Seasonal variations of VOC emissions and concentrations in a mixed temperate forest consisting of beech and Douglas fir

Seasonal variations of VOC emissions and concentrations in a mixed temperate forest consisting of beech and Douglas fir

X. Shi¹, H. Li¹, Y. Li¹, M. Menon¹, A. Orphal¹, U. Ezenobi¹, T. Leisner¹, H. Saathoff¹

¹Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany

Biogenic volatile organic compounds (BVOCs) play a dominating role in the formation of secondary pollution due to their large emissions and high reactivity (Carslaw et al., 2010, Emanuelsson et al., 2013). Secondary organic aerosol (SOA) generated from oxidation of monoterpenes results in the formation of oxygenated volatile organic compounds (OVOC) with a wide range of volatility. Highly oxygenated organic molecules (HOMs) are a subset of OVOCs, which play an important role in new particle formation and the growth of newly formed particles to cloud condensation nuclei (CCN). Although forest vegetation is known to be a significant source of BVOCs, the role of soil and especially seasonal variations remains uncertain due to limited observations (Vermeul et al., 2023).

Therefore, we studied VOC levels and emissions in a healthy mixed temperate forest consisting of beech and Douglas fir on the slopes of the upper Rhine valley (476 m a.s.l.) in southwest Germany at two different seasons. The study in autumn lasted from September 6^th to October 16^th, 2024 and the study in summer from July 5^th to August 29^th 2025. VOC emissions were observed with a time resolution of 1.5h in autumn and 3 h in summer at 10 different individual beech leaves or bundles of fir needles within the canopy. Furthermore, VOC concentrations were measured at different heights between ground level and 46 m, which is about 18 m above the canopy top. The VOC were measured by proton-transfer-reaction mass spectrometry (PTR-MS 4000, IONICON) including also a fast GC to separate individual monoterpenes.

The primary objective of this experiment is to understand VOC emission seasonal patterns depending on tree species and environmental parameters. For example, we will show diurnal cycles of monoterpene emission at different seasons and for different tree types and positions. Overall, the monoterpene emission rates from Douglas fir needles were higher than those from beech leaves in summer but not in autumn. The monoterpene emission rates of Douglas fir in summer were much higher than those in autumn but this wasn’t the case for the beech. This indicates that the monoterpene emissions of Douglas fir show a higher temperature dependence.

In this contribution we will discuss the influence of tree type and environmental parameters on VOC emissions at individual leaves and the resulting vertical gradients.

This work was supported by the China Scholarship Council.

Carslaw et al., Atmos. Chem. Phys.,10(4): 1701-1737, 2010

Emanuelsson et al., Atmos. Chem. Phys., 13(5): 2837-2855, 2013

Vermeuel et al., Atmos. Chem. Phys., 23, 4123–4148, 2023