Understanding an Observed Enhancement of Formaldehyde within Forest Canopies: Implications for Formaldehyde as an Oxidative Tracer of Gas-Phase Chemistry

Formaldehyde (HCHO) is a common tracer of gas-phase oxidation chemistry in the atmosphere since it is generally formed when volatile organic compounds (VOCs) are oxidized by OH, O₃, and NO₃. In fact, HCHO has been used to constrain anthropogenic and biogenic emissions in atmospheric models, and it tests our understanding of VOC oxidation chemistry in chemical mechanisms. Thus, it is imperative to have proper source attribution (i.e., whether HCHO is from direct emission or gas-phase oxidation chemistry) when using HCHO as a constraint in models.

During the PROPHET-AMOS 2016 field campaign (located in a mixed hardwood/boreal forest near Pellston, MI), distinct vertical gradients of HCHO were measured by Harvard and NASA laser-induced fluorescence instruments at several heights (5, 17, 21, and 30 m) above and below the forest canopy. Overnight HCHO mixing ratios above the canopy were generally higher than HCHO within the trunk space of the canopy, which suggests a dominant ground deposition HCHO sink at night. This is consistent with previous measurements at the PROPHET site and at Ponderosa Pine forests in Colorado and California. In addition, a mid-day enhancement of HCHO mixing ratios ( 0.5 ppbv) was consistently measured within the crown space of the forest canopy (17 and 21 m) during the days that data was collected.

A 1-D model for simulating the exchange of VOCs and their oxidation products in forest canopies (FORCAsT) was utilized to help in understanding the observed crown space enhancement of HCHO. In addition to accounting for the chemistry, emission, deposition, and transport of VOCs and OVOCs, FORCAsT also has parameters for plant photosynthesis and respiration. Gas-phase chemistry alone has not been able to reproduce the enhanced levels of HCHO, even with the implementation of two different chemical mechanisms (CACM and RACM-MIM) in FORCAsT. This has direct implications for HCHO source attribution since it suggests that other sources (such as unknown gas-phase chemistry not implemented in the chemical mechanisms or direct emission via a stomatal-dependent mechanism, etc.) are responsible for the elevated HCHO levels.