Can Isoprene Oxidation Explain High Concentrations of Atmospheric Formic and Acetic Acid over Forests?

Abstract

Formic and acetic acid concentrations are particularly high over forested areas of the world. However, the gas-phase mechanisms for producing these acids are poorly understood even for isoprene, the globally dominant biogenic hydrocarbon. We quantified formic and acetic acid production from reactions of hydroxyl radical (OH) (between high and low ranges of nitric oxide (NO) levels) with isoprene, methacrolein (MACR), isoprene epoxydiol (IEPOX), isoprene hydroxy hydroperoxide (ISOPOOH), and α-pinene from the focused isoprene experiments at California Institute of Technology (FIXCIT) laboratory chamber study. We find that (i) OH oxidation of MACR, IEPOX, and ISOPOOH are sources of formic acid, (ii) isoprene peroxy radical isomerization and associated photolysis oxidation products are potentially important sources of organic acids, and (iii) high levels of NO generally suppress organic acid formation from OH oxidation of isoprene. We modified existing chemical mechanisms for isoprene oxidation to account for organic acid production pathways observed in the FIXCIT study. We simulated organic acid production during the Southeastern Oxidant and Aerosol Study using the updated chemical mechanisms and represented acetic acid within a factor of 2 but still underpredicted formic acid by a factor of 6. While we cannot explain ambient formic acid with explicit chemical mechanisms, the FIXCIT results suggest that the oxidation of isoprene could account for as much as 70% of the global annual production of formic acid from gas-phase reactions.

Simultaneous Measurements of O3 and HCOOH Vertical Fluxes Indicate Rapid In‐Canopy Terpene Chemistry Enhances O3 Removal Over Mixed Temperate Forests

“…MT or NO), k oxidation is the bimolecular rate constant for the oxidation reaction taken from the Master Chemical Mechanism v 3.3.1 or literature (Calogirou et al., 1999; Saunders et al., 2003), C oxidant is the oxidant concentration, either measured [O 3 ] or an estimated [OH] as taken from base GEOS‐Chem (v9.2) runs of a forested site (Nguyen et al., 2015), is the residence time of an air parcel within the forest canopy, approximated at 5 min, and Y is the molar yield as estimated from literature (). In the case of HCOOH, product yields are estimated as: Y MT+OH = 0.08, Y iso+OH = 0.08, Y SQT+OH = 0.06 (Atkinson & Arey, 2003; Lee et al., 2006b; Link et al., 2020). Although the used is an estimate, it is based on calculations of for parcels released at less than 0.5X the canopy height for mixed forests of comparable canopy height and LAI (Fuentes et al., 2007; Strong et al., 2004).…”

“…One of those oxidation products, formic acid (HCOOH), is an important regulator of rainwater pH and gas‐particle partitioning (Millet, 2012; Metzger et al., 2006). It is found in high concentrations in forests due in part to large product yields in MT, SQT, and isoprene ozonolysis ( Y HCOOH,MT = 0.04–0.20, Y HCOOH,SQT = 0.04, Y HCOOH, isoprene = 0.14) (Atkinson & Arey, 2003; Lee, et al., 2006b, 2006a; Link et al., 2020). The relatively short lifetimes of MT and SQT against O 3 ( = 13 min–1.1 day, = 2 min–14 h under 30 ppbv O 3 ) (Atkinson & Arey, 2003) and the high concentrations of isoprene in northern temperate mixed forests (>3 ppbv) (Alwe et al., 2019; J. D. Fuentes & Wang, 1999) suggest that HCOOH is a tracer of in‐canopy BVOC + O 3 chemistry in these regions.…”

“…One of those oxidation products, formic acid (HCOOH), is an important regulator of rainwater pH and gas-particle partitioning (Millet, 2012;Metzger et al, 2006). It is found in high concentrations in forests due in part to large product yields in MT, SQT, and isoprene ozonolysis (Y HCOOH,MT = 0.04-0.20, Y HCOOH,SQT = 0.04, Y HCOOH, isoprene = 0.14) (Atkinson & Arey, 2003;Lee, et al, 2006bLee, et al, , 2006aLink et al, 2020). The relatively short VERMEUEL ET AL.…”

Volatile organic compound fluxes over a winter wheat field by PTR-Qi-TOF-MS and eddy covariance

“…Value under brackets are standard deviations 685 m/z 61.028 (tentatively attributed to acetic acid) was the third most depositing compound. Acetic acid is formed in the atmosphere by oxidation of isoprene, and its oxidation products, with O3 and OH at low NO (Link et al, 2020). However acetic acid can be directly emitted from soils (Mielnik et al, 2018), and from plants especially under stress conditions and during senescence (Portillo-Estrada et al, 2020).…”

“…This suggests a possible link with the senescence of the plant, which started on week 3 (Figure 5 m/z 47.013 (formic acid, CH2O2) was indeed also depositing substantially and had high deposition velocities. Link et al (2020) showed that it is formed from the oxidation of isoprene degradation products (MACR, isoprene epoxydiol, IEPOX, isoprene hydroxy hydroperoxide, ISOPOOH) with the OH radical. The daily pattern of Vdep 730 showed clear deposition during the day, and the smallest deposition rate at the end of the night.…”