Broadly speaking, the oxidation of volatile organic compounds (VOCs) in the atmosphere aids in
the formation of (1) ozone and (2) secondary organic aerosol. Both of these products have air quality
as well as health and climate impacts. Additionally, studying the oxidation of VOCs under pristine
conditions, such as those found in rural forests, is crucial for our understanding of atmospheric
chemistry under pre-industrial conditions. Given that many climate models use the pre-industrial
era as a baseline for their simulations, it is imperative to understand atmospheric chemistry with
little anthropogenic influence.
Since formaldehyde (HCHO) is a ubiquitous oxidation product of VOCs, it is commonly used as
a constraint in models to test our latest understanding of oxidation chemistry in the atmosphere.
However, using HCHO as a tracer of oxidation chemistry, such as in a pristine forest, requires
knowing the magnitude of its sources and sinks. This work analyzes the source and sink budget
of HCHO in a pristine forest located near Pellston, Michigan, during the PROPHET-AMOS field
campaign that took place in July 2016.
The HCHO budget is approached in three ways. First, vertically-resolved gradients of HCHO
were measured at the field site using an upgraded laser-induced fluorescence (LIF) instrument for
measuring HCHO. Second, leaf cuvette measurements conducted in the laboratory on Northern
red oak (Quercu rubra) and Leyland cypress (Cupressu × leylandii) saplings were performed to
understand the bidirectional exchange of HCHO with foliage as well as quantify the HCHO yield
from the heterogeneous conversion of isoprene hydroxy hydroperoxides (e.g., 1,2-ISOPOOH) depositing on leaves. Finally, a 1-D forest canopy model (FORCAsT) was updated with laboratory
measurements and a modified version of the Reduced Caltech Isoprene Mechanism (RCIM) to simulate the HCHO budget on a photochemically-active day in the forest. Discrepancies between modeled and measured HCHO thus help to show gaps in our understanding of oxidative and transport
processes within pristine forested environments.