Canopy fluxes of monoterpene, isoprene and isoprene oxidation products in a pine-oak forest

Monoterpenes and isoprene emitted from forest ecosystems contribute to the formation of secondary organic aerosols (SOAs) and photochemical oxidants (Ox) and affect an ecosystem’s carbon budget. Initial oxidation products of isoprene, methacrolein (MACR) and methyl vinyl ketone (MVK), are key intermediate compounds for the formation of SOAs and Ox, but the production and loss processes of MACR and MVK and its controlling factors within a forest have not been revealed. To address them within a forest and the behavior of related compounds, we measured vertical concentrations and fluxes of monoterpenes, isoprene, and MACR+MVK in a pine-oak forest during summer. Monoterpene concentrations were the highest near the forest floor. A higher isoprene concentration was observed at the height of the Quercus trees. High positive fluxes of monoterpenes and isoprene were observed during the day. The average flux of isoprene during the measurement period was 2.6 times higher than that of monoterpene. Quercus in the lower layer of the forest can be an important source of isoprene, even though the light intensity was estimated much lower than that of red pine canopy. The MACR+MVK concentrations did not show clear vertical gradient patterns. Both positive and negative MACR+MVK fluxes were observed and large positive MACR+MVK fluxes were occasionally observed under a relatively high O3 concentration and isoprene flux around noon or during the afternoon, suggesting that they are produced more frequently by reaction with reactive species including O3 at a higher temperature. Our results demonstrate that, to investigate sink and source dynamics of MACR+MVK above a forest, it is necessary to separately estimate production rate of MACR+MVK, which depends on isoprene emission from the target and surrounding forests, O3 concentration, temperature, and its deposition rate, which is controlled by its concentration and micrometeorological factors.