Uptake of Formic Acid on Thin Ice Films and on Ice Doped with Nitric Acid between 195 and 211 K

• Published in: Chemphyschem 2010-12, Vol.11 (18), p.4042-4052
• Main Authors: Romanias, Manolis N., Zogka, Antonia G., Stefanopoulos, Vassileios G., Papadimitriou, Vassileios C., Papagiannakopoulos, Panos
• Format: Article
• Language: English
• Subjects: adsorption; atmospheric chemistry; Chemical composition and interactions. Ionic interactions and processes; Earth, ocean, space; Exact sciences and technology; External geophysics; formic acid; ice; Meteorology; thin films
• ISSN: 1439-4235; 1439-7641
• DOI: 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)×1011 molecule cm−3. Experiments are performed in a Knudsen flow reactor coupled with a quadrupole mass spectrometer. The initial uptake coefficients γ are strongly and inversely dependent on the ice temperature. Initial uptake is determined 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 HNO3 is a reversible process, and the adsorption isotherms exhibit Langmuir behaviour. Nmax(1) is (2.94±0.67)×1014 molecule cm−2, in good agreement with previous measurements. The temperature dependence of KLin is very well represented by the expression: KLin(1)=(1.43±0.32)×10−8 exp[(4720±520)/T] cm3 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 HNO3 (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 removal of formic acid by adsorption are estimated under conditions related to the upper troposphere. The adsorption of formic acid on pure ice films and those lightly doped with HNO3 is reversible and depends on the temperature. The atmospheric impact of formic acid uptake by cirrus clouds in the upper troposphere/lower stratosphere region is examined (see picture).