Formation and Stability of Bulk Carbonic Acid (H2CO3) by Protonation of Tropospheric Calcite

Organic acids play an important role in the acidification of our atmosphere. These weak acids can contribute up to 60% of the free airborne acidity. By far the most abundant organic acids are the C1 and C2 monocarboxylic acids, formic acid (HCOOH) and acetic acid (CH3COOH), which show mixing ratios in the gas phase ranging up to 20 ppb over land and down to 0.2 ppb in the remote oceanic boundary layer or troposphere. These acids are partitioned between the gas phase and the particulate phase, where roughly one half to two thirds can be found in particulate matter (PM2.5). The most important removal mechanism is dry deposition, which accounts for more than 90% of the total organic acid deposition budget. The remaining fraction is removed by rain as particulate-phase acids, whereas removal by chemical reactions is negligible. 5] In addition to the two most important organic acids, C3–C10 aliphatic monocarboxylic acids and C2–C11 aliphatic dicarboxylic acids as well as aromatic carboxylic acids have also been observed in air. The water-soluble fraction of organic carbon can on average consist of 35% monoand dicarboxylic acids. While the C2-dicarboxylic acid, oxalic acid (COOH)2, is commonly observed in all field studies, the C1-dicarboxylic acid, carbonic acid (H2CO3), has barely received any attention, mainly because it is thought that it immediately decomposes to water and carbon dioxide. However, it has previously been shown that gaseous, water-free carbonic acid is surprisingly stable, that amorphous and crystalline solids of pure carbonic acid can be produced and stored without decomposition at temperatures up to 230 K even in the presence of water and that this solid can be sublimed at, for example, 220 K and recondensed in vacuo at surfaces of lower temperature. Formation of bulk carbonic acid in the laboratory has so far been achieved 1) by high-energy irradiation of CO2 or CO2/H2O mixtures, 2) through surface reactions of CO molecules with hydroxyl (OH) radicals at 10–40 K and 3) by protonation of aqueous or methanolic solutions of KHCO3 or K2CO3 at low temperatures of ~140–180 K. Mechanism (1) is believed to be of astrophysical relevance, and carbonic acid is hence supposed to be present, for example, on comets, the Galilean satellites, on Venus and on the Martian surface. For mechanisms (2) and (3) no relevance in nature, and in particular in the Earth’s atmosphere, is envisioned because in the case of (2) temperatures are too low and in case of (3) aqueous/ methanolic solutions of potassium(bi)carbonate are not present in the atmosphere at T<180 K. A possible mechanism (4) of carbonic acid formation in nature was outlined by Grassian et al. , who studied the interaction of gas-phase acids such as formic acid, acetic acid, sulphur dioxide and nitric acid with calcium carbonate particles (CaCO3) at ambient temperature. Under dry conditions of <1% relative humidity these acids react with the Ca(OH)(CO3H) surface layer of calcium carbonate and produce surface-adsorbed carbonic acid, which is stable even at 296 K under dry vacuum conditions, but decomposes at higher relative humidities. According to this mechanism, carbonic acid is an important, but short-lived intermediate in the surface chemistry of calcium carbonate even at room temperature. Here, we propose a novel mechanism (5), which produces bulk carbonic acid under conditions relevant to our atmosphere. This mechanism involves protonation of mineral dust particles, such as CaCO3, in the troposphere by acids, for example droplets containing HCl, at 200 K. Using FT-IR spectroscopy we demonstrate that high-surface-area CaCO3 particles are consumed on a time scale of hours at 200 K in a humid atmosphere, while a non-volatile component, possibly Ca(HCO3)2, and aqueous/amorphous H2CO3 (that crystallizes at 220 K to the b-polymorph of carbonic acid) are produced. By contrast, as shown by Santschi et al. , reaction of CaCO3 with HCl at 300 K involves Ca(OH)(HCO3) and presumably CaCl2·2H2O, but not carbonic acid and not Ca(HCO3)2. Furthermore, we show that H2CO3 does not decompose readily even at 250 K in a humid atmosphere (0.6–0.8 mbar water vapour pressure, 60–100% relative humidity, 100–450 mbar total pressure). At higher temperatures H2CO3 sublimes and/or decomposes. We therefore suggest that some of the CaCO3 and also the MgCO3 fraction of mineral dust, such as Saharan or Asian dust, is converted in the midand high troposphere at 200–250 K to aqueous/amorphous H2CO3 and may even crystallize and exist as H2CO3, more precisely as the b-polymorph. [22–25] Because of the long-term stability of H2CO3 at conditions relevant to the troposphere we suggest that carbonic acid contributes to the acidity in the troposphere. Figure 1 depicts the IR spectrum of the CaCO3 powder in the optical window of the range of 4000–500 cm . The spectrum clearly shows calcite bands, in particular n4(E’) at 713 cm , n2(A2’’) at 877 cm 1 and the broad and strong n3(E’) [a] J. Bernard, M. Seidl, Prof. Dr. T. Loerting Institute of Physical Chemistry University of Innsbruck Innrain 52a, 6020 Innsbruck (Austria) Fax: (+43)5125072925 E-mail : [email protected] Homepage: http://homepage.uibk.ac.at/~c724117/ [b] Prof. Dr. E. Mayer Institute of General, Inorganic and Theoretical Chemistry University of Innsbruck (Austria) Re-use of this article is permitted in accordance with the Terms and Conditions set out at http://onlinelibrary.wiley.com/journal/ 10.1002/(ISSN)1439-7641/homepage/2267_onlineopen.html.