25]
Total OH reactivity measurements were conducted by the comparative reactivity method (CRM)[27,28], which is based on the competitive reaction of OH with a non-atmospheric molecule and all atmospheric OH sink compounds
Summary
The hydroxyl radical (OH) removes most atmospheric pollutants from air. The loss frequency of OH radicals due to the combined effect of all gas-phase OH reactive species is a measureable quantity termed total OH reactivity. The missing reactivity has been shown to be dependent on the ecosystem, season, time of the day and environmental conditions These unknown and undetected atmospheric OH sinks are likely to be either forest emissions[20,22], the oxidation products of such[13,23], or a complex mixture of both[21,24]. High missing OH reactivity implies an incomplete understanding of the processes occurring at the forest canopy–atmosphere interface such as biogenic emissions, gas-to-particle partitioning of oxidation products, atmospheric ozone formation potential, trace gas deposition inside a forest canopy, canopy dynamics, and the photooxidation of global pollutants and greenhouse gases. We assess the composition and chemical nature of the total atmospheric OH reactivity through comparison with co-measured volatile organic compounds (VOC)
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