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| Titel |
A large and ubiquitous source of atmospheric formic acid |
| VerfasserIn |
D. B. Millet, M. Baasandorj, D. K. Farmer, J. A. Thornton, K. Baumann, P. Brophy, S. Chaliyakunnel, J. A. de Gouw, M. Graus, L. Hu, A. Koss, B. H. Lee, F. D. Lopez-Hilfiker, J. A. Neuman, F. Paulot, J. Peischl, I. B. Pollack, T. B. Ryerson, C. Warneke, B. J. Williams, J. Xu |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 11 ; Nr. 15, no. 11 (2015-06-09), S.6283-6304 |
| Datensatznummer |
250119798
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| Publikation (Nr.) |
 copernicus.org/acp-15-6283-2015.pdf |
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| Zusammenfassung |
| 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. |
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