DOI: 10.1002/cphc.201000434 Uptake of Formic Acid on Thin Ice Films and on Ice Doped with Nitric Acid between 195 and 211 K Manolis N. Romanias, Antonia G. Zogka, Vassileios G. Stefanopoulos, Vassileios C. Papadimitriou, and Panos Papagiannakopoulos* [a] 1. Introduction The heterogeneous chemistry of halogenated and nitrated trace gases on stratospheric ice particles has been an impor- tant scientific issue in the last decade, due to its significance to stratospheric ozone depletion. [1] Several experimental studies have shown that these molecules interact strongly with pure or doped (HNO 3 ,H 2 SO 4 ) ice particles and therefore they may initiate new heterogeneous processes in polar stratospheric clouds. [2] However, less attention has been given to the hetero- geneous interaction of hydrocarbons, including oxygenated volatile organic compounds (OVOCs), on ice surfaces, although such species are in considerable mixing ratios in the upper tro- posphere (UT) and may play an important role in tropospheric chemistry. [1] Carboxylic acids are among the most abundant OVOCs with mixing ratios up to 792 ppt, which are generated via several homogeneous and heterogeneous photochemical processes or directly emitted by vegetation and biomass burn- ing. [3] They are considered as potential sources of HO x radicals and therefore they may also participate in the photochemical cycle of tropospheric ozone. Furthermore, carboxylic acids are polar, weak and quite soluble organic acids, which are formed in the atmospheric and aqueous environment via oxidation of hydrocarbons. [4] Moreover, nitric acid traces have been detected in the mid and upper troposphere with mixing ratios of 0.1–2 ppb, [5] and in the lower stratosphere (LS) as crystalline nitric acid tri- hydrate (NAT). [6] In addition, condensed-phase HNO 3 has been observed in cirrus clouds in the UT at levels higher than 0.8 ppbv, and the mean percentage of total HNO 3 condensed on cirrus cloud particles was 16 %. [7] Although the partitioning of nitric acid gas in ice cloud particles is not well understood, it appears that nitric acid molecules are trapped during the ice particle growth in the UT/LS region, which results in ice parti- cles doped with HNO 3 or NAT particles. [6, 7] It is therefore impor- tant to investigate the adsorption of carboxylic acids on ice films doped with HNO 3 . In the literature, there are only a few experimental studies that have investigated the interaction of small oxygenated gas- eous molecules with pure ice films or with ice films doped with HNO 3 at low temperatures. [1,8–11] In particular, the hetero- geneous interaction of formic acid with pure ice surfaces has been studied by Hessberg et al. [12] in the temperature range 187–221 K, and more recently by Symington et al. [13] in the temperature range 208–228 K, both using the coated-wall lam- inar flow tube (CWLFT) technique. However, there are no ex- perimental data for the adsorption of formic acid on ice surfa- ces doped with HNO 3 , which are encountered in the UT/LS region of the atmosphere. There are also no experimental data for the initial uptake coefficients g of formic acid on pure ice films or on ice films doped with HNO 3 . [a] Dr. M. N. Romanias, A. G. Zogka, Dr. V. G. Stefanopoulos, Dr. V. C. Papadimitriou, Prof. Dr. P. Papagiannakopoulos Laboratory of Photochemistry and Kinetics Department of Chemistry, University of Crete 71003 Heraklion, Crete (Greece) Fax: (+ 30) 2810-545-001 E-mail: panosp@chemistry.uoc.gr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000434. The adsorption of formic acid on thin ice films and on ice doped with nitric acid (1.96, 7.69 and 53.8 wt %) is studied as a function of temperature T = 195–211 K and gas concentration (0.33–10.6)  10 11 molecule cm 3 . Experiments are performed in a Knudsen flow reactor coupled with a quadrupole mass spec- trometer. The initial uptake coefficients g are strongly and in- versely dependent on the ice temperature. Initial uptake is de- termined at low surface coverages and ranges from (0.65–3.78)  10 3 . The adsorption uptake of formic acid on pure ice films and on ice lightly doped with HNO 3 is a reversi- ble process, and the adsorption isotherms exhibit Langmuir behaviour. N max (1) is (2.94  0.67)  10 14 molecule cm 2 , in good agreement with previous measurements. The temperature de- pendence of K Lin is very well represented by the expression: K Lin (1) = (1.43  0.32)  10 8 exp[(4720520)/T] cm 3 molecule 1 ; the quoted uncertainty is at the 95 % level of confidence and includes systematic uncertainties. Formic acid uptakes on ice films highly doped with HNO 3 (53.8 wt %) are two orders of magnitude higher than those measured on pure ice films and irreversible, thus indicating the formation of a supercooled liquid layer on the ice films upon which dissolution of formic acid occurs. Finally, the atmospheric lifetime of formic acid due to heterogeneous loss on cirrus cloud ice particles and the re- moval of formic acid by adsorption are estimated under condi- tions related to the upper troposphere. 4042  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 4042 – 4052