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

Abstract

Monoterpenes and isoprene emitted from forest ecosystems contribute to the formation of secondary organic aerosols SOAs and photochemical oxidants O x 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 O x , 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 O 3 concentration and isoprene flux around noon or during the afternoon, suggesting that they are produced more frequently by reaction with reactive species including O 3 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, O 3 concentration, temperature, and its deposition rate, which is controlled by its concentration and micrometeorological factors.

Review: Exchanges of O₃, NO, and NO₂ between forest ecosystems and the atmosphere

“…The REA method involves taking an air sample and separating it into two bags or tubes based on the direction of the vertical wind and then measuring the average concentration difference between the upward and downward vertical winds ( Mochizuki et al, 2020b;Tani and Mochizuki, 2021). The REA method is more practical for measuring trace gas concentrations than the eddy covariance method, and the fluxes obtained are more accurate than those obtained using the gradient method.…”

“…Analyzing observed and simulated O 3 , NO, and NO 2 fluxes in a forest is useful in understanding the deposition and emission mechanisms of O 3 , NO, and NO 2 . Extensive modeling studies have been conducted to assess O 3 fluxes and deposition velocities ( Meyers et al, 1988Meyers et al, , 1998Wesely ML, 1989;Padro et al, 1992;Finkelstein et al, 2000;Stroud et al, 2005;Katata et al, 2011;Wu et al, 2011;Rannik et al, 2012;Launiainen et al, 2013;Zhou et al, 2017;Wada et al, 2020b). Flux models primarily comprise single layer resistance models and multi-layer canopy models (e.g., Meyers et al, 1988).…”

“…Model studies on NO and NO 2 fluxes in forests have also been reported (Wesely, 1989;Gao et al, 1991Gao et al, , 1993Wesely and Hicks, 2000;Zhang et al, 2002Zhang et al, , 2003Wu et al, 2011;Saylor, 2013;Seok et. al., 2013;Zhang et al, 2017;Wada et al, 2020b). Gao et al (1991) calculated NO and NO 2 fluxes using one-dimensional models coupled with second-order equations considering the chemical reaction of O 3 within the atmospheric surface layer.…”

“…The deposition velocities of NO 2 near noon (11 00 -13 00) were estimated to be approximately 0.6 -0.7 cm s -1 , and the deposition velocity of NO was approximately zero. Wada et al (2020b) applied inverse multi-layer models ( Ueyama et al, 2014) to estimate the NO flux above the canopy and the vertical sink and source distribution within the canopy. The model estimated that there was a strong NO sink in the trunk space, where loss by reaction with O 3 is expected.…”

“…estimated O 3 flux and concentration profiles using a multi-layer model that incorporated chemical reactions. They calculated the contribution of O 3 loss and production by chemical reactions in a forest canopy, particularly with BVOCs, and showed that chemistry generally plays a minor role in O 3 uptake inside the canopy Wada et al (2020b). applied inverse multi-layer models (Ueyama et al, 2014) to estimate the O 3 flux above the canopy and the vertical sink and source distribution within the canopy using the concentration profile of the forest and micrometeorological data.…”