Seasonal shifts in regional and local controls on ambient isoprene in a semi-arid African savanna

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Abstract A key challenge in semiarid savannas is determining whether ambient isoprene directly traces local biogenic emissions or represents an integrated signal shaped by chemistry and atmospheric dynamics. Here we use hourly measurements of isoprene, methyl vinyl ketone, and methacrolein from August to December 2024 at a savanna site near the Okavango Delta, Botswana, spanning dry to early wet seasons. During the early-wet seasons, daytime isoprene increased with vegetation growth and followed the expected daytime pattern of biogenic emission, suggesting a stronger local contribution. During the dry seasons, isoprene often increased at night, with patterns consistent with stronger atmospheric processing and shallow nighttime boundary-layer influence. Back-trajectory analyses suggested a preferred upwind sector, consistent with possible recent transport. These results suggest that seasonal state affects both isoprene abundance and its representativeness as a proxy for local biogenic emissions in semiarid savannas.

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