Vegetation Phenology Contributes to Seasonal Patterns of Isoprene above a Rain Forest in Central Amazonia

The Amazon rainforest is a major global source of biogenic volatile organic compounds (BVOCs), among which isoprene is most abundantly emitted. Isoprene largely influences the atmospheric oxidative capacity due to its high reactivity with hydroxyl radicals (OH). Additionally, the resulting oxidation products serve as a significant source of secondary organic aerosols (SOA) in the clean rainforest, which may contribute to the cloud formation and consequently influence regional weather and climate. Hence, it is necessary to investigate long-term isoprene budgets in the tropical rainforest to establish regional OH and SOA budgets. However, little is known about its seasonal concentration patterns. In this study we aim to (i) investigate the degree of seasonality in isoprene concentration using field experiments and (ii) identify the main processes explaining such seasonality via a zeroth dimensional model that includes sources and sinks of isoprene as well as atmospheric boundary layer dynamics. Field measurements showed that daytime median isoprene mixing ratios varied throughout the year by a factor of two. This pattern was not solely driven by light and temperature, which showed minor seasonal variation. Instead, leaf age and quantity consistently tracked the seasonal variations of isoprene concentrations, suggesting leaf phenology was a crucial variable needed to correctly estimate isoprene emissions. The zero-dimensional model shows that deposition was the major sink of isoprene during the nighttime. During the daytime chemical reactions destroyed 56, 77, 69, and 69 % of the emitted isoprene in June, September, December, and January, respectively. Entrainment fluxes from the residual layer contributed 34 % to the early-morning isoprene mixing ratios. Sensitivity analysis showed that hydroxyl radical (HO) recycling and segregation of isoprene-HO played relatively lesser roles in regulating ambient isoprene levels. Nitric oxide (NO) levels dominated isoprene chemical reaction pathways associated with consumption and production of HO under low-NO and high VOC conditions.