Abstract
We present a detailed budget of formic and acetic acids, two of the most abundant trace gases in the atmosphere. Our bottom-up estimate of the global source of formic and acetic acids are ∼1200 and ∼1400Gmolyr−1, dominated by photochemical oxidation of biogenic volatile organic compounds, in particular isoprene. Their sinks are dominated by wet and dry deposition. We use the GEOS-Chem chemical transport model to evaluate this budget against an extensive suite of measurements from ground, ship and satellite-based Fourier transform spectrometers, as well as from several aircraft campaigns over North America. The model captures the seasonality of formic and acetic acids well but generally underestimates their concentration, particularly in the Northern midlatitudes. We infer that the source of both carboxylic acids may be up to 50% greater than our estimate and report evidence for a long-lived missing secondary source of carboxylic acids that may be associated with the aging of organic aerosols. Vertical profiles of formic acid in the upper troposphere support a negative temperature dependence of the reaction between formic acid and the hydroxyl radical as suggested by several theoretical studies.
Highlights
Formic (HCOOH, hereafter FA) and acetic (CH3COOH, hereafter a FA (AA)) acids are among the most abundant and ubiquitous trace gases in the atmosphere
In Wollongong, the anomalously high FA total column measured in December 2006, the largest on the record, is associated with biomass burning, since (a) it clearly precedes the biogenic peak in FA observed in other years, (b) very large bush fires burned over 1 million acres from December 2006 to January 2007 in the Victorian Alps ∼450 km southwest of Wollongong, and (c) anomalously high CO and HCN columns are recorded during the same time period (Fig. S5)
Since our goal is to examine the sensitivity of free tropospheric FA to biomass burning injection height, modifications of the injection height are applied globally, an important simplification (Val Martin et al, 2010, and references therein)
Summary
Formic (HCOOH, hereafter FA) and acetic (CH3COOH, hereafter AA) acids are among the most abundant and ubiquitous trace gases in the atmosphere They have been detected in remote, rural, polar, marine and urban environments in the gas-phase as well as in clouds and in aerosols (Keene and Galloway, 1988; Chebbi and Carlier, 1996; Khare et al, 1999). Sinks of FA and AA are better understood Both acids are relatively long-lived in the gas-phase with respect to OH photooxidation (τFA 25 days and τAA 10 days at T = 260 K and [OH]=106 molec cm−3). Because both gases are very soluble, their primary atmospheric sink is thought to be deposition (Chebbi and Carlier, 1996). Major discrepancies between the model and the measurements are investigated and several avenues for further research are discussed
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