1. Introduction The understanding of the uptake and incorporation of atmospheric trace gases in water ice as well as their interactions with water molecules is very important for the understanding of processes at the air/ice interface. Trace gases trapped in snow, in ice particles or at ice surfaces may be subject of photochemical reactions when irradiated with solar UV radiation. In the study reported here, we focused on glyoxal (HCOCHO), its interaction with H 2 O molecules and ice, and its photochemistry when trapped in an ice or argon matrix. Glyoxal, an atmospheric relevant carbonyl compound is the simplest  -dicarbonyl and is formed as product in the photooxidation of simple volatile organic compounds in air in the presence of NOx. Few measurements of glyoxal have been reported with mixing ratios ranging from 100 ppt to a few ppb. Glyoxal exhibits an absorption spectrum consisting of two main absorption bands: a broad UV band between 220 and 350 nm and a stronger structured band in the range 350-480 nm.Theoretically, the photolysis of glyoxal can occur through the following channels: HCOCHO + h  2 HCO  H° 298 = 68.5 kcal/mol, threshold < 417 nm HCOCHO + h  H 2 + 2 CO  H° 298 = -2.1 kcal/mol;all HCOCHO + h  H 2 CO + CO  H° 298 = -1.7 kcal/mol;all HCOCHO + h  H + CO + HCO  H° 298 = -85.4 kcal/mol; threshold < 334 nm Theoretical results Adsorption of glyoxal molecules on atmospheric water ice nanoparticles O. Schrems 1, S. K. Ignatov 1,2, and O. B. Gadzhiev 1,2 (1)Alfred Wegener Institute for Polar and Marine Research in der Helmholtz-Gemeinschaft, Bremerhaven (Germany) (2)Department of Chemistry, N.I. Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod (Russia) Experimental American Geophysical Union, General Assembly. 10-20 December 2012, San Francisco (USA) References Tadic, J., Moortgat, G. K., Wirtz, K.(2006). Photolysis of glyoxal in air, J. Photochem. Photobiol. A: Chem. 177, 116-124. Horowitz, A., Meller, R., Moortgat, G. K. (2001). The UV-VIS absorption cross sections of the a-dicarbonyl compound: Pyruvic acid, biacetyl and glyoxal, J. Photochem. Photobiol. A: Chem. 146, 19-27. Diem, M., MaDonald, B.G., Lee, E.K.C. (1981). Photolysis and Laser-Excited Fluroescence and Phosphorescence Emission of trans-Glyoxal in Argon Matrix at 13 K, J. Phys. Chem. 85, 2227-2232 Shapiro, E.L., Szprengiel, Sareen, N., Jen, C.N., Giordano, M.R., McNeill, V.F. (2009). Light-absorbing secondary organic material formed by glyoxal in aqueous aerosol mimics, Atmos. Chem. Phys. 9,2289-2300. Larsen, R. W., Hegelund, F., Ceponkus, J., Nelander, B. (2002). A High-Resolution FT-IR Study of the Fundamental Bands, , , and  of trans-Glyoxal, J. Mol. Spectrosc. 211,127-134 Manohar. S. N., (2008). FTIR Spectroscopic Studies of Atmospheric Molecules in Ice and on Ice Surfaces, Master Thesis, University of Bremen Glyoxal was prepared by decomposing its trimer hydrate at 150-160^C in the presence of P 2 O 5 and under a N 2 flow. It was collected as yellow crystals at -20°C. and stored at -20°C. In laboratory experiments we have simulated the UV photochemistry of glyoxal (HCOCHO) trapped in ice (at 14 K) and for comparison in the gas phase and in solid argon matrices. The low temperatures have been obtained by means of a closed-cycle helium refrigerator. The photoproducts formed in the ice have been identified by means of FTIR spectroscopy. The photolysis experiments were carried out with a Xe/Hg deep UV lamp (Ushio UXM -502 MD) which is operated by a 1000W arc lamp power supply (ORIEL).The light from the arc- lamp is collected by a mirror and leaves the lamp housing through a condenser with a diaphragm. A water filter is installed in front of the condenser to block the IR-light. Quantum chemical calculations Adsorption of glyoxal and its possible photolytic products on water ice nanoparticles was modelled using the water clusters (H 2 O) 48, (H 2 O) 72, (H 2 O) 216, and (H 2 O) 270. Among them the largest one is the cluster corresponding the minimal nanoparticle size (n=275) to be crystalline as observed experimentally. The clusters up to (H 2 O) 72 were studied at the B3LYP/6-31++G(d,p) and B3LYP/6-311++G(2d,2p) levels. The larger clusters were studied using DFTBA and DFTB+ methods. Optimized structures, adsorption energies and vibrational frequencies were obtained in the calculations. Experimental Results Figure 3 below shows a section of the recorded FTIR spectra (difference spectra) of glyoxal trapped in ice at 14K and the major photoproducts which are growing with incrasing photolsyis time. For glyoxal CO, CO 2 and HCHO were the main photoproducts observed during the photodissociation studies at 14 K in in H 2 O-ice and also in a solid argon matrix. 160017001800190020002100220023002400 Wavenumber cm -1 0.00 0.05 0.10 0.15 0.20 Absorbance Difference spectra: before /after UV photolyis 65 min CO 2 CO HCOCHO 45 min 10 min 5 min Table 1: Calculated energies and thermodynamic parameters of adsorption of glyoxal and its photolytic products at the (0001) plane of (H 2 O) 72 nanoparticle. B3LYP/6-31++G(d,p) and B3LYP/6-311++G(2d,2p) (in parentheses) calculation results. Figure 1: Adsorption complexes of glyoxal and its UV photolysis products on the ice Ih nanoparticles. BLYP/6-31++G(d,p) optimized structures.. HCHO Glyoxal HCHO CO CO 2 HCHO Glyoxal + Glyoxal + hv (350 nm) 16.0 А Ice Ih nanoparticle 10.5 А Figure 2: Ice nanoparticle (H 2 O) 270 with 12 glyoxal molecules adsorbed at different planes and sites. DFTB+ optimized structure. Left panel – top view; right panel – side view. Calculated average energy of adsorption among 12 sites: –32.7 kJ/mol. Acknowledgements: SKI and OBG are thankful to AWI for the fellowship support. The work was supported by the Eastpartnership program of DAAD and the Russian foundation for basic research (project No. 11-03-00085). Figure 4: Difference spectra (before and after UV photolysis) of glyoxal isolated in a solid argon matrix at 14K. Figure 3: FTIR difference spectra of glyoxal (HCOCHO) in H 2 O ice at 14K (before/after photolyis)