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Abstract

Speciated monoterpene measurements in the Amazon rainforest air are scarce, but important in order to understand their contribution to the overall reactivity of volatile organic compound (VOCs) emissions towards the main atmospheric oxidants, such as hydroxyl radical (OH), ozone (O3) and nitrate radical (NO3). In this study, we present the chemical speciation of gas phase monoterpenes measured in the tropical rainforest at the Amazon Tall Tower Observatory (ATTO, Amazonas, Brazil). Samples of VOCs were collected by two automatic sampling systems positioned on a tower at 12 and 24 m height and analysed using Gas Chromatography Flame Ionization Detection (GC-FID). The samples were collected in October 2015, representing the dry season, and compared with previous wet and dry season studies at the site. In addition, vertical profile measurements (at 12 and 24 m) of total monoterpene mixing ratios were made using Proton-Transfer Reaction Mass Spectrometry (PTR-MS). The results showed a distinctly different chemical speciation between day and night. For instance, α-pinene was more abundant during the day, whereas limonene was more abundant at night. Reactivity calculations showed that the most abundant compounds may not be the most atmospheric chemically relevant compounds. Furthermore, inter- and intra-annual results demonstrate similar chemodiversity during the dry seasons analysed. Simulations with a canopy exchange modelling system compare relatively well with the observed temporal variability in speciated monoterpene mixing ratios, but also indicate the necessity of more experiments to enhance our understanding of in-canopy sinks of these monoterpenes.