Formaldehyde (HCHO) Budget in a Pristine Forest in Northern Michigan

Abstract

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 (Quercus rubra) and Leyland cypress (Cupressus x 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.