A large and ubiquitous source of atmospheric formic acid

Abstract

Abstract. Formic acid (HCOOH) is one of the most abundant acids in the atmosphere, with an important influence on precipitation chemistry and acidity. Here we employ a chemical transport model (GEOS-Chem CTM) to interpret recent airborne and ground-based measurements over the US Southeast in terms of the constraints they provide on HCOOH sources and sinks. Summertime boundary layer concentrations average several parts-per-billion, 2–3× larger than can be explained based on known production and loss pathways. This indicates one or more large missing HCOOH sources, and suggests either a key gap in current understanding of hydrocarbon oxidation or a large, unidentified, direct flux of HCOOH. Model-measurement comparisons implicate biogenic sources (e.g., isoprene oxidation) as the predominant HCOOH source. Resolving the unexplained boundary layer concentrations based (i) solely on isoprene oxidation would require a 3× increase in the model HCOOH yield, or (ii) solely on direct HCOOH emissions would require approximately a 25× increase in its biogenic flux. However, neither of these can explain the high HCOOH amounts seen in anthropogenic air masses and in the free troposphere. The overall indication is of a large biogenic source combined with ubiquitous chemical production of HCOOH across a range of precursors. Laboratory work is needed to better quantify the rates and mechanisms of carboxylic acid production from isoprene and other prevalent organics. Stabilized Criegee intermediates (SCIs) provide a large model source of HCOOH, while acetaldehyde tautomerization accounts for ~ 15% of the simulated global burden. Because carboxylic acids also react with SCIs and catalyze the reverse tautomerization reaction, HCOOH buffers against its own production by both of these pathways. Based on recent laboratory results, reaction between CH3O2 and OH could provide a major source of atmospheric HCOOH; however, including this chemistry degrades the model simulation of CH3OOH and NOx : CH3OOH. Developing better constraints on SCI and RO2 + OH chemistry is a high priority for future work. The model neither captures the large diurnal amplitude in HCOOH seen in surface air, nor its inverted vertical gradient at night. This implies a substantial bias in our current representation of deposition as modulated by boundary layer dynamics, and may indicate an HCOOH sink underestimate and thus an even larger missing source. A more robust treatment of surface deposition is a key need for improving simulations of HCOOH and related trace gases, and our understanding of their budgets.

Tropospheric sources and sinks of gas-phase acids in the Colorado Front Range

“…Despite their ubiquity, our understanding of tropospheric organic acid sources and sinks is incomplete. This is especially apparent for formic acid -measured tropospheric concentrations are often several times higher than modeled values (Paulot et al, 2011;Yuan et al, 2015;Millet et al, 2015;Schobesberger et al, 2016). Model simulations have also failed to capture the temporal variation and vertical gradients of formic acid .…”

“…Formic acid is produced during ozonolysis of ethene and propene (Atkinson et al, 2006;Millet et al, 2015), both of which have known combustion sources (Gilman et al, 2013), and during OH oxidation of diesel emissions (Friedman et al, 2017). ONG wells were dominantly to the east of the site (Fig.…”

“…These data point to a photochemical source of alkanoic acids, consistent with known reaction mechanisms. For example, ozonolysis of alkenes and photooxidation of isoprene are photochemical sources of formic acid in the troposphere Jacob and Wofsy, 1988;Paulot et al, 2009Paulot et al, , 2011Millet et al, 2015). Alkanoic acids are also produced during photooxidation of diesel exhaust (Friedman et al, 2017).…”

“…Sources and sinks determine tropospheric concentrations of gas-phase organic acids, and thus their impacts on biological health and air quality. However, several model-measurement comparisons for tropospheric formic and acetic acid indicate missing sources, potentially coupled to missing sinks (Paulot et al, 2011;Yuan et al, 2015;Millet et al, 2015;Schobesberger et al, 2016). Model-measurement comparisons for other tropospheric organic acids are lacking.…”

Estimating the contribution of organic acids to northern hemispheric continental organic aerosol

“…Oxidation of VOCs is thought to be a key source of organic acids with recent studies providing new insight into their formation pathways and properties [Paulot et al, 2011;Sommariva et al, 2011;Andrews et al, 2012;Millet et al, 2015;Yuan et al, 2015]. Recent studies indicate organic acids can be a large fraction of SOA at locations highly influenced by biogenic emissions [Vogel et al, 2013], and it has been hypothesized that their fractional contribution to OA increases with increasing photochemical age [Heald et al, 2010;Levin et al, 2014].…”