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.
Highlights
Formic acid (HCOOH) is, along with acetic acid (CH3COOH), the dominant carboxylic acid in the troposphere
Millet et al.: Sources and sinks of atmospheric formic acid whether this is the case, we carried out a sensitivity analysis with the HCOOH deposition velocity for each time and model location set to the corresponding value computed for HNO3
The strong temporal decline (Fig. 9) and vertical gradient (Fig. 8) of HCOOH at night measured during Southern Oxidant and Aerosol Study (SOAS) and SENEX are not captured by the model, perhaps indicating an underestimate of the HCOOH deposition velocity
Summary
Formic acid (HCOOH) is, along with acetic acid (CH3COOH), the dominant carboxylic acid in the troposphere. Recent advances in remote sensing (Cady-Pereira et al, 2014; Stavrakou et al, 2012; Zander et al, 2010) and in situ (Baasandorj et al, 2015; Le Breton et al, 2012; Liu et al, 2012; Veres et al, 2011; Yuan et al, 2015) measurement capabilities have led to the realization that atmospheric HCOOH concentrations are much too high to be consistent with present estimates of the source and sink magnitudes This points to a key gap in present understanding of the atmospheric reactive carbon budget. Millet et al.: Sources and sinks of atmospheric formic acid budget, and (ii) there is an undefined and widespread chemical source of HCOOH from a range of different precursor species
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