Importance of secondary sources in the atmospheric budgets of formic and acetic acids

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 ~1400 Gmol yr<sup>−1</sup>, 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

Five years of CO, HCN, C<sub>2</sub>H<sub>6</sub>, C<sub>2</sub>H<sub>2</sub>, CH<sub>3</sub>OH, HCOOH and H<sub>2</sub>CO total columns measured in the Canadian high Arctic

“…Concerning HCOOH, several studies have highlighted the underestimation of emissions in the models by a factor of nine in the marine boundary layer (Baboukas et al, 2000) and an order of magnitude for the free troposphere (Von Kuhlmann et al, 2003). Recently, Paulot et al (2011) investigated an underestimation in the model by a factor of two to five compared to polar FTIR measurements at Thule (Greenland; 76 • N, 69 • W), confirming the missing local sources in the HCOOH budget simulated in the model. Finally, HCOOH and H 2 CO spectroscopic parameters have been refined recently (HITRAN 2008, http: //www.cfa.harvard.edu/hitran/updates.html, Rothman et al, 2009.…”

“…HCOOH is removed through oxidation by OH as well as by dry and wet depositions . However, a recent study suggested a missing source in the HCOOH budget of the northern latitudes and inferred its lifetime to be about three to four days (Paulot et al, 2011).…”

“…For HCOOH and H 2 CO, the a priori VMRs in the boundary layer are about 8 and 30 pptv and then decrease to 1 and 10 pptv around 30 km for HCOOH and H 2 CO, respectively. However, the sinks and sources are not as well understood (Paulot et al, 2011;Jones et al, 2009), especially in the polar region, so the interpretation of the a priori profiles has to be done carefully.…”

“…For HCOOH, we use one MW between 1104.650 and 1105.600 cm −1 , which corresponds to the band used in Zander et al (2010), Paulot et al (2011) and . The spectroscopic parameters of this band have been updated (Perrin and Vander Auwera, 2007).…”

“…CH 3 OH and HCOOH have been measured in a limited number of ground-based FTIR studies (Paton-Walsh et al, 2005Rinsland et al, , 2009Zander et al, 2010;Paulot et al, 2011). The first FTIR data sets of H 2 CO were obtained in remote areas (Mahieu et al, 1997;Notholt et al, 1997a, b) and were then extended to urban areas (Hak et al, 2005) and the Southern Hemisphere (Jones et al, 2009;Vigouroux et al, 2009;Paton-Walsh et al, 2010).…”