Eddy covariance (EC) fluxes of monoterpene oxidation products from a boreal forest canopy

Oxidation of volatile organic compounds (VOC) by ozone (O₃), hydroxyl radicals (OH) and nitrogen oxide radicals (NO₃, NOx) reduces their volatility and leads to the formation of secondary organic aerosols (SOA) through gas-particle partitioning. Recent studies have shown that monoterpene (C₁₀H₁₆) oxidation products can participate in all stages of aerosol formation, especially in forested boreal environments. However, deposition of these semi-volatile and (extremely) low-volatility organic compounds (SVOC, LVOC, ELVOC) to surfaces in the canopy directly competes with the gas-particle partitioning and has a substantial effect (~50%) on organic aerosol loading. Hence understanding the fate of these oxidation products is crucial in determining the organic aerosol budget and thereby constraining their contribution to climate-relevant processes such as new-particle formation and cloud formation.

Oxidation products of monoterpenes were measured at the station for measuring ecosystem atmosphere relations (SMEAR II), a boreal forest research station in Hyytiälä, Finland, in spring/summer 2019. The forest is dominated by Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst) which are well known high monoterpene emitters. Eddy covariance (EC) flux measurements of oxygenated organic compounds in the gas phase were performed using an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (I-CIMS) with high frequency (5 Hz) co-located with a sonic anemometer (METEK USA-1) on a tower, 35 m above the forest floor. The ion-molecule reaction (IMR) chamber of I-CIMS was actively humidified to mitigate the dependence of the sensitivity of the measurements on the ambient relative humidity. The EC data were analysed following standard correction procedures like lag correction, coordinate rotation and uncertainty analysis. VOCs and oxygenated VOCs were also measured at ground level using a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-MS), which is sensitive also to the majority of compounds measured by I-CIMS.

We present the first continuous I-CIMS dataset at high time resolution (5 Hz) from a tall tower and calculate the Eddy covariance fluxes of a wide range of monoterpene oxidation products during the primary plant-growth season in a boreal forest. Bidirectional fluxes for formic acid (HCOOH) were observed at a higher temporal resolution than reported in earlier studies. We found an increasing trend in the deposition velocity for heavier monoterpene oxidation products which enables us to constrain the net flow of organics between the atmosphere and the canopy layer using the continuity/mass balance equation. When coupled to ground-based measurements using Vocus-PTR, our EC flux measurements will give further insight about the abundance of organics above the canopy vs near ground-level. We also plan to integrate our observations with a chemical transport model containing details of monoterpene oxidation chemistry (ADCHEM) to simulate the sources and sinks and to derive parameterizations for representing the dry deposition rates of monoterpene oxidation products in the boreal forested environments.